Chapter 17: Special Senses --Smell, taste, vision, hearing, and equilibrium --Chemical Senses *interaction of molecules with receptor cells *Olfaction: smell *Gustation: taste *both project to cerebral cortex and limbic system (evoke strong emotional reactions) Olfaction --Olfactory Epithelium *1 sq. inch of membrane holding 10-100 million receptors *covers superior nasal cavity and cribiform plate *3 Types of Cells a. Olfactory receptors b. supporting cells c. basal stem cells *odor producing molecules are small atoms with relatively high water and lipid solubility --Cells of the Olfactory Membrane a. Olfactory receptors *bipolar neurons with cilia or olfactory hairs b. Supporting cells *columnar epithelium *provide physical support, nourishment, electrical insulation, and detoxification c. Basal Cells/Stem cells *replace receptors monthly d. Olfactory/Bowman s glands *produce mucus, dissolve odorants **both epithelium and glands are innervated by CN VII --Olfaction: Sense of Smell *odorants bind to receptors *lipophylic odorants may use ODP (odorant-binding proteins) in mucous *activated receptors are coupled to G proteins which actiate 2 nd messengers (camp, phospholipase C) *Na+ or Ca++ channels open *Depolarization occurs *Nerve impulse is triggered *Rate of impulse firing is ~ to [odorant] --Adaptation and Odor Thresholds *Adaptation = decreasing sensitivity *Olfactory adaptation is rapid sometimes beneficial a. 50% in 1 second, complete in 1 minute b. occurs, at least partially, at higher levels hyperpolarization of cortical neurons c. specific to odor smelled threshold of other odors is unchanged *Low threshold (very sensitive to smell) a. only a few molecules need to be present; role of OBP in concentrating odorants b. methyl mercaptan added to natural gas as warning *Olfaction discrimination over 10,000 odors can be distinguished (TREMENDOUS)
*Differentiation of stimulus intensity is poor 30% change needed to be detected *about 3% of genes dedicated to olfaction --Olfactory Pathway *Axons from olfactory receptors form the olfactory nerves (CN I) that synapse in the olfactory bulb; olfactory bulb is part of the telencephalon (the only sense which has 1 st synapse in telecephalon) pass through 40 foramina in the cribiform plate *Second order neurons w/i the olfactory bulb form the olfactory tract that synapses on primary olfactory area of temporal lobe conscous awareness of smell begins *Other pathways lead to the frontal lobe (Brodmann area 11) where identification of the odor occurs, hypothalamus and the limbic system (memory and emotional responses to smell) *1000 different olfactory receptors, we can distinguish 10,000 different odors due to different patterns of activation of olfactory glomeruli --Pain Receptors *naked endings of trigeminal nerves stimulated by irritating substances *responsible for initiating sneezing, lacrimation, respiratory inhibition --Vomeronasal organ *perception of odor in the form of pheromones: role in reproduction and ingestion *has 30 serpentine receptors that differ from the olfactory receptors (perfumes) --Abnormalities *Anosmia: associated with hypogonadism (Kallmann s syndrome) *Hyposmia: can be age associated *Dysosmia: age related increase in olfactory threshold Gustatory Sensation: Taste --Detected by taste buds (10,000 found on tongue, soft palate, and larynx) --Papillae a. (Circum)Vallate: 100-300 buds, taste buds on side b. Fungiform: over entire tongue, taste buds on sides, most numerous c. Foliate: only in childhood d. Filiform: only tactile, no taste --Histology a. Support Cells b. Basal Cells c. Gustatory cells not primary neuron *Hairs --Function *receptors on hairs detect dissolved substances --Taste types (5 modalities) a. Sour: H+ b. Salty: Ha+ c. Bitter: cations (Mg++, Ca++, ammonium, urea, caffeine, nicotine, morphine (no commone feature) d. Sweet: organic substances, 3-D structure is important e. Umami: savory, meat, amino acids --Taste requires dissolving of substances --Anatomy of Taste Buds
