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The Senses

In this unit we study the sensory receptors. All receptors are transducers, that is they respond to a stimulus by changing (transducing) it into a generator or receptor potential. A receptor potential is like the graded potentials which occur at a synapse. Graded potentials can result in an action potential produced in a neuron leading to the brain. Various stimuli can often produce a generator potential including electrical, physical, chemical, and others. But receptors can be classified according to the stimulus  to which they normally respond: mechanoreceptors - respond to a mechanical stimulus: examples are touch, pressure, stretch, hearing, balance, position and movement, vibration, muscle contraction, as well as pressoreceptors and baroreceptors. [muscle spindle] thermoreceptors - respond to temperature change: example heat and cold. photoreceptors - respond to light: example vision chemoreceptors - respond to various chemicals such as glucose, oxygen, carbon dioxide, hormones and many, many more. [taste buds][olfactory cells] nociceptors - pain receptors from any noxious stimulus [free nerve endings] We tend to classify receptors according to the location or origin of the stimulus: Exteroceptors respond to stimuli from outside the body - vision, sound, touch, smell, temperature, pain etc. Interoceptors or visceroceptors respond to stimuli arising within the body such as chemical stimuli, deep pressure, and many others. Proprioceptors respond to muscle or tendon stretch and help the body monitor body position (body sense). The idea of the "five senses" refers only to certain of the exteroceptors. But in fact we have dozens of senses which keep our nervous system consciously and unconsciously aware of our internal and external environment.

General vs Special Senses: (See Table 13.1) Although all sensory receptors basically do the same thing, they can be classified as to their complexity. Special sense receptors are the most complex. Unencapsulated receptors - these have no special structure and are basically free nerve endings. Examples are pain receptors, temperature receptors, Merkel disks (touch), hair root plexus. Encapsulated receptors have a special capsule which encloses a nerve ending. Meissner's corpuscles - light touch Pacinian corpuscles - deep pressure, vibration Muscle spindle receptors - muscle stretch Golgi tendon organs - tendon stretch, muscle contraction.

The Special Senses - some sense organs are more complex than these and are considered "special" senses. Taste cells are found in groups called taste buds located on the tongue, epiglottis, hard palate and a few other places in the mouth. Taste cells are neuroepithelial cells possessing microvilli that respond to chemicals taken into the mouth. They are called neuroepithelial cells because they respond to stimuli by releasing neurotransmitters from vesicles they contain. These neurotransmitters cause depolarization of nerve fibers which lie adjacent to the taste cells. Taste cells replace themselves about every ten days from the basal cells nearby, and therefore taste buds recover quickly from damage. There are four basic tastes and a fifth which appears to make the others more intense. The four basic tastes are sweet, salt, sour, and bitter. These tastes are unevenly distributed on the tongue (See Figure 16.1), and there is a lot of overlap. Taste cells appear to be able to respond to several. The fifth taste is a response to glutamate, dubbed "yummy" or deliciousness. It tends to accentuate the other tastes, which forms the basis for the use of monosodium glutamate in flavor enhancers. Taste buds are located on the sides of papillae of the tongue. There are three types of papillae: Fungiform papillae cover most of the tongue and contain most of the taste buds. Circumvallate papillae are large, contain taste buds, and are found only at the back of the tongue. Filiform papillae are very slender, do not contain taste buds, and are found only near the tip of the tongue. They are believed to help sense touch. Taste stimuli travel through the facial and glossopharyngeal nerves, and some from the vagus nerve, to the thalamus and gustatory center in the uncus of the inner parietal lobe. Much of what we perceive as taste is actually olfactory. Olfactory cells (See Figure 16.2) also respond to chemicals, those dissolved in air. They are modified neurons found in mucosa in the roof of the nasal cavity. Olfactory cells are the only neurons known to replace themselves, about every 60 days. Although all olfactory cells look the same, they apparently respond in varying measures to the many smells. Smells have not been as precisely classified as tastes, although some have tried, and we can perceive thousands of them. Olfactory memories are among the most intense and specific and are stored in the hippocampus and mamillary body. Olfactory neurons produce depolarization when chemical smells stimulate receptor proteins which produce a second messenger that opens sodium channels. (See Figure 16.3)

