FIGURES C. The interneurons form the olfactory tracts, which extend to the olfactory cortex in the temporal lobes. FIGURES 15.4 and 15.

Transcription

1 OLFACTION 1. Olfaction is the sense of smell. 2. Neural components. FIGURES A. The olfactory neurons are bipolar neurons located in the olfactory epithelium in the superior part of the nasal cavity. The ends of the dendrites of the olfactory neurons have enlarged bulbs, called olfactory vesicles, from which cilia called olfactory hairs extend. B. The axons of the olfactory neurons form the olfactory nerves, which extend through the cribriform plate to the olfactory bulbs where they synapse with interneurons. C. The interneurons form the olfactory tracts, which extend to the olfactory cortex in the temporal lobes. 3. Detecting odors. A. Air borne molecules enter the nasal cavity, dissolve into the mucous layer, and combine with chemoreceptors (membrane bound receptors) in the plasma membrane of the olfactory hairs. As a result, action potentials are produced and conducted to the olfactory cortex. B. The chemoreceptors are specialized to combine with certain molecules. There may be as many as 50 different classes of receptors. The activation of a particular class of receptors or the activation of several different classes of receptors results in the all the different smells that can be recognized. TASTE FIGURES 15.4 and Neural components. A. The sensory structures that detect taste are called taste buds. They are located (mainly) in papillae, which are raised bumps on the surface of the tongue. However, most papillae do not have taste buds. B. Within taste buds there are taste cells that have microvilli called gustatory hairs. C. The anterior two thirds of the tongue is supplied by the facial (VII) nerve and the posterior one third by the glossopharyngeal (IX) nerve. 15-1

2 2. Detecting tastes. A. Substances dissolve or become mixed in saliva and enter the taste cells or bind to receptors in the plasma membrane of the gustatory hairs. As a result, the taste cell releases a neurotransmitter, causing the production of an action potential in a sensory neuron of a cranial nerve. Action potentials are then conducted to the taste areas of the cerebral cortex. B. The five basic tastes are sour, salty, bitter, sweet, and umami (savory). The umami receptor binds to glutamate (an amino acid). Monosodium glutamate has been used for many years as a flavor enhancer. What happens to your sense of taste when you have a cold? What do you conclude from this observation? 15-2

3 VISUAL SYSTEM Accessory Structures FIGURES 15.7 and The eyebrows prevent perspiration from running down the forehead into the eyes. 2. The eyelids function to protect the eye (blink reflex), remove foreign substances (corneal reflex), keep the exposed surfaces of the eye moist by moving tears, and regulate incoming light (squinting). 3. The conjunctiva is a mucous membrane (stratified columnar epithelium) that covers the white of the eyes and the insides of the eyelids. 4. Lacrimal (L. tear) apparatus. FIGURE 15.9 A. Anatomy. 1) The lacrimal gland produces about 1 ml of tear per day. 2) The tears flow across the conjunctiva and enter the puncta (L. a point), the openings of the lacrimal canaliculi. 3) The lacrimal canals empty into the lacrimal sac, which continues as the nasolacrimal duct into the nasal cavity. In what passageway of the skull is the nasolacrimal duct located? B. The tears contain water and salts (keep the eye moist and wash away foreign objects) and lysozyme (kills bacteria). C. Normally tears evaporate or are carried away through the duct system. A cold can cause inflammation and obstruction of the ducts, resulting in watery eyes. 15-3

4 Why does crying cause a runny nose? 5. Extrinsic eye muscles. FIGURES and A. The extrinsic eye muscles attach to the exterior of the eyeball. There are six skeletal muscles per eye. B. These muscle control voluntary and involuntary movements of the eyeball. Involuntary movements occur when the eyes fix on an object and stay fixed despite movement of the object or the head. C. Disorders. 1) In strabismus, light does not enter both eyes in the same way because one eye (or both eyes) turn in or out. This produces a deviated gaze. When the gaze is deviated inward, it is sometimes called cross-eyed. When the gaze is deviated outward, it is sometimes called cockeyed. 2) Ambylopia (lazy eye) can be caused by strabismus in which the image formed by the deviated eye may be suppressed by the brain and cause blindness in that eye. Note that the eye is functional, but the brain no longer responds to the action potentials generated by the eye. 2) Diplopia or double vision is seeing two images. Normally only one image is perceived when looking an at object because of the way in which the image lines up on the retina. If one eye is out of alignment, double vision results. Check this out by manually moving the eyeball. Anatomy of the Eye FIGURE The eye consists of three layers or tunics: the fibrous, vascular, and nervous tunics. 15-4

