Physiological Psychology

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Physiological Psychology
Physiological Psychology
B.Sc. In Counselling Psychology: A Job Oriented Course
2011 Admission onwards
III Semester
School of Distance Education
B.Sc. In Counselling Psychology: A Job Oriented Course
III Semester
Prepared by:
Department of Psychology,
University of Calicut
Layout & Settings
Computer Section, SDE
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Auditory system
Anatomy of the human ear.
The auditory system is the sensory system for the sense of hearing.
Outer ear
The folds of cartilage surrounding the ear canal are called the pinna. Sound waves
are reflected and attenuated when they hit the pinna, and these changes provide
additional information that will help the brain determine the direction from which the
sounds came.
The sound waves enter the auditory canal, a deceptively simple tube. The ear
canal amplifies sounds that are between 3 and 12 kHz. At the far end of the ear canal is
the eardrum (or tympanic membrane), which marks the beginning of the middle ear.
Middle ear
Sound waves traveling through the ear canal will hit the tympanic membrane, or
eardrum. This wave information travels across the air-filled middle ear cavity via a series
of delicate bones: the malleus (hammer), incus (anvil) and stapes (stirrup). These ossicles
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act as a lever and a teletype, converting the lower-pressure eardrum sound vibrations
into higher-pressure sound vibrations at another, smaller membrane called the oval (or
elliptical) window. The malleus articulates with the tympanic membrane via the
manubrium, where the stapes articulates with the oval window via its footplate. Higher
pressure is necessary because the inner ear beyond the oval window contains liquid
rather than air. The sound is not amplified uniformly across the ossicular chain. The
stapedius reflex of the middle ear muscles helps protect the inner ear from damage. The
middle ear still contains the sound information in wave form; it is converted to nerve
impulses in the cochlea.
Inner ear
The inner ear consists of the cochlea and several non-auditory structures. The
cochlea has three fluid-filled sections, and supports a fluid wave driven by pressure
across the basilar membrane separating two of the sections. Strikingly, one section, called
the cochlear duct or scala media, contains an extracellular fluid similar in composition to
endolymph, which is usually found inside of cells. The organ of Corti is located at this
duct, and transforms mechanical waves to electric signals in neurons. The other two
sections are known as the scala tympani and the scala vestibuli; these are located within
the bony labyrinth which is filled with fluid called perilymph. The chemical difference
between the two fluids (endolymph & perilymph) is important for the function of the
inner ear.
Organ of Corti
The organ of Corti located at the scala media.
The organ of Corti forms a ribbon of sensory epithelium which runs lengthwise
down the entire cochlea. The hair cells of the organ of Corti transform the fluid waves
into nerve signals. The journey of a billion nerves begins with this first step; from here
further processing leads to a panoply of auditory reactions and sensations.
Hair cell
Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at the
top, for which they are named. These cilia are the mechanosensors for hearing. Lightly
resting atop the longest cilia is the tectorial membrane, which moves back and forth with
each cycle of sound, tilting the cilia and allowing electric current into the hair cell.
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Hair cells, like the photoreceptors of the eye, show a graded response, instead of
the spikes typical of other neurons. These graded potentials are not bound by the “all or
none” properties of an action potential.
At this point, one may ask how such a wiggle of a hair bundle triggers a
difference in membrane potential. The current model is that cilia are attached to one
another by “tip links”, structures which link the tips of one cilium to another. Stretching
and compressing, the tip links may open an ion channel and produce the receptor
potential in the hair cell. Recently it has been shown that cdh23 and pchh15 are the
adhesion molecules associated with these tip links. It is thought that a calcium driven
motor causes a shortening of these links to regenerate tensions. This regeneration of
tension allows for apprehension of prolonged auditory stimulation.
Afferent neurons innervate cochlear inner hair cells, at synapses where the
neurotransmitter glutamate communicates signals from the hair cells to the dendrites of
the primary auditory neurons.
There are far fewer inner hair cells in the cochlea than afferent nerve fibers. The
neural dendrites belong to neurons of the auditory nerve, which in turn joins the
vestibular nerve to form the vestibulocochlear nerve, or cranial nerve number VIII.
Efferent projections from the brain to the cochlea also play a role in the perception
of sound. Efferent synapses occur on outer hair cells and on afferent (towards the brain)
dendrites under inner hair cells.
Central auditory system
This sound information, now re-encoded, travels down the vestibulocochlear
nerve, through intermediate stations such as the cochlear nuclei and superior olivary
complex of the brainstem and the inferior colliculus of the midbrain, being further
processed at each waypoint. The information eventually reaches the thalamus, and from
there it is relayed to the cortex. In the human brain, the primary auditory cortex is
located in the temporal lobe
Associated anatomical structures include:
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Cochlear nucleus
The cochlear nucleus is the first site of the neuronal processing of the newly
converted “digital” data from the inner ear. This region is anatomically and
physiologically split into two regions, the dorsal cochlear nucleus (DCN), and ventral
cochlear nucleus (VCN).
Trapezoid body
The Trapezoid body is a bundle of decussating fibers in the ventral pons that
carry information used for binaural computations in the brainstem.
Superior olivary complex
The superior olivary complex is located in the pons, and receives projections
predominantly from the ventral cochlear nucleus, although the posterior cochlear
nucleus projects there as well, via the ventral acoustic stria. Within the superior olivary
complex lies the lateral superior olive (LSO) and the medial superior olive (MSO). The
former is important in detecting interaural level differences while the latter is important
in distinguishing interaural time difference.
Lateral lemniscus
Main article: lateral lemniscus
The lateral lemniscus is a tract of axons in the brainstem that carries information
about sound from the cochlear nucleus to various brainstem nuclei and ultimately the
contralateral inferior colliculus of the midbrain.
Lateral lemniscus in red, as it connects the cochlear nucleus, superior olivary nucleus and the
inferior colliculus. Seen from behind.
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Inferior colliculi
The IC are located just below the visual processing centers known as the superior
colliculi. The central nucleus of the IC is a nearly obligatory relay in the ascending
auditory system, and most likely acts to integrate information (specifically regarding
sound source localization from the superior olivary complex and dorsal cochlear
nucleus) before sending it to the thalamus and cortex
Medial geniculate nucleus
The medial geniculate nucleus is part of the thalamic relay system.
Primary auditory cortex
The primary auditory cortex is the first region of cerebral cortex to receive
auditory input.
Perception of sound is associated with the right posterior superior temporal gyrus
(STG). The superior temporal gyrus contains several important structures of the brain,
including Brodmann areas 41 and 42, marking the location of the primary auditory
cortex, the cortical region responsible for the sensation of basic characteristics of sound
such as pitch and rhythm.
The auditory association area is located within the temporal lobe of the brain, in
an area called the Wernicke's area, or area 22. This area, near the lateral cerebral sulcus,
is an important region for the processing of acoustic signals so that they can be
distinguished as speech, music, or noise.
Auditory localization
Auditory localization or Sound localization is a listener's ability to identify the
location or origin of a detected sound.
There are two general methods for sound localization, binaural cues and
monaural cues.
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Binaural cues
Binaural localization relies on the comparison of auditory input from two separate
detectors. Therefore, most auditory systems feature two ears, one on each side of the head.
The primary biological binaural cue is the split-second delay between the time when sound
from a single source reaches the near ear and when it reaches the far ear. This is often
technically referred to as the "interaural time difference" (ITD). ITDmax = 0.63 ms. Another
binaural cue, less significant in ground dwelling animals, is the reduction in loudness when
the sound reaches the far ear, or the "interaural amplitude difference" (IAD) or (ILD) as
"interaural level difference". This is also referred to as the frequency dependent "interaural
level difference" (ILD) (or "interaural intensity difference" (IID)). Our eardrums are only
sensitive to the sound pressure level differences.
Note that these cues will only aid in localizing the sound source's azimuth (the angle
between the source and the sagittal plane), not its elevation (the angle between the source and
the horizontal plane through both ears), unless the two detectors are positioned at different
heights in addition to being separated in the horizontal plane. In animals, however, rough
elevation information is gained simply by tilting the head, provided that the sound lasts long
enough to complete the movement. This explains the innate behavior of cocking the head to
one side when trying to localize a sound precisely. To get instantaneous localization in more
than two dimensions from time-difference or amplitude-difference cues requires more than
two detectors. However, many animals have quite complex variations in the degree of
attenuation of a sound receives in travelling from the source to the eardrum: there are
variations in the frequency-dependent attenuation with both azimuthal angle and elevation.
These can be summarised in the head-related transfer function, or HRTF. As a result, where
the sound is wideband (that is, has its energy spread over the audible spectrum), it is possible
for an animal to estimate both angle and elevation simultaneously without tilting its head. Of
course, additional information can be found by moving the head, so that the HRTF for both
ears changes in a way known (implicitly!) by the animal.
In vertebrates, inter-aural time differences are known to be calculated in the superior
olivary nucleus of the brainstem. According to Jeffress, this calculation relies on delay lines:
neurons in the superior olive which accept innervation from each ear with different
connecting axon lengths. Some cells are more directly connected to one ear than the other,
thus they are specific for a particular inter-aural time difference. This theory is equivalent to
the mathematical procedure of cross-correlation. However, because Jeffress' theory is unable
to account for the precedence effect, in which only the first of multiple identical sounds is used
to determine the sounds' location (thus avoiding confusion caused by echoes), it cannot be
entirely correct, as pointed out by Gaskell.
The tiny parasitic fly Ormia ochracea has become a model organism in sound
localization experiments because of its unique ear. The animal is too small for the time
difference of sound arriving at the two ears to be calculated in the usual way, yet it can
determine the direction of sound sources with exquisite precision. The tympanic membranes
of opposite ears are directly connected mechanically, allowing resolution of nanosecond time
differences and requiring a new neural coding strategy. Here showed that the coupledeardrum system in frogs can produce increased interaural vibration disparities when only
small arrival time and intensity differences were available to the animal’s head. Efforts to
build directional microphones based on the coupled-eardrum structure are underway.
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Monaural (filtering) cues
Monaural localization mostly depends on the filtering effects of external
structures. In advanced auditory systems, these external filters include the head,
shoulders, torso, and outer ear or "pinna", and can be summarized as the head-related
transfer function. Sounds are frequency filtered specifically depending on the angle from
which they strike the various external filters. The most significant filtering cue for
biological sound localization is the pinna notch, a notch filtering effect resulting from
destructive interference of waves reflected from the outer ear. The frequency that is
selectively notch filtered depends on the angle from which the sound strikes the outer
ear. Instantaneous localization of sound source elevation in advanced systems primarily
depends on the pinna notch and other head-related filtering. These monaural effects also
provide azimuth information, but it is inferior to that gained from binaural cues.
In order to enhance filtering information, many animals have large, specially
shaped outer ears. Many also have the ability to turn the outer ear at will, which allows
for better sound localization and also better sound detection. Bats and barn owls are
paragons of monaural localization in the animal kingdom, and have thus become model
Processing of head-related transfer functions for biological sound localization
occurs in the auditory cortex.