Sensory Adaptation Many sensory receptors stop responding when a stimulus is constant. We all experience the phenomenon of adaptation when we put on our clothes. At first we feel them of course, but soon we no longer notice their presence. A room may have an odor when we first enter, but after a short time we are no longer aware of it, until we leave the room. We say these senses are "fast adapting" because they only respond when the stimulus changes and stop responding when it's continuous. Other receptors such as those to sight and sound are "slowly adapting" because their response is nearly constant. (See below for the use of the term adaptation for the rods and cones).

The Eyes:  (See Figure 16.7 modified, also Labeled Eye)      The physical structure of the eye functions much like the supporting structure of the eye. The outer structure which supports the eye is the sclera, part of the eye's fibrous tunic. The part of the sclera visible anteriorly we call the "white of the eye". The conjunctiva is a thin tissue which covers what we see of the sclera and which has blood vessels running through it. These vessels bring the blood supply to the vicinity of the sclera and cornea, which themselves are avascular (without blood vessels). When the vessels in the conjunctiva become inflamed we say our eyes are "bloodshot". Drugs which constrict these vessels are used to reduce the inflammation.       The anterior portion of the fibrous tunic is the cornea. Unlike the sclera the cornea is transparent because its collagen fibers are finely divided and very regularly spaced. The surface of the cornea is epithelium and can repair itself when scratched. Scar tissue forms when the fibrous portion of the cornea is damaged. The cornea's shape bends entering light rays toward the pupil. (See The Refractive Apparatus below). Behind the cornea is the anterior chamber or anterior segment. This cavity is filled with aqueous humor, (See Figure 16.11) which helps to hold the other structures in their normal position and shape. Aqueous humor is produced by secretion by cells in the ciliary body. It must be absorbed at the same rate it is filtered and this is accomplished by absorption into the canal of Schlemm (scleral venous sinus). If filtration exceeds absorption, increased pressure builds in the anterior segment producing a condition known as glaucoma. This can result from high blood pressure, but mostly results from conditions which block the Canal of Schlemm. These include disease (or the body's response to it) or congenital narrowing of the drainage angle. It is treated with vasodilators, or sometimes surgery. If left untreated it can damage the structures of the eye, including ultimately the retina, causing blindness.

The middle layer of the eye is called the vascular tunic because it is highly vascularized. These vessels serve other layers like the retina and sclera. The vascular tunic is also highly pigmented. This pigment serves to absorb excess light. The most anterior part of this layer is the iris. We are familiar with the front of the iris having the color of the eye: brown, blue, etc. This pigment helps to prevent excess light from entering the eye and disrupting the image. Individuals with very lightly pigmented irises have difficulty seeing well in very bright light. The opening in the iris is the pupil and serves to direct the incoming light rays toward the center of the lens, its most effective part for refraction. Muscles on the posterior side of the iris enable the pupil to be constricted, or dilated. (See Figure 16.7) There are two sets of iris muscles, the circular and the radial or radiating. The circular muscles run in a circle around the pupil, and when they contract the pupil constricts. The radiating muscles run in rays from the pupil to the outer edge of the iris. When they contract they dilate the pupil. Circular muscles are innervated by the parasympathetic fibers of nerve III, while radiating muscles are innervated by sympathetic fibers from the otic ganglion. Another part of the vascular tunic is the ciliary body. The ciliary body attaches to the suspensory ligament which holds the lens in place and maintains the shape of the lens. The ciliary muscles (located within the ciliary body) are circular muscles which act to regulate the tension on the lens and its shape. The posterior portion of the vascular tunic is the choroid coat. This is the middle layer between the sclera and the retina and its blood vessels help to supply their needs. Its pigment absorbs excess light after it has passed through the retina, preventing it from reflecting back and forth in the eye and disrupting the image. In many animals, but not man, a very shiny part of the choroid called the tapetum lucidum allows some of the light to re-stimulate the retina, thus allowing those animals to see better in dim light.      The lens is made of flexible proteins called crystallins which are produced and maintained by cells called lens fibers. The lens is flexible, but its natural shape is very convex. (See Figure 16.15 modified) A convex lens is one which is thicker in the center that at the edges. It causes convergence of light rays. Any given point on an object reflects light rays in all directions. These rays strike the lens at different angles and points, but for an image to be properly focused the lens must cause all rays from the same point on the image to converge at the same point on the receptor surface. In doing this, the image is inverted, i.e. turned upside down and backwards. This is true for your eye, for a camera, for a microscope, and for most other lens applications. For far objects the light rays are nearly parallel and don't require much convergence. But for near objects the rays are not parallel and require more convergence the closer the object is to the lens. Since the natural shape of the lens is convex, when there is less tension put on the lens by the suspensory ligament it becomes more convex for focusing near objects. This is achieved by contracting the ciliary muscles! Since they are circular muscles contracting them squeezes the suspensory ligaments and relieves their tension. Relaxing the ciliary muscles restores the tension produced by the suspensory ligaments and causes the lens to be pulled flat, less convex, for distant objects. This is why close work which requires ciliary muscle contraction can lead to eye strain, while looking at distant objects relaxes and rests these muscles.