5 Fibrous Tunic 1. The fibrous tunic is the outermost (most superficial) layer of the eye. 2. The sclera is the posterior five sixths of the fibrous tunic. The sclera is dense connective tissue. A. The sclera maintains the shape of the eye, protects inner eye structures, and is the site of attachment of the extrinsic eye muscles, which move the eyeball. B. When seen from the front, the sclera forms the whites of the eye. The sclera may have a bluish tinge in infants (due to melanin in the vascular tunic showing through) and may appear yellowish in adults (due to accumulation of fat). 3. The cornea is the anterior one sixth of the fibrous tunic. It is layers of collagen fibers covered on the outer surface by stratified squamous epithelium and on the inner surface by simple squamous epithelium. It is clear and does not contain blood vessels. A. The cornea allows light to enter the eye and refracts (bends) the light that enters the eye. B. If damaged (abrasion, chemicals, disease), the cornea can be replaced by a transplant from another human. How does the lack of blood vessels in the cornea relate to the cornea s function? Vascular Tunic 1. The vascular tunic is the middle layer of the eye and contains most of the blood vessels of the eye. 2. The choroid is the posterior portion of the vascular tunic. A. The choroid is darkly pigmented with many blood vessels. The choroid functions to absorb light so that it does not scatter inside the eye. B. In some animals (not humans), part of the choroid is modified to reflect light back out of the eye. This is the tapetum lucidum (L. shining carpet). 15-5

6 3. The ciliary body is located at the anterior margin of the choroid and consists of two parts. FIGURE A. The ciliary ring is composed of the ciliary muscles (smooth muscle), which control the shape of the lens during focusing. The ciliary muscles are innervated by the oculomotor nerve (parasympathetic system). B. The ciliary process produce aqueous humor, a fluid that circulates within the eye, and the suspensory ligaments, which attach the lens of the eye to the ciliary body. 4. The iris is the most anterior portion of the vascular tunic. The center of the iris is a hole, the pupil, through which light enters the eye. A. The iris consists of two groups of smooth muscle. 1) The sphincter pupillae are circular muscles that decrease the size of the pupil. They are controlled through the parasympathetic division (oculomotor nerve). 2) The dilator pupillae are radial muscles that increase the size of the pupil. They are controlled through the sympathetic division. 3) The ciliary and iris smooth muscles collectively are called the intrinsic eye muscles. B. Pigments in the iris are responsible for eye color. Eye color is genetically determined and is due to melanin (the same pigment responsible for skin color). The amount and distribution of melanin produces the different eye colors. Retina 1. The retina or nervous tunic is the innermost layer of the eye. 2. The retina contains photoreceptor cells, called rods and cones, which respond to light. 3. The macula lutea (L. yellow spot) is a small (4 mm diameter) area on the posterior region of the retina. In the center of the macula lutea is a depression, the fovea centralis (L. central pit). Normally light is focused on the fovea centralis and the macula lutea. 15-6

7 4. The optic disc is located medial to the macula lutea. It is the site of exit of the optic nerve (axons from the sensory retina) from the eye. Since it contains no photoreceptor cells no vision is possible at this site, which is also called the blind spot. FIGURE Macula lutea Medial Lateral Optic disk (blind spot) Fovea centralis Looking Into the Left Eye Test for the Blind Spot 15-7

