Chapter 15 The Special Senses Lecture Outline PDF

Chapter 15 The Special Senses Lecture Outline  Lecture Outline The special senses include olfaction, taste, vision, hearing, and balance. 2  Special Senses The special senses include: Smell. Taste. Sight....

Chapter 15 The Special Senses Lecture Outline  Lecture Outline The special senses include olfaction, taste, vision, hearing, and balance. 2  Special Senses The special senses include: Smell. Taste. Sight. Hearing. Balance. In contrast to general senses: pressure, touch, pain, and others.  15.1 Olfaction Olfaction: sense of smell. Olfactory epithelium found in superior nasal cavity. 10 million olfactory neurons. Dendrites of olfactory neurons have enlarged ends called olfactory vesicles. Olfactory hairs are cilia of olfactory neuron embedded in mucus. Odorants dissolve in mucus. Odorants attach to receptors, cilia depolarize and initiate action potentials in olfactory neurons. One receptor may respond to more than one type of odor. Olfactory epithelium is replaced as it wears down. Olfactory neurons are replaced by basal cells every two months.  Olfactory Region, Epithelium, and Bulb Action of Odorant Binding to Membrane Receptor of Olfactory Hair 1. Each odorant receptor molecule is associated with a G protein. 2. Binding of an odorant to the receptor molecule activates the G protein. 3. The G protein activates adenylate cyclase. 4. Adenylate cyclase is an enzyme that catalyzes the formation of cyclic AMP (cAMP) from ATP. 5. cAMP in these cells causes Na+ and Ca2+ channels to open. The influx of ions into the olfactory hairs results in depolarization and the production of action potentials in the olfactory neurons. 6  Olfactory Receptors Receptor molecules vary in structure to allow for about 1000 different odorant receptor molecules that react to odorants of different sizes, shapes, and functional groups. Use multiple intracellular pathways involving G proteins, adenylate cyclase, and ion channels. People can detect about 4000 different smells. Threshold for detecting odors is very low and adaptation occurs quickly. Replaced about every 2 months from basal cells in the olfactory epithelium 7  Primary Classes of Odors Camphorous (mothballs) Musky Floral Pepperminty Ethereal (fresh pears) Pungent Putrid May be as many as 50 primary odors. 8  Neuronal Pathways for Olfaction Olfactory sensory pathway: olfactory neurons (bipolar) in the olfactory epithelium pass through cribiform plate to olfactory bulbs and synapse with tufted cells or mitral cells. These extend to the olfactory tract and synapse with association neurons. Association neurons also receive input from brain, so information can be modified before it reaches the brain. Information goes to olfactory cortex of the frontal lobe without going through thalamus (only major sense that does not go through thalamus).  Olfactory Cortex and Neuronal Pathways 1. Axons from the olfactory neurons, which form the olfactory nerves (cranial nerve I), project through numerous small foramina in the bony cribriform plate to the olfactory bulb. 2. Within the olfactory bulbs, the olfactory neurons synapse with secondary neurons, which relay olfactory information to the brain through the olfactory tracts. Olfactory bulb neurons also receive input from nerve cell processes entering the olfactory bulb from the brain, enhancing adaptation that occurs along this first part of the olfactory pathway. 10  Olfactory Processing Majority of neurons in the olfactory tracts project to the central olfactory cortex areas in the temporal and frontal lobes where they are processed to allow us to perceive odors. Includes the piriform cortex, amygdala, and orbitofrontal cortex Secondary olfactory areas involved with emotional and autonomic responses to smell. Includes the hypothalamus, hippocampus, and limbic system. 11  15.2 Taste/Gustation Taste bud: supporting cells surrounding taste (gustatory) cells. Taste cells have microvilli (gustatory hairs) extending into taste pores. Types of gustatory or glossal papillae: Filiform. Filament-shaped. Most numerous. No taste buds. Vallate. Largest, least numerous. 8 to 12 in V along border between anterior and posterior parts of the tongue. Have taste buds. Foliate. Leaf-shaped. In folds on the sides of the tongue. Contain most sensitive taste buds. Decrease in number with age. Fungiform. Mushroom-shaped. Scattered irregularly over the superior surface of tongue. Look like small red dots interspersed among the filiform. Have taste buds.  