Individuals whose eyeballs are too long for the natural convexity of their lens have myopia or nearsightedness. In this condition the focal plane of the image occurs in front of the retina. The correction is to put diverging lenses, concave lenses, in front of the eyeball.      Individuals whose eyeballs are too short or whose ciliary muscles are weak have farsightedness or hyperopia. Their lens cannot be made convex enough so convex lenses are used for correction.      As individuals get older the muscles and ligaments, and the lens itself, become less flexible and able to focus at different distances. This is called presbyopia and literally means "old eyes". These individuals must wear bifocals or different lenses for different conditions.      Astigmatism is the condition in which the cornea or lens is irregular and misshapen which causes a portion of the image to be out of focus. Lenses are placed in front of the eye which compensate for the irregularities.      New techniques for correcting vision include 1) RK (radial keratotomy) in which minor cuts are made in the cornea near its margin. When these heal it causes the cornea to flatten and reduces the convergence of light rays. 2) LASIC surgery which shaves small portions from the cornea thus reshaping it. 3) a technique which places lenses over the cornea which cause it to adapt to the desired shape. These techniques are not always successful and a significant portion of patients must repeat the procedure.

The Refractive Apparatus (also Refractory Apparatus )      All the structures which allow passage of light process it in some way and are part of the refractive apparatus, or "light bending" part. First the cornea bends the light toward the pupil and center of the lens. The lens center is the most accurate at focusing the light. The pupil itself allows some bending. This is the same principle as a pinhole camera in which the physical structure of the small pinhole bends and focuses the light image. A smaller aperture (opening) produces a sharper image with a greater range of focus. This is true with a camera and with the eye. The aqueous humor is clear and, although light passes through it, it does not normally bend the rays. Neither does the vitreous humor, a jelly-like mass of filamentous proteoglycans which occupies the posterior segment. This mass is crystal clear to allow light to pass through. It functions to support and maintain the position of the retina helping to prevent retinal detachment.

The Sensory Tunic: The Retina      The retina consists of a neural layer and a pigmented epithelial layer. The pigmented layer abuts the choroid coat and serves the same purpose, to absorb excess light preventing it from reflecting back toward the receptor cells. The neural layer contains the sensory receptors for vision, the rods and cones, together with other neurons which are part of the processing of signals.      The structure of the retina is curious (See Figure 16.9, modified) having the receptor cells at the back next to the pigment. The processing bipolar and ganglion cells lie in front of the receptors, so that light must pass through them before striking the receptors. Except in the fovea centralis! Since the fovea is a cone-shaped area the bipolar cells and ganglion cells are bent to the side and out of the light path. This makes the images produced much sharper. There are three reasons the fovea centralis is the area of sharpest vision. 1) the point mentioned above, light strikes the receptors directly without passing through the other cells; 2) There is a 1:1 ratio of receptors to optic nerve fibers. Outside the retina, several receptor cells may converge into only one nerve fiber. In that case different points of light striking the different receptors would be interpreted as the same point on the image. 3) Only cones are found in the fovea. Cones provide color daylight vision which makes the image clearer.