Distance cues
Neither inter-aural time differences nor monaural filtering information provides
good distance localization. Distance can theoretically be approximated through interaural amplitude differences or by comparing the relative head-related filtering in each
ear: a combination of binaural and filtering information. The most direct cue to distance
is sound amplitude, which decays with increasing distance. However, this is not a
reliable cue, because in general it is not known how strong the sound source is. In case of
familiar sounds, such as speech, there is an implicit knowledge of how strong the sound
source should be, which enables a rough distance judgment to be made.
In general, humans are best at judging sound source azimuth, then elevation, and
worst at judging distance. Source distance is qualitatively obvious to a human observer when
a sound is extremely close (the mosquito in the ear effect), or when sound is echoed by large
structures in the environment (such as walls and ceiling). Such echoes provide reasonable
cues to the distance of a sound source, in particular because the strength of echoes does not
depend on the distance of the source, while the strength of the sound that arrives directly
from the sound source becomes weaker with distance. As a result, the ratio of direct-to-echo
strength alters the quality of the sound in such a way to which humans are sensitive. In this
way consistent, although not very accurate, distance judgments are possible. This method
generally fails outdoors, due to a lack of echoes. Still, there are a number of outdoor
environments that also generate strong, discrete echoes, such as mountains. On the other
hand, distance evaluation outdoors is largely based on the received timbre of sound: short
soundwaves (high-pitched sounds) die out sooner, due to their relatively smaller kinetic
energy, and thus distant sounds appear duller than normal (lacking in treble).
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Coding auditory information
The auditory neural code must serve a wide range of auditory tasks that require
great sensitivity in time and frequency and be effective over the diverse array of sounds
present in natural acoustic environments. It has been suggested that sensory systems
might have evolved highly efficient coding strategies to maximize the information
conveyed to the brain while minimizing the required energy and neural resources. Here
we show that, for natural sounds, the complete acoustic wave form can be represented
efficiently with a nonlinear model based on a population spike code. In this model,
idealized spikes encode the precise temporal positions and magnitudes of underlying
acoustic features. We find that when the features are optimized for coding either natural
sounds or speech, they show striking similarities to time-domain cochlear filter
estimates, have a frequency-bandwidth dependence similar to that of auditory nerve
fibres, and yield significantly greater coding efficiency than conventional signal
representations. These results indicate that the auditory code might approach an
information theoretic optimum and that the acoustic structure of speech might be
adapted to the coding capacity of the mammalian auditory system.
Taste (or, more formally, gustation; adjectival form: "gustatory") is a form of
direct chemoreception and is one of the traditional five senses. It refers to the ability to
detect the flavor of substances such as food, certain minerals, and poisons. In humans
and many other vertebrate animals the sense of taste partners with the less direct sense
of smell, in the brain's perception of flavor. In the West, experts traditionally identified
four taste sensations: sweet, salty, sour, and bitter. In the Eastern hemisphere, piquance
(the sensation provided by, among other things, chili peppers) and savoriness (also
known as umami) have been traditionally identified as basic tastes as well. More
recently, psychophysicists and neuroscientists have suggested other taste categories
(fatty acid taste most prominently, as well as the sensation of metallic and water tastes,
although the latter is commonly disregarded due to the phenomenon of taste
adaptation.[citation needed]) Taste is a sensory function of the central nervous system.
The receptor cells for taste in humans are found on the surface of the tongue, along the
soft palate, and in the epithelium of the pharynx and epiglottis.
Psychophysicists have long suggested the existence of four taste 'primaries',
referred to as the basic tastes: sweetness, bitterness, sourness and saltiness. Although
first described in 1908, savoriness (also called "umami" in Japanese) has been only
recently recognized as the fifth basic taste since the cloning of a specific amino acid taste
receptor in 2002. The savory taste is exemplified by the non-salty sensations evoked by
some free amino acids such as monosodium glutamate.
Other possible categories have been suggested, such as a taste exemplified by
certain fatty acids such as linoleic acid. Some researchers still argue against the notion of
primaries at all and instead favor a continuum of percepts, similar to color vision.
All of these taste sensations arise from all regions of the oral cavity, despite the
common misconception of a "taste map" of sensitivity to different tastes thought to
correspond to specific areas of the tongue. This myth is generally attributed to the
mistranslation of a German text, and perpetuated in North American schools since the
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early twentieth century. Very slight regional differences in sensitivity to compounds
exist, though these regional differences are subtle and do not conform exactly to the
mythical tongue map. Individual taste buds (which contain approximately 100 taste
receptor cells), in fact, typically respond to compounds evoking each of the five basic
The "basic tastes" are those commonly recognized types of taste sensed by
humans. Humans receive tastes through sensory organs called "taste buds" or "gustatory
calyculi", concentrated on the upper surface of the tongue, but a few are also found on
the roof of one's mouth, furthering the taste sensations we can receive. Scientists describe
five basic tastes: bitter, salty, sour, sweet, and savory. The basic tastes are only one
component that contributes to the sensation of food in the mouth—other factors include
the food's smell, detected by the olfactory epithelium of the nose, its texture, detected by
mechanoreceptors, and its temperature, detected by thermoreceptors. Taste and smell
are subsumed under the term "flavor".
Taste bud
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In Western culture, the concept of basic tastes can be traced back at least to
Aristotle, who cited "sweet" and "bitter", with "succulent", "salt", "pungent", "harsh",
"puckery" and "sour" as elaborations of those two basics. The ancient Chinese Five
Elements philosophy lists slightly different five basic tastes: bitter, salty, sour, sweet and
spicy. Ayurveda, the ancient Indian healing science refers astringent as the sixth taste.
Japanese culture also adds its own sixth taste to the basic five.
For many years, books on the physiology of human taste contained diagrams of
the tongue showing levels of sensitivity to different tastes in different regions. In fact,
taste qualities are found in all areas of the tongue, in contrast with the popular view that
different tastes map to different areas of the tongue.
Recent discoveries
The receptors for all known basic tastes have been identified. The receptors for
sour and salty are ion channels while the receptors for sweet, bitter and savory belong to
the class of G protein coupled receptors.
In November 2005, a team of researchers experimenting on rodents claimed to
have evidence for a sixth taste, for fatty substances. It is speculated that humans may
also have the same receptors. Fat has occasionally been raised as a possible basic taste in
the past (Bravo 1592, Linnaeus 1751) but later classifications abandoned fat as a separate
taste (Haller 1751 and 1763).
Basic tastes
For a long period, it was commonly accepted that there is a finite and small
number of "basic tastes" of which all seemingly complex tastes are ultimately composed.
Just as with primary colors, the "basic" quality of those sensations derives chiefly from
the nature of human perception, in this case the different sorts of tastes the human
tongue can identify. Until the 2000s, the number of "basic" tastes was considered to be
four. More recently, a fifth taste, savory, has been proposed by a large number of
authorities associated with this field.
Bitterness is the most sensitive of the tastes, and is perceived by many to be
unpleasant, sharp, or disagreeable. Common bitter foods and beverages include coffee,
unsweetened cocoa, South American mate, marmalade, bitter melon, beer, bitters, olives,
citrus peel, many plants in the Brassicaceae family, dandelion greens, wild chicory,
escarole and lemons. Quinine is also known for its bitter taste and is found in tonic
water. The threshold for stimulation of bitter taste by quinine averages 0.000008 M. The
taste thresholds of other bitter substances are rated relative to quinine, which is given an
index of 1. For example, Brucine has an index of 11, is thus perceived as intensely more
bitter than quinine, and is detected at a much lower solution threshold. The most bitter
substance known is the synthetic chemical denatonium, which has an index of 1,000. It is
used as an aversive agent that is added to toxic substances to prevent accidental
ingestion. This was discovered in 1958 during research on lignocaine, a local anesthetic,
by Macfarlan Smith of Edinburgh, Scotland.
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Research has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs)
such as TAS2R38 coupled to the G protein gustducin are responsible for the human
ability to taste bitter substances. They are identified not only by their ability to taste for
certain "bitter" ligands, but also by the morphology of the receptor itself (surface bound,
monomeric). Researchers use two synthetic substances, phenylthiocarbamide (PTC) and
6-n-propylthiouracil (PROP) to study the genetics of bitter perception. These two
substances taste bitter to some people, but are virtually tasteless to others. Among the
tasters, some are so-called "supertasters" to whom PTC and PROP are extremely bitter.
The variation in sensitivity is determined by two common alleles at the TAS2R38 locus..
This genetic variation in the ability to taste a substance has been a source of great interest
to those who study genetics.
In addition, it is of interest to those who study evolution, as well as various health
researchers since PTC-tasting is associated with the ability to taste numerous natural
bitter compounds, a large number of which are known to be toxic. The ability to detect
bitter-tasting, toxic compounds at low thresholds is considered to provide an important
protective function. Plant leaves often contain toxic compounds, yet even amongst leafeating primates, there is a tendency to prefer immature leaves, which tend to be higher
in protein and lower in fiber and poisons than mature leaves. Amongst humans, various
food processing techniques are used worldwide to detoxify otherwise inedible foods and
make them palatable. Recently it is speculated that the selective constraints on the
TAS2R family have been weakened due to the relatively high rate of mutation and
Saltiness is a taste produced primarily by the presence of sodium ions. Other ions of
the alkali metals group also taste salty, but the further from sodium the less salty the
sensation is. The size of lithium and potassium ions most closely resemble those of sodium
and thus the saltiness is most similar. In contrast rubidium and cesium ions are far larger so
their salty taste differs accordingly. The saltiness of substances is rated relative to sodium
chloride (NaCl), which has an index of 1. Potassium, as potassium chloride - KCl, is the
principal ingredient in salt substitutes, and has a saltiness index of 0.6.
Other monovalent cations, e.g. ammonium, NH4+, and divalent cations of the
alkali earth metal group of the periodic table, e.g. calcium, Ca2+, ions generally elicit a
bitter rather than a salty taste even though they, too, can pass directly through ion
channels in the tongue, generating an action potential.
Sourness is the taste that detects acidity. The sourness of substances is rated
relative to dilute hydrochloric acid, which has a sourness index of 1. By comparison,
tartaric acid has a sourness index of 0.7, citric acid an index of 0.46, and carbonic acid an
index of 0.06. The mechanism for detecting sour taste is similar to that which detects salt
taste. Hydrogen ion channels detect the concentration of hydronium ions that are formed
from acids and water. Additionally, the taste receptor PKD2L1 has been found to be
involved in tasting sourness.
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Hydrogen ions are capable of permeating the amiloride-sensitive channels, but
this is not the only mechanism involved in detecting the quality of sourness. Other
channels have also been proposed in the literature. Hydrogen ions also inhibit the
potassium channel, which normally functions to hyperpolarize the cell. By a combination
of direct intake of hydrogen ions (which itself depolarizes the cell) and the inhibition of
the hyperpolarizing channel, sourness causes the taste cell to fire in this specific manner.
