Download gustatory and olfactory senses

UNIT IV
APPLIED NEUROBIOLOGY
Sensory Reception
Animals respond to the messages they receive from the world around them. Their
reactions to the outside world depend on how the data collected from their surroundings
are correctly coded into signals that can be received and processed by neurons in the
brain. The sensory organs provide the only means of communication from the
environment to the nervous system. Sensations arise when signals detected by sensory
receptor cells are transmitted through the nervous system to the designated part of the
brain. Various organs and cells are designated to receive specific stimuli. The major
categories of sensory reception addressed here are chemoreception, mechanoreception,
and photoreception.
What are the general properties of sensory reception and how are these messages
transmitted to the central nervous system?
Animals require a constant detection of information from their surroundings. Such
information is the animal’s link to the outside world. Sensory input is initially detected by
sensory receptors. Some receptors are very complex, with many individual receptors
along with other structures being organized into sensory organs such as the vertebrate
eye.
Sensory receptors are transducers; they convert stimuli into electric signals. In most
cases, they do not directly generate action potentials. Instead, sensory receptors generate
receptor potentials, which vary in intensity with the intensity of the stimulus. These
changes in membrane potential are passed to adjacent sensory neurons, which may
generate an action potential if the incoming stimuli are sufficient for the neuron to reach
threshold (see section on Communication - the nervous system for further details about
how this occurs). Increases in receptor potential intensity are translated into a higher
frequency of action potentials in the sensory neurons.
Sensory receptors are specialized to respond to only certain stimuli, which will activate
the receptor with weak or moderate levels of intensity. The signal is then chemically
amplified within the receptor cells. In order for the signal to be effective the intracellular
chemical signal must cause membrane channels to open. This produces an electrical
signal that will be transmitted to the central nervous system.
How can sensory systems detect such a broad range of stimuli intensity?
The sensitivity range of a sensory organ is much broader than the range of a single
receptor cell. This is because individual afferent fibers of the sensory system cover
different parts of the sensitivity spectrum. For example, only the most sensitive receptor
cells will respond to a low level stimulus. As the stimulus intensity continues to increase,
the receptors become fully activated (saturated), but a group of less sensitive receptor
becomes stimulated. This recruitment of additional receptors continues as the stimulus
intensity increases, until all receptors are fully saturated. This subdivision of the total
range of response by receptor cells of different sensitivities is called range fractionation
because individual receptors cover only a fraction of the total range of the sensory
system. When all receptors are fully active, the system is not capable of detecting any
further increase in stimulus intensity.
Some sensory systems (receptors and their neurons) generate a rather constant
"background" rate of action potentials. If stimulated further, the rate of action potential
generation increases due to increase levels of depolarization of the neurons. Therefore,
the system does not rely on a minimal level of sensory input in order to respond, which
makes the system more sensitive.
Chemoreception
Chemoreception is the ability to perceive specific molecules in the air or in water. These
molecules are important clues to the presence of specific objects in the environment. It is
essential to many animals in finding food, locating a mate, and avoiding danger.
Chemoreception is divided into two main categories: gustation (taste) and olfaction
(smell). Gustatory receptors respond to dissolved molecules that come in contact with the
receptors. Olfactory receptors respond to airborne molecules from sources a distance
away.
Differences in chemoreception in invertebrates and in vertebrates
Vertebrates detect chemicals using general receptors and two types of specialized
receptors, gustatory and olfactory. Many aquatic vertebrates have generalized chemical
receptors scattered over their body surface. Vertebrates usually accomplish
chemoreception by moving chemically rich air or water into a canal or sac that contains
the chemical receptors.
Chemoreception is much different in invertebrates than in vertebrates. For example,
planarians find food by following chemical gradients in their surroundings. Their simple
chemoreceptors are found in pits on their bodies, over which they move water with cilia.
Insects have chemoreceptors in their body surface, mouthparts, antennae, forelegs, and, in
some cases, the ovipositor. Moths, for example, smell with thousands of sensory hairs on
their antennae. About 70 percent of the adult male receptors are made to respond to one
molecule called bombkyol, a sex attractant released by females of the species. The
molecules enter the tiny pores of the hair, or sensillum, where the olfactory receptors are
found.
Olfactory (smell) mechanisms
The receptors for the olfactory nerves are located in the upper part of the nasal cavitity.
The olfactory sense organ consists of hair-like cells at the end of a neuron and is simple
compared to the complex visual and auditory organs. The olfactory receptors are very
sensitive to stimuli; however, they also become very fatigued. This explains why odors
seem to go away after being easily noticeable. Canals lined with sheets of receptors with
the nasal cavity are called turbinates. Protruding from the end of the nerve are thin cilia
that are covered by mucus. Molecules are absorbed into the mucous layer and passed to
the cilia where the chemical is detected. Notice the chemicals must be dissolved in the
mucus and absorbed in order for the olfactory receptors to react. This is a lot like the
gustatory mechanisms.
Gustatory (taste) mechanisms
The receptors for the gustatory nerves are known as taste buds located on the tongue and
the roof of the mouth. Sweet, sour, bitter, and salty are the four basic taste sensations
resulting from stimulation of the taste buds and the stimulation of the olfactory receptor.
This is why it is harder to taste when one has a cold. These four basic tastes may
evolutionarily developed to show some basic food properties. Sweet taste signals foods
high in calories, salty foods signal for food that helps maintain water balance, sour tastes
may help to signal foods that could be dangerous if eaten in excess, and bitter taste
sensations signal toxic foods.
Taste is also referred to as contact chemoreception for obvious reasons. For example,
insects have contact receptors called taste hairs or sensilla. At the tip of each sensillum is
a tiny pore that allows molecules to reach the sensory cells. Each cell is sensitive to a
different chemical. Sensilla can be located in a variety of locations on the body. Flies,
for example, have sensilla on their tarsi (feet).
Mechanoreception
Mechanoreception is sensing physical contact on the surface of the skin or movement of
the surrounding environment (such as sound waves in air or water). The simplest
mechanoreceptors are nerve endings of skin’s connective tissue. The most complex
example of mechanoreception occurs in the middle and inner ear of vertebrates. The hair
cell is the basic unit of vertebrate mechanoreception.
Structural mechanisms of the vertebrate ear
Sound waves enter the external ear of a vertebrate aided by the pinna and the tragus. The
entire external structure has a function similar to that of a funnel, amplifying and then
concentrating sound waves. Vibrations from sound waves cause changes in air pressure,
which travel from the external ear, down the auditory canal, and then move the eardrum
(tympanum). This energy is then conducted through the malleus, incus, and stapes, the
three small bones that constitute the rest of the middle ear. These three bones are key in
the conversion from airborne vibrations to fluid movements. Beneath the stapes is a
membrane called the oval window, which opens into the choclea of the spiral shaped,
fluid filled inner ear. This entire process serves to amplify sound stimuli up to 22 times
before it reaches the cochlea.
How does the ear then change vibration waves to mechanical sound?
The ear converts energy of sound into nerve impulses. This process begins at the
tympanic membrane. The vibrations that move the eardrum, and then consequently the
three additional bones of the middle ear, are transmitted to the oval window. These
vibrations in turn move the fluid of the cochlea. The cochlea is divided into three
longitudinal chambers. The two outer chambers are called the scala tympani, and the
scala vestubuli, and they are both filled with a liquid perilymph that contains high sodium
concentrations. The scala media is the compartment located between these outer two
chambers. The scala media is filled with a fluid endolymph that had high concentrations
of potassium. It also contains the organ of corti.
The sound vibrations that pass by the oval window into the chochlear chambers and
vibrate the tectorial and basilar membranes, eventually dissipate through the membrane
of the round window.
The floor of the chochlea contains the previously mentioned basilar membrane, and the
scala media, containing the organ of corti is where these vibrations undergo the
conversion to neuronal impulses. The organ of corti contain sensory hair cells, and the
waves of fluid in the cochlea press the hair cells against an overhanging tectorial
membrane, and then pull them away. These hair cells are just across synapses from
sensory neurons, and this action provides a stimulus that opens sodium channels in the
sensory cell membranes. This provides for an action potential in the environment of high
potassium concentrations that the endolymph has. Auditory nerves located in a spiral
ganglion carry the action potential to the brain.
The frequency of impulses from action potentials relays information on sound to the
brain. The louder a sound is the greater height or amplitude of the vibrations in the sound
wave, the more movement of hair cells, and thus the more action potentials. Pitch can be
distinguished through differences in sound wave frequencies. Different areas of the
basilar membrane are sensitive to different pitches due to different levels of flexibility
along the membrane. Higher frequencies stimulate the basilar membrane closest to the
oval window, lower frequencies stimulate areas further along. These regions then
stimulate neurons to send the sound signals to specific areas of the brain, and that leads to
the perception of a certain pitch.