Rods and Cones: (See Figure 16.18)      The names given to these receptors result from their general shape. Each contains disks of visual pigment, the chemicals which are affected by light. All of the visual pigments are based on the molecule called rhodopsin or visual purple. Rhodopsin consists of a molecule of retinal , also known as retinaldehyde or retinene, (See Figure 16.19) bound to a protein called opsin. The opsin protein varies from one receptor type to another and determines the molecule's light absorbing characteristics. In the retinal-opsin form retinal is in is 11-cis isomer form. When light strikes this molecule the retinal detaches from opsin and changes to its all trans form. This is called bleaching. (See Figure 16.20, and Diagram) The opsin and retinal come back together to form unbleached rhodopsin, a process necessary for continued ability to respond to light. (Retinal is derived from and replenished from vitamin A, and lack of vitamin A causes night blindness, the reduced ability to see in low light conditions) The recovery process occurs in the area surrounding the receptors and rhodopsin is transported into the receptors. Bleaching creates a change in polarity which can result in impulses subsequently perceived as points of light. (See Figure 16.21).      The different types of receptors respond differently to light. Rods are dim light receptors, essentially only functioning in dark or near dark conditions. Being very sensitive to light, in daylight conditions the rhodopsin in rods is virtually totally bleached and unable to respond. The pigment recovers in the process known as dark adaptation over a period of 10 to 15 minutes (See [Dark Adaptation]). This is exemplified by the experience of walking into a darkened room and at first being unable to see, but very slowly vision improves as more rhodopsin is regenerated. Rods are found around the periphery of the retina, outside the macula lutea. Parts of the visual field in this area is unable or poorly able to perceive color, but works much better than the fovea in dim light.       The cones are bright light color receptors. They actually have several pigment forms which respond differentially to different wavelengths (colors) of light (See Figure 16.13) They require much more light than rods to be stimulated, but also do not tend to become excessively bleached except in very bright light. This accounts for the brief inability to see when extremely bright light is encountered. Cones are concentrated in the macula lutea, being the only receptors found in the fovea centralis. This partly accounts for the acuity of vision in these areas.

The process by which light stimulation is converted into nerve impulses is complicated, and still somewhat speculative. [See Diagram] When it's dark the receptors are actually releasing an inhibitory neurotransmitter at the bipolar cell, preventing it from becoming activated. When light strikes the receptor it causes the closing of the sodium channels, preventing entrance of sodium into the cell and causing hyperpolarization. This hyperpolarization stops the secretion of inhibitory transmitters allowing the bipolar cells to become excited (depolarized). In order for an action potential to be generated and perceived as light, many such depolarizations must sum at a ganglion cell. Horizontal cells and amacrine cells participate in the integration of all excitatory and inhibitory stimuli, which then produce (or don't produce) a threshold depolarization at the ganglion cell. Lateral divergence of stimuli from horizontal and amacrine cells produce "on center" or "off center" fields" in which excitation depends on stimulation of adjacent areas. This is an integration system that works to enhance contrast.

Action potentials produced in the ganglion cell travel through nerve fibers in the optic nerve to the optic chiasma. (See Figure 16.22) In the optic chiasma fibers from the medial area of the retina cross to the other side of the brain, while lateral fibers remain on the same side. Visual impulses travel through the optic tract to the thalamus (lateral geniculate body) and from there to the visual cortex in the occipital lobe. They also go to the superior colliculi in the midbrain where visual reflexes are centered. Images from both eyes are combined to produce the three dimensional image we perceive.

The Ear

(See Figure 16.25 and Labeled Ear) 

    The ear is a three-chambered sensory structure that functions in the perception of sound (auditory system) and in the maintenance of balance (vestibular system). Each of the three divisions of the ear, the external ear, the middle ear, and the inner ear, is an essential part of the auditory system.  The external and middle ear collect and conduct sound energy to the inner ear, where auditory sensory receptors transduce that energy into the electrical energy of nerve impulses.  The sensory receptors of the vestibular system are also located in the inner ear.  These receptors respond to gravity and movement of the head.