8 Compartments of the Eye 1. Compartments of the eye. The eye is divided into the three chambers. FIGURE and A. The anterior chamber is between the cornea and the iris. B. The posterior chamber is between the iris and the lens. C. The vitreous chamber is posterior to the lens. 2. Aqueous humor. A. The ciliary processes produce aqueous humor from the blood. 1) The aqueous humor flows into the posterior chamber, through the pupil into the anterior chamber. 2) The aqueous humor returns to the blood through the canal of Schlemm or the scleral venous sinus, which is a venous sinus that forms a ring around the outer edge of the iris. 3) About 5 to 6 ml. of fluid are produced per day from the blood. 4) The aqueous humor is analogous to cerebrospinal fluid. It provides nutrients to the cornea and lens, neither of which have blood vessels. 5) Pressure produced by the aqueous humor is called intraocular pressure. The pressure ensures that the retina is pushed onto the choroid. B. Glaucoma can occur when the buildup of aqueous humor exceeds the removal of aqueous humor. 1) Intraocular pressure increases, placing pressure on the retina and destroys neurons. 2) Glaucoma is the second most common cause of blindness in the USA and mostly affects people over 40 years of age. 3) Treatment includes drugs or surgery to decrease the production or increase the drainage of the aqueous humor. 2. Vitreous humor A. The vitreous chamber is filled with a jellylike substance the vitreous humor. B. The vitreous humor maintains the shape of the eye, helps to keep the retina in place, and contributes to intraocular pressure. C. Unlike the aqueous humor, the vitreous humor does not change very much. The origin of the vitreous humor is not clear. D. Aqueous humor can also enter the vitreous chamber. 15-8

9 3. Lens. A. The lens is a clear, biconvex, elastic structure made-up of proteins called crystallines. It does not contain blood vessels. It functions to focus light onto the retina. B. The lens is surrounded by a capsule, which attaches to the ciliary muscles by the suspensory ligaments. Contraction of the ciliary muscles causes the lens to change shape. C. A cataract is a clouding of the lens due to the buildup of proteins. The lens loses its transparency and blocks light transmission into the eye. Cataracts are the most common cause of blindness in the USA. Cataracts can be corrected by surgical removal of the lens. Eyeglasses or a plastic lens implant help to compensate for the removed lens. Functions of the Complete Eye Light FIGURE The electromagnetic spectrum is radiation of different wavelengths. Gamma rays are the shortest and radio waves are longest. 2. The visible spectrum or visible light is the portion of the electromagnetic spectrum detected by the human eye. Light Refraction and Reflection FIGURE When light passes from one media to another of different density (as from air into the eye), the light can be bent]. The bending of the light is called refraction. The more curved the surface boundary between the two media, the greater the bending of the light. 2. If the surface is concave the light diverges, whereas if the surface is convex the light converges. The point where converging light rays come together is called the focal point and causing the light to converge is called focusing. 15-9

10 3. Reflection occurs when light rays bounce off an object. We see objects because the light reflected off of them enters our eyes. Focusing of Images on the Retina 1. Eye components responsible for refraction. A. The cornea is responsible for most of the refraction. B. The lens, by changing shape (curvature), regulates how much the light is refracted. The lens "fine tunes" the refraction produced by the cornea. C. The aqueous humor and vitreous humor refract light slightly. 2. As light passes through the lens of the eye and is focused, the image is focused upside down and is reversed from right to left (mirror image). The brain interprets this image as normal. Note that the focused image does not occur at the focal point. 3. Far vision. A. Objects that are at a distance of 20 feet or more are clearly focused on the back of the retina (macula lutea). B. This condition is called emmetropia (G. measure of the eye), and is passive because the lens does not change shape to cause the image to be clearly focused. That is, the image is focused simply because of the arrangement of all the elements in the eye. C. The distance at which the eye is passive is called the far point of vision. For a normal eye this distance is 20 feet. 4. Near vision A. When objects are closer to the eye that 20 feet, the eye must adjust in order to keep the image clearly focused on the retina. This adjustment includes: 1) Accommodation. The ciliary muscles contract and the lens bulges. This creates a greater curvature of the lens and more refraction of the light. FIGURE ) Pupil constriction. Reducing the diameter of the pupil increases the depth of focus, which is the greatest distance through which an object can be moved and still be in focus. Why does it help to have more light when you are looking at the fine details of close up objects? 15-10