In addition, it has also been suggested that weak acids, such as CO2 which is converted
into the bicarbonate ion by the enzyme carbonic anhydrase, to mediate weak acid
transport. The most common food group that contains naturally sour foods is the fruit,
with examples such as the lemon, grape, orange, and sometimes the melon. Wine also
usually has a sour tinge to its flavor. If not kept correctly, milk can spoil and contain a
sour taste. Sour candy is especially popular in North America[30] including Cry Babies,
Warheads, Lemon drops, Shock tarts and Sour Skittles and Starburst. Many of these
candies contain citric acid.
Sweetness, usually regarded as a pleasurable sensation, is produced by the
presence of sugars, some proteins and a few other substances. Sweetness is often
connected to aldehydes and ketones, which contain a carbonyl group. Sweetness is
detected by a variety of G protein coupled receptors coupled to the G protein gustducin
found on the taste buds. At least two different variants of the "sweetness receptors" need
to be activated for the brain to register sweetness. The compounds which the brain
senses as sweet are thus compounds that can bind with varying bond strength to two
different sweetness receptors. These receptors are T1R2+3 (heterodimer) and T1R3
(homodimer), which are shown to be accountable for all sweet sensing in humans and
animals. Taste detection thresholds for sweet substances are rated relative to sucrose,
which has an index of 1. The average human detection threshold for sucrose is 10
millimoles per litre. For lactose it is 30 millimoles per litre, with a sweetness index of 0.3,
and 5-Nitro-2-propoxyaniline 0.002 millimoles per litre.
Savoriness is the name for the taste sensation produced by amino acids such as
glutamate. The compounds that generate savoriness are commonly found in fermented
and aged foods. It is also described as "meatiness", "relish", or having a "rich" taste.
Savoriness is considered a fundamental taste in Chinese, Japanese, Thai and Korean
cooking, but is not discussed as much in Western cuisine, at least prior to the
introduction of the umami concept in the West.
Humans have taste receptors specifically for the detection of the amino acids, e.g.,
glutamic acid. Amino acids are the building blocks of proteins and are found in meats,
cheese, fish, and other protein-heavy foods. Examples of food containing glutamate (and
thus strong in savoriness) are beef, lamb, parmesan, and roquefort cheese as well as soy
sauce and fish sauce. The glutamate taste sensation is most intense in combination with
sodium ions, as found in table salt. Sauces with savory and salty tastes are very popular
for cooking, such as Worcestershire sauce for Western cuisines and soy sauce and fish
sauce for Oriental (East Asian) cuisines.
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The additive monosodium glutamate (MSG), which was developed as a food
additive in 1907 by Kikunae Ikeda, produces a strong savory taste. Savoriness is also
provided by the nucleotides 5’-inosine monophosphate (IMP) and 5’-guanosine
monophosphate (GMP). These are naturally present in many protein-rich foods. IMP is
present in high concentrations in many foods, including dried skipjack tuna flakes and
kombu used to make "dashi", a Japanese broth. GMP is present in high concentration in
dried shiitake mushrooms, used in much of the cuisine of Asia. There is a synergistic
effect between MSG, IMP, and GMP which together in certain ratios produce a strong
savory taste.
Some savory taste buds respond specifically to glutamate in the same way that
"sweet" ones respond to sugar. Glutamate binds to a variant of G protein coupled
glutamate receptors.
Nature of Chemoreceptors
Because the ability to detect, distinguish, and respond to chemicals in the external
environment is essentially universal among all living things, chemoreception almost
certainly arose contemporaneously with life itself. Changes in the nature of
chemoreceptors should have occurred in concert with major transitions in the history of
life, including the origins of eukaryotes and multicellular organisms, and the transition
to land.
Fundamental similarities in the biology of chemoreception have been identified
among disparate groups of metazoans. A key question is the extent to which these
similarities represent homology or homoplasy. Unfortunately, in terms of both overall
number of phyla represented, as well as diversity within phyla, the number of groups
studied remains small. This clearly limits our ability to conduct comparative studies to
test hypotheses about the evolution of chemoreceptors.
Structure of Taste buds
Each taste bud is flask-like in shape, its broad base resting on the corium, and its
neck opening, the gustatory pore, between the cells of the epithelium.
The bud is formed by two kinds of cells: supporting cells and gustatory cells.
The supporting (sustentacular) cells are mostly arranged like the staves of a cask, and form
an outer envelope for the bud. Some, however, are found in the interior of the bud
between the gustatory cells.
The gustatory (taste) cells, a chemoreceptor, occupy the central portion of the bud; they are
spindle-shaped, and each possesses a large spherical nucleus near the middle of the cell.
The peripheral end of the cell terminates at the gustatory pore in a fine hair
filament, the gustatory hair. Some early experimental studies (Kirk and Grills, 1992)it
was shown that subjects who were genetically predisposed to baldness were found to be
78% more likely to experience taste loss sensations in 5 out of 5 taste trials (p < 0.05). It
was hypothesized that this was due to 'balding' of the tongue.
The central process passes toward the deep extremity of the bud, and there ends
in single or bifurcated varicosities.
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The nerve fibrils after losing their medullary sheaths enter the taste bud, and end
in fine extremities between the gustatory cells; other nerve fibrils ramify between the
supporting cells and terminate in fine extremities; these, however, are believed to be
nerves of ordinary sensation and not gustatory.
Neural Pathways in Sensory Systems
A single afferent neuron with all its receptor endings makes a sensory unit. When
stimulated, this is the portion of body that leads to activity in a particular afferent
neuron is called the receptive field of that neuron.
Afferent neurons enter the CNS, diverge and synapse upon many interneurons.
These afferent neurons are called sensory or ascending pathways and specific ascending
pathways if they carry information about a single type of stimulus. The ascending
pathways reach the cerebral cortex on the side opposite to where their sensory receptors
are located.
Specific ascending pathways that transmit information from somatic receptors
and taste buds go to somatosensory cortex (parietal lobe), the ones from eyes go to visual
cortex (occipital lobe), and the ones from ears go to auditory cortex (temporal lobe).
With many taste receptors now identified, researchers are turning to a longstanding question in taste perception: how is taste coded? When we eat, our tongue is
bombarded with tastants. How is their detection and transduction of information
organised so that the appropriate response is elicited? Taste physiologist Sue Kinnamon
(Colorado State University, Fort Collins, Colorado, United States) explains the two
theories of taste-coding. In the ‘labelled-line’ model, sweet-sensitive cells, for example,
are hooked up to sweet-sensitive nerve fibres that go to the brain and code sweet. If you
stimulate that pathway, says Kinnamon, ‘you should elicit the appropriate behavioural
response without any input from other cell types’. In the ‘cross-fibre’ model, the pattern
of activity over many receptors codes taste. This model predicts that taste receptor cells
are broadly tuned, responding to many tastants. Support for this theory, says Kinnamon,
comes from electrical recordings from receptor cells and from nerves innervating the
taste buds that show that one cell can respond to more than one taste quality.
Zuker and Ryba's recent work strongly suggests that taste-coding for bitter, sweet,
and umami fits the labelled-line model in the periphery of the taste system. Their
expression data show that receptors for these qualities are expressed in distinct
populations of taste cells. In addition, in early 2003, they reported that, as in other
sensory systems, a single signalling pathway involving the ion channel TRPM5 and
PLCβ2, a phospholipase that produces a TRPM5 activator, lies downstream of the bitter,
sweet, and umami receptors. When the UCSD–NIDCR researchers took PLCβ2 knockout
mice, which did not respond to bitter, sweet, or umami, and engineered them so that
PLCβ2 was only expressed in bitter receptor-expressing cells, only the ability to respond
to bitter tastants was regained. These data, says Zuker, support the labelled-line model.
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The latest data supporting the labelled-line model came last October when Zuker
and colleagues described mice in which a non-taste receptor—a modified κ-opioid
receptor that can only be activated by a synthetic ligand—was expressed only in cells
expressing T1R2, sweet-responsive cells. The mice were attracted to the synthetic ligand,
which they normally ignore, indicating that dedicated pathways mediate attractive
behaviours. The researchers plan similar experiments to see whether the same is true for
aversive behaviours.
Even with all these molecular data, the cross-fibre model of taste-coding still has
its supporters—just how many depends on whom one talks to. Both Damak and
Kinnamon, for example, believe that there is at least some involvement of cross-fibre
patterning even in the taste receptor cells. But, says neurobiologist and olfaction expert
Lawrence C. Katz (Duke University, Durham, North Carolina, United States), ‘the onus
is now on people who believe otherwise [than the labelled-line model] to provide
compelling proof for the cross-fibre theory because now, at least at the periphery, the
evidence is compelling for a labelled line for bitter, sweet, and umami’. Bartoshuk also
says the debate is decided in favour of the labelled-line model in the periphery. The
crossfibre model is an interesting historical footnote, she comments.
Whether this putative link between taste perception and health can be confirmed
and whether it will be possible to manipulate food preferences to improve health remain
to be seen. However, it seems certain that, as in the past five years, the next five years
will see large advances in our knowledge of many aspects of taste, a fascinating and
important sensory system.
This sense is mediated by specialized sensory cells of the nasal cavity of
vertebrates, and, by analogy, sensory cells of the antennae of invertebrates. Many
vertebrates, including most mammals and reptiles, have two distinct olfactory systems the main olfactory system, and the accessory olfactory system (mainly used to detect
pheremones). For air-breathing animals, the main olfactory system detects volatile
chemicals, and the accessory olfactory system detects fluid-phase chemicals.[ For waterdwelling organisms, e.g., fish or crustaceans, the chemicals are present in the
surrounding aqueous medium. Olfaction, along with taste, is a form of chemoreception.
The chemicals themselves which activate the olfactory system, generally at very low
concentrations, are called odorants.
Olfactory receptors expressed in the cell membranes of olfactory receptor neurons
are responsible for the detection of odor molecules. Activated olfactory receptors are the
initial player in a signal transduction cascade which ultimately produces a nerve impulse
which is transmitted to the brain. These receptors are members of the class A rhodopsinlike family of G protein-coupled receptors (GPCRs).
In vertebrates, the olfactory receptors are located in the cilia of the olfactory
sensory neurons. In insects, olfactory receptors are located on the antennae and other
chemosensory organs. Sperm cells also express odor receptors, which are thought to be
involved in chemotaxis to find the egg cell.
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Rather than binding specific ligands like most receptors, olfactory receptors
display affinity for a range of odor molecules, and conversely a single odorant molecule
may bind to a number of olfactory receptors with varying affinities. Once the odorant
has bound to the odor receptor, the receptor undergoes structural changes and it binds
and activates the olfactory-type G protein on the inside of the olfactory receptor neuron.