EXTERNAL EAR:   The external ear is composed of an auricle and an external auditory meatus. The auricle (pinna) is the appendage that projects from the lateral surface of the head, i.e., the "ear." The characteristic shape of the auricle is determined by an internal supporting structure of elastic cartilage.  Thin skin with hair follicles, sweat glands, and sebaceous glands covers the auricle.  The auricle is considered to be a nearly vestigial structure in humans, compared with its development and role in other animals.  However, it is an essential component in sound localization and amplification. The external auditory canal (meatus) follows a slightly S-shaped course for about 25 mm to the tympanic membrane (eardrum).  The lateral one-third of the canal is cartilage and is continuous with the elastic cartilage of the auricle, the medial two-thirds of the canal is contained within the temporal bone. The lateral part of the canal is lined by skin that contains hair follicles, sebaceous glands, and ceruminous  (wax) glands. The coiled tubular apocrine ceruminous glands are modified sweat glands.  Their secretion mixes with that of the sebaceous glands and with exfoliated cells to form cerumen or earwax. The cerumen lubricates the skin and coats hairs near the opening to impede the entry of foreign particles into the ear.  Excessive accumulation of cerumen can plug the meatus, however, resulting in conductive hearing toss.  The medial part of the canal, within the temporal bone, has thinner skin and fewer hairs and glands.

MIDDLE EAR:  The middle car is an air-filled mucus-membrane-lined space in the temporal bone, the tympanic cavity (Fig. 16.26). It is spanned by three small bones, the auditory ossicles, that are connected by two movable joints.  The middle ear also contains the internal auditory canal (Eustachian canal) as well as the muscles that move the ossicles.  The middle ear is bounded anteriorly by the auditory tube, posteriorly by the spongy bone of the mastoid process, laterally by the tympanic membrane, and medially by the bony wall of the inner ear. The primary function of the middle ear is to convert sound waves (air vibrations) arriving from the external auditory meatus into mechanical vibrations that are transmitted to the inner ear.  Two openings in the medial wall of the middle ear, the vestibular (oval) window and the cochlear (round) window, are essential components in this conversion process.

    The tympanic membrane (Eardrum) separates the external auditory canal from the middle ear. It consists of a framework of collagen fibers covered by skin on the lateral side and mucosa on the side of the middle ear cavity.  One of the auditory ossicles, the malleus, is attached to the tympanic membrane.  Sound in the form of pressure waves causes the tympanic membrane to vibrate, and these vibrations are transmitted to the attached auditory ossicles that link the external ear to the inner ear.  Perforation of the tympanic membrane may cause transient or permanent hearing impairment.

    The three small bones known as the ossicles, the malleus, the incus, and the stapes, cross the space of the middle ear in series and connect the tympanic membrane to the oval window.  These bones help to convert sound waves, i.e., vibrations in air, to mechanical vibrations in tissues and fluid-filled chambers of the inner ear. In the process the strong vibrations are magnified and the weak ones are overshadowed.  1) The malleus (hammer) is attached to the tympanic membrane, 2) The  incus (anvil), links the malleus to the stapes,  3) The stapes (stirrup), whose footplate fits into the oval window leading to the inner ear.

    Muscles attach to the ossicles and affect their movement.  The tendon of the tensor tympani  inserts on the malleus.  Contraction of this muscle increases tension on the tympanic membrane.  The stapedius tendon inserts on the stapes.  Contraction of the stapedius tends to dampen the movement of the stapes at the oval window.  The stapedius, only a few millimeters in length, is the smallest of all the skeletal muscles. The two muscles of the middle ear are responsible for a protective reflex called the attenuation reflex centered in the inferior colliculi of the midbrain. Contraction of the muscles makes the chain of ossicles more rigid, thus reducing the transmission of vibrations to the inner ear.  This protects the inner ear from the damaging effects of very loud sound.