How does the insect’s system of mechanoreception compare to that of the
vertebrate’s?
Most insects have ‘ears’ in their legs. A common structure consists of respiratory tracts
called tracheae that lead to a membrane stretched over an internal air chamber. Similar to
a mammal, sound waves stimulate the membrane to vibrate, but in the insect, this directly
activates nerve impulses in attached receptor cells. These nerve impulses then travel to
the central nervous system. Some insects also have a related tracheal system that directs
information on air pressure changes, inside the insect, to the eardrum. If the right
tympanum is stimulated, it will send the signal through the tracheae to the left tympanum.
The delay in stimulus between the left and the right ear helps the insect locate the
direction from which the sound came.
Some insects such as the noctuid moths have ears specially adapted to avoid their
predators; such as bats. The ear structure consists of a tympanic cavity, a membrane, and
three neurons in a scolopida formation. The system is stimulated by the ultrasonic
vibrations of bat cries. One specific neuron is sensitive to the low intensity vibrations
picked up from distant predators, and a different neuron is sensitive to the strong
vibrations of a nearby predator. When the neurons are stimulated they send an action
potential along the tympanic nerve, and the moth can move according to which neurons
have been stimulated.
Some insects also use their sense of mechanoreception to attract mates. At dusk, their
setae stand upright, and they vibrate in accordance to the sound waves sent out by the
hum of the female.
System of mechanoreception in a fish
Many fishes and amphibians have a lateral line system enabling them to experience
mechanoreception. Pores run up both sides of the fish, through which moving water
enters the lateral line system. This leads to stimulation of neuromasts, the receptor cells
of fishes. These neuromasts are located throughout the skin, in channels beneath the
scales of the main body, and in the dermal bones of the head. These function like sensory
hair cells, and vibrations in the water indicating nearby objects or organisms can be
detected. Similar to in the vertebrate, their stimulation leads to an action potential.
Eventually these nerve impulses travel to the brain through sensory neurons.
Fish also have inner ears systems to extend their hearing to higher frequencies. Sound
waves in the water surrounding the fish are conducted as vibrations through the skull.
Then, they travel to chambers similar to those located in the cochlea of the vertebrate,
and move small granules called otoliths. These granules then stimulate sensory hair cells.
Most fish tissue has the same approximate density as water, so vibrations in the water
travel right through a fish’s body. Any structures in the fish that have a significantly
different density vibrate differently. The otolith provides this sensory detection in the
inner ear of a fish. It’s membranes pass the signal on the neighboring sensory hair cells,
and eventually trigger action potentials in the neurons of the auditory nerve.
The gas bladders of some fish also provide an area of variable density. Vibrations pass
through the gas bladder, and travel through a pathway of small bones called Weberian
ossicles. These serve to connect the gas bladder directly with the inner ear of the fish.
Photoreception
Photoreception is the translation of photons of light into electrical and then neuronal
signals.
Structural mechanisms of the vertebrate eye
In vertebrates such as humans, the surface of the eyeball is made up of the sclera, a white
connective tissue, and under that a thin pigmented layer called the choroid. The sclera
contains the cornea which is transparent, and is where light initially enters the eye, and
the choroid contains the iris which contracts and expands to regulate the amount of light
entering the hole in its center, known as the pupil. The rear internal surface of the eye is
the retina, which contains the actual photoreception cells. Between the cornea and the rest
of the eyeball is a clear protein lens. The rest of the eyeball consists of a mass of ‘jelly
like’ vitreous humor, which functions as an additional liquid lens through which to focus
light images.
How does the vertebrate eye operate?
Visual Perception in all animals is based on a conserved mechanism. Specific protein
molecules make up an optical pathway in which light is directed towards a certain
photoreceptive surface in which photoreceptors capture photons. Light initially enters the
eye through the cornea. During this process, light rays are bent, and are then further
refracted upon passage through the lens to form an inverted image on the retina. To focus
on images, most vertebrates change the curve and thickness of the lens. This action is
controlled through ciliary muscles surrounding the lens. They relax, and the lens flattens
out when the organism is viewing a distant object, and they contract to provide a rounded
lens through which to view closer images. Strong ocular muscles direct both left and right
eyes so that images received by each eye travel to the same spots on the two retinas;
producing binocular convergence.
In the retina, there are two types of receptor cells, rods and cones. Rods are for dim light,
and cones are for bright light and color. Rods and cones contain visual pigments made up
of light absorbing retinal molecules. These are bound to proteins called opsins, which
control which pigments are absorbed by each receptor cell. In the rods, this protein is
called rhodopsin, and in bright light, the opsin and retina separate thus making the rods
inactive. Rods are more sensitive to light than cones, which is why they work better in
dim light. This sensitivity is due to the connections with neuronal cells that are
significantly closer than the receptor cell/ neuronal cell connection in cones. This
increased convergence leads to greater magnification of a weak stimuli. When only our
rods are stimulated, such as in dim light, we only see in black and white. Additionally,
because our rods are not part of the fovea, where images are best focused, we can see
images better at night when we don’t look directly at them. In the human there are three
kinds of cones; blue, green, and orange. Each type of cone has specific photopigment
molecules, and each molecule experiences maximum absorption at a different wave
length. All other colors are perceived by the stimulation of two or more cone types.
In vertebrates such as human beings, there is a specialized portion of the retina called the
fovea. This area provides for our high visual acuity. This region only contains cones, and
this enables the human to see in great detail. This area of the eye is most efficiently taken
advantage of during the day when the photoreceptor cells of the cones that dominate the
fovea, are best able to absorb light. The light hitting the receptor cells, rods or cones,
produces a charge gradient across the membrane, but this is not an action potential. These
cells, however, synapse with neurons that then synapse with ganglion cells. These convey
the image message as an action potential to the brain along optic nerves. The optic nerves
from the right and left eyes meet at an optic chiasma in the brain.
To demonstrate the blind spot, cover your left eye and look at the "O" below directly with
your right eye. You should also be able to see the "+" even though you aren't looking
directly at it. Now slowly move closer to the screen (or page), keeping you right eye
focused on the "O". The image of the "+" should disappear, then reappear as you
continue to move closer. This is because the image of the "+" moved across your blind
spot as you moved closer..
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Why do some animals see in black and white?
Many animals are nocturnal, and have increased amounts of rods in their optical systems.
The cones that control color vision, are really unnecessary or are needed in extremely
small quantities.
What kind of photoreception systems do insects have?
Vertebrate and insect eyes have vastly different morphology and structure, although they
operate under very similar photochemical systems. The compound eye of most insects
has many facets. Behind the corneal lens of each facet, there are functional units called
ommatidium. The receptor cells within the ommatidium each detect a very small fraction
of the spectrum of light that the eye as a whole is exposed to; like the rods and cones of
the vertebrate eye. In compound eyes, the photoreception cells are called retinular cells,
and they surround a single eccentric cell. The receptor cells have a specific portion of
membrane, designated as a rhabdomere, which has a high density of microvilli.
Rhodopsin, a photoreceptive pigment molecule, is contained in this rhabdomere, and this
protein absorbs the photons of light energy that enter the eye. This then provides for
amplification of this light signal through a G-Protein directed reaction. Ion channels in
the cells open, allowing calcium ions to enter the cell. This is the basis for a current
traveling down the receptor cell axon, which crosses gap junctions and reaches the
dendrite of the adjacent eccentric cell. This eccentric cell then depolarized and generates
action potentials. These travel through the optic nerve to the Central Nervous System.
Within each ommatiduim, different retinular cells are sensitive to different colors due to
protein variations with in the rhodopsin. Most insects are equipped to see further along
the short wavelength end of the color spectrum, towards ultra-violet, however, they don’t
see into the reds, which make up the longer wavelengths that vertebrates can see.
How do fish see?
The optical system in fish is very similar to that of the land vertebrates, however, there
are some important differences. The fish has a more spherical shaped lens than the land
dwellers. Fish focus by changing the relative distance between the lens and the retina,
where as other vertebrates change the curvature of their more flexible lens. Fish have
choroids which contain a special structure, the tapetum lucidum, and this contains very
reflective guanine crystals to aid in dim light vision. This is very important because of the
lowered amount of light that penetrates the fish’s watery environment. Additionally,
many deep-sea fish have only these and rods, for increased low light sensitivity. They
even have epithelial layers for the specific purpose of protection from bright light.
Fish with cones generally have four types, red, green, blue, and ultraviolet. Some only
have two or three of these possibilities; fish with all four usually live close to the water’s
surface, and may have further special adaptations. Some fish have upwardly directed
eyes, especially those who are preyed upon by birds. Some deep sea fishes have tubular
eyes, which help to concentrate the limited light that penetrates to great depths. The
South American "four-eyed fish " swims along the surface, with it's eyes protruding
partly out of the water. Each of its two eyes is split into an upper half for vision in the air
and a lower half for underwater vision.