The G protein (Golf and/or Gs) in turn activates the lyase - adenylate cyclase - which
converts ATP into cyclic AMP (cAMP). The cAMP opens cyclic nucleotide-gated ion
channels which allow calcium and sodium ions to enter into the cell, depolarizing the
olfactory receptor neuron and beginning an action potential which carries the
information to the brain.
There are a large number of different odor receptors, with as many as 1,000 in the
mammalian genome which represents approximately 3% of the genes in the genome.
However not all of these potential odor receptor genes are expressed and are functional.
According to an analysis of data derived from the human genome project, humans have
approximately 400 functional genes coding for olfactory receptors and the remaining 600
candidates are pseudogenes.
The reason for the large number of different odor receptors is to provide a system
for discriminating between as many different odors as possible. Even so, each odor
receptor does not detect a single odor. Rather each individual odor receptor is broadly
tuned to be activated by a number of similar odorant structures. Analogous to the
immune system, the diversity that exists within the olfactory receptor family allows
molecules that have never been encountered before to be characterized. However, unlike
the immune system, which generates diversity through in-situ recombination, every
single olfactory receptor is translated from a specific gene; hence the large portion of the
genome devoted to encoding OR genes. Furthermore most odors activate more than one
type of odor receptor. Since the number of combinations and permutations of olfactory
receptors is almost limitless, the olfactory receptor system is capable of detecting and
distinguishing between a practically infinite number of odorant molecules.
The Importance of Smell
Taste and smell together are the so-called chemical senses, meaning that stimuli
associated with them are chemically based. In many respects the sense of smell is mysterious-not only because little is known about its operation as yet, but also because most people are
insufficiently aware of its importance. When people are asked what sense they would be
prepared to do without if necessary, smell comes at the top of the list and sight at the bottom.
This is a debatable choice, given that smell plays a significant part in many psychic processes
and behavior patterns. Smell is essential for the operation of the sense of taste; it affects one's
sex life, motivation and memory processes (including learning, health and feelings of security
and well-being); and it has an alarm function in life-threatening situations (for instance, in
detecting gas fumes, etc.). What is more, in "competition" (that is, when several senses are
stimulated simultaneously), the nose often comes out on top. A beautiful-looking apple that
smells rotten does not whet our appetite.
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Smell and Science
The scientific world is still not very much interested in the olfactory organ: the
number of researchers worldwide is a few hundred at most. There are various possible
explanations for this.
Scents and associated olfactory sensations are not nearly as easy to measure or
map as stimuli and observations based on light and sound: after all, a scent has no
wavelength or other easily measurable property. Moreover, olfactory sensations are
triggered by chemical substances of very different kinds, which are difficult to group
under a single common denominator. Our knowledge of the operation of the sense of
smell is so poor that we do not know exactly what properties of chemical substances
cause the sensations. Strictly speaking, we don't even know whether chemical
characteristics of substances are responsible for olfactory sensations, or whether, to
mention just one possibility, the shape of the molecule is responsible (the key-lock
principle or the so-called stereochemical theory). A researcher has expressed this
uncertainty as follows: "It is still impossible to predict with any degree of accuracy
whether a chemical compound will have a smell, and if so, what qualitative properties
that smell will have." That is no small admission, certainly compared with our
knowledge of other senses.
Olfactory research also has many technical problems to contend with. Odors can
interact with their environment in all kinds of ways before we perceive them. This means
that the experimental area and the equipment used must be odor-free, and that the
researcher must be very familiar with the doses used. Only in the second half of the
twentieth century have researchers developed good "olfactometers"--apparatus for
administering carefully calculated quantities of odors.
Research is further complicated by the fact that people display wide differences
both in their sensitivity to smells and in their appreciation of smells. All kinds of diseases
or congenital defects may underlie these differences, but even among normal, healthy
people the sense of smell varies enormously. Two extremes are general anosmia, an
inability to smell, and hyperosmia, an oversensitivity to olfactory stimuli. Moreover,
depending on the circumstances there is also a great deal of variation within the same
individual: one processes the smell of fried eggs differently the morning after a drinking
binge than on the evening of the same day after a healthy ramble through the woods.
"There are days when I am moved by the slightest smell; on others, far more numerous, I
smell nothing," Maine de Biran wrote in 1815, noting the favorable days, like May 13 of
that year, when "the wonderfully perfumed air that I breathe in makes me glad to be
In general, women have a keener sense of smell than men and older people regain
their olfactory capacity less quickly than younger people after an "olfactory
bombardment." The range of smells on offer also varies from country to country and
village to village. As a result, it is possible for people to lose their ability to distinguish
certain smells to a greater or lesser extent through conditioning; members of a particular
culture, for example, may develop extreme sensitivity to certain (say, dangerous) smells.
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Finally, the world of smells is difficult to pin down in concrete terms. The
available vocabulary for describing smells is very limited. Often smells are simply
related back to their supposed source. "This smells of coffee" or "It smells like after a
thunderstorm in August here." The results of a recent experiment demonstrate this
relative inability: different respondents described the smell of isobutyraldehyde as that
of "chocolate," "peanut butter sandwich," "sickly and dry," "sour milk," "codfish,"
"endives" or "cocoa"; strikingly, a third of those involved could not describe the smell in
any terms at all.
This phenomenon can be understood partly on the basis of evolution. In
evolutionary terms the sense of smell is an old one, with relatively few direct
connections with the youngest part of this brain--namely, the left neocortex, a system
which houses, for example, "language centers." It does have many well-developed
connections with older brain structures that regulate emotions and motivation, including
the so-called limbic system, the brain stem or the "neural chassis," together with the
"president" of the hormonal system, the hypophysis or pituitary gland. Via the pituitary
gland, smell influences general bodily function (hormone production). One result of this
construction is that we do not in the first instance rationalize and verbalize what we
smell, but have an immediate reaction to a smell and a tendency to act in accordance
with it. In other words, people do not generally convert as olfactory perception into a
considered intellectual judgment followed by consciously controlled behavior; smelling
something generally leads to emotionally colored and sometimes even instinctive
The commercial interests of the cosmetics and food industries play important
parts in olfactory research, and olfactory researchers generally profit from government
and industrial support. Over and against this support (and perhaps because of it), no
research of importance has been carried out into topics like olfactory disorders, which
often have serious consequences (such as memory problems and depressions), or into
substances that may affect our moods, performance and possibly diseases. For example,
there are indications that Alzheimer's disease originates with a decline in the olfactory
capacity, a process which might be preventable.
It is both reprehensible and strange that so little attention is paid to the way in
which smells (apart from perfumes) can affect our behavior, our social interaction and
our well-being.
The detection of small molecules plays an important role in the survival of most
animals, which use odor for identifying and evaluating their food, predators, and
territory. For many years, scientists have been very successful in synthesizing fragrances.
Many types of industry today have synthetic fragrances in their materials; for example,
inks, paints, soaps, cleaning products, and foods. However, the detection and processing
of odor by the body is not well understood. Despite the importance of olfaction (sense of
smell) to our daily lives, the chemical aspect to olfaction was not given much attention
by the scientific community until the 1980's.
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What happens when we inhale?
The initial detection of odors takes place at the back of the nose in a small region
known as the olfactory epithelium. Dissolved molecules interact with specialized
receptors called odorant-binding proteins. (Odorants are molecules that stimulate the
olfactory receptors.) The binding of the molecules to these receptors initiate an electrical
signal that transmits to the olfactory bulbs and higher brain centers for processing of the
olfactory information. Precisely how the olfactory system detects and processes the
numerous scents is still being explored.
In 1991, researchers used gene-cloning techniques and isolated genes encoding
the odor receptors. This research suggests that odor discrimination involves a large
number of distinct olfactory receptors (as many as 1000). Each of these receptors seems
capable of responding to a small number of odorants and each odor must bind to several
receptors. Scientists believe that various receptors respond to discrete parts of an odor's
structure and that an odor usually consists of several chemical groups that each activates
a characteristic receptor. To distinguish the smell, the brain must then determine the
precise combination of receptors that are activated by a particular odor.
The process that we are going to examine is the physiology of the sense of smell.
There is a variety of theories proposed to explain olfaction, but there is no general
agreement. Most evidence supports a stereochemical theory of odor. Amoore et al. (in
1952) proposed that the sense of smell is based largely on the geometry of the odorant
molecules. In this theory, there are a small number of primary odors that are detected by
complementary receptor sites in the nose. Molecules that fit into a similar primary odor
family do not necessarily have similar structural formulas, but they do have roughly the
same molecular shape and size. Sometimes, charge is an additional important
component. When a molecule of the correct size and shape fits into a complementary
receptor site, an impulse is initiated .Complex odors result when a molecule fits into
more than one kind of site (i.e., sideways into a wide receptor site and end-on into a
narrow receptor site). X-ray diffraction, infrared spectroscopy, and electron-beam probes
have enabled scientists to build models of the seven primary odors.
The cutaneous sense or sense of touch involves three different types of receptors.
One-the basket cell -consists of neural fibers wrapped around the base of a hair cell. You
can verify that a single basket cell, when stimulated, produces a detectable sensation.
Find a single hair on your arm, carefully bend it without disturbing other hairs or
pressing on your skin, and note the sensation. When the hair is bent by pressure, the
neuron sends impulses to the brain.
On hairless skin such as fingertips, a receptor called the Pacinian corpuscle senses
physical pressure to the skin. It consists of a multi-layered bag surrounding a sensitive
nerve ending. Pressure to the bag triggers a nerve impulse.
A third type of touch receptor is the free nerve ending. It is simply a nerve fiber in
the skin. Free nerve endings are sensitive to any sort of distortion, from pressure to
tissue damage. Free nerve endings are the most numerous of the cutaneous receptors.
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Basket cells and Pacinian corpuscles are pressure receptors. Free nerve endings
are pressure receptors and also temperature receptors. Pressure and temperature are the
basic ingredients of the cutaneous sense.
Neural coding is a neuroscience-related field concerned with how sensory and
other information is represented in the brain by networks of neurons. The main goal of
studying neural coding is to characterize the relationship between the stimulus and the
individual or ensemble neuronal responses and the relationship among electrical activity
of the neurons in the ensemble 1
Neurons are remarkable among the cells of the body in their ability to propagate
signals rapidly over large distances. They do this by generating characteristic electrical
pulses called action potentials or, more simply, spikes that can travel down nerve fibers.
Sensory neurons change their activities by firing sequences of action potentials in
various temporal patterns, with the presence of external sensory stimuli, such as light,
sound, taste, smell and touch. It is known that information about the stimulus is encoded
in this pattern of action potentials and transmitted into and around the brain.
Although action potentials can vary somewhat in duration, amplitude and shape,
they are typically treated as identical stereotyped events in neural coding studies. If the
brief duration of an action potential (about 1ms) is ignored, an action potential sequence,
or spike train, can be characterized simply by a series of all-or-none point events in time .