The internal auditory canal, commonly known as the eustachian canal, a narrow flattened channel lined with ciliated pseudostratified columnar epithelium, is approximately 3.5 cm long and connects to the nasopharynx. It allows pressure in the middle ear to equilibrate with atmospheric pressure.  The walls of the tube are normally pressed together but separate during yawning and swallowing to allow equalization of pressure.  It is common for infections to spread from the pharynx to the middle ear via the auditory tube (causing otitis media). A small mass of lymphatic tissue, the tubal tonsil. is often found at the pharyngeal opening of the auditory tube to help protect against infection.

INNER EAR:   (See Figure 16.27) The inner ear Consists of two compartments or labyrinths, one contained within the other. The bony (osseous) labyrinth is a complex system of interconnected cavities and canals in the temporal bone.  The membranous labyrinth lies within the bony labyrinth and consists of a complex system of small sacs and tubules that also form a continuous space enclosed within a wall of epithelium and connective tissue. There are three fluids in the inner ear:  1) the endolymph, similar to intrcellular fluid, contained within the membranous labyrinth, 2) the perilymph, similar to extracellular fluid, lying between the bony labyrinth and the membranous labyrinth,  3) and a lesser known fluid space, the cortilymph lying within the organ of Corti.

 The three components of the inner ear are: 1) Semicircular canals,  2) Vestibule,   3) Cochlea.

The vestibule is the central space of the bony labyrinth. The utricle and saccule of the membranous labyrinth lie in an elliptical and spherical recess, respectively.  The semicircular canals extend from the vestibule posteriorly, and the cochlea extends from the vestibule anteriorly.  The oval window into which the footplate of the stapes inserts lies in the lateral wall of the vestibule. Two groups of sense cells of the utricle and saccule sense the position of the head and linear movement.

The Semicircular Canals, three narrow bony-walled tubes, each forming about three-quarters of a circle,  lie at approximately right angles to each other in superior, posterior, and horizontal planes.  At the lateral end of each semicircular canal, close to the vestibule, is an enlargement called an ampulla. Each inner ear has three ampullae.  The three canals open into the vestibule through five openings, with the superior and posterior semicircular canals sharing a common ampulla medially. Three cristae ampularis located in the ampullae of the semicircular ducts respond to angular acceleration of the head, e.g. turning, flexing the neck, etc. 

  The Cochlea is a conically shaped helix connected to the vestibule. The lumen of the cochlea, like that of the semicircular canals, is continuous with that of the vestibule.  It connects to the vestibule on the side opposite the semicircular canals.  Between its base and the apex, the cochlea makes about 2-3/4 turns around a central bony core called the modiolus. A sensory ganglion, the spiral ganglion, lies in the modiolus.  One opening of the canal, the cochlear round window on its inferior surface near the base, is covered by a thin membrane (the secondary tympanic membrane) by which it absorbs or damps vibrations reaching it. The organ of Corti that projects into the endolymph of the cochlear duct is the sense organ for hearing.

     Despite having different specific functions, all receptors mentioned above share similar structural specializations and characteristics. The several different functions of the receptors of the inner ear are performed by hair cells that are remarkably similar in structure.  They also have a common basis of function in the initiation of nerve impulses.

Several important characteristics are common to these hair cells: 1) All are epithelial cells; 2)  Each possesses numerous stereocilia, modified microvilli, called sensory "hairs";  3) In the vestibular system, each hair cell possesses a single true cilium called a kinocilium; 4) In the auditory system, hair cells lose their cilium during development but do have a residual basal body. 5) All hair cells are associated with both afferent and efferent nerve endings;  6) All hair cells are transducers; i.e., they convert mechanical energy to electrical energy that can be transmitted via the vestibulocochlear nerve to the brain.

All receptor (hair) cells of the inner ear appear to function by the bending or flexing of their stereocilia (sensory hairs).  The means by which the stereocilia are bent varies from receptor to receptor.  Stretching of the plasma membrane caused by the bending of the stereocilia generates trans-membrane potential changes in the receptor cell that are conveyed to the afferent nerve ending(s) associated with each hair cell.  When a kinocilium is present, its location relative to the bending of the stereocilia is important.  Stereocilia that are bent away from the kinocilium cause hyperpolarization of the receptor cell; stereocilia that are bent toward the kinocilium cause depolarization of the receptor cell and consequent generation of an action potential.  In the crista ampullaris a gelatinous mass called the cupula adds inertia to the stereocilia making them bend slowly when movement of the head creates a differential between the walls of the semicircular canals and the endolymph inside.  Bending of the stereocilia in the narrow space between the hair calls and the cupula leads to the generation of nerve impulses in the associated nerve endings.