Specific mechanisms of the conversion of light stimulation to neuronal impulses
Visual pigment molecules are the specific structures that absorb photons of light. These
pigment molecules are made up of an opsin such as rhodopsin, and an actual light
absorbing component, which is usually retinal. When a photon of light hits this molecule,
the normal cis-configuration of the retinal is isomerized into a trans-configuration. This
in turn leads to a separation between the opsin and the retinal molecule, and eventually to
changes in the opsin’s conformation too. When the light is absorbed by the retinal,
proteins that are associated with the cell membrane are activated. This alters the cell's
membrane potential, and can eventually lead to an action potential, which is carried to the
brain via a sensory neuron.
At a cell’s resting state, there is a certain concentration of each ion in and outside of the
cell membrane. This provides for potential diffusions across the membrane. However,
there are also charges on each of these ions, and there are polar gradients that do no
necessarily correlate to the diffusion gradients. There is usually a stronger negative
charge within the cell and a stronger positive charge outside of the cell. Positive ions are
attracted to the cell membrane because of the negative interior, and vice versa. The only
way that these ions can travel through the selectively permeable membrane, though, is
through specific ion channels. Some of the channels are normally open, and stimulation
of the cell closes them, and others are closed at resting state, and open in response to
stimulation. Such a stimulus could be the absorption of a photon of light by retinal, and
when this occurs, there are two possible results. The cell can become hyperpolarized, or
depolarized. In retinal for example, sodium channels close in the presence of light, and
potassium continues to move out of the cell. This makes the cell environment even more
positive, and the inside more negative. This is hyperpolarization. A lessening in the
charge of difference between the in and outside of the cell would conversely be
depolarization. Each of these conditions leads to charges that travel across synapses to
neuronal cells. This alters the firing rate of action potentials in the adjacent neurons.
Thermoreception in snakes
Temperature receptors in some snakes can be extremely sensitive. The infrared detectors
in the facial pits of rattlesnakes are an excellent example. The sensory axons from the pit
organs increase the rate of action potentials when the temperature inside the facial pit
increases by 0.002 0 C. A rattlesnake can detect the body heat of a mouse standing 40cm
away if the mouse’s body temperature is at least 10 0C above the surrounding air
temperature. Because the snakes have a pit on each side of its head, they can tell the
direction of the source of heat.
Muscle Tissues
Muscle fiber generates tension through the action of actin and myosin cross-bridge
cycling. While under tension, the muscle may lengthen, shorten or remain the same.
Though the term 'contraction' implies shortening, when referring to the muscular system
it means muscle fibers generating tension with the help of motor neurons (the terms
twitch tension, twitch force and fiber contraction are also used).
Voluntary muscle contraction is controlled by the central nervous system. Voluntary
muscle contraction occurs as a result of conscious effort originating in the brain. The
brain sends signals, in the form of action potentials, through the nervous system to the
motor neuron that innervates several muscle fibers. In the case of some reflexes, the
signal to contract can originate in the spinal cord through a feedback loop with the grey
matter. Involuntary muscles such as the heart or smooth muscles in the gut and vascular
system contract as a result of non-conscious brain activity or stimuli proceeding in the
body to the muscle itself.
There are three general types of muscle tissues:
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Skeletal muscle responsible for movement
Cardiac muscle responsible for pumping blood
Smooth muscle responsible for sustained contractions in the blood vessels,
gastrointestinal tract and other areas in the body
Skeletal and cardiac muscles are called striated muscle because of their striped
appearance under a microscope which is due to the highly organized alternating pattern
of A band and I band.
While nerve impulse profiles are, for the most part, always the same, skeletal muscles are
able to produce varying levels of contractile force. This phenomenon can be best
explained by Force Summation. Force Summation describes the addition of individual
twitch contractions to increase the intensity of overall muscle contraction. This can be
achieved in two ways: (1) by increasing the number and size of contractile units
simultaneously, called multiple fiber summation, and (2) by increasing the frequency at
which action potentials are sent to muscle fibers, called frequency summation.
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Multiple fiber summation – When a weak signal is sent by the CNS to contract a
muscle, the smaller motor units, being more excitable than the larger ones, are
stimulated first. As the strength of the signal increases, more motor units are
excited in addition to larger ones, with the largest motor units having as much as
50 times the contractile strength as the smaller ones. As more and larger motor
units are activated, the force of muscle contraction becomes progressively
stronger. A concept known as the size principle allows for a gradation of muscle
force during weak contraction to occur in small steps, which then become
progressively larger when greater amounts of force are required.
Frequency summation - For skeletal muscles, the force exerted by the muscle is
controlled by varying the frequency at which action potentials are sent to muscle
fibers. Action potentials do not arrive at muscles synchronously, and during a
contraction some fraction of the fibers in the muscle will be firing at any given
time. Typically when a human is exerting a muscle as hard as they are
consciously able, roughly one-third of the fibers in that muscle will be firing at
once, but various physiological and psychological factors (including Golgi tendon
organs and Renshaw cells) can affect that. This 'low' level of contraction is a
protective mechanism to prevent avulsion of the tendon - the force generated by a
95% contraction of all fibers is sufficient to damage the body.
Skeletal muscle contractions
Skeletal muscles contract according to the sliding filament model:
1. An action potential originating in the CNS reaches an alpha motor neuron, which
then transmits an action potential down its own axon.
2. The action potential propagates by activating sodium dependent channels along
the axon toward the synaptic cleft. Eventually, the action potential reaches the
motor neuron terminal and causes a calcium ion influx through the calciumdependent channels.
3. The Ca2+ influx causes vesicles containing the neurotransmitter acetylcholine to
fuse with the plasma membrane, releasing acetylcholine out into the extracellular
space between the motor neuron terminal and the motor end plate of the skeletal
muscle fiber.
4. The acetylcholine diffuses across the synapse and binds to and activates nicotinic
acetylcholine receptors on the motor end plate of the muscle cell. Activation of
the nicotinic receptor opens its intrinsic sodium/potassium channel, causing
sodium to rush in and potassium to trickle out. Because the channel is more
permeable to sodium, the muscle fiber membrane becomes more positively
charged, triggering an action potential.
5. The action potential spreads through the muscle fiber's network of T-tubules,
depolarizing the inner portion of the muscle fiber.
6. The depolarization activates L-type voltage-dependent calcium channels
(dihydropyridine receptors) in the T tubule membrane, which are in close
proximity to calcium-release channels (ryanodine receptors) in the adjacent
sarcoplasmic reticulum.
7. Activated voltage-gated calcium channels physically interact with calcium-release
channels to activate them, causing the sarcoplasmic reticulum to release calcium.
8. The calcium binds to the troponin C present on the actin-containing thin filaments
of the myofibrils. The troponin then allosterically modulates the tropomyosin.
Normally the tropomyosin sterically obstructs binding sites for myosin on the thin
filament; once calcium binds to the troponin C and causes an allosteric change in
the troponin protein, troponin T allows tropomyosin to move, unblocking the
binding sites.
9. Myosin (which has ADP and inorganic phosphate bound to its nucleotide binding
pocket and is in a ready state) binds to the newly uncovered binding sites on the
thin filament (binding to the thin filament is very tightly coupled to the release of
inorganic phosphate). Myosin is now bound to actin in the strong binding state.
The release of ADP and inorganic phosphate are tightly coupled to the power
stroke (actin acts as a cofactor in the release of inorganic phosphate, expediting
the release). This will pull the Z-bands towards each other, thus shortening the
sarcomere and the I-band.
10. ATP binds myosin, allowing it to release actin and be in the weak binding state (a
lack of ATP makes this step impossible, resulting in the rigor state characteristic
of rigor mortis). The myosin then hydrolyzes the ATP and uses the energy to
move into the "cocked back" conformation. In general, evidence (predicted and in
vivo) indicates that each skeletal muscle myosin head moves 10-12 nm each
power stroke, however there is also evidence (in vitro) of variations (smaller and
larger) that appear specific to the myosin isoform.
11. Steps 9 and 10 repeat as long as ATP is available and calcium is present on thin
filament.
12. While the above steps are occurring, calcium is actively pumped back into the
sarcoplasmic reticulum. When calcium is no longer present on the thin filament,
the tropomyosin changes conformation back to its previous state so as to block the
binding sites again. The myosin ceases binding to the thin filament, and the
contractions cease.
The calcium ions leave the troponin molecule in order to maintain the calcium ion
concentration in the sarcoplasm. The active pumping of calcium ions into the
sarcoplasmic reticulum creates a deficiency in the fluid around the myofibrils. This
causes the removal of calcium ions from the troponin. Thus the tropomyosin-troponin
complex again covers the binding sites on the actin filaments and contraction ceases.