*an oval body consisting of 50 receptor cells surrounded by supporting cells *a single gustatory hair (a single long microvillus, unlike the few cilia of olfactory receptors) projects upward through the taste pore *basal cells develop into supporting cells and then into new receptor cells *receptor cells have life span of 10 days *taste receptors are not neurons but specialized receptors --Physiology of Taste *complete adaptation in 1-5 minutes *thresholds vary based on 5 tastes a. most sensitive to bitter (on back of tongue circumvallate) b. least sensitive to salty and sweet *Mechanism a. dissolved substances contacts gustatory hairs b. receptor potential results in NT release c. nerve impulse formed in 1 st order neuron --Action of Major Tastants a. Salt: Na+ channel protein b. Acid/Sour: H+, K+, Positive ion through receptors c. Bitter: bitter tastant through 2 nd ary messenger(g protein with GDP) d. Glutamate/Umami: through 2ndary messenger e. Sweet: sugar/sweetener membrane receptors/secondary messenger system --Characteristics a. Taste threshold: varies with stimuli *high threshold: sweet/salty *lowest: biter b. Intensity discrimination: 30% c. Abnormalities: ageusia, hypogeusia, disgeusia (temporary, permanent) d. Genetic: autosomal recessive; phenylthiourea (bitter), taster/nontaster e. Taste aversion learning f. After reactions and contrast phenomena --Gustatory Pathway a. First order gustatory givers found in cranial nerves *CN VII (facial): serves anterior 2/3 of tongue *CN IX (glossopharyngeal): serves posterior 1/3 of tongue *CN X (vagus): serves palate and epiglottis b. Signal travel to the medulla, and then to the thalamus or limbic system (strong association with emotions) and hypothalamus c. Taste fibers extend from the thalamus to the primary gustatory area on PARIETAL lobe of the cerebral cortex (providing conscious perception of taste) Visual System --Accessory structures *protects eyes from sunlight and damaging particles *Eyelids/Palpebrae: a. protect and lubricate b. epidermis, dermis, CT, orbicularis oculi m., tarsal plate, tarsal glands, and conjunctivia *Tarsal Glands
a. oily secretions keep lids from sticking together *Conjunctiva a. palpebral and bulbar b. stops at corneal edge c. dilated BV = bloodshot *Eyelashes and eyebrows a. eyelashes and eyebrows help protect from foreign objects, perspiration, and sunlight b. sebaceous glands are found at base of eyelashes (sty) c. palpebral fissure is gap b/w eyelids *Lacrimal apparatus a. about 1mL of tears produced/day b. spread over eye by blinking c. contains lysozyme --Optic Nerve (CN II) *tracts, pathways --Eyes respond to light and initiate afferent APs --Extraocular Muscles a. six muscles that insert on the exterior surface of the eyeball b. have the smallest motor units of any skeletal muscle (2-3 ms fibers/neuron) for precision and smoothness of motion c. Innervated by CN III (oculomotor), CN IV (trochlear), and CN VI (abducens) *CN III: inferior oblique, medial, superior, inferior rectus mm. *CN IV: superior oblique m. *CN VI: lateral rectus m. --Anatomy of the Eye *Three coats or tunics a. Fibrous: outer layer ~Sclera: white outer layer --F: provides shape, support, and muscle attachment --at junction of sclera and cornea is an opening (scleral venous sinus) --posteriorly pierced by CN II ~Cornea: avascular, transparent --F: allows light to enter eye and bends and refracts light (astigmatism) --nourished by tears and aqueous humor b. Vascular: middle layer ~Choroid --pigmented epithelial cells (melanocytes) and blood vessels --provides nutrients to retina --black pigment in melanocytes absorb scattered light ~Iris: multi unit smooth muscle --F: regulate amount of light entering eye --Autonomic reflexes: i. Circular/Constrictor pupillae m: contract in bright light to shrink pupil, inn. by parasympathetic fibers ii. Radial/dilator pupillae: contract in dim light to enlarge pupil, inn by sympathetic fibers ~Ciliary Body --Ciliary muscles: control lens shape; smooth muscle (contols tension on
ligaments and lens) --ciliary processes: fold on ciliary body, secrete aqueous humor c. Nervous: inner layer ~Retina: contains neurons sensitive to light --macula lutea or fovea centralis (area of greatest visual acuity) --optic disc: blind spot, where CN II exits back of eyeball *Compartments a. Anterior Cavity: ~anterior to lens ~filled with aqueous humor (produced by ciliary processes, replaced every 90 min) ~2 chambers i. Anterior chamber: b/w cornea and iris ii. posterior chamber: b/w iris and lens b. Posterior Cavity: ~filled with vitreous humor/body ~formed once during embryonic life *Lens a. held by suspensory ligaments attached to ciliary mm. ~Suspensory Ligaments: attach lens to ciliary process i. increased tension: oval/flattened lens ii. decreased tension: rounded lens b. transparent, biconvex, avascular c. focuses light on fovea --Aqueous Humor *continuously produced by ciliary body *Flows form posterior chamber into anterior through the pupil *Scleral venous sinus/canal of Schlemm: opening in white of eye at junction