The maculae are innervated sensory thickenings of the epithelium facing the endolymph in the saccule and utricle of the vestibule. As in the cristae, each macula consists of hair cells and nerve endings associated with the hair cells.  The maculae of the utricle and saccule are oriented at right angles to one another.  When a person is standing, the macula utriculi is in a horizontal plane, and the macula sacculi is in a vertical plane. The gelatinous material that overlies the maculae is called the otolithic membrane.  It contains 3-5 mm crystalline particles of calcium carbonate and protein, the otoliths, on its outer surface. This surface of the otolithic membrane lies opposite to the surface in which the stereocilia of the hair cells are embedded.  The otolithic membrane moves on the macula in a manner analogous to that by which the cupula moves on the crista.  Stereocilia of the hair cells are bent by gravity in the stationary individual when the otolithic membrane and its otolith pull on the stereocilia.  They are also bent during linear movement when the individual is moving in a straight line and the otolithic membrane drags on the stereocilia because of inertia.

  The Organ of Corti is the sensor of sound vibrations. (See Figure 16.28)   The cochlear duct divides the cochlear canal into three parallel canals or scalae: 1) Scala media, the middle compartment in the cochlear canal; 2)  Scala vestibule or vestibular canal;  3) Scala tympani or tympanic canal.  The cochlear duct, itself, is the scala media.  The scala vestibuli and scala tympani are the spaces above and below, respectively.  The scala media is an endolymph-containing space that is continuous with the lumen of the saccule and contains the Organ of Corti, which rests on its lower wall. The scala vestibule and the scala tympani are perilymph containing spaces and communicate with each other at the apex of the cochlea through a small channel called the helicotrema.  The scala vestibule is described as beginning at the oval window, and the scala tympani is described as ending at the round window. The upper wall of the scala media, which separates it from the scala vestibuli, is the vestibular (Reissner's) membrane. The lower wall or floor of the scala media is the basilar membrane. The organ of Corti rests on the basilar membrane and is overlain by the tectorial membrane.

 The Organ of Corti is composed of hair cells and supporting cells.  Sound waves striking the tympanic membrane are translated into simple mechanical vibrations.  The ossicles of the middle ear convey these vibrations to the cochlea. Movement of the stapes in the oval window of the vestibule sets up vibrations or traveling waves in the perilymph of the vestibular canal.  The vibrations are transmitted through the vestibular membrane to the scala media (cochlear duct), which contains endolymph, and are also propagated to the perilymph of the tympanic canal.  Pressure changes in this closed system are reflected in movements of the membrane that covers the round window in the base of the cochlea.

As a result of sound vibrations entering the inner ear, a traveling wave is set up in the basilar membrane. A sound of specified frequency causes displacement of a relatively long segment of the basilar membrane, but the region of maximal displacement is narrow.  High-frequency sounds cause maximal vibration of the basilar membrane near the base of the cochlea; low-frequency sounds cause maximal displacement nearer the apex.  The point of maximal displacement of the basilar membrane is specified for a given frequency of sound, and this is the basis of frequency discrimination.  Perception of sound intensity or loudness depends on the degree of displacement of the basilar membrane at any given frequency range.  

Hair cells are attached, through other cells, to the basilar membrane, which vibrates during sound reception.  The stereocilia of these hair cells are, in turn, attached to the tectorial membrane, which also vibrates.  The shearing effect between the basilar membrane and the tectorial membrane distorts the stereocilia of the hair cells and this distortion generates membrane potentials that, when conveyed to the brain via the cochlear nerve (cochlear division of the vestibulocochlear nerve, cranial nerve VIII), are perceived as sound.

THE END OF BIOL 237!