Histology of Taste Buds Taste buds consist of three major cell types: Taste (gustatory) cells Basal cells Supporting cells A taste bud has about 50 taste cells, each having several microvilli called taste hairs. Taste hairs extend from apex of the taste cell through a taste pore. Taste cells are replaced every 10 days. 13 Taste Bud Histology & Glossal Papillae Filiform papilla Fungi form papilla 14  Taste Types Sour. Most sensitive receptors on lateral aspects of the tongue. Salty. Most sensitive receptors on tip of tongue. Shares lowest sensitivity with sweet. Anything with Na+ causes depolarization plus other metal ions. Craved by humans. Bitter. Most sensitive receptors on posterior aspect. Highest sensitivity. Sensation produced by alkaloids, which are toxic. Sweet. Most sensitive receptors on tip of tongue. Shares lowest sensitivity with salty. Sugars, some carbohydrates, and some proteins (NutraSweet: aspartame). Craved by humans. Umami (Glutamate). Scattered sensitivity. Caused by amino acids. Craved by humans.  Taste Mechanism Substances called tastants, dissolve in saliva, enter the taste pores, then stimulate the taste cells. Texture and temperature affect the perception of taste. Very rapid adaptation, both at level of taste bud and within the C NS. Taste influenced by olfaction. Different tastes have different thresholds with bitter being the taste to which we are most sensitive. Many alkaloids (bitter) are poisonous. All taste buds can detect all five tastes but are usually more sensitive to one tastant. 16  Stimulation of Taste Receptors 1 Salt: Sodium ions diffuse through Na + channels, resulting in depolarization. Sweet: Sugars, such as glucose, or artificial sweeteners bind to receptors and cause the cell to depolarize by means of a G protein mechanism. The  subunit of the G protein activates adenylate cyclase, which produces cAMP. cAMP activates a kinase that phosphorylates K + channels. The K + channels close, resulting in depolarization. 17  Stimulation of Taste Receptors 2 Sour: Hydrogen ions of acids cause depolarization of taste cells by one of three mechanisms: Enter the cells directly through H+ channels Bind to ligand-gated K+ channels and block exit of K+ from cell Open ligand-gated channels for other positive ions and allow them to diffuse into the cell. Bitter: stimulate through G protein mechanism Umami: amino acids like glutamate bind to receptors and depolarize through a G protein mechanism. 18  Neuronal Pathways for Taste Carried by three cranial nerves: Chorda tympani (part of Facial nerve, VII): carry sensations from anterior two-third of tongue (except from circumvallate papillae. Glossopharyngeal nerve (IX): carries taste sensations from posterior one-third tongue, the vallate papillae, and superior pharynx. Vagus nerve (X): carries a few fibers for taste sensation from the tongue root and epiglottis. 19  Pathways for the Sense of Taste 1. Axons of cranial nerves extend from the taste buds to the tractus solitarius of the medulla oblongata. 2. Fibers from this nucleus extend to the thalamus decussating at the level of the midbrain (not shown in figure). 3. Neurons from the thalamus project bilaterally to the taste areas of both hemispheres of the cerebrum. The taste areas are located in the insula, deep within the lateral fissure between the temporal and parietal lobes. 20  15.3 Visual System Includes the eyes, accessory structures, and optic nerves, tracts, and pathways. Accessory structures: Eyebrows: shade; inhibit sweat. Eyelids (palpebrae) with conjunctiva. Palpebral fissure: space between eyelids. Canthi: lateral and medial, eyelids meet. Medial canthus has caruncle with modified sweat and sebaceous glands. Five layers of tissues including a dense connective tissue tarsal plate that helps maintain shape of lid. Eyelashes: double/triple row of hairs Ciliary glands (modified sweat glands) empty into hair follicles. Meibomian glands at inner margins produce sebum. Conjunctiva: thin transparent mucous membrane. Palpebral conjunctiva: inner surface eyelids. Bulbar conjunctiva: anterior surface of eye except over pupil. Accessory Structures of the Eye ©Eric A. Wise 22 Structures of the Visual System 23  Lacrimal Apparatus 1. Tears are produced in the lacrimal gland and exit the gland through several lacrimal ducts onto the surface of the eyeball. 2. Most of the fluid produced by the lacrimal glands evaporates from the surface 3. Excess tears are collected in the medial corner of the eye by the lacrimal canaliculi. The opening of each lacrimal canaliculus is called a punctum (PU NGK-tum; pl. puncta). The upper and lower eyelids each have a punctum located near the medial canthus on a small lump called a lacrimal papilla. The lacrimal canaliculi open into a lacrimal sac. 4. Tears flow from the lacrimal sac through the nasolacrimal duct into the nasal cavity. Specifically, the nasolacrimal duct opens into the inferior meatus of the nasal cavity beneath the inferior nasal concha. 