11 3) Convergence. Medial rotation of the eyeballs ensures that the image falls on the macula lutea, which prevents double vision. B. At a distance of about 6 inches or less, the eye can no longer compensate and the image becomes blurred. This distance is called the near point of vision. Visual Acuity 1. Visual acuity is the ability to tell apart two closely placed objects. Visual acuity is mostly a function of cones (which are very close together) located in the macula lutea. 2. Visual acuity for far vision is measured with an eye chart. A. 20/20 - see at twenty feet what a normal eye would see at twenty feet. B. 20/30 - see at thirty feet what a normal eye would see at thirty feet. This is less than normal visual acuity. 20/200 (uncorrectable) is considered to be legally blind. C. 20/15 - see at twenty feet what a normal eye would see at fifteen feet. Very good visual acuity. D. A nearsighted person (myopia) would not be able to clearly see at twenty feet what a normal eye would. A farsighted person (hyperopia) would be able to see clearly at twenty feet what a normal eye would. Eye Disorders That Produce Blurred Vision 1. With age, the lens loses its elasticity (the ability to bulge). The result is an increase in the near point. Objects are in focus only if they are some distance away from the eye. This condition is called presbyopia (old man eye). 2. Myopia (G. short-sighted) or nearsightness is the ability to see close objects but not distant ones. The image is focused in front of the retina because the eyeball is too long or the cornea and lens are too powerful (too much refraction of light). It is correctable with concave lens. FIGURE 15.B, p. 536 See Clinical Focus: Eye Disorders 3. Hyperopia (G. over-sighted) or farsightness is the ability to see distant objects but not close ones. The image is focused behind the retina because the eyeball is too short or the cornea and lens is too weak (too little refraction of light). It is correctable with a convex lens. Is presbyopia more like myopia or hyperopia? Explain. How would you correct for presbyopia? 15-11

12 4. Astigmatism is due to an uneven surface of the cornea or lens, and light is not uniformly refracted. The result is that part of the image is focused in front of the retina or behind the retina, causing distorted vision. Corrected with lens that are curved in one plane more than another plane. Structure and Function of the Retina FIGURE The retina has two parts: the sensory retina and the pigmented retina. 2. The outermost layer of the retina is the pigmented retina. A. The pigmented retina is a single layer of epithelial cells with melanin. B. The pigmented retina and the choroid form a black background that absorbs light. 3. The innermost layer of the retina is the sensory retina. It contains neurons. A. The photoreceptor layer consists of rods and cones and responds to light by producing action potentials. B. The bipolar layer and the ganglionic layer conduct the action potentials toward the brain. C. A detached retina is the separation of the sensory retina from the pigmented retina. Detached retinas can result from a blow to the head. 1) Fluid accumulates between the sensory and pigmented retinas and causes the sensory retina to bulge or detach from the pigmented retina. 2) The sensory retina is too far from the blood vessels in the choroid and blindness can occur in the affected part of the retina. 3) The retina can sometimes be reattached surgically. Rods FIGURE Rods are responsible for vision in low light. They are bipolar neurons with modified dendrites. The dendrites have discs that contain visual pigments which can respond to light. 2. The visual pigment in rods is rhodopsin, which consists of opsin (a protein) and retinal (a derivative of vitamin A). FIGURE A. In the dark, retinal is in the cis state (bent form)

13 B. When light strikes retinal, it changes shape to the trans state (straight form). C. The shape change of retinal causes opsin to change shape. Opsin, in turn activates a G protein, called transducin. Transducin causes some other reactions that cause Na + channels to close, resulting in hyperpolarization of the rod plasma membrane. As described below, this will result in stimulation of the bipolar cells with which rods synapse. D. Retinal separates from opsin. With an expenditure of energy (ATP), retinal can resume its original shape and reattach to opsin. E. The cycle can be repeated. 3. How hyperpolarization of rods results in action potentials in bipolar cells. FIGURE A. In the dark, 1) Rods are unstimulated, gated Na + channels are open, and Na + diffuse into rods, producing a local potential that is a depolarization. 2) Rods continuously releases the neurotransmitter glutamate when depolarized. 3) Glutamate inhibits the bipolar cells by causing hyperpolarization of bipolar cell membranes (produces IPSPs). The inhibited bipolar cells do not stimulate ganglionic cells. B. In the light, 1) Rods are stimulated by light, causing Na + channels to close. As a result, the rod hyperpolarizes. 2) Less glutamate is released from rods when they hyperpolarize. 3) Bipolar cells are no longer inhibited because of the decrease in glutamate. The bipolar cells depolarize (EPSPs) and release neurotransmitters that stimulate the production of action potentials in the ganglionic cells. 4. Dark adaptation is an increased sensitivity of the eye to light, allowing vision in dim light. A. The pupil of the eye dilates to let more light enter the eye. B. In dim light, opsin and retinal combine to form rhodopsin faster than light breaks it down. This increases the amount of rhodopsin in the rods and makes the rods very sensitive to light. The process takes 20 to 40 minutes