Classification of voluntary muscular contractions
Voluntary muscular contractions can be classified according to either length changes or
force levels. In spite of the fact that the muscle only actually shortens in concentric
contractions, all are typically referred to as "contractions".
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In concentric contraction, the force generated is sufficient to overcome the
resistance, and the muscle shortens as it contracts. This is what most people think
of as a muscle contraction.
In eccentric contraction, the force generated is insufficient to overcome the
external load on the muscle and the muscle fibers lengthen as they contract. An
eccentric contraction is used as a means of decelerating a body part or object, or
lowering a load gently rather than letting it drop.
In isometric contraction, the muscle remains the same length. An example would
be holding an object up without moving it; the muscular force precisely matches
the load, and no movement results.
In isotonic contraction, the tension in the muscle remains constant despite a
change in muscle length. This can occur only when a muscle's maximal force of
contraction exceeds the total load on the muscle.
In isovelocity contraction (sometimes called "isokinetic"), the muscle contraction
velocity remains constant, while force is allowed to vary. True isovelocity
contractions are rare in the body, and are primarily an analysis method used in
experiments on isolated muscles which have been dissected out of the organism.
Smooth muscle contraction
The interaction of sliding actin and myosin filaments is similar in smooth muscle. There
are differences in the proteins involved in contraction in vertebrate smooth muscle
compared to cardiac and skeletal muscle. Smooth muscle does not contain troponin, but
does contain the thin filament protein tropomyosin and other notable proteins-caldesmon
and calponin. Contractions are initiated by the calcium activated phosphorylation of
myosin rather than calcium binding to troponin. Contractions in vertebrate smooth
muscle are initiated by agents that increase intracellular calcium. This is a process of
depolarizing the sarcolemma and extracellular calcium entering through L type calcium
channels, and intracellular calcium release predominately from the sarcoplasmic
reticulum. Calcium release from the sarcoplasmic reticulum is from Ryanodine receptor
channels (calcium sparks) by a redox process and Inositol triphosphate receptor channels
by the second messenger inositol triphosphate. The intracellular calcium binds with
calmodulin which then binds and activates myosin-light chain kinase. The calciumcalmodulin-myosin light chain kinase complex phosphorylates myosin, specifically on
the 20 kilodalton (kDa) myosin light chains on amino acid residue-serine 19 to initiate
contraction and activate the myosin ATPase. The phosphorylation of caldesmon and
calponin by various kinases is suspected to play a role in smooth muscle contraction.
Phosphorylation of the 20 kDa myosin light chains correlates well with the shortening
velocity of smooth muscle. During this period there is a rapid burst of energy utilization
as measured by oxygen consumption. Within a few minutes of initiation the calcium level
markedly decrease, the 20 kDa myosin light chains phosphorylation decreases, and
energy utilization decreases, however there is a sustained maintenance of force in tonic
smooth muscle. During contraction of muscle, rapidly cycling crossbridges form between
activated actin and phosphorylated myosin generating force. The maintenance of force is
hypothesized to result from dephosphorylated "latch-bridges" that slowly cycle and
maintain force. A number of kinases such as ROCK, Zip kinase, and Protein Kinase C are
believed to participate in the sustained phase of contraction, and calcium flux may be
significant.
SOMESTHESIA: PERIPHERAL MECHANISMS
The broadest definition of somesthesia is the awareness of having a body and the ability
to sense the contact it has with its surroundings. Receptors are generally put into two
broad classes: the exteroceptors, that sense stimuli from outside the body and signal
what is happening in the outside world, and the enteroceptors, that receive stimuli from
inside the body and tell us what is happening in the inside world. The broad class of
exteroceptors includes, in addition to receptors in the skin, receptors for light in the eye,
sound in the ear, and for chemical substances in the nasal mucosa and tongue.
The Exteroceptors
The skin serves many functions:





as protection from injury and dehydration
as a radiation surface and regulator in temperature maintenance
in secretion of chemical substances, such as pheromones that function as
attractants or repellents
as camouflage due to coloration in some species
in reception of mechanical, thermal and, to some extent, chemical stimulation
From our present
point of view, we
may think of the
skin as a sheet of
sensory receptors
held together and
supported by a
network
of
connective tissue
and blood vessels.
Figure 5-1 shows
a cross section
through
a
transitional region
between glabrous
and hairy skin.
The outer layer or
epidermis
is
composed of four
to five layers of
cells
and
connective tissue
and is devoid of Fig. 5-1. A section through a trasitional region between glabrous and
blood vessels. The hairy skin showing the locations and arrangements of various dermal and
epidermis receives epidermal receptors (Warwick R and Williams PL [ed]: Gray's Anatomy,
its nutrients from 35th ed. Philadelphia, WB Saunders, 1973).
the
dermis
immediately
beneath it. The dermis consists mainly of loose connective tissue. Nerve fibers course
into the skin through the dermis, and many of them end at the dermal-epidermal border
where many of the sensory receptor structures are located. Figure 5-1 shows several of
the types of receptors that are typical of skin. Structure A is a typical hair follicle-note
that all hairs are innervated and thus serve as sensory receptors. The nerve fibers
associated with a hair enter the follicle and follow a wandering course up and down along
the root sheath and also around it. This winding pattern of the nerve fiber may determine
how the receptor responds to hair movement, but as yet we do not know how. In addition
to hair follicles, there are many encapsulated nerve endings found at the dermalepidermal border. These are endings surrounded by specialized structures; a few of the
types are shown in the figure (B-F). These structures vary somewhat in form so that it is
not always clear in which class a particular structure belongs. The largest class of
receptors is that with no specialized structure at all, the free nerve endings (G). Near their
termination, the nerve fibers simply branch many times, and the many tiny terminal
"twigs" lie in the dermis, near the border between the dermis and epidermis, or
sometimes penetrate into the epidermis itself.
Many attempts have been made to associate different receptor structures with particular
sensations, but there appears to be no clear relationship between structure and sensation.
One problem is that the sensations associated with skin are surprisingly complex. Nearly
everyone allows that there are (1) mechanical sensations, (2) thermal sensations, and
(3) nociceptive or pain sensations, but only some will divide mechanical sensations into
touch, pressure, and pinch, whereas others maintain that the list should also include
vibration, tickle, itch, and perhaps others. Clearly, we may have more describable
sensations than we have receptor types to account for them.
The problem is further compounded if we realize that we experience all of the normal
skin sensations on the pinna or auricle, the external part of the ear, yet the pinna probably
has only free nerve endings. Similarly, the cornea of the eye can sense temperature and
pain, but has only free nerve endings. Although there is not a one-to-one relationship
between receptor structure and sensation, that is not to say that there is no relationship at
all. Free nerve endings are usually associated with the sensations of pain, temperature,
and what many call crude touch, a sensation that requires firm pressure to elicit and is
difficult to localize. The encapsulated endings are associated with light touch and
pressure when they lie superficially within the skin and with deep pressure and tissue
deformation when they lie deep within the tissue. Hair receptors, of course, can be
associated with a class of sensations that accompany hair movement; these sensations
have no special terminology.
Mechanical sensations
If recordings are made from the
sensory nerve fibers innervating
particular cutaneous receptors, the
stimuli that best excite each type of
receptor, the adequate stimuli, can
be identified. Examples
of
recordings from four different
primary afferent fibers serving four
different kinds of receptors are
shown in Figure 5-2. A monitor of
the mechanical displacement of the
structure is shown in trace 5-the
probe indented the receptors in
traces 1, 3 and 4, and pushed the
hair laterally in trace 2. Traces 1 to
4 show the spike discharges
recorded from the fibers, the
primary afferent fibers. The
bottom trace is a time scale, with
each division representing 100 Fig. 5-2. Responses of cutaneous primary afferent
msec. The discharge pattern of the fibers. A mechanical indentation or displacement was
Pacinian corpuscle should already applied to four different types of receptors, the monitor
of the movement being shown in trace 5 (numbered
be familiar. The fiber discharges from the top down). The action potential responses of
when the receptor is compressed two rapidly adapting fibers are shown in traces 1 and 2:
and again when the receptor is They respond only at the onset or offset of the stimulus.
restored to its resting state-the Responses of the two slowly adapting fibers are shown
discharge is rapidly adapting or in traces 3 and 4: They discharge throughout the
phasic. The same kind of discharge stimulus. A 100-msec time base is shown in trace 6.
pattern is seen in recordings from
afferent fibers associated with hairs
when a hair (trace 2) is displaced. The hair receptors are all rapidly adapting, i.e., they are
incapable of signaling sustained stimulation. On the other hand, the slowly adapting
receptors, types I and II, can signal the presence of a sustained stimulus. They begin
discharging with the indentation and continue to discharge until the stimulus is removed.