The lengths of interspike intervals (ISIs) between two successive spikes in a spike train
often vary, apparently randomly . The study of neural coding involves measuring and
characterizing how stimulus attributes, such as light or sound intensity, or motor actions,
such as the direction of an arm movement, are represented by neuron action potentials
or spikes. In order to describe and analyze neuronal firing, statistical methods and
methods of probability theory and stochastic point processes have been widely applied.
Encoding and decoding
The link between stimulus and response can be studied from two opposite points
of view. Neural encoding refers to the map from stimulus to response. The main focus is
to understand how neurons respond to a wide variety of stimuli, and to accurately
construct models that attempt to predict responses to other stimuli. Neural decoding
refers to the reverse map, from response to stimulus, and the challenge is to reconstruct a
stimulus, or certain aspects of that stimulus, from the spike sequences it evokes.
Coding schemes
A sequence, or 'train', of spikes may contain information based on different
coding schemes. In motor neurons, for example, the strength at which an innervated
muscle is flexed depends solely on the 'firing rate', the average number of spikes per unit
time (a 'rate code'). At the other end, a complex 'temporal code' is based on the precise
timing of single spikes. They may be locked to an external stimulus such as in the
auditory system or be generated intrinsically by the neural circuitry .
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Whether neurons use rate coding or temporal coding is a topic of intense debate
within the neuroscience community, even though there is no clear definition of what
these terms mean.
Paradoxical cold, specifically is when you increase the temperature or decrease the
temperature to a certain point (depending on species) at which you feel the opposing sensation
to the temperature. ie. Contact with an object approximately 50 degrees will evoke some cold
receptors to be activated, yielding a feeling of coldness. This situation is also the same for warm
receptors and cold temperatures except not as evident due to the fact that warm receptors are
"Synthetic heat", also known as the heat grill illusion, occurs when contact with spatially
adjacent warm and cold stimuli produce a sensation of "heat". This phenomenon has been
explained as a painful perception that occurs when warm stimulation inhibits cold-sensitive
neurons in the spinothalamic tract (STT), which in turn unmasks activity in the pain pathway
caused by stimulation of C-polymodal nociceptors (CPNs).
Proprioception , from Latin proprius, meaning "one's own" and perception, is the
sense of the relative position of neighbouring parts of the body. Unlike the exteroceptive
senses by which we perceive the outside world, and interoceptive senses, by which we
perceive the pain and movement of internal organs, proprioception is a third distinct
sensory modality that provides feedback solely on the status of the body internally. It is
the sense that indicates whether the body is moving with required effort, as well as
where the various parts of the body are located in relation to each other.
Proprioception vs. kinesthesia
Kinesthesia is another term that is often used interchangeably with
proprioception, though use of the term "kinesthesia" can place a greater emphasis on
Some differentiate the kinesthetic sense from proprioception by excluding the
sense of equilibrium or balance from kinesthesia. An inner ear infection, for example,
might degrade the sense of balance. This would degrade the proprioceptive sense, but
not the kinesthetic sense. The affected individual would be able to walk, but only by
using the sense of sight to maintain balance; the person would be unable to walk with
eyes closed.
Proprioception and kinesthesia are seen as interrelated and there is considerable
disagreement regarding the definition of these terms. Some of this difficulty stems from
Sherrington's original description of joint position sense (or the ability to determine
exactly where a particular body part is in space) and kinesthesia (or the sensation that
the body part has moved) under a more general heading of proprioception. Clinical
aspects of proprioception are measured in tests that measure a subject's ability to detect
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an externally imposed passive movement, or the ability to reposition a joint to a
predetermined position. Often it is assumed that the ability of one of these aspects will
be related to another; however, experimental evidence suggests there is no strong
relation between these two aspects. This suggests that, while these components may well
be related in a cognitive manner, they seem to be separate physiologically.
Much of the foregoing work is dependent on the notion that proprioception is, in
essence, a feedback mechanism; that is, the body moves (or is moved) and then the
information about this is returned to the brain, whereby subsequent adjustments could
be made. More recent work into the mechanism of ankle sprains suggests that the role of
reflexes may be more limited due to their long latencies (even at the spinal cord level), as
ankle sprain events occur in perhaps 100 msec or less. In accordance, a model has been
proposed to include a 'feedforward' component of proprioception, whereby the subject
will also have central information about the body's position prior to attaining it.
Kinesthesia is a key component in muscle memory and hand-eye coordination,
and training can improve this sense (see blind contour drawing). The ability to swing a
golf club or to catch a ball requires a finely-tuned sense of the position of the joints. This
sense needs to become automatic through training to enable a person to concentrate on
other aspects of performance, such as maintaining motivation or seeing where other
people are.
Basis of proprioceptive sense
The initiation of proprioception is the activation of a proprioreceptor in the
periphery. The proprioceptive sense is believed to be composed of information from
sensory neurons located in the inner ear (motion and orientation) and in the stretch
receptors located in the muscles and the joint-supporting ligaments (stance). There are
specific nerve receptors for this form of perception termed "proprioreceptors," just as
there are specific receptors for pressure, light, temperature, sound, and other sensory
experiences. Proprioreceptors are sometimes known as adequate stimuli receptors.
Although it was known that finger kinesthesia relies on skin sensation, recent
research has found that kinesthesia-based haptic perception relies strongly on the forces
experienced during touch. This research allows the creation of "virtual", illusory haptic
shapes with different perceived qualities.
Conscious and unconscious proprioception
In humans, a distinction is made between conscious proprioception and
unconscious proprioception:
* Conscious proprioception is communicated by the posterior column-medial
lemniscus pathway to the cerebrum.
* Unconscious proprioception is
spinocerebellar tract, to the cerebellum.
O Such an unconscious reaction is seen in the human proprioceptive reflex. This
remarkable proprioceptive reflex (Law of Righting), in the event that the body tilts in
any direction, will cock the head back to level the eyes against the horizon.[8] This is
seen even in infants as soon as they gain control of their neck muscles. This control
comes from the cerebellum, the part of the brain affecting balance.
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Law enforcement
Proprioception is tested by American police officers using the field sobriety test,
wherein the subject is required to touch his or her nose with eyes closed. People with
normal proprioception may make an error of no more than 20 millimeters. People
suffering from impaired proprioception (a symptom of moderate to severe alcohol
intoxication) fail this test due to difficulty locating their limbs in space relative to their
There are several relatively specific tests of the subject's ability to propriorecept.
These tests are used in the diagnosis of neurological disorders. They include the visual
and tactile placing reflexes.
Learning new skills
Proprioception is what allows someone to learn to walk in complete darkness
without losing balance. During the learning of any new skill, sport, or art, it is usually
necessary to become familiar with some proprioceptive tasks specific to that activity.
Without the appropriate integration of proprioceptive input, an artist would not be able
to brush paint onto a canvas without looking at the hand as it moved the brush over the
canvas; it would be impossible to drive an automobile because a motorist would not be
able to steer or use the foot pedals while looking at the road ahead; a person could not
touch type or perform ballet; and people would not even be able to walk without
watching where they put their feet.
Oliver Sacks once reported the case of a young woman who lost her
proprioception due to a viral infection of her spinal cord. At first she was not able to
move properly at all or even control her tone of voice (as voice modulation is primarily
proprioceptive). Later she relearned by using her sight (watching her feet) and inner ear
only for movement while using hearing to judge voice modulation. She eventually
acquired a stiff and slow movement and nearly normal speech, which is believed to be
the best possible in the absence of this sense. She could not judge effort involved in
picking up objects and would grip them painfully to be sure she did not drop them.
The proprioceptive sense can be sharpened through study of many disciplines.
The Alexander Technique uses the study of movement to enhance kinesthetic judgment
of effort and location. Juggling trains reaction time, spatial location, and efficient
movement. Standing on a wobble board or balance board is often used to retrain or
increase proprioception abilities, particularly as physical therapy for ankle or knee
injuries. Standing on one leg (stork standing) and various other body-position challenges
are also used in such disciplines as Yoga or Wing Chun. In addition, the slow, focused
movements of Tai Chi practice provide an environment, whereby the proprioceptive
information being fed back to the brain stimulates an intense, dynamic "listening
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environment" to further enhance mind/body integration.[citation needed] Several
studies have shown that the efficacy of these types of training is challenged by closing
the eyes,[citation needed] because the eyes give invaluable feedback to establishing the
moment-to-moment information of balance. There are even specific devices designed for
proprioception training, such as the exercise ball, which works on balancing the
abdominal and back muscles.
It has been seen that temporary loss or impairment of proprioception may happen
periodically during growth, mostly during adolescence. Growth that might also
influence this would be large increases or drops in bodyweight/size due to fluctuations
of fat (liposuction, rapid fat loss, rapid fat gain) and muscle content (bodybuilding,
anabolic steroids, catabolisis/starvation). It can also occur in those that gain new levels
of flexibility, stretching, and contortion. A limb's being in a new range of motion never
experienced (or at least, not for a long time since youth perhaps) can disrupt one's sense
of location of that limb. Possible experiences include suddenly feeling that feet or legs
are missing from one's mental self-image; needing to look down at one's limbs to be sure
they are still there; and falling down while walking, especially when attention is focused
upon something other than the act of walking.
Proprioception is occasionally impaired spontaneously, especially when one is
tired. One's body may appear too large or too small, or parts of the body may appear
distorted in size. Similar effects can sometimes occur during epilepsy or migraine auras.
These effects are presumed to arise from abnormal stimulation of the part of the parietal
cortex of the brain involved with integrating information from different parts of the
Proprioceptive illusions can also be induced, such as the Pinocchio illusion.
The proprioceptive sense is often unnoticed because humans will adapt to a
continuously-present stimulus; this is called habituation, desensitization, or adaptation.
The effect is that proprioceptive sensory impressions disappear, just as a scent can
disappear over time. One practical advantage of this is that unnoticed actions or
sensation continue in the background while an individual's attention can move to
another concern. The Alexander Technique addresses these issues.
People that have a limb amputated may still have a confused sense of that limb
existence on their body, known as phantom limb syndrome. Phantom sensations can
occur as passive proprioceptive sensations of the limb's presence, or more active
sensations such as perceived movement, pressure, pain, itching, or temperature. The
etiology of the phantom limb phenomenon was disputed in 2006, but some consensus
existed in favour of neurological (e.g., neural signal bleed across a preexisting sensory
map, as posited by V.S. Ramachandran) over psychological explanations. Phantom
sensations and phantom pain may also occur after the removal of body parts other than
the limbs, such as after amputation of the breast, extraction of a tooth (phantom tooth
pain), or removal of an eye (phantom eye syndrome).
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Temporary impairment of proprioception has also been known to occur from an
overdose of vitamin B6 (pyridoxine and pyridoxamine). Most of the impaired function
returns to normal shortly after the intake of vitamins returns to normal. Impairment can
also be caused by cytotoxic factors such as chemotherapy.