14 How could a lack of dietary vitamin A result in a temporary night blindness (an inability to see at night)? C. Cones do not function in dim light, so vision in dim light depends upon the number of functional rods. 5. Light adaptation is a decreased sensitivity of the eye to light, allowing vision in bright light. A. The pupil of the eye constricts and restricts the amount of light that enters the eye. B. Rhodopsin is broken down to opsin and retinal faster that it is reformed. This decreases the amount of rhodopsin in the rods and makes the rods less sensitive to light. This process takes 5 to 10 minutes. C. Cones, which function at higher light intensities than rods, also become more active. Cones FIGURE Cones are responsible for color vision, visual acuity, and vision in bright light (cones do not function in dim light). They are bipolar neurons with modified dendrites. The dendrites have discs that contain visual pigments which can respond to light. 2. The visual pigment in cones is called iodopsin. It consist of retinal combined with different kinds of opsins than in rods. FIGURE A. There are three different opsins, found in different cones, that make the retinal sensitive to different wavelengths of light. B. Blue cones are most sensitive to blue light, green cones to green light, and red cones to red light

15 C. The number of each type of cone that is activated is interpreted by the brain as a different color, e.g., activation of red and green cones is interpreted as yellow. D. Color blindness results when there is a deficiency of one or more types of cones. 1) The most common type of color blindness is red-green blindness due to a lack of red cones. Therefore the individual cannot distinguish between red and green. 2) Most color blindness is a recessive X-linked trait. At night we do not see in color, but only in black and white. This type of "color blindness" is normal. Can you explain how it happens? Distribution of Rods and Cones in the Retina 1. Light normally is focused on the fovea centralis and macula lutea A. The fovea centralis contains only cones and has the highest concentration of cones in the eye. The macula lutea contains mostly cones. B. The fovea centralis and macula lutea are responsible for visual acuity, the ability to tell apart two closely spaced objects. This is possible because of the high concentration of cones. 2. Away from the macula lutea and continuing around toward the periphery of the retina, the number of cones is relatively low (about 15 times less concentrated than in the fovea centralis). Within the macula lutea, the concentration of rods is relatively low. Away from the macula lutea, the number of rods increases (about 4 to 8 times more concentrated than in the macula lutea)]. Fovea centralis (only cones) Macula lutea (mostly cones, some rods) Periphery of retina (some cones, mostly rods) Explain why at night a person may notice a movement "out of the corner of his eye," but when he tries to look directly at the place where the movement was noticed, it appears as if nothing is there

16 Inner Layers of the Retina 1. The outer layer of the sensory retina consists of rods and cones. The rods and cones synapse with bipolar cells, which form a middle retinal layer. The bipolar cells synapse with ganglionic cells, which form an inner retinal layer. FIGURE A. The afferent pathway for rods exhibits considerable convergence, whereas the afferent pathway for cones does not. Rod Bipolar cell Ganglionic cell Rod Pathway Cone Cone Pathway Why is the cone system better for visual acuity, but the rod system is better for dim light? B. The ganglionic cell axons converge at the optic disc and exit the eye as the optic nerve. Because there are no photoreceptor cells at this location, this area is also called the blind spot. Neuronal Pathways for Vision FIGURE The optic nerve from each eye connects to each other at the optic chiasm