The type I receptor is a receptor associated with a Merkel's disk, whereas the type II is
associated with a Ruffini ending.
A receptive field is the area of skin over
which the application of a stimulus excites
a primary afferent fiber.
Receptive fields
The area of skin over which the application of a stimulus excites a given primary afferent
fiber is called the receptive field of that fiber. As far as we know, a primary afferent
neuron only innervates one particular type of receptor, though it may innervate a number
of individual receptors of that type. For example, a hair afferent neuron may innervate
anywhere from a few to 100 hairs and a given hair may receive innervation from 2 to 20
different fibers. Thus, there is considerable overlap in the receptive fields of different
fibers. The size of a receptive field varies over the body surface, with those located on the
extremities being the smallest, of the order of a few square millimeters on the digits,
growing in size along the leg or arm, and reaching a maximum size on the trunk. This
arrangement might account, in part, for the observed distribution in two-point
thresholds, a commonly used measure of touch sensitivity. Two-point thresholds can be
tested by using an ordinary pair of dividers. When the closed dividers are touched to the
skin, the perception is of being touched with only a single point. As the dividers are
opened more and more on successive applications to the skin, a separation of the points is
reached at which the perception is of being touched with two points. The separation at
which this first happens is the two-point threshold.
Temperature sensations
Because of the high touch receptor density in some areas, touch sensitivity sometimes
appears to be uniformly distributed. In contrast, temperature sensitivity is always
punctate or localized to small spots on the skin. We speak of "warm spots" and "cold
spots" on the skin that are areas sensitive to upward and downward changes in skin
temperature, surrounded by areas of virtual insensitivity to changes in temperature. The
low density of temperature-sensitive spots is indicated in Table 5-1. At no place is the
density of temperature spots as high as is the lowest touch-spot density. Note also the low
density of warm spots compared to cold spots.
Table 5-1
Sensitive Spots Per Square Centimetera
Touchb
Pain
Ball of thumb
120
60
Tip of nose
100
Forehead
Chest
Cold
Warmth
44
13
1.0
50
184
8
0.6
29
196
9
0.3
Volar side of forearm
15
203
6
0.4
Back of hand
14
188
7
0.5
a
Data from Woodworth RS, Schlosberg H: Experimental Psychology. New
York, Holt, Rinehart and Winston, 1965.
b
Arranged in descending touch-spot density.
Pain sensations
The experience of pain is influenced by prior experience; by the meaning of the situation
in which it occurs; by attention, anxiety and suggestion; and by the sensory adaptation
level of the individual.
Prior experience with a stimulus can cause that stimulus to be perceived as either more or
less painful, depending upon the nature of the experience. Painful stimulation, repeated in
a psychological trauma-producing situation, may tend to make similar stimulation in the
future more painful, whereas painful stimulation, repeated in otherwise pleasant
surroundings, may tend to make future stimulation less painful.
Pricking pain is a short-duration pain; burning pain is a cutaneous pain
that continues.
All pain has two psychological aspects: one discriminative, that is, we can objectively
gauge its intensity, location, and quality, and the other affective or emotional, pain causes
suffering. It is important to distinguish the discriminative aspect from the affective aspect
of pain. The importance of this distinction is highlighted by the fact that the two aspects
can be dissociated by the proper clinical maneuvers, suggesting that different parts of the
nervous system are involved. For example, separation of the prefrontal lobes from the
rest of the cerebral cortex in the patient with intractable pain leaves the patient with his
pain sensation intact, but the pain no longer bothers him. The suffering is eliminated even
though the pain is not. The affective aspect of pain depends upon the integrity of cerebral
cortical function; the discriminative aspect apparently does not.
The Enteroceptors
Joint sensations
Though joints differ in the range and direction of their movement, most have an enclosed
cavity filled with synovial fluid and are surrounded by cartilage. Free nerve endings are
abundant in the articular cartilage and nearly everywhere around the joint. In addition,
there are spray-like endings in the joint capsule and encapsulated corpuscles both on and
in the capsule. Free nerve endings arise from both myelinated and unmyelinated fibers in
the articular nerves, whereas spray-like endings and corpuscles arise from myelinated
fibers only.
Originally, it was thought that the sense of the position of the joint, that is, the angle
between the bones of the joint, was signaled by the myelinated fibers of the articular
nerve leaving that joint. Recent studies indicate that most of these fibers do not, in fact,
signal the static position of the limb. This is because they fail to discharge at any position
but the extremes of flexion and extension. In addition, anesthetizing the human knee joint
does not diminish position sense for that joint. It appears that the most likely candidate
for signaling joint angle would be the muscle spindle receptors or group Ia or II afferent
fibers.
Vision
Most objects reflect light, and because light travels at high speed, it is possible to nearly
instantly assess their shape, size, position, speed, and direction of movement. The light
rays emanating from an object are gathered and focused onto an array of photoreceptors.
Activities generated in the different photoreceptors by the light interact to produce a twodimensional representation of the object which is transmitted to the brain. The brain then
reconstructs a three-dimensional representation using information received from the two
eyes. The end-products of the activity of the visual system are sensations that represent
the object and its surroundings. These sensations can be used to guide our immediate
behavior, or they can be
stored
for
future
reference.
Figure 7-1 shows a cross
section
through
the
human eye. It consists of
two fluid-filled chambers
separated by a transparent
structure, the lens. Nearly
the entire eye is covered
with a tough, fibrous
coating called the sclera
that is modified anteriorly
to form the transparent
cornea.
The
human
cornea is about 12 mm in
diameter, about 0.5 mm
thick in the center and
0.75 to 1 mm thick on the
edge, and it is made of the Fig. 7-1. A section through the human eye illustrating the major
same
collagenous structures. (Walls GL: The Vertebrate Eye and its Adaptive
connective
tissue Radiations. New York, Hafner, 1967)
substance as is the sclera, but the fibers of the cornea are oriented in parallel arrays that
let light pass through with minimal scatter, whereas fibers of the sclera are random and
light rays are scattered when passing through. The result is that light passes easily
through the cornea, but not through the sclera. Lining the inside of the posterior twothirds of the sclera are two membranes: the choroid, a pigment layer containing the
vascular supply for the eyeball as well as mechanisms for maintaining the integrity of the
photoreceptors, and the retina that contains the photoreceptors and other neural elements
essential to our visual process.
Visual neurons in the retina
The receptor cells and the bipolar cells of the retina respond to light with graded,
electrotonic responses rather than all-or-nothing action potentials. The graded responses
in the receptors are the result of the photochemical process, but those in the bipolar cells
are synaptically driven. Furthermore, it may be surprising that the receptors respond to
light with an hyperpolarizing receptor potential, that is accompanied by an increase in
membrane resistance. Figure 7-17 shows a schematic diagram of a section of the retina
with two receptors, one illuminated, the other unilluminated. The responses of the
receptors, bipolar, ganglion, horizontal and amacrine cells are shown in circles
representing each cell type. The time when a small spot of light was turned on is
indicated by the upward deflection of the lower trace of each pair and the response of the
cell by the upper trace. The hyperpolarizing responses of the illuminated receptor and its
subjacent bipolar cell are illustrated as are the action potentials generated by the ganglion
cell. It is now known that in the dark there is a constant inward Na+ current (dark current)
flowing through the outer segment membrane and an outward current near the junction
with the bipolar cell. This keeps the cell partly hypopolarized, and transmitter substance
is continuously released onto the bipolar cell hypopolarizing it. The light flash decreases
the dark current by the action of calcium to reduce membrane conductance,
hyperpolarizes the cell relative to its dark state and decreases the amount of transmitter
released onto the bipolar cell. If this scenario is correct, then adding excess magnesium to
the bathing solution should cause bipolar and ganglion cells to behave as if their receptors
were illuminated because excess magnesium blocks the release of transmitter substances
at chemical synapses. A release of transmitter substance blocked by magnesium should
be the same as one blocked by light, and it is. This scheme accounts mechanistically for
the curious hyperpolarizing receptor potentials.
Fig. 7-17. Synaptic organization of the vertebrate retina and responses of retinal neurons. The
receptor on the left is illuminated, that on the right is not. The intracellularly recorded response
from each cell is illustrated in the circle corresponding to its cell body (upper trace) along with a
monitor of when the illumination was present over the left receptor. The responses were recorded
in the retina of Necturus. (Dowling JE, Werblin FS: Vision Res Suppl 3:1-15, 1971)
The response recorded from ganglion cells following light stimulation of the receptors is
more conventional. Hypopolarizing synaptic potentials initiate trains of action potentials
that propagate along the ganglion cell's axon. Ganglion cell responses are of three types:
(1) they respond (either discharge or increase their rate of discharge) only when the light
is turned on: (2) they respond only when the light is turned off; or (3) they discharge only
at the beginning and end of a light period. These are called on-responses, off-responses
and on-off-responses, respectively. The receptive fields of ganglion cells are usually
circular areas of retina, as shown in Figure 7-18.