of cornea and sclera, drainage of aqueous humor from eye to bloodstream *Glaucoma: increased intraocular pressure that could produce blindness, problem with drainage of aqueous humor (inc. pressure of aqueous humor in anterior cavity) --Functions of the Complete Eye a. Functions like a camera b. Iris allows light into eye c. Lens, cornea, humors focus light onto retina d. Light striking retina is converted into APs relayed to the brain --Light *Visible light: portion of electromagnetic spectrum detected by human eye *Refraction: bending of light as it passes from one substance (air) into a 2 nd substance of a different density (light is refracted by the anterior and posterior surfaces of the cornea and the lens) a. Divergence: light striking a concave surface b. Convergence: light striking a convex surface *Focal point: point where light rays converge and cross --Major Processes of Image Formation a. Refraction of light *by cornea and lens
*light must fall upon the retina b. Accommodation of the lens *changing shape of lens so that light is focused c. Constriction of the pupil *less light enters the eye --Refraction by the Cornea and Lens a. Image focused on retina is inverted and reversed from left to right *Brain learns to work with that information b. 75% of refraction is done by cornea rest done by lens c. Light rays from >20 ft are nearly parallel and only need to be bend enough to focus on retina d. Light rays <6 ft are more divergent and need more refraction (extra process needed to get additional bending of light is called accommodation) --Accommodation and the Lens a. Convex lens refract light rays towards each other b. Concave lens diverge light rays from each other c. Lens of eye is convex on both surfaces d. Viewing a distant object *lens is nearly flat by pulling of suspensory ligaments e. Viewing a close object *ciliary m. is contracted and decreases the pull of the suspensory ligaments on the lens *elastic lens thickens as the tension is removed from it *incrase in curvature of lens is called accommodation --Focus and Accommodation *Emmetropia: normal resting condition of lens *Far vision: 20 ft or more from eye *Near vision: closer than 20 ft (accommodation, pupil constriction, convergence) --Near Point of Vision and Presbyopia *Near point is the closest distance from the eye an object can be in clear focus a. 4 inches in a young adult b. 8 inches in a 40 year old c. 31 inches in a 60-80 year old (lens become less elastic) *Presbyopia: inc. in near point with age, inability of lens to become more round --Correction for Refraction Problems *Emmetropic eye (normal): can refract light from 20 ft away *Myopia (nearsighted): can t see distant objects, eyeball is too long or lens too thick, glasses concave *Hypermyopia (farsighted): can t see near objects, eyeball is too short or lens is too thin, glasses convex *Astigmatism: corneal surface wavy, parts of image out of focus --Constriction of the Pupil *Constrictor pupillae muscle contracts *Narrows beam of light that enters the eye *Prevents light rays from entering the eye through the edge of the lens *Sharpens vision by preventing blurry edges *Protects retina from very excessively bright light
--Convergence of the Eyes *Binocular vision in humans has both eyes looking at the same object *As you look at an object close to your face, both eyeballs must turn inward (CONVERGENCE) a. required so that light rays from the object will strike both retinas at the same relative point b. extrinsic eye muscles must coordinate this action (medial rectus) --The Retina *Pigmented retina: provides black backdrop for increasing visual acuity (absorbs light) *Sensory *Photoreceptors a. Rods: noncolor vision ~Rhodopsin reduction: light adaptation ~Rhodopsin production: dark adaptation b. cones: color vision *Layers of the Retina a. Pigmented epithelium --nonvisiual portion, absorbs stray light and helps keep image clear b. 3 layers of neurons (outgrowth of brain) --photoreceptor layer --bipolar neuron layer --ganglion neuron layer c. 2 other cells types that modify the signal --horizontal cells (outer synaptic layer) --amacrine cells (inner synaptic layer) --Pathway of Nerve Signal in Retina a. light penetrates retina b. rods and cones transduce light into APs c. rods and cones excite bipolar cells d. bipolars excite ganglion cells e. axons of ganglion cells form optic nerve leaving the eyeball (blind spot) f. to thalamus and then primary visual cortex in occipital lobe **Stimulus: ganglion bipolar photoreceptor** **Signal: photoreceptor bipolar ganglion neuron optic nerve thalamus occipital lobe** --Rods and Cones Photoreceptors a. Rods: *rod shaped *shades of gray in dim light *120 million rod cells *discriminate shapes and movements *distributed along periphery b. Cones: *cone shaped *sharp, color vision *6 million *fovea of macula lutea (densely packed region, at exact visual axis of eye, 2 nd cells d/n cover