24  Extrinsic Eye Muscles Six attached to each eye: Superior, inferior, medial, lateral rectus muscles. Superior and inferior oblique muscles. 25 Photograph of the Eye and Its Associated Structures 26  Anatomy of the Eye Hollow, fluid-filled sphere. Wall is composed of three tunics: Fibrous: sclera and cornea. Vascular: choroid, ciliary body, iris. Nervous: retina. Tunics and Structures of the Eyeball, Sagittal Section 28  Fibrous Tunic Sclera: white outer layer. Maintains shape, protects internal structures, provides muscle attachment point, continuous with cornea. Dense collagenous connective tissue with elastic fibers. Collagen fibers are large and opaque. Cornea: transparent window continuous anteriorly with sclera. Connective tissue matrix containing collagen, elastic fibers and proteoglycans. Layer of stratified squamous epithelium on the outer surface. Collagen fibers are small, thus transparent. More proteoglycans than sclera, low water content (water would scatter light). Avascular, transparent, allows light to enter eye; bends and refracts light.  Vascular Tunic Middle layer. Contains most of the blood vessels of the eye: branches off the internal carotid arteries. Contains melanin. Iris: colored part of the eye. Controls light entering the pupil. Smooth muscle determines size of pupil. Sphincter pupillae: parasympathetic. Dilator pupillae: sympathetic. Ciliary body: produces aqueous humor that fills anterior chamber. Ciliary muscles: control lens shape; smooth muscle. Ciliary processes attached to suspensory ligaments of lens. Choroid: associated with sclera. Very thin, pigmented. Lens, Cornea, Iris, and Ciliary Body 31  Retina 1 Two layers. Pigmented retina: outer, pigmented layer; pigmented simple cuboidal epithelium. Pigment of this layer and choroid help to separate sensory cells and reduce light scattering. Sensory retina: inner layer of rod and cone cells sensitive to light. Opthalmoscopic View of the Left Retina Lens focuses light on macula and fovea centralis. Macula: small yellow spot. Fovea centralis: area of greatest visual acuity; photoreceptor cells tightly packed. Optic disc “blind spot”: Area through which blood vessels enter eye, where nerve processes from sensory retina meet and exit from eye. ©Steve Allen/Getty Images  Chambers of the Eye Anterior “aqueous” chamber: anterior to lens; filled with aqueous humor. Anterior aqueous chamber: between cornea and iris. Posterior aqueous chamber: between iris and lens. Helps maintain intraocular pressure; supplies nutrients to structures bathed by it; contributes to refraction of light. Produced by ciliary process; returned to venous circulation through scleral venous sinus. Glaucoma: abnormal increase in intraocular pressure. Vitreous chamber: posterior to lens. Filled with jelly- like vitreous humor. Helps maintain intraocular pressure, holds lens and retina in place, refracts light.  Lens Transparent and biconvex. Anterior surface lined with simple cuboidal epithelial cells. Posterior region contains long, columnar epithelial cells called lens fibers. Lose nuclei and other organelles. Accumulate proteins called crystallines. Covered by elastic, transparent capsule.  Functions of the Eye 1. As light passes through the pupil of the iris, it is focused on the retina by the cornea, lens, and humors. 2. The light striking the retina is converted into action potentials. 3. The optic nerve conveys these action potentials to the brain. 36 Electromagnetic Spectrum Electromagnetic spectrum is the entire range of wavelengths or frequencies of electromagnetic radiation. Visible light: portion of electromagnetic spectrum detected by human eye (380 to 750 nm). 37  Functions of the Eye- Refraction Refraction – the bending of light rays as they pass through materials of different densities. If the surface of a lens is concave, the light rays diverge as a result of refraction. If the surface of a lens is convex, the light rays converge as a result of refraction. Focal point: point where light rays converge and cross. Focusing: causing light to converge. Lens changes shape causing adjustment of focal point on the retina; produces an inverted image on the retina. 38  Functions of the Eye - Convergence Light rays converge as they move from the air to pass through the convex cornea; greatest contrast in density here causes the most refraction. Additional convergence occurs as light passes through the aqueous humor, lens, and vitreous humor. Fine adjustments to focus light on the retina are made by the changing the shape of the lens. 39 Focusing Emmetropia (a): normal resting condition of lens. Ciliary muscle is relaxed. Lens is flat for far vision. Far point of vision: point at which lens does not have to thicken to focus. 