Fig. 7-18. Receptive fields of two retinal ganglion cells. Fields are circuluar areas of the retina
surrounded by an annulus of different properties. The cell in the upper part of the figure responds
when the center is illuminated (on-center, a) and when the surround is darkened (off surround, b).
The cell in the lower part of the figure responds when the center is darkened (off-center, d) and
when the surround is illuminated (on-surround, e). Both cells give on- and off- responses when
both center and surround are illuminated (c and f), but neither response is as strong as when only
center or surround is illuminated. (Hubel DH: Sci Amer 209:54-62, 1963)
When a ganglion cell fails to discharge at an on- or off-transient of the light, it is not
because it is not being excited, but because it is being inhibited, as is indicated by the fact
that illumination of both center and surround simultaneously produces both a reduced onresponse and a reduced off-response (c and e) compared to those produced by
illuminating either center or surround alone. This effect is termed lateral inhibition or
surround inhibition.
Visual neurons outside the retina
The axons of the ganglion cells form the optic nerves, which, after leaving the eyeball,
proceed toward the brain until they come to the optic chiasm, where the optic nerves
divide. Fibers from the nasal half of the retina cross to the opposite side of the brain;
fibers from the temporal half go to the same side of the brain (Fig. 7-19). Past the chiasm,
crossed fibers from the contralateral eye join the uncrossed fibers from the ipsilateral eye
to form the optic tract. Fibers in the optic tract, which are still the axons of retinal
ganglion cells, then proceed to the thalamus, where they end on cells of the lateral
geniculate nucleus. The fibers of this nucleus project to neurons of the calcarine area of
the occipital cortex.
Fig. 7-19. The anatomic organization of the visual pathway from the retina to the visual cortex.
Lesions of the visual pathway (a-g) produce defects in the visual fields as indicated at the right.
(Homans A: Textbook of Surgery, 5th ed. Springfield, IL. C.C. Thomas, 1941)
The retinal receptive fields of neurons in the occipital cortex are complicated. Some
visual cortical neurons (non-oriented neurons) possess receptive fields not particularly
different from those of geniculate neurons; they have circular receptive fields and
respond equally to stimuli of all orientations. However, the receptive fields of most
cortical neurons are not circular, most are arranged as parallel barlike excitatory and
inhibitory areas with straight, rather than circular borders. Cortical cells with this
characteristic receptive field are termed simple cells. In one sample of cortical neurons,
two-thirds of the cells had simple receptive fields. These cells can have two inhibitory
regions flanking an excitatory region, as in Figure 7-20; the reverse situation can occur;
or there can be just two regions side by side, one excitatory, the other inhibitory. As a
result the cells respond best to narrow bars of light oriented in a particular direction
across the retina. Sample recordings are shown for three different orientations of such a
bar in Figure 7-20. Rotation of the bar has two effects: (1) it reduces the excitation,
because less of the excitatory area is illuminated and (2) it increases the inhibition,
because more of the inhibitory area is illuminated. The result is that the cortical neuron
responds less well when bars are not in their preferred orientation.
Fig. 7-20. The receptive field on the retina (ellipse) of a simple cell in the visual cortex is shown
at the left with a bar of light superimposed on it at various angles. The vertically striped portion
of the receptive field is the excitatory area (excitatory receptive field), illumination of which
excites the cell; the unhatched portion is the inhibitory area (inhibitory receptive field),
illumination of which inhibits the cell. The bar of light is striped across its length. The responses
of the cell to the three different orientations of the light are shown at the right. (Hubel DH: Sci
Amer 209:54-62, 1963)
Audition
An object vibrating in air sets up motion of the molecules in the air around it so that when
the object moves in the direction of an observer, it compresses the air and when it moves
away, it produces a rarefaction. This sequence of compressions and rarefactions is
transmitted in a straight line through the air at a characteristic speed. Sound waves, unlike
light, cannot travel through a vacuum, but require some medium, gaseous, liquid or solid,
each medium with a different speed of conduction. The ear is a structure specialized to
receive these vibrations of the air, transduce them into nervous impulses, encoding
important features, and transmit the impulses to the central nervous system; the result is
what we call hearing.
Highly sensitive mechanoreceptors in the ear are capable of sensing amplitudes of
vibrations of the air as small as 10-8 centimeters (the diameter of a hydrogen ion is 2 x 10-
8
cm), yet the ear is structured in such a way that it can withstand sound so intense that it
vibrates the whole body.
The ear is normally considered to have three parts, an outer ear, a middle ear, and an
inner ear, as illustrated in Figure 8-3.
Fig. 8-3. The peripheral auditory apparatus with cochlea rotated slightly to show coils and
auditory nerve. (Davis H, Silverman SR: Hearing and Deafness, 3rd ed. New York, Holt,
Rinehart, Winston, 1970)
Central auditory pathways
Primary auditory fibers enter the brain stem and immediately make connections with
secondary neurons in the cochlear nucleus, as illustrated in Figure 8-14. From here, the
auditory information goes to a remarkable number of places in the central nervous
system. Fibers arising from the cochlear nuclei ascend in both a crossed and an uncrossed
projection, which either enters the lateral lemniscus directly or first relays in the nucleus
of the trapezoid body or the superior olive before joining the lateral lemniscus.
The lateral lemniscus contains both second and third-order neurons that project either
directly or indirectly to the inferior colliculus, which is an obligatory relay for all
auditory fibers. Cells of the inferior colliculus project to the medial geniculate nucleus
bilaterally, and the medial geniculate nucleus projects to the primary auditory cerebral
cortex, located on the superior and medial aspect of the temporal lobe, as indicated in
Figure 8-15. The responses of neurons in the various nuclei of the central auditory system
resemble those of primary auditory neurons in many ways, but they also differ in
important ways. As shown in Figure 8-13, the cells of the trapezoid body (b), inferior
colliculus (c), medial geniculate nucleus (d), and primary auditory cortex (e) respond to
quite a wide range of sound frequencies and exhibit tuning curves reminiscent of those
for auditory nerve fibers. The curves are narrower (i.e., the range of frequencies that
causes the cell to discharge is smaller) for higher order neurons (indicated by numbers in
Fig. 8-14) up to the level of the medial geniculate nucleus, and then they get wider again.
This narrowing represents a sharpening of the cells' frequency discrimination abilities and
probably results from a process like lateral inhibition, where cells with close best
frequencies inhibit each other. Some investigators have concluded on the basis of this
observation that frequency and intensity discriminations are accomplished at or before
the medial geniculate level, because cortical neurons simply do not have fine
discriminative behavior (i.e., they have wide tuning curves). This is probably true, at least
in animals.
Neurons in the auditory nerve
change their frequency of discharge
with changes in sound intensity.
Sample intensity functions for these
and other cells of the auditory
system are illustrated in Figure 816. The relationship between
intensity and discharge frequency is
sigmoid for auditory nerve fibers
and cells of the trapezoid body and
the superior olive. These cells
increase
their
frequency
of
discharge with increasing sound
intensity, slowly at first and then
more rapidly, to a certain intensity,
at which the frequency reaches a
maximum.
At
even
greater
intensities, there is no further
increase in frequency. Cells of the
medial geniculate nucleus and the
auditory cortex do not increase their
frequency of discharge with
increasing intensity of sound. (This
is the reason for claiming that
intensity discriminations occur
below this level.) Neurons at
successively higher levels of the
Fig. 8-14. Central auditory pathways. First, second,
auditory system discharge fewer
third, and fourth-order cells are indicated by the
spikes in response to a standard
numerals.
tone. Below the level of the inferior
colliculus, neurons give repetitive responses to maintained tone stimuli, but at and above
the level of the colliculus, they more often give on-, off-, or on-off responses (these terms
are used here in the same manner as for visual cells. In the auditory cortex, there are
seldom continuous responses to sustained tones; most cells signal sound onset or offset or
both.
However, sounds that we hear
are seldom pure tones (i.e.,
tones of a single frequency) as
were those employed in the
preceding experiments. More
often, they are composites of
sine waves of different
frequencies and amplitudes.