17 2. Some of the fibers in the optic nerves cross to the opposite side of the brain, whereas others continue on the same side. After the optic chiasm the fibers are called the optic tracts. 3. Most of the fibers synapse with neurons in the thalamus. Axons from the thalamic neurons form the optic radiations which project to the visual cortex in the occipital lobe, where vision is interpreted. 4. Some of the fibers from the optic tract go to the superior colliculi, which are involved in visual reflexes. 5. A visual field is everything that can be seen by one eye. Normally, a person has a right and left visual field. A. Each visual field can be subdivided into a nasal and temporal portion. B. The right half of each visual field (i.e., right temporal and left nasal visual fields) projects to the left side of the brain, whereas the left half of each visual field (i.e., left temporal and right nasal visual fields) projects to the right side of the brain. In other words, what you see to the right goes to the left side of the brain, and what you see to the left goes to the right side of the brain. Left visual field Right visual field Superior view of visual pathway 15-17

18 What would happen if a tumor interrupted the conduction of action potentials in the following locations? Indicate the loss of vision by darkening the appropriate part of each visual field. 1) The right optic nerve. Left visual field T N T N Right visual field 2) The right optic tract. Left visual field T N T N Right visual field 3) The optic chiasm. Left visual field T N T N Right visual field 15-18

19 6. Binocular vision and depth perception FIGURE 15.22c A. Normally a person has two eyes and each eye has its own visual field. B. Normally the right and left visual fields overlap. The region of overlap is the area of binocular vision and is responsible for depth perception (the ability to distinguish between near and far objects and to judge their distance). C. Because there are two eyes slightly apart, each eye receives a slightly different image. The brain interprets the slight difference as distance between objects, that is, as depth perception. D. With one eye there is no depth perception except for learned cues, such as knowing that distant objects appear smaller. HEARING AND BALANCE FIGURE The organs of hearing and balance can be divided into three parts: the external, middle, and inner ears. 2. The sense of hearing involves all three parts, whereas the sense of balance involves only the inner ear. Auditory Structures and Their Functions FIGURE External Ear 1. The pinna or auricle is composed of elastic cartilage covered by skin. It functions to funnel sound into the ear. 2. The external acoustic meatus is a curved passageway (about an inch in length) from the auricle to the tympanic membrane. A. It is lined with skin that has hairs and ceruminous glands. B. The ceruminous glands secrete cerumen (ear wax). The cerumen and hairs prevent foreign objects from entering the ear

20 3. The tympanic membrane (eardrum) is a membrane composed of epithelial and connective tissue that stretches across the external acoustic meatus. A. Sound waves strike the tympanic membrane and cause it to vibrate. B. A broken eardrum occurs when the tympanic membrane is ruptured. This most commonly occurs in auto accidents or due to infections. It can also happen in diving or sudden decompression in an airplane. The result is impairment of hearing. Middle Ear 1. The middle ear is an air-filled chamber within the temporal bone. 2. Ossicles (ear bones). A. There are three ossicles in each middle ear. They connect the external ear to the inner ear. 1) The malleus (hammer) is attached to the tympanic membrane. 2) The incus (anvil) is attached to the malleus and stapes. 3) The stapes (stirrup) is attached to the oval window of the inner ear. B. Vibrations of the tympanic membrane are transferred to the inner ear by the ossicles. 1) The ossicles not only transfer the vibrations, but also magnify them. 2) If the ear ossicles do not function properly (e.g., ossification or infections), there is a reduction in the ability to transfer vibrations to the inner ear. This can result in conduction deafness. Hearing aids can help conduction deafness by producing amplified sound waves (louder sound) that is transmitted past the conductive blockage. 3) Skeletal muscles attach to the malleus and stapes. When a loud sound enters the ear, the muscles reflexively contract (attenuation reflex), dampening the movement of the ossicles and thus protect the ear from damage. This works for continually loud sounds but provides little protection for sudden loud sounds. FIGURE Why have one system that magnifies sounds (ossicles) and another that dampens sounds (muscles)? 15-20