20 feet or more from eye. Near point of vision: (b) Closer than 20 feet. Changes occur in lens, size of pupil, and distance between pupils. Lens is spherical for close vision. 40 Distant Vision  Near Point of Vision Closer than 20 feet. Changes occur in lens, size of pupil, and distance between pupils. Accommodation: ciliary muscles contract due to parasympathetic input via cranial nerve III. Pulls choroid toward lens reducing tension on suspensory ligaments. Lens becomes more spherical, greater refraction of light, used for near vision.  Near Point of Vision Pupil constriction: varies depth of focus. Convergence: as objects move close to the eye, eyes are rotated medially. Reflex contraction of the medial rectus muscles.  Visual Acuity Ability to focus an image on the retina. Myopia: nearsightedness; cornea/lens too powerful or eyeball too long. Corrected by concave lens (glasses/contacts). Mild versions can be corrected by radial keratotomy and LASIK. Hyperopia: farsightedness; cornea/lens too weak or eyeball too short. Corrected by convex lens (glasses/contacts). Presbyopia: increase in near point of vision with age due to less flexible eye lens. Astigmatism: cornea/lens not uniformly curved so light rays do not focus at single focal point. Visual Disorders and Their Correction 46  Structure and Function of the Retina Neural layer: three layers of neurons: photoreceptor cells, bipolar cells, and ganglionic cells. Cell bodies form nuclear layers separated by plexiform layers, where neurons of adjacent layers synapse with each other. Pigmented layer: single layer of cells; filled with melanin. With choroid, enhances visual acuity by isolating individual photoreceptors, reducing light scattering. Retina 2 (b) Steve Gschmeissner/Science Source 48  Rods 1 Bipolar photoreceptor cells; black and white vision (contrast). Found over most of retina, but not in fovea. More sensitive to light than cones. Protein rhodopsin changes shape when struck by light; and eventually separates into its two components: opsin and retinal. Retinal can be converted to Vitamin A from which it was originally derived. In absence of light, opsin and retinal recombine to form rhodopsin.  Rods 2 Rods are unusual sensory cells: when not stimulated they are depolarized. Light causes them to hyperpolarize. Depolarization of rods causes depolarization of bipolar cells causing depolarization of ganglion cells. Light and dark adaptation: adjustment of eyes to changes in light. Happens because of changes in amount of available rhodopsin, pupil reflexes, and changes in level of photoreceptor function. Sensory Receptor Cells of the Retina 51 Rhodopsin Cycle 52 Rod Cell Hyperpolarization 1 1. When photoreceptor cells are not exposed to light and are in a resting, nonactivated state, gated Na + channels in their membranes are open, and Na + flows into the cell. 2. This influx of Na +, referred to as the dark current, causes the photoreceptor cells to release the neurotransmitter glutamate from their presynaptic terminals. 3. Glutamate binds to receptors on the postsynaptic membranes of the bipolar cells of the retina, causing them to hyperpolarize. Thus, glutamate causes an inhibitory postsynaptic potential (IPSP) in the bipolar cells. The influx of Na + is offset by + the efflux of K + through nongated K channels. Equilibrium of Na + and K + in the cell is maintained by a sodium-potassium pump. 53 Rod Cell Hyperpolarization 2 4. Exposure to light stimulates the rod cell. Rhodopsin and the attached transducing are also activated. Transducin activates cGMP phosphodiesterase, which catalyzes the conversion of cGMP to GMP, closing Na + channels. 5. As less Na + enters the cell, glutamate release from the presynaptic terminal decreases. 6. The hyperpolarization in the bipolar cells decreases, and they depolarize sufficiently to release neurotransmitters. 7. Bipolar cells release neurotransmitters that stimulate ganglionic cells to generate action potentials. 54  Light and Dark Adaptation Mechanisms involved: Changes in amount of available rhodopsin: In bright light, excess rhodopsin is broken down, so that not as much is available to initiate action potentials, and the eyes become adapted to bright light. Conversely, in a dark room more rhodopsin is produced, making the retina more light-sensitive. Pupil reflexes: In dim light, the pupil enlarges to allow more light into the eye; in bright light, the pupil constricts to allow less light into the eye. Changes in level of photoreceptor function: During light conditions, rod function decreases and cone function increases, whereas the opposite happens during dark conditions. This occurs because rod cells are more sensitive to light than cone cells and because the rhodopsin in rods is depleted more rapidly than the visual pigment in cones. 