cones, sharpest resolution of acuity) c. Characteristics of Photoreceptors *named for shape of outer segment *transduction of light energy into a receptor potential in outer segment *photopigment is integral membrane protein of outer segment membrane (photopigment membrane folded into discs and replaced at a very rapid rate) *Photopigments = scotopsin (protein) + retinal (derivative of vitamin A) -Rods contain rhodopsin -Cone photopigments contain 3 different opsin proteins permitting the absorption of 3 different wavelengths (colors) of light --Photopigments a. Isomerization: light cause cis-retinal to straighten and become trans-retinal shape b. Bleaching: enzymes separate the trans-retinal from the opsin, colorless final products c. Regeneration: in darkness, an enzyme converts trnas-retinal back to cis-retinal (resynthesis of a photopigment) --Formation of Receptor Potentials *In darkness a. Na+ channels are help open and photoreceptor is always partially depolarized (-30mV), Na+ moves in via diffusion (dark current) b. continuous release of inhibitory NT onto bipolar cells (glutamate) *In light a. enzymes cause the closing of Na+ channels producing a hyperpolarized receptor potential (-70mV) b. release of inhibitory NT is stopped c. bipolar cells become excited and a nerve impulse will travel towards the brain --Release of NTs *In Darkness a. cgmp-gated Na+ channels open inflow of Na+ (dark current) membrane potential of -30mV glutamate released at synaptic terminals inhibits bipolar cell *In light a. isomerization of retinal activates enzyme that breaks down cgmp cgmp gated Na+ channels close inflow of Na+ slows hyperpolarizing receptor potential glutamate release is turned off, which excites bipolar cell --Regeneration of Photopigments *pigmented epithelium near the photoreceptors contains large amounts of vitamin A and helps the regeneration process *after complete bleaching, it takes 5 minutes to regenerate ½ of the rhodopsin but only 90 seconds to regenerate the cone photopigments *full regeneration of bleached rhodopsin takes 30 to 40 minutes *rods contribute little to daylight vision, since they are bleached as fast as they regenerate --Light and Dark Adaptation a. Light Adaptation *adjustments when emerge from the dark into the light b. Dark adaptation *adjustments when enter a dark from a birght situation *light sensitivity increases as photopigments regenerate -during first 8 min of dark adaptation, only cone pigments are regenerated, so
threshold burst of light is seen as color -after sufficient time, sensitivity will increase so that a flash of a single photon of light will be seen as gray-white (rods have regenerated the photopigment) --Color Blindness and Night Blindness a. Color blindness (daltonism) *inability to distinguish between certain colors *absence of certain cone photopigments *red-green color blind person cannot tell red from green b. Night blindness (nyctalopia) *difficulty seeing in low light *inability to make normal amount of rhodopsin *possibly due to deficiency of vitamin A --Transmission of most signals occur in the retinal neurons by electrotonic conduction (not APs) --The only retinal neurons that fire APs are the ganglion cells --Electrotonic conduction: cytoplasmic spreading of direct current (hyperpolarization) from site of generation (outer segments) all the way to site of release of synaptic vesicle without abatement --The significance of electrotonic conduction is that it allows for graded conduction of signal strength by rods and cones --Retinal Processing of Visual Information a. Convergence *one cone cell synapses onto one bipolar cell produces best visual acuity *600 rod cells synapse on single bipolar cell increasing light sensitivity although slightly blurry image results (convergence of signal lots to little) *126 million photoreceptors converge on 1 million ganglion cells b. Horizontal and amacrine cells *Horizontal cells enhance contrasts in visual scene by lateral inhibition of bipolar cells in the area (sharpens image) *Amacrine cells excite bipolar cells if levels of illumination change --Transmission of changes of light intensity the O-Off