21 3. The middle ear is also connected to other structures. A. The middle ear connects to the mastoid sinus. B. The middle ear connects to the back of the throat through the auditory tube (Eustachian tube). Normal Go up in elevation Go down in elevation 1) The auditory tube functions to maintain equal pressure between the external and middle ear. This permits proper operation of the tympanic membrane. 2) In adults the auditory tube is normally closed. Swallowing can open the auditory tube and equalize the pressure. 3) In children the auditory tube is normally open. Children are more likely to develop middle ear infections than adults. 4) Infections in the middle ear can spread to the mastoid sinus to cause mastoiditis. Rarely, infections can sometimes spread into the brain. 5) Infections in the middle ear can sometimes cause a buildup of fluid that causes a temporary reduction of hearing. Inner Ear FIGURE The inner ear consists of an osseous and membranous labyrinth A labyrinth is a complex structure with winding passages. A. The osseous labyrinth forms a complex, interconnected set of tunnels within the temporal bone. B. The membranous labyrinth follows the course of the osseous labyrinth. C. The osseous labyrinth contains a fluid called perilymph, and the membranous labyrinth contains a fluid called endolymph. Perilymph and endolymph are much like cerebrospinal fluid

22 2. The osseous labyrinth is divided into three areas. A. The cochlea which is involved in hearing. B. The vestibule and the semicircular canals, which are involved with balance. The Cochlea FIGURE The cochlea (L. snail shell) is a tunnel that wraps around two and one half times to form a snaillike structure. 2. The membranous labyrinth extends through the cochlea to divide it into three compartments. A. The vestibular membrane separates the scala vestibuli from the cochlear duct. The basilar membrane separates the scala tympani from the cochlear duct. B. The cochlear duct contains endolymph. The scala vestibuli and the scala tympani contain perilymph. 3. The spiral organ is found within the cochlear duct. A. The spiral organ, or organ of Corti, consists of specialized hair cells that rest on the basilar membrane. FIGURES and ) If a hair cell were analogous to a person, the hair cell would be standing on the basilar membrane and the hair would be on the top of the cell. The hairs are very long microvilli called stereocilia. 2) The hair cells are of different heights. It is like lining up people by height, with the shortest person at the beginning of the line and the tallest person at the end of the line. The stereocilia of the tallest (longest) hair cell is embedded within the tectorial membrane, which is attached to bone. 3) A tip link connects the tip of each stereocilia to the next taller stereocilia. 4) Each tip link is a gating spring (microtubules) that connects to the gate of a gated K + channel. 5) The base of the hair cells are associated with neurons

23 B Mode of operation of the spiral organ. FIGURE ) In the absence of sound, the gating spring is relaxed and the gated K + channel is closed. 2) Sound waves (as we shall see) cause movement of the basilar membrane. This causes the hairs to be bent as the basilar membrane moves toward the tectorial membrane, which remains stationary. 3) As the shorter hair cells bend toward the taller hair cells, the gating spring is stretched and the gate of the K + channel is pulled open. 4) The endolymph has a high K + concentration and when the gates open, K + move into the cell causing depolarization. Remember that the movement of positive ions into a cell, such as Na + and Ca 2+, causes depolarization. 5) Depolarization causes the release or neurotransmitters from the hair cells that stimulate action potential production in the neurons. Auditory Function 1. Sound is that which is heard. Sound results from the vibration of molecules that travel in sound waves (pressure waves). A. Sound waves can pass from air to solid objects. Sound waves cause structures within the ear to vibrate and the movement results in the production of action potentials that are interpreted as sound. B. Without molecules, as in a vacuum, there is no sound. If there is no one in a forest, does a tree make a sound when it falls? FIGURE Loudness is the amplitude of the sound wave. 3. Pitch is the frequency of the sound waves. 4. Timbre is the characteristic quality of a sound. For example, if a piano and a violin produce a sound of the same intensity and pitch, we could still tell them apart