55  Cones 1 Bipolar receptor cells with a conical light- sensitive part that tapers. Outer segments consist of double- layered discs that are more numerous and more closely stacked than in rods. Requires bright light to function. The visual pigment is iodopsin, which consists of retinal combined with an opsin protein. Function in the same way as the rhodopsin cycle. 56  Cones 2 Responsible for color vision and visual acuity. Numerous in fovea and macula lutea; fewer over rest of retina. As light intensity decreases, so does our ability to see color. Visual pigment is iodopsin: three types that respond to blue, red, and green light. Overlap in response to light, thus interpretations of gradation of color possible: several millions. 57  Inner Layers of the Retina Rods and cones synapse with bipolar cells that synapse with ganglion cells in all areas except the fovea. Except in fovea centralis, ganglion cell axons converge at optic disc, then exit via optic nerve then impulses travel to visual cortex. Fovea centralis: highest visual acuity. Rods: spatial summation. One bipolar cell receives input from numerous rods, one ganglion cell receives input from several bipolar cells. Cones exhibit little or no convergence.  Receptive Fields Area from which a ganglion cell receives input. Those in fovea centralis smaller than in other parts of retina. Two types of receptive fields. On-center ganglion cells: generate more action potentials when light is directed onto the receptive field. Respond to intensity of light. Off-center ganglion cells: more action potentials when light is off or when light does not hit center of field. Respond to contrasts in light. Interneurons present in inner layers and modify signal before signal leaves retina. Enhance borders and contours, increasing intensity at borders. Horizontal, amacrine, and interplexiform cells.  Visual Pathways 1 1. Each visual field is divided into a temporal and a nasal half. 2. After passing through the lens, light from each half of a visual field projects to the opposite side of the retina. 3. An optic nerve consists of axons extending from the retina to the optic chiasm. 4. In the optic chiasm, axons from the nasal part of the retina cross and project to the opposite side of the brain. Axons from the temporal part of the retina do not cross.  Visual Pathways 2 5. An optic tract consists of axons that have passed through the optic chiasm (with or without crossing) to the thalamus. 6. The axons synapse in the lateral geniculate nuclei of the thalamus. Collateral branches of the axons in the optic tracts synapse in the superior colliculi. 7. An optic radiation consists of axons from thalamic neurons that project to the visual cortex.  Visual Pathways 3 8. The right part of each visual field (dark green and light blue) projects to the left side of the brain, and the left part of each visual field (light green and dark blue) projects to the right side of the brain.  Visual Pathways 4 Binocular vision: visual fields partially overlap yielding depth perception. Organization of the Visual Field The projections of ganglion cells from the retina of each eye are related to the visual field for each eye. Can be seen by observing with one eye closed. Divided into temporal (lateral) and nasal (medial) parts. Images from the right visual field projects to the left side of the brain Binocular vision: visual fields partially overlap yielding depth perception. 64 Myopia: Nearsightedness. Eye Disorders Focal point too near lens, image focused in front of retina. Corrected by concave Retinal detachment. lenses. Can result in complete blindness. Hyperopia: Farsightedness. Glaucoma. Image focused behind retina. Corrected by Increased intraocular pressure by convex lenses. aqueous humor buildup. Presbyopia. Cataract. Degeneration of accommodation, corrected Clouding of lens. by reading glasses. Macular degeneration. Astigmatism: Cornea or lens not Common in older people, loss in acute uniformly curved. vision. Ptosis: drooping of the upper eyelid due Diabetes. to paralysis or disease, or as a congenital Dysfunction of peripheral circulation. condition  15.4 Hearing and Balance Divided into external, middle, and inner ear. External and middle: hearing. Internal: hearing and equilibrium. External ear. Auricle or pinna: elastic cartilage covered with skin. External auditory canal: lined with hairs and ceruminous glands. Produce cerumen. Tympanic membrane. Thin membrane of two layers of epithelium with connective tissue