We know little about how the
nervous system deals with
such complex sounds, but a
beginning has been made. In
the cochlear and medial
geniculate
nuclei,
the
response to a tone is
unaffected by a second tone Fig. 8-15. Diagram to indicate the location of the primary
(of
different
frequency) auditory cortex on the superior lip of the temporal lobe.
presented simultaneously at (Guyton A: Textbook of Physiology. Philadelphia, WB
weak intensity. As the second Saunders, 1976)
tone is increased in intensity,
the response to the first tone is gradually suppressed until it finally fails completely. At
this intensity, only responses to the second tone are seen if its frequency lies within the
response range of the neuron. In the cerebral cortex, the same maneuver performed on a
cell with high best frequency results in enhancement of the cell's response if the tones are
harmonically related (i.e., if the ratio of their frequencies is 1:2, 1:3, 1:4) or if the
difference in frequency is such that it results in 50 to 200 beats/sec. The greater the
frequency of beats, the larger the discharge up to 100 beats/sec. Above 100, the response
falls off again. It is tempting to speculate that cortical neurons are playing some role in
decoding complex sounds or tone patterns, somehow using beats and harmonics.
Fig. 8-16. Intensity functions for neurons in the cochlear nerve, trapezoid body, superior olivary
complx, auditory cortex, and medial geniculate nucleus of the cat. Frequency of discharge of the
neurons is plotted on the ordinate against sound intensity on the abscissa. (Katsuki Y: Neural
mechanism of auditory sensation in cats. In Rosenblith WA [ed]: Sensory Communication.
Cambridge MA, MIT Press, 1961)
GUSTATORY AND OLFACTORY SENSES
Taste
The gustatory system is much simpler than the olfactory system. Four primary taste
submodalities are generally recognized: sweet, sour, salty, and bitter. Different regions on
the tongue exhibit different maximal sensitivities to the four taste submodalities (Figure
10-1 which also shows the pattern of innervation of the tongue). The tip of the tongue is
the most sensitive to sweetness and saltiness. The sensation of sourness is experienced
best on the lateral aspects of the tongue, and bitterness is experienced best and perhaps
only on the back of the tongue. Next time you put some bitter substance such as tonic
water (quinine) into your
mouth, you can verify this
for yourself.
Fig. 10-1. The distribution of gustatory papillae, their
innervation, and the regions of maximum sensitivity to different
submodalities of taste on the human tongue. (Altner H:
Physiology of taste. In Schmidt RF [ed]: Fundamentals of
Sensory Physiology. New York, Springer-Verlag, 1978)
Taste neurons normally respond to several different kinds of chemicals so that chorda
tympani taste fibers typically respond to substances that are salty, bitter, sweet, and sour.
That is, they appear to respond to stimuli of two or three or even four different taste
submodalities. An example of the discharges evoked in a single taste fiber in the chorda
tympani nerve by substances flowing over the tongue is shown in Figure 10-3. This
particular nerve cell gives a brisk discharge when NaCl and saccharin flow over the
tongue, but it is only minimally, if at all, excited by sucrose, HCl, or quinine. It is also not
especially sensitive to changes in the temperature of the fluid bathing the tongue.
Figure 10-3. Impulse discharges in a single chorda tympani nerve fiber of a
rat. Responses elicited by application to the tongue of 0.1 M NaCl, 0.5 M
sucrose, 0.01 N HCl, 0.02 M quinine hydrochloride, 0.02 M sodium
saccharin, 40°C water and 20°C water. Spontaneous discharges are shown
in the bottom trace. Between traces the tongue was rinsed with 25°C water.
(Ogawa H, Sato M, Yamashita S: J. Physiol (Lond) 199:223-240, 1968)
It appears that most taste receptors do not signal single submodalities uniquely, but the
submodality, i.e., the quality of the taste, must be determined by the central nervous
system from the discharge pattern across the ensemble of sensory nerve fibers. To see
this, examine the histograms of Figure 10-4, which are plots of the number of spikes
discharged by 28 different chorda tympani nerve fibers in the first five seconds after
various solutions were allowed to flow over the tongue of a hamster.
In Figure 10-4, the data plotted are from actual experiments, and they illustrate that taste
nerve fibers respond better to one or two of the taste submodalities than to others. Few
receptors signal a single submodality uniquely; therefore, submodality must be signaled
in the form of the ensemble code.
Figure 10-4. Response profiles of 28 hamster chorda tympani
fibers. Stimuli were 0.1 M NaCl, 0.5 M sucrose, 0.01 N HCl,
0.02 M quinine hydrochloride, 20 C water and 40 C water. The
response of a single fiber to each stimulus is found by reading
up columns indicated by the letters on the abscissa. (OgawaH,
Sato M, Yamashita J: J Physiol (Lond) 199:223-240, 1968)
Taste intensity, on the other hand, seems to be signaled in terms of the total number of
impulses discharged per second (i.e., the frequency of discharge) in the ensemble of
primary taste fibers. Single cells increase their discharge frequencies with increasing
concentration of taste substances, as illustrated in Figure 10-5. The graphs indicate the
responses of three different chorda tympani fibers to increasing concentrations of taste
stimuli. The number of impulses evoked in five sec is plotted against the concentration of
the solution (on the abscissa). Fiber #1 was quite sensitive to NaCl, less so to sucrose.
However, the discharge of the cell increased steadily as the concentration of either NaCl
or sucrose was increased. Fiber #2 was more sensitive to sucrose than NaCl, but it still
increased its discharge frequency when either substance was in higher concentration.
Fiber #3 was sensitive to NaCl, quinine and HCl, and it showed the same sort of
increased response to increasing concentration of any of the three substances. The total
number of impulses per second increased in all the fibers, but their relative discharge for
each substance was the same.
Figure 10-5. Concentration-response magnitude relationships in
three typical chorda tympani fibers of rats. Ordinate indicates the
number of impulses discharged by the fiber in the first 5 sec after
the substance was applied. Fiber #1 was predominantly sensitive to
NaCl, fiber #2 was more sensitive to sucrose than NaCl, and fiber
#3 was sensitive to NaCl, quinine, and HCl. (O Ogawa H, Sato M,
Yamashita J: J Physiol (Lond) 199:223-240, 1968)
In an attempt to correlate psychophysical data for taste intensity with neuron responses to
stimulus concentration, subjective intensities for citric acid and sucrose were estimated
by two human subjects for six different concentrations of each substance and responses
of chorda tympani nerve fibers were recorded at the same six concentrations. The results
of these two experiments are plotted together on the same log-log plot in Figure 10-6.
The magnitudes of taste sensations are plotted as open circles; the magnitudes of neural
responses are plotted as filled circles. The plot on the left is for citric acid, that on the
right for sucrose. There is remarkable agreement between the psychophysical and neural
data, both showing a power function relation between stimulus intensity and response
magnitude.
Figure 10-6. Dependence of subjective intensity of taste sensations (open circles)
and of the frequency of discharge in fibers of the chorda tympani nerve (filled
circles) upon the concentration of citric acid (red) and sucrose (green) solutions.
Log-log plot. The slopes of the lines correspond to the exponents, k, of power
functions with k=0.85 and 1.1. (Borg G, Diamant H, Strom L et al: J Physiol (Lond)
192:13-20, 1967)
Smell
The human can distinguish the odors of a vast number of different molecules and
describes them as aromatic, fragrant, repulsive, ethereal, resinous, spicy, burned, putrid,
and so forth. Whether any of these can be considered primary is a point of debate.
Odorous substances have in common that they are either gases or volatile liquids. This is
the form in which the odorant reaches the sensory epithelium, either through the nostrils
with inspired air or by the back door through the mouth and throat. The receptor
structures for olfaction are covered with mucus so that aqueous solubility is an asset to an
odorant.
Of all the chemical elements, only 16 seem to play any role in the production of odor
sensation. These are hydrogen, carbon, silicon, nitrogen, phosphorus, arsenic, antimony,
bismuth, oxygen, sulfur, selenium, tellurium, fluorine, chlorine, bromine, and iodine.
Only the halogens and ozone, O3, are odorous as elements
Smell anatomy
The olfactory receptors are located in the posterior portion of the nasal cavity in a
mucosal area that measures about 5 cm2. The location is shown in Figure 10-8, which is a
sagittal section of the human nasal passages. The sensory epithelium sits at the top of the
nasal cavity above the superior turbinate bones. It is yellow in color and has no
synchronously beating cilia and therefore is distinguishable from the surrounding
respiratory epithelia. Within this sensory region are found Bowman's glands that contain
most of the yellow pigment and secrete much of the mucus that covers the sensory
epithelium in a 10- to 40-m layer. The odorant must dissolve in this mucus layer in order
to reach the olfactory receptors.
Figure 10-8. Sagittal section of the human nasal
passages, showing the nasal fossae, the location of
the olfactory epithelium and olfactory bulb, and the
direction of travel of inspired air (small arrows).
(Douek E: The Sense of Smell and Its Abnormalities.