24 FIGURE The arrangement and function of structures within the inner ear are best understood when the cochlea is unwound. External Ear 1. The pinna funnels sound waves into the external acoustic meatus. 2. The sound waves cause the tympanic membrane to vibrate. Middle Ear 1. The movement of the tympanic membrane causes the ossicles to move. 2. The stapes moves within the oval window. The oval window is the opening into the scala vestibuli. The stapes fills the oval window and is sealed in place by a ligament. Inner Ear 1. When the stapes moves within the oval window, sound waves (vibrations) are produced in the perilymph of the scala vestibuli. 2. Sound waves in the perilymph can travel two different routes. A. To the round window. 1) The scala vestibuli is connected to the scala tympani by the helicotrema (G. a hole in a spiral structure). 2) The scala tympani is separated from the middle ear by the round window. The round window is covered by a membrane that bulges out into the middle ear when sound waves reach it. This functions to absorb the sound waves and prevent them from bouncing around within the cochlea. It also protects the inner ear from a buildup of pressure when the stapes pushes into the inner ear. B. To the spiral organ. 1) Sound waves in the perilymph of the scala vestibuli cause movement of the vestibular membrane. 2) Movement of the vestibular membrane produces sound waves in the endolymph of the cochlear duct. 3) Sound waves in the endolymph cause movement of the basilar membrane. 4) Movement of the basilar membrane causes the hair cells of the spiral organ to bend. As a result, action potentials are produced that are conducted to the brain

25 3. Interpretation of loudness and pitch. FIGURE A. Loud sounds produce greater vibration of the basilar membrane and thus produce more action potentials than soft sounds. B. Sounds of different pitch caused different parts of the basilar membrane to vibrate. Vibrations near the base of the membrane are interpreted as high pitch sounds and vibrations at the apex are interpreted as low pitch sounds. Neuronal Pathways for Hearing FIGURE Action potentials generated in the spiral organ are conducted in the cochlear branch of the vestibulocochlear nerve (cranial nerve VIII). 2. From the vestibulocochlear nerve to the medulla, to the inferior colliculus, to the thalamus, and then to the auditory cortex of the cerebrum. 3. Damage to any of the neural elements, from the hair cells of the spiral organ to the auditory cortex can produce sensorineural hearing loss. A. Hearing aids can help with sensorineural hearing loss because they produce amplified sound waves that produce greater than normal stimulation of the spiral organ. Sound clarity improves with amplification, but may never be perceived as normal. B. The term deaf refers to sensorineural hearing loss so severe that the sense of hearing is nonfunctional, with or without amplification, for the ordinary purposes of life. Balance FIGURE The vestibule (L. entrance hall). A. The membranous labyrinth in the vestibule make up the utricle and the saccule. The utricle and the saccule provide information about the position of the head relative to gravity, and they detect linear acceleration or deceleration, such as when a car speeds up or slows down. This is called static balance

26 B. The receptor structures of the utricle and saccule are the maculae. 1) A macula contains hair cells embedded within a gelatinous mass. 2) The gelatinous mass contains otoliths (G. ear stones), which are particles of calcium carbonate. 3) Endolymph fills the interior of the macula and the gelatinous substance floats in the endolymph. C. Mode of operation of the macula. FIGURE ) Changing the orientation of the head relative to the ground causes movement of the gelatinous mass. 2) Bending or stretching of the hair cells causes the production of action potentials that are conducted to the brain. 2. The semicircular canals. FIGURE A. The semicircular canals are three canals, each oriented at approximately right angles to each other. Thus, a canal is located in each major plane of movement. B. The semicircular canals detect angular acceleration or deceleration of the head. The three major movements of the head that are detected are rotation, left/right, and forward/backward. This is called dynamic balance. C. The receptor structure of the semicircular canals is the crista ampullaris. 1) At the base of each semicircular canal is an enlargement, the ampulla. 2) Inside the ampulla is a group of hair cells called the crista ampullaris. The hair cells are embedded in a gelatinous mass, the cupula. 3. The semicircular canals are filled with endolymph that can move around the semicircular canal system. D. Mode of operation of the crista ampullaris. FIGURE ) During head movement, the endolymph pushes against the crista ampullaris, causing the hair cells to bend. This results in the production of action potentials that are conducted to the brain

27 2) Depending on which cristae ampullares are stimulated, the brain interprets the movement of the head. Neuronal Pathways for Balance FIGURE Action potentials generated by the maculae or cristae ampullares go out the vestibular branch of the vestibulocochlear nerve (cranial nerve VIII). 2. The vestibulocochlear nerve extends to the medulla oblongata. From there the fibers concerned with balance take several different routes. A. Through the thalamus to the parietal lobe of the cortex (adjacent to the primary sensory area). B. To the cerebellum. C. To nuclei that control eye movement (oculomotor, abducens, and trochlear)