It has been proposed that even common tinnitus and the attendant hearing
frequency-gaps masked by the perceived sounds may cause erroneous proprioceptive
information to the balance and comprehension centers of the brain, precipitating mild
Proprioception is permanently impaired in patients that suffer from joint
hypermobility or Ehlers-Danlos Syndrome (a genetic condition that results in weak
connective tissue throughout the body). It can also be permanently impaired from viral
infections as reported by Sacks. The catastrophic effect of major proprioceptive loss is
reviewed by Robles-De-La-Torre (2006).
Pain is an unpleasant sensory and emotional experience associated with actual or
potential tissue damage, or described in terms of such damage. It is the feeling common
to such experiences as stubbing a toe, burning a finger, putting iodine on a cut, and
bumping the "funny bone".
Pain motivates us to withdraw from damaging or potentially damaging
situations, protect the damaged body part while it heals, and avoid those situations in
the future. It is initiated by stimulation of nociceptors in the peripheral nervous system,
or by damage to or malfunction of the peripheral or central nervous systems.
Most pain resolves promptly once the painful stimulus is removed and the body
has healed, but sometimes pain persists despite removal of the stimulus and apparent
healing of the body; and sometimes pain arises in the absence of any detectable stimulus,
damage or pathology.
The International Association for the Study of Pain (IASP) classification system
recommends describing pain according to five categories: duration and severity,
anatomical location, body system involved, cause, and temporal characteristics
(intermittent, constant, etc
Pain is usually transitory, lasting only until the noxious stimulus is removed or
the underlying damage or pathology has healed, but some painful conditions, such as
rheumatoid arthritis, peripheral neuropathy, cancer and idiopathic pain, may persist for
years. Pain that lasts a long time is called chronic, and pain that resolves quickly is called
acute. Traditionally, the distinction between acute and chronic pain has relied upon an
arbitrary interval of time from onset; the two most commonly used markers being 3
months and 6 months since the onset of pain, though some theorists and researchers
have placed the transition from acute to chronic pain at 12 months. Others apply acute to
pain that lasts less than 30 days, chronic to pain of more than six months duration, and
subacute to pain that lasts from one to six months.
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Region and system
Pain can be classed according to its location in the body, as in headache, low back
pain and pelvic pain; or according to the body system involved, i.e., myofascial pain
(emanating from skeletal muscles or the fibrous sheath surrounding them), rheumatic
(emanating from the joints and surrounding tissue), causalgia ("burning" pain in the skin
of the arms or, sometimes, legs; thought to be the product of peripheral nerve damage),
neuropathic pain (caused by damage to or malfunction of any part of the nervous
system), or vascular (pain from blood vessels).
Nociceptive pain is initiated by stimulation of nociceptors, and may be classified
according to the mode of noxious stimulation; the most common categories being
"thermal" (heat or cold), "mechanical" (crushing, tearing, etc.) and "chemical" (iodine in a
cut, chili powder in the eyes).
Nociceptive pain may also be divided into "superficial somatic" and "deep", and
deep pain into "deep somatic" and "visceral". Superficial somatic pain is initiated by
activation of nociceptors in the skin or superficial tissues, and is sharp, well-defined and
clearly located. Examples of injuries that produce superficial somatic pain include minor
wounds and minor (first degree) burns. Deep somatic pain is initiated by stimulation of
nociceptors in ligaments, tendons, bones, blood vessels, fasciae and muscles, and is dull,
aching, poorly-localized pain; examples include sprains and broken bones. Visceral pain
originates in the viscera (organs) and often is extremely difficult to locate, and several
visceral regions produce "referred" pain when injured, where the sensation is located in
an area distant from the site of injury or pathology.
Neuropathic pain is caused by damage to or malfunction of the nervous system,
and is divided into "peripheral" (originating in the peripheral nervous system) and
"central" (originating in the brain or spinal cord). Peripheral neuropathic pain is often
described as “burning,” “tingling,” “electrical,” “stabbing,” or “pins and needles.”
Bumping the "funny bone" elicits peripheral neuropathic pain.
Psychogenic pain, also called psychalgia or somatoform pain, is a sensation of
pain caused, increased, or prolonged by mental, emotional, or behavioral factors.
Headache, back pain, and stomach pain are sometimes diagnosed as psychogenic.
Sufferers are often stigmatized, because both medical professionals and the general
public tend to think that pain from a psychological source is not "real". However,
specialists consider that it is no less actual or hurtful than pain from any other source.
People with long term pain frequently display psychological disturbance, with
elevated scores on the Minnesota Multiphasic Personality Inventory scales of hysteria,
depression and hypochondriasis (the "neurotic triad"). Some investigators have argued
that it is this neuroticism that causes acute injuries to turn chronic, but clinical evidence
points the other way, to chronic pain causing neuroticism. When long term pain is
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relieved by therapeutic intervention, scores on the neurotic triad and anxiety fall, often
to normal levels. Self-esteem, often low in chronic pain patients, also shows striking
improvement once pain has resolved.
“The term 'psychogenic' assumes that medical diagnosis is so perfect that all
organic causes of pain can be detected; regrettably, we are far from such infallability...
All too often, the diagnosis of neurosis as the cause of pain hides our ignorance of many
aspects of pain medicine.” Ronald Melzack, 1996.
Phantom pain
Phantom pain is the sensation of pain from a part of the body that has been lost or
from which the brain no longer receives physical signals. It is a type of neuropathic pain.
Phantom limb pain is a common experience of amputees. One study found that eight
days after amputation, 72 per cent of patients had phantom limb pain, and six months
later, 65 percent reported it. Some experience continuous pain that varies in intensity or
quality; others experience several bouts a day, or it may occur only once every week or
two. It is described as shooting, crushing, burning or cramping. If the pain is continuous
for a long period, parts of the intact body may become sensitized, so that touching them
evokes pain in the phantom limb, or phantom limb pain may accompany urination or
Local anesthetic injections into the nerves or sensitive areas of the stump may
relieve pain for days, weeks or, sometimes permanently, despite the drug wearing off in
a matter of hours; and small injections of hypertonic saline into the soft tissue between
vertebrae produces local pain that radiates into the phantom limb for ten minutes or so
and may be followed by hours, weeks or even longer of partial or total relief from
phantom pain. Vigorous vibration or electrical stimulation of the stump, or current from
electrodes surgically implanted onto the spinal cord all produce relief in some patients.
Paraplegia, the loss of sensation and voluntary motor control after serious spinal
cord damage, may be accompanied by root ("girdle") pain at the level of the spinal cord
damage, visceral pain evoked by a filling bladder or bowel, or, in five to ten per cent of
paraplegics, phantom body pain in areas of complete sensory loss. Phantom body pain is
initially described as burning or tingling but may evolve into severe crushing or
pinching pain, fire running down the legs, or a knife twisting in the flesh. Onset may be
immediate or may not occur until years after the disabling injury. Surgical treatment
rarely provides lasting relief.
Pain asymbolia
Pain science acknowledges, in a puzzling challenge to IASP definition, that pain
may be experienced as a sensation devoid of any unpleasantness: this happens in a
syndrome called pain asymbolia or pain dissociation, caused by conditions like
lobotomy, cingulotomy or morphine analgesia. Typically, such patients report that they
have pain but are not bothered by it, they recognize the sensation of pain but are mostly
or completely immune to suffering from it.
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Insensitivity to pain
The ability to experience pain is essential for protection from injury, and
recognition of the presence of injury. Episodic analgesia may occur under special
circumstances, such as in the excitement of sport or war: a soldier on the battlefield may
feel no pain for many hours from a traumatic amputation or other severe injury.
However, insensitivity to pain may also be acquired following conditions such as spinal
cord injury, diabetes mellitus, or more rarely leprosy. A small number of people suffer
from congenital analgesia ("congenital insensitivity to pain"), a genetic defect that puts
these individuals at constant risk from the consequences of unrecognized injury or
illness. Children with this condition suffer carelessly repeated damage to their tongue,
eyes, joints, skin, and muscles. They may attain adulthood, but have a shortened life
Experimental subjects challenged by acute pain and patients in chronic pain
experience impairments in attention control, working memory, mental flexibility,
problem solving, and information processing speed.
Pain receptors are any one of the many free nerve endings throughout the body
that warn of potentially harmful changes in the environment, such as excessive pressure
or temperature. The free nerve endings constituting most of the pain receptors are
located chiefly in the epidermis and in the epithelial covering of certain mucous
membranes. They also appear in the stratified squamous epithelium of the cornea, in the
root sheaths and the papillae of the hairs, and around the bodies of sudoriferous glands.
The terminal ends of pain receptors consist of unmyelinated nerve fibers that often
anastomose into small knobs between the epithelial cells. Any kind of stimulus, if it is
intense enough, can stimulate the pain receptors in the skin and the mucosa, but only
radical changes in pressure and certain chemicals can stimulate the pain receptors in the
viscera. Referred pain results only from stimulation of pain receptors located in deep
structures, such as the viscera, the joints, and the skeletal muscles, and never from pain
receptors in the skin.
Chronic pain has several different meanings in medicine. Traditionally, the
distinction between acute and chronic pain has relied upon an arbitrary interval of time
from onset; the two most commonly used markers being 3 months and 6 months since
the initiation of pain, though some theorists and researchers have placed the transition
from acute to chronic pain at 12 months. Others apply acute to pain that lasts less than 30
days, chronic to pain of more than six months duration, and subacute to pain that lasts
from one to six months. A popular alternative definition of chronic pain, involving no
arbitrarily fixed durations is "pain that extends beyond the expected period of healing."
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Chronic pain may be divided into "nociceptive" (caused by activation of
nociceptors), and "neuropathic" (caused by damage to or malfunction of the nervous
Nociceptive pain may be divided into "superficial somatic" and "deep", and deep
pain into "deep somatic" and "visceral". Superficial somatic pain is initiated by activation
of nociceptors in the skin or superficial tissues. Deep somatic pain is initiated by
stimulation of nociceptors in ligaments, tendons, bones, blood vessels, fasciae and
muscles, and is dull, aching, poorly-localized pain. Visceral pain originates in the viscera
(organs). Visceral pain may be well-localized, but often it is extremely difficult to locate,
and several visceral regions produce "referred" pain when injured, where the sensation is
located in an area distant from the site of pathology or injury.
Neuropathic pain is divided into "peripheral" (originating in the peripheral
nervous system) and "central" (originiting in the brain or spinal cord).[6] Peripheral
neuropathic pain is often described as “burning,” “tingling,” “electrical,” “stabbing,” or
“pins and needles.” Bumping the "funny bone" elicits peripheral neuropathic pain.
Under persistent activation nociceptive transmission to the dorsal horn may
induce a wind up phenomenon. This induces pathological changes that lower the
threshold for pain signals to be transmitted. In addition it may generate nonnociceptive
nerve fibers to respond to pain signals. Nonnociceptive nerve fibers may also be able to
generate and transmit pain signals. In chronic pain this process is difficult to reverse or
eradicate once established.