between. Sound waves cause it to vibrate. Border between external and middle ear. External, Middle, and Inner Ears 67  Auditory Structures and Their Functions Middle ear. Separated from the inner ear by the oval and round windows. Two passages for air. Auditory or eustachian tube: opens into pharynx, equalizes pressure. Passage to mastoid air cells in mastoid process. Ossicles: malleus, incus, stapes: transmit vibrations from eardrum to oval window. Oval window: connection between middle and inner ear. Foot of the stapes rests here and is held in place by annular ligament. Auditory Ossicles and Muscles of the Middle Ear 69 Muscles of the Middle Ear Tensor tympani: inserts on malleus; innervated by cranial nerve V Stapedius: inserts on stapes and innervated by cranial nerve VII Attenuation reflex: muscles contract during loud noises and prevent damaging vibrations Inner Ear 1  Inner Ear 1 Bony labyrinth: tunnels and chambers in the temporal bone. Cochlea: hearing. Vestibule: balance. Semicircular canals: balance. Membranous labyrinth: membranous tunnels and chambers suspended in the bony labyrinth. Fluids. Endolymph: in membranous labyrinth. Perilymph: in spaces between membranous labyrinth and periosteum of bony labyrinth. Inner Ear: Bony and Membranous Labyrinths Oval window communicates with vestibule, which communicates with the scala vestibuli of the cochlea. Scala vestibuli extends from oval window to helicotrema at cochlear apex. Second cochlea chamber (scala tympani) from helicotrema to round window. Scala vestibuli and scala tympani filled with perilymph.  Inner Ear 4 Wall of scala vestibuli is vestibular membrane. Wall of scala tympani is basilar membrane. Cochlear duct (scala media): space between vestibular and basilar membranes. Filled with endolymph. Width of basilar membrane increases from 0.04 mm near oval window to 0.5 mm near helicotrema. Near oval window basilar membrane responds to high- frequency vibrations. Near helicotrema responds to low- frequency vibrations.  Inner Ear 5 Spiral organ (organ of Corti): cells in cochlear duct. Contain hair cells (sensory cells) with hair-like projections at the apical ends. These are microvilli called stereocilia. Tips of inner hair cells embedded in tectorial membrane. Basilar region of hair cells covered by synaptic terminals of sensory neurons. Cell bodies of afferent neurons grouped into cochlear (spiral) ganglion. Afferent fibers form the cochlear nerve. Cochlea 1 Cochlea 2 Scanning Electron Micrograph of Cochlear Hair Cell Stereocilia (a) A. J. Hudspeth; (b) Steve Gschmeissner/Science Source (c) A. J. Hudspeth; (d) David Corey Access the text alternative for slide images. 80  Inner Ear 6 Hair cells arranged in rows of inner hair cells (responsible for hearing) and outer hair cells (regulate tension on basilar membrane). Hair bundle: stereocilia of one inner hair cell. Tip link (gating spring) attaches tip of each stereocilium in a hair bundle to the side of the next longer stereocilium. As stereocilia bend, they open K+ gates (mechanically gated ion channel).  Auditory Function Vibrations produce sound waves. Volume or loudness: function of wave amplitude. Pitch: function of wave frequency. Timbre: resonance quality or overtones of sound. Sound Waves  The Process of Hearing 1 External ear. Collects sound waves, conducts through external auditory canal. Middle ear. Tympanic membrane vibrates, ossicles vibrate, vibrations transferred to oval window. Tensor tympani and stapedius muscles reflexively dampen excessively loud sounds (sound attenuation reflex).  The Process of Hearing 2 Inner ear. Vibration of perilymph causes vestibular membrane to vibrate, which causes vibrations in endolymph. Basilar membrane displaced, detected by hair cells. Vibrations in scala tympani dissipated by movement of round window.  Effect of Sound Waves on Points Along the Basilar Membrane Different frequency sounds cause maximum vibrations of different regions of basilar membrane. High pitch near base. Low pitch near apex.  Sensitivity of Hearing Fine-tuning tension on basilar membrane. More than 90% of afferent axons of the cochlear ganglion synapse with inner hair cells, 10 to 30/hair cells. A few small-diameter afferent axons synapse with rows of outer hair cells. Outer hair cells receive efferent input causing them to shorten. Tuning hair cells to specific frequencies. Actin filaments in hair cells attach to K+ gated channels and can move them along the cell membrane and tighten or loosen the spring. Hair cells tuned to very specific frequencies. Localization of pitch along the cochlea. Afferent cochlear nerve fibers send action potentials to superior olivary nucleus in medulla oblongata. These are compared to one another and strongest is taken as standard. Efferent action potentials inhibit other action potentials. Action potentials from maximum vibration go to cortex and are perceived.  