Edinburgh, Livingstone, 1974)
The olfactory epithelium contains the receptor cells, as shown in the diagram of Figure
10-9, as well as sustentacular and basal cells. The receptor cells are really bipolar nerve
cells and therefore unlike the receptor cells of the eye, ear, and tongue which are
specialized epithelial cells. The olfactory receptor cells have their nuclei in the lower 2/3
of the epithelium, but their apical ends project to the surface. The apical ends are slightly
enlarged, and they possess numerous cilia that project 50 to 150 m out into the mucus.
The location of the receptor sites for odorant molecules on the olfactory receptor cell is
unknown. Many people believe they are on the cilia, but the existence of olfactory cells
without cilia and the persistence of olfactory sensation after the cilia have been removed
argue against this idea. The receptor cells are arranged in interconnected rings, one of
which is illustrated in the figure. Gap junctions are thought to connect adjacent receptor
cells both between spines and less frequently at their apical expansions. The centers of
the rings are apparently filled with supporting cells.
At its basal end, each receptor cell gives rise to an axon that forms a part of the olfactory
nerve. Olfactory nerve fibers are unmyelinated and average about 0.2 m in diameter. In
the rabbit, the olfactory epithelium contains about 100 million receptor cells, and thus
there are about the same number of fibers in the olfactory nerve. In man, the olfactory
nerve is short, merely perforating the cribriform plate into the brain cavity and
terminating in the olfactory bulb. Olfactory nerve fibers converge onto mitral cells in the
bulb, which send their axons (about 100,000 of them) in the olfactory tract to the piriform
cortex, the periamygdaloid area, and the olfactory tubercle. The olfactory bulb contains
interneurons, the granule cells that mediate a form of lateral inhibition between mitral
cells; an active mitral cell inhibits (reduces) activity in its neighbors. There are also
centrifugal fibers to the olfactory bulb that primarily mediate inhibition of mitral cell
activity. The functions of this connection and of the lateral inhibition are not known.
Unlike other sensory pathways to the cerebral cortex, the olfactory pathway does not
relay in the thalamus as we have just seen. However, fibers do leave the olfactory cortical
areas and relay in the thalamus on their way to the hypothalamus or other areas where
they perhaps play a role in the regulation of the intake of food and other behaviors that
depend upon olfactory information.
Figure 10-9. The olfactory mucosa showing arrangement of
receptor cells in interconnected rings, one of which is illustrated.
Receptor cells are connected together by gap junctions both
between spines and less frequently at their apical expansions (two
shown) The centers of the rings are apparently filled with
supporting cells. (Graziadei PPC: Z Zellforsch 118:449-466, 1971)
Smell physiology
The coding mechanism for olfactory quality (submodality) has been just as elusive as the
identification of the qualities themselves. Microelectrode recordings from the olfactory
epithelium yield two sorts of responses to olfactory stimulation. Both can be seen in
Figure 10-10. First, there is a slow-wave potential reminiscent of the generator potentials
of mechanoreceptors. However, this potential is much larger than generator potentials
recorded extracellularly from single receptors elsewhere. It also does not have a fixed
relationship to the time of spike initiation, as can be seen in the figure. This potential
probably is the summation of the generator potentials of a number of nearby receptor
cells and perhaps potentials generated by supporting cells.
Second, there are the familiar spike discharges in the receptor cell axons or somata. It can
be seen from this typical example that receptors are not highly specific in their responses
to odorants. This particular cell responded to camphor, limonene, carbon disulfide, ethyl
butyrate, and musk xylene, but it was insensitive to nitrobenzene, benzaldehyde, nbutanol and pyridine. In general, each olfactory receptor responds to a variety of
odorants, suggesting a very complex ensemble code.
Figure 10-10. Responses of a single cell in the olfactory epithelium to
stimulation with camphor, limonene, carbon disulfide, ethyl butyrate,
and musk xylene. (Gesteland RC, Lettvin JY, Pitts WH et al.: Odor
specificities of frog's olfactory receptors. In Zotterman Y [ed]:
Olfaction and Taste. New York, Macmillan, 1963)
UNIT V
BEHAVIOUR SCIENCE
Refer
1. Guyton
2. Ganong
Disorders associated with the nervous system
Term
Definition
Cause
Effect
Bell's Palsy
A form of Neuritis
that
involves
paralysis of the
facial nerve causing
weakness of the
muscles of one side
of the face and an
inability to close the
eye.
Unknown.
Paralysis of the
(Recovery
may facial
nerve;
occur
weakness of the
spontaneously.)
muscles of one side
of
the
face;
may
result
in
inability to close the
eye.
(In some cases the
patient's hearing may
also be affected in
such a way that
sounds seem to
him/her
to
be
abnormally
loud.
Loss
of
taste
sensation may also
occur.)
Cerebal Palsy
A
nonprogressive
disorder
of
movement resulting
from damage to the
brain before, during,
or immediately after
birth.
progressive
Motor Neurone A
degenerative disease
Disease
of the motor system
occurring in middle
Cerebal Palsy is
attributed to damage
to
the
brain,
generally occuring
before, during, or
immediately
after
birth.
It is often associated
with
other
neurological
and
mental
problems.There are
many
causes
including
birth
injury,
hypoxia,
hypoglycaemia,
jaundice
and
infection.
The most common
disability is a spastic
paralysis.
Sensation is often
affected, leading to a
lack of balance, and
intelligence, posture
and
speech
are
frequently impaired.
Contractures of the
limbs may cause
fixed abnormalities.
Other
associated
features
include
epilepsy,
visual
impairment, squint,
reduced hearing, and
behavioural
problems.
Some
forms
of
Motor
Neurone
Disease
are
inherited.
Motor
Neurone
disease
primarily
affects the cells of
the anterior horn of
age and causing
muscle
weakness
and wasting.
the spinal cord, the
motor nuclei in the
brainstem, and the
corticospinal fibres.
Multiple Sclerosis A chronic disease of
the nervous system
that can affect young
and
middle-aged
adults.
The course of this
illness
usually
involves
recurrent
relapses followed by
remissions, but some
patients experience a
chronic progressive
course.
The myelin sheaths
surrounding nerves
in the brain and
spinal
cord
are
damaged,
which
affects the function
of
the
nerves
involved.
A
condition
Myalgic
by
Encephalomyelitis characterized
extreme
disabling
(ME)
fatigue that has
lasted for at least six
months, is made
worse by physical or
mental
exertion,
does not resolve with
bed rest, and cannot
be attributed to other
disorders.
Unknown.
The
underlying
cause of the nerve
damage
remains
unknown.
Often occurs as a
sequel to such viral
infections
as
glandular fever.
Multiple
Scerosis
affects different parts
of the brain and
spinal cord, resulting
in typically scattered
symptoms.
These can include:
Unsteady gait and
shaky movement
of
the
limbs
(ataxia);
Rapid involuntary
movements of the
eyes (nystagmus);
Defects in speech
pronunciation
(dysarthria);
Spastic weakness
and
retrobulbar
neuritis
(=
inflammation of
the optic nerve).
Extreme disabling
fatigue that has
lasted for at least six
months, is made
worse by physical or
mental
exertion,
does not resolve with
bed rest, and cannot
be attributed to other
disorders.
The
fatigue
is
accompanied by at
least some of the
following:
Muscle pain or
weakness;
Poor
co-
Neuralgia
Neuritis
Parkinson's
Disease
ordination;
Joint pain;
Sore throat;
Slight fever;
Painful
lymph
nodes in the neck
and armpits;
Depression;
Inability
to
concentrate;
General malaise.
Maybe
due
to A severe burning or
previous attack of stabbing pain often
shingles
following the course
(Postherpetic
of a nerve.
Neuralgia).
A disease of the
Inflammation of the
peripheral
nerves
nerves, which may
showing
the
be painful.
pathological changes
of
inflammation.
(This term may also
be less precisely
used to refer to any
disease
of
the
peripheral
nerves,
usually
causing
weakness
and
numbness.)
Degenerative disease Associated with a Tremor, rigidity and
process (associated deficiency of the poverty
of
with aging) that neurotransmitter
spontaneous
affects the basal dopamine.
movements.
ganglia of the brain.
Also associated with The
commonest
aging.
symptom is tremor,
which often affects
one hand, spreading
first to the leg on the
same side then to the
other limbs. It is
most profound in
resting
limbs,
interfering with such
actions as holding a
cup.
Sciatica
A common condition
arising
from
compression of, or
damage to, a nerve
or nerve root.
Usually caused by
degeneration of an
intervertebral disc,
which
protrudes
laterally to compress
a lower lumbar or an
upper sacral spinal
nerve root.The onset
may be sudden,
brought on by an
awkward lifting or
twisting movement.
The patient has an
expressionless face,
an
unmodulated
voice, an increasing
tendency to stoop,
and a shuffling walk.
Pain felt down the
back and outer side
of the thigh, leg, and
foot. The back is
stiff and painful.
There
may
be
numbness
and
weakness in the leg.