Chronic pain of different etiologies has been characterized as a disease affecting
brain structure and function. Magnetic Resonance Imaging studies have shown
abnormal anatomical and functional connectivity involving areas related to the
processing of pain. Also, persistent pain has been shown to cause grey matter loss,
reversible once the pain has resolved.
Complete and sustained remission of many neuropathies and most idiopathic
chronic pain (pain that extends beyond the expected period of healing, or chronic pain
that has no known underlying pathology) is rarely achieved, but much can be done to
reduce suffering and improve quality of life.
Pain management (also called pain medicine) is that branch of medicine
employing an interdisciplinary approach to the relief of pain and improvement in the
quality of life of those living with pain. The typical pain management team includes
medical practitioners, clinical psychologists, physiotherapists, occupational therapists,
and nurse practitioners. Acute pain usually resolves with the efforts of one practitioner;
however, the management of chronic pain frequently requires the coordinated efforts of
the treatment team.
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Behavioral model
The behavioral model of chronic pain comes from the applied behavior analysis
literature. The model focuses on decreasing pain behaviors . This model has shown
effectiveness in reducing pain responses though operant based interventions.. More
recently a [behavioral activation] model has been generated for pain . This model has
research support and has been replicated.
Chronic pain may cause other symptoms or conditions, including depression and
anxiety. It may also contribute to decreased physical activity given the apprehension of
exacerbating pain. Very little work has been done on the cognitive effects of chronic
pain, with most of the publications focussing on the effects of cognition on pain but only
5% examining the effects of pain on cognition.
People with high-intensity chronic pain have significantly reduced ability to
perform attention-demanding tasks. Pain appears to strongly capture the attention of
people with chronic pain; tests assessing the ability to attend show poorer performance
than pain-free people on all tests demanding attention. The exception is found with tasks
that are highly demanding of attention, where performance between the two groups is
equivalent. In experimental testing, two-thirds of individuals with chronic pain
demonstrate clinically significant impairment of attention, independent of age,
education, medication and sleep disruption. Individuals with the highest levels of pain
showed greatest disruption of memory traces, suggesting that pain diminishes working
The ICD-9-CM Official Guidelines for Coding and Reporting were updated,
effective November 15, 2006, with a new section for the proper sequencing and usage of
the pain codes The following is a summary of the guidelines.
It is appropriate to assign other codes with codes from category 338 to further
describe the acute or chronic pain and the neoplasm-related pain. “If the pain is not
specified as acute or chronic, do not assign codes from category 338, except for
postthoracotomy pain, postoperative pain, or neoplasm related pain”
Do not assign a code from subcategories 338.1 and 338.2 if the underlying
(definitive) diagnosis is known, unless the reason for the encounter is pain
control/management and not management of the underlying condition.
A code from category 338 should be sequenced as the principal diagnosis when:
• The related definitive diagnosis has not been established by the physician.
• The reason for admission is for pain control or pain management.
It is appropriate to assign a code from category 338 with another site-specific code
if the code from category 338 provides additional information. Sequencing of these two
codes will depend on the circumstances of admission with the following two exceptions.
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If the reason for admission is for pain control/management, then the code from
category 338 is sequenced as the principal diagnosis followed by the site-specific pain
If the reason for admission is for any reason other than pain control/management
and a related definitive diagnosis has not been established, then the site-specific pain
code will be sequenced as the principal diagnosis with the code from category 338
sequenced as a secondary diagnosis.
Pain due to a device or foreign body left in a surgical site is not assigned to
category 338. Instead, assign the appropriate code from Chapter 17, “Injury and
Poisoning.” However, if the patient was admitted for pain control/management because
of pain due to a device or foreign body left in a surgical site, then a code from category
338 is assigned and sequenced as the principal diagnosis.
Postoperative pain and postthoracotomy pain not specified as acute or chronic
defaults to the code for the acute type. Sequence the postoperative pain code as the
principal diagnosis when the patient is admitted for postoperative pain control or pain
management. Sequence the postoperative pain code as a secondary diagnosis when the
patient develops an “unusual or inordinate amount of postoperative pain” after
outpatient surgery. Do not assign a code for the postoperative pain if it is routine or
expected after surgery.
There is no time frame that identifies when the pain can be defined as chronic.
Code assignment is based on the physician’s documentation.
Neoplasm-related pain (338.3) is for pain due to, related to, or associated with
primary or secondary malignancy or tumor, regardless if the pain is acute or chronic.
Code 338.3 is sequenced as the principal diagnosis when the patient is admitted for pain
control/management. If the patient is admitted for management of the neoplasm and
pain is also documented, code 338.3 may be sequenced as a secondary diagnosis.
The diagnosis of chronic pain syndrome is not the same as chronic pain. Assign
the code for chronic pain syndrome only when that diagnosis has been documented by
the physician.
Coding and sequencing for pain are dependent on the physician documentation
in the medical record and application of the Official Coding Guidelines for inpatient
The process of providing medical care that alleviates or reduces pain. Pain
management is an extremely important part of health care, as patients forced to remain
in severe pain often become agitated and/or depressed and have poorer treatment
outcomes. Mild to moderate pain can usually be treated with analgesic medications,
such as aspirin or ibuprofen. For chronic or severe pain, opiates and other narcotics are
often used, sometimes in concert with analgesics; with steroids or non-steroidal antiinflammatory drugs when the pain is related to inflammation; or with anti-depressants,
which can potentiate some pain medications without raising the actual dose of the drug,
and which affect the brain's perception of pain. Narcotics carry with them a potential for
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side effects and addiction, so patients and caregivers must weigh the level of pain
against these dangers in the pain management process. The risk of addiction is not
normally a concern in the care of terminal patients.
For hospitalized patients with severe pain, devices for self- administration of
narcotics are now frequently used. Other procedures can also be useful in pain
management programs. For bedridden patients, simply changing position regularly or
using pillows to support a more comfortable posture can be effective. Massage,
acupuncture, acupressure, and biofeedback have also shown some validity for increased
pain control in some patients.
Mechanisms of Movement
Bones provide the frame that holds up muscles. Signals are sent from the locomotion
center of the brain (medulla) along nerves which synapse with certain muscles. This
synapse causes myosin and actin filaments to slide over each other, causing the muscle
to contract and resulting in locomotion.
Main structures in the human elbow joint.
bicep - flexor muscle, used to bend arm at the elbow
humerus bone - provides firm anchor for muscles
triceps - extensor muscle, used to straighten the arm
synovial fluid - lubricates the joint to reduce friction
capsule - seals the joint
tendon - attaches muscle to bone
radius - upper bone in forearm, transmits forces from bicep through
ulna - lower bone in forearm, transmits forces from triceps through the
ligament - tough cords of tissue, links bone to bone, prevents dislocation
cartilage - a layer of smooth and tough tissue that covers the ends of the
bone where bones meet to reduce friction
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skeletal muscle contraction by the sliding of filaments.
Using ATP, actin and myosin filaments slide over each other, causing sarcomeres
in the myofibrils to become shorter, ultimately creating a contraction in skeletal muscles.
1. Myosin forms a bridge with binding sites on actin filaments
2. ATP binds with myosin heads causing them to break the cross bridge by
detaching from the binding site
3. ATP loses a phosphate molecule, releasing energy, leaving ADP + P+ and
changing the angle of the myosin head. This is known as a cocked myosin
4. The angled head attaches to a new binding site on actin that are further
back in the line of actin binding sites
5. ADP + P+ is released, causing the heads push filaments inwards toward
the sarcomere, resulting in a sliding of myosin and actin filaments over one
The simplest way a message can get from a sense organ to an effector organ, such
as a muscle is by way of the reflex arc. This involves just two neurons (nerve cells)
between sense organ and effector. The majority of animal movements, however, are the
result of nerve impulses passing along a line of nerve cells. Between the sensory nerve
and the motor nerves there are a number of connecting nerves called association
neurons. These are not in contact with either sense organ or effector organ and merely
pass on the impulses to the next nerve in the chain. The association neurons increase in
number as the nervous system increases in complexity.
Multipolar and bipolar neurons
The simple, unmodified nerve cell consists of the cell body with the nucleus and a
number of fine processes which radiate in all directions, making contact with
neighboring cells to receive and transmit impulses. The cells are said to be multipolar.
Most of the cells in the cndarian (coelenterate) nerve net are of this type but nerve
impulses do not necessarily travel all over the net in all directions. Other cells in the
nerve net are bipolar, i.e., the processes are on two sides only. In all animals with a
definite pattern to their nervous system, the typical conducting nerve cells are elongated.
One of the processes grows out to form the axon of the nerve cell and in some motor
neurons may be several feet long, (e.g., a nerve may pass from the spinal cord to the foot
of an animal without interruption). Neurons of this type are said to be polarized.
Impulses normally travel only in one direction along the axon, although they can travel
in both directions.
How a nerve impulse travels
An impulse is picked up by the dendrites (fine branches) at the end of the neuron
and passes through the cell into the fine endings at the other end. These make contact
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with those of neighboring neurons but the cytoplasm does not join across the gap. The
junction is called a synapse.
Although the structure and arrangement of the neurons varies, the nature of the
nerve impulse appears to be the same in all animals. When a receptor is stimulated (e.g.
when hot water burns the heat-sensitive cells in your hand) and electrical disturbance is
set up and transmitted in its nerve, as a series of electric currents which can be detected
and measured by placing tiny electrodes on the nerve concerned and connecting them to
an amplifying device and a meter.
The nerve cell is covered on the outside by a thin sheet of tissue (a membrane).
When the cell is at rest (i.e. when there is no current moving through it) the membrane
allows potassium and chloride ions to pass in and out of the nerve but not sodium ions.
The latter are very abundant in the surrounding tissues. Organic ions, negatively
charged, balance the potassium ions in the nerve and create a negative charge on the
inside of the membrane. The sodium ions create a positive charge on the outside and the
difference between the two charges is called the resting potential and is of the order of 80
millivolts (eighty thousandths of a volt). When the nerve fiber is stimulated by the
receptor the properties of the membrane are altered and sodium ions are allowed to pass
inward. When they do this they neutralize the excess organic ions within the nerve cell
and cause a negative charge to occur on the outer surface. Sodium ions on neighboring
parts of the surface then move along and the resting potential breaks down on the next
part of the nerve fiber and the inflowing sodium ions set up a current there. In this way a
current is set up all the way along a fiber.