Central Nervous System Pathways for Hearing 1 1. Sensory axons from the cochlear ganglion terminate in the cochlear nucleus in the brainstem. 2. Axons from the neurons in the cochlear nucleus project to the superior olivary nucleus or to the inferior colliculus. 3. Axons from the inferior colliculus project to the medial geniculate nucleus of the thalamus.  Central Nervous System Pathways for Hearing 2 4. Thalamic neurons project to the the temporal auditory cortex. 5. Neurons in the superior olivary nucleus send axons to the inferior colliculus, back to the inner ear, or to motor nuclei in the brainstem that send efferent fibers to the middle ear muscles.  Balance Static labyrinth: utricle and saccule of the vestibule. Evaluates position of head relative to gravity. Detects linear acceleration and deceleration (as in a car). Dynamic labyrinth: semicircular canals. Evaluates movement of the head in three dimensional space.  Static Labyrinth Utricle has macula oriented parallel to base of skull. Saccula has macula oriented perpendicular to base of skull. Macula: specialized epithelium of supporting columnar cells and hair cells with numerous stereocilia (microvilli) and one cilium (kinocilium) embedded in gelatinous mass weighted by otoliths. Gelatinous mass moves in response to gravity bending hair cells and initiating action potentials. Otoliths stimulate hair cells with varying frequencies. Patterns of stimulation translated by brain into specific information about head position or acceleration. Structure of the Utricular and Saccular Maculae (d) Susumu Nishinag/Science Source 92 Function of the Vestibule in Maintaining Balance (Top both) Trent Stephens 93  Dynamic Labyrinth Three semicircular canals filled with endolymph: transverse plane, coronal plane, sagittal plane. Base of each expanded into ampulla with sensory epithelium (crista ampullaris). Cupula suspended over crista hair cells. Acts as a float displaced by fluid movements within semicircular canals. Displacement of the cupula is most intense when the rate of head movement changes, thus this system detects changes in the rate of movement rather than movement alone. 94 Function of the Semicircular Canals 95 Depolarization of Maculae 1. Stereocilia have tiplinks connected to gated K + channels. 2. Deflection of the hairs toward the kinocilium results in depolarization of the hair cells, whereas deflections away from the kinocilium results in hyperpolarization. 3. Hair cells have no axons but synapse with neurons of C N VIII. Hairs cells release neurotransmitters, including glutamate 96  Central Nervous System Pathways for Balance 1 1. Sensory axons from the vestibular ganglion pass through the vestibular nerve to the vestibular nucleus, which also receives input from several other sources, such as proprioception from the legs. 2. Vestibular neurons send axons to the cerebellum, which influences postural muscles. Central Nervous System Pathways for Balance 2 3. Vestibular neurons also send axons to motor nuclei (oculomotor, trochlear, and abducens), which control extrinsic eye muscles. 4. Vestibular neurons also send axons to the posterior ventral nucleus of the thalamus. 5. Thalamic neurons project to the vestibular area of the cortex.  Hearing and Balance Disorders Tinnitus: Phantom sound sensations, such as ringing in ears; a common problem Otitis media: Middle ear infection; low-grade fever, lethargy, irritability brane;, and pulling at ear; in extreme cases, can damage or rupture tympanic membrane; common in young children Meniere disease: Vertigo, hearing loss, tinnitus, and feeling of fullness in the affected ear; most common disease involving dizziness from inner ear but may involve a fluid abnormality in ears. Motion sickness: nausea, weakness, and other dysfunction due to overstimulation of the semicircular canals during motion. Conductive hearing loss: mechanical deficiency in transmission of sound waves from external ear to spiral organ. Sensorineural hearing loss: involves the spiral organ and neuronal pathways; aided with sound amplification and hearing aids. 99  Effects of Aging on the Special Senses Slight loss in ability to detect odors. Decreased sense of taste. Lenses of eyes lose flexibility - presbyopia Development of cataracts, macular degeneration, glaucoma, diabetic retinopathy. Decline in visual acuity and color perception. Presbyacusis – age-related hearing loss Hair cells in the cochlea, utricle, saccule, and ampulla decrease. More falls due to instability and vertigo. 100