response *rapid impulses when light is turned on and rapidly diminishing (within a sec) *lateral inhibition of neighboring ganglion cell (same but reverse pattern) --Lateral inhibition: provides contrast detection and enhancement *Assume that light strikes the central and one of the neighboring cells (the other is in darkness), then: a. the central receptor excites the bipolar cell b. the one in darkness inhibits the horizontal cell which in turn loses its inhibitory effect in the bipolar cell causing even greater excitation of the bipolar cell --Visual Fields *left occipital lobe receives visual images from right side of an object through impulses from nasal ½ of the right eye and temporal ½ of the left eye *left occipital lobe sees right ½ of the world *fibers from nasal ½ of each retina cross in optic chiasm --Determination of Distance a. sizes of retinal images of known objects b. moving paralax monocular, relative distance of different objects (closer on object, faster it moves across retina) c. binocular parallax (strereopsis)
--Eye Disorders a. Myopia: nearsightedness, focal point too near lens, image focused in front of retina b. Hyperopia: farsightedness, image focused behind retina c. Presbyopia: degeneration of accommodation, corrected by reading glasses d. Astigmatism: cornea or lens not uniformly curved e. Strabismus: lack of parallelism of light paths through eyes f. Retinal detachment: can result in complete blindness g. Glaucoma: increased intraocular pressure by aqueous humor buildup h. Cataract: clouding of lens i. Macular degeneration: common in older people, loss in acute vision j. Diabetes: dysfunction of peripheral circulation Auditory System/The Ear --External ear: hearing, terminates at eardrum --Middle ear: hearing, contains auditory ossicles --Inner ear: hearing and balance; interconnecting fluid-filled tunnels and chambers --External ear *Function: collect sounds *Structures a. auricle or pinna: elastic cartilage covered with skin b. external auditory canal: curved one inch tube of cartilage and bone leading into temporal bone, ceruminous glands produce cerumen (ear wax) c. tympanic membrane/eardrum: epidermis, collagen and elastic fibers, simple cuboidal epithelium *Perforated eardrum (hole is present) a. at time of injury (pain, ringing, hearing loss, dizziness) b. caused by explosion, scuba diving, or ear infection --Middle Ear Cavity *air filled cavity in the temporal bone *separated from external ear by eardrum and from internal ear by oval and round window *3 ear ossicles connected by synovial joints a. malleus attached to eardrum, incus, and stapes attached by a foot plate to membrane of oval window b. stapedius and tensor tympani muscles attach to ossicles *auditory/eustachian tube leads to nasopharynx (helps to equalize pressure on both sides of eardrum) *Connection to mastoid bone; mastoiditis --Muscles of the Ear *Tensor tympani: attached to malleus a. limits movements of malleus and stiffens eardrum to prevent damage b. innervated by the mandibular branch of the trigeminal nerve (CN V) *Stapedius m. inserts onto stapes a. prevents very large vibrations of stapes from loud noises b. innervated by CN VII
--Inner Ear Bony Labyrinth *Bony labyrinth: set of lubelike cavities in temporal bone a. semicircular canals, vestibule, and cochlea lined with periosteum and filled with perilymph b. surrounds and protects membranous labyrinth *Membranous labyrinth: set of membranous tubes containing sensory receptors for hearing and balance and filled with endolymph a. utricle, saccule, ampulla, 3 semicircular ducts and cochlea --Inner Ear *Labyrinths a. Bony: cochlea (hearing), vestibule (balance), semicircular canals (balance) b. Membranous *Lymphs a. Endolymph: similar to cytosol in its high level of K+ b. Perilymph: similar to CSF, space b/w membranous and bony labyrinth --Cranial Nerves of the Ear Region *Vestibulocochlear nerve (CN VIII) a. ampullary, utricular, and saccular branches form vestibular branch b. cochlear branch has spiral ganglion in bony modiolus --Cochlear Anatomy *3 fluid filled channels found within the cochlea (scala vestibuli, scala tympani, and cochlear duct) *vibration of the stapes upon the oval window sends vibrations into the fluid of the scala vestibuli *partitions that separate the channels are Y shaped a. bony shelf of central modiolus b. vestibular membrane above and basilar membrane below form the central fluid filled chamber (cochlear duct) *fluid vibrations affect hair cells in cochlear duct --Anatomy of the Organ of