As the surface potential is destroyed in the process of conducting an impulse, the
nerve fiber cannot conduct further impulses the resting state has been regained by
pumping the sodium out of the nerve cells. The period required for this is called the
refractory period and in mammalian nerve fiber is about 2 milliseconds. Because of the
need for this recharging period, the mammalian nerve cell cannot conduct impulses
more frequently than about 500 per second. In practice the frequency will be much
lower, 50 to 100 impulses per second. The sense organ itself regulates the frequency with
which it sends impulses. All impulses in a particular nerve fiber are of the same size. A
stronger impulse will cause more frequent impulses not larger ones. If a sense organ is
stimulated it will produce a reaction in the nerve but unless the stimulus is large enough
there will be no impulse. If an impulse is set up it will be complete and of the standard
size. This is known as the all or nothing law of nerve conduction. The size of an impulse
does vary, however, with the condition of the nerve. A thick nerve cell will conduct a
larger impulse than a smaller process of the nerve. A nerve fiber affected by drugs will
also carry only a small impulse. The size of an impulse does not decrease as the distance
traveled increases. It travels without decrement.
Crossing a synapse
On the way to the effector organ the nerve impulse has to cross one or more
synapses. When stimulated by the impulse, the nerve fiber ending produces a minute
amount of a chemical called acetylcholine. This acts on the endings of the adjacent
neuron and changes the permeability of the membrane. Sodium ions enter and the chain
of electrical disturbances continues through this neighboring nerve cell. As soon as it is
produced, acetylcholine is attacked by the enzyme acetylcholinesterase and is destroyed.
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If this did not happen the nerves would effectively be poisoned. Some insecticides work
by destroying acetylcholinesterase and so poisoning the nervous system of the insects.
Muscle contraction
At the motor end plate the fine branches of the nerve come into close contact with
the muscle fibers .When the nerve impulse reaches a muscle its electrical energy must be
made to liberate mechanical energy in muscle contraction. Most of the work on
contraction is done with striped muscle, which makes up most of the body's
musculature. The muscles are made up of large numbers of cells each of which has one
or more connections with a motor nerve ending. In vertebrates there is only one
connection but arthropod muscle fibers have several motor nerve endings. The junction
between nerve fiber and muscle fiber is known as a motor end plate.
A nerve impulse, arriving at the junction causes the release of acetylcholine which
affects the muscle membrane. Ions are allowed to pass in and out in much the same way
as in nerve cells and the current set up triggers the many hundreds of nerve fibers run
from the spinal cord to each muscle in vertebrates .
When a single impulse reaches a vertebrate muscle fiber the fiber will contract
and then relax. This response is called the muscle twitch. A series of quick impulses will
keep the muscle fiber contracted since it will have no time to relax. This prolonged
contraction is called a tetanus. Tonic contraction is the term applied to partial contraction
of a muscle which can be maintained for a long period. This is possible in vertebrate
striped muscle because there are hundreds of nerve fibers supplying each muscle. Each
fiber branches two or three times and each ending supplies one muscle fiber. Impulses in
a few nerve fibers will therefore cause only part of the muscle to contract. There is still
complete contraction in each individual fiber affected. Tetanic contraction of striped
muscle requires much energy, but tonic contraction of unstriped muscle can be
maintained for long periods without fatigue.
Whereas vertebrate muscle is innervated by a large number of nerve fibers,
arthropod muscle is supplied by only a few (1-5) nerve fibers. Each fiber branches many
times and has several nerve endings. Local contraction of a fiber is thus possible.
Arthropod muscles (and some other invertebrate muscles) are supplied by an
inhibitory nerve fiber as well as by the excitatory nerve fibers. The inhibitory nerve
impulses stop or modify muscle contraction. Inhibition in vertebrate nerves is
accomplished by stopping the nerve impulses. The muscle then returns to its relaxed
The brain's motor cortex is the area that controls motor movements. The region
controlling the fine motor skills controls the muscles of the elbow, shoulder and hands.
This part of the brain evolved later than the rest of the motor cortex in humans and is
next to the older part of the motor cortex that control the other muscles used during
voluntary movement. The motor cortex is at the back of the frontal lobe. Other areas of
the brain also contribute to movement, providing sense information, for instance, to
allow you to make appropriately sized movements within your space.
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The pyramidal system is a major motor system comprised of the axons that run
through the corticospinal and corticobulbar tracts. The corticospinal neurons are
involved with the movement of fingers, hands, arms torso, legs, and feet. The
corticobulbar neurons are involved with the movement of the neck, face, oral cavity, and
larynx. Damage to either the corticospinal or corticobulbar tracts above the level of
efferent cranial nerve nuclei will result in problems with the motor activity on the
opposite side. For instance, if there is damage to the lateral area of one precentral gyrus,
the muscle function of the face and oral cavities on the opposite side could be affected.
In human anatomy, the extrapyramidal system is a neural network located in the
brain that is part of the motor system involved in the coordination of movement. The
system is called "extrapyramidal" to distinguish it from the tracts of the motor cortex that
reach their targets by traveling through the "pyramids" of the medulla. The pyramidal
pathways (corticospinal and some corticobulbar tracts) may directly innervate motor
neurons of the spinal cord or brainstem (anterior (ventral) horn cells or certain cranial
nerve nuclei), whereas the extrapyramidal system centers around the modulation and
regulation (indirect control) of anterior (ventral) horn cells.
Extrapyramidal tracts are chiefly found in the reticular formation of the pons and
medulla, and target neurons in the spinal cord involved in reflexes, locomotion, complex
movements, and postural control. These tracts are in turn modulated by various parts of
the central nervous system, including the nigrostriatal pathway, the basal ganglia, the
cerebellum, the vestibular nuclei, and different sensory areas of the cerebral cortex. All of
these regulatory components can be considered part of the extrapyramidal system, in
that they modulate motor activity without directly innervating motor neurons.
The Glandular Systems
The body has two types of glandular systems, the endocrine, which generally
secrete hormones through the bloodstream, and the exocrene which secrete fluids to the
outer surfaces of the body, such as sweating.
Endocrine glands
The Endocrine System combines neural and glandular mechanisms which control
physiological functions/behavior via the secretion of hormones. Hormones are chemical
signaling molecules which play an integral role during development (organizational
effects) and day-to-day functioning (activational effects) of target tissues at critical times.
Secretory cells of a particular type are often clumped together into a well defined gland
(e.g. pituitary, thyroid, adrenal, testes, ovaries). Secreted at that site they distribute
throughout the body via the blood stream, and cause physiological changes at any other
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Steroids: derived from cholesterol
Amines: derived from amino acids
Peptides: short chains of amino acids via protein synthesis
Sex hormones, largely steroids, are secreted from gonads and adrenal cortex.
Androgens (e.g., testosterone) are usually higher in male mammals while levels of
estrogens (e.g., estradiol) in female mammals exceed those in males. Circulating levels of
sex hormones then provide the basic organization for gender phenotypes.
Neurohormone refers to a compound that is released into the bloodstream at
specialized neurohemal release sites. It binds to receptors anywhere in the body and
thereby coordinates disparate biochemical responses.They are released from glands,
transported via the circulatory system and influence the activity of target organs.
Functionally hormones are categorized as Effector hormones (e.g. Vasopressin,
Oxytocin) or Tropic hormones, releasing factors (e.g. Gonadotropin Releasing Hormone GnRH, Growth Hormone Releasing Hormone - GHRH). Target Organs receive
hormones via blood stream, respond directly or release their own hormones in response
(steroid hormones), and these hormones circulate back to turn off hormonal secretion:
endocrine feed back loops. Compensatory hypertrophy results from feedback loops
control levels of activity where systems are upregulated until they achieve sufficient
functional effects (e.g. thyroxine and goiter). Activational Effects and Organizational
Sex Determination
Hormonal influences during critical periods produce fairly permanent changes in
nervous and endocrine systems. Sex-determination may be chromosomal or
Mammals: hormonal cascade of events takes an individual down one of two paths
(default is female). testes determining factor (tdf) in mammals or Sexchromosomal
Abnormalities: Turner syndrome (XO monosomy), Klinefelter syndrome (XXY trisomy),
XYY syndrome ("supermale"), Multi-X syndrome ("superfemale"), XX Male syndrome
(SRY gene transference) XY Female syndrome (SRY gene missing)
Birds: Males are homogametic (ZZ), females are heterogametic (ZW)
Turtles: temperature
Fish: social stimuli
Internal sex organs: Precursors for both internal sex organs are present in the embryo:
the Mullerian system for the female sex; the Wolffian system for males. Controlled by the
levels of circulating hormones, only one set of organs develops while the other shrinks.
Secreted by the testes, Mullerian inhibiting hormone (i.e., a peptide hormone) has
defeminizing effects while androgens (i.e., steroid hormones) exert a masculinizing
effect. Gonads: testes or ovaries, first to develop. External genitalia: Androgens are
essential for the development of primary sexual characteristics.
Androgen insensitivity syndrome: A genetic male lacking functional androgen
receptors develops testes, secretes Mullerian inhibiting substance and androgens.
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Internal sex organs progress onto the intended male path, but female external genitalia
result from the inability to respond to androgens correctly. Turner’s syndrome, with
genotype XO (i.e., a single X chromosome and no Y), lacks testes and ovaries.
Maternal effects explain influenced by effects of the mother that are not due to direct
genetic inheritance. Positional effects in the mammalian uterus
Hormonal secretion or its inhibition may bring about some rapid physiological
changes in a variety of target tissues. Vasopressin.
Hormones influence sensory perception (human visual, rat odor, preference in
castrated vs. intact). Hypothalamus neurons monitor the internal state (thirst, hunger)
and send neurosecretory cell axons to the posterior pituitary. In females, Oxytocin,
released from the posterior pituitary, triggers milk let-down in mammary glands or
contraction in the uterus during child-birth. It is also associated with powerful emotional
effects (affective states) in parental behavior and maternal competence. Moreover, the
hypothalamus secretes releasing factors to stimulate or inhibit manufacture and
secretion of hormones in anterior pituitary portal system: Prolactin, Follicle stimulating
hormone (FSH), Luteinizing hormone (LH); receives, stores and releases neurohormones
from hypothalamus. Sex behaviors: gonadotropin releasing hormone (GnRH) ->
gonadotropin -> gonad maturation and gonadal steroid production -> feed back to brain.
Sex steroids initiate the basic female and male anatomies (i.e., organizing effects in
sexual differentiation cascade) but also allow animals to later respond to activational
effects. Expression of sex differences in behavior also requires that steroids activate
many aspects of the phenotype during maturation. Reproductive Neuroendocrinology of
Ring Doves (Streptopelia risoria): monogamous, sexually monomorphic, parental care
from both male and female (incubate eggs, crop "milk"); Lordosis in female hamsters:
female soliciting behavior. Lee-Boot effect - estrous cycles slow and stop in female mice
housed together; Whitten effect - individuals start cycling again in synchrony if exposed
to male odor; Vandenbergh effect - acceleration of onset of puberty in female rat when
exposed to odor of male; Bruce effect - failure of recent pregnancy of female rat when
exposed to male who is not the father.
Scheider, A.M. & Tatshis, B.(1998), Physiological Psychology(3rd ed),
Random House, N.Y.
Leukal ,F.(2000), Introduction of Physiological Psychology(3rd ed), CBS
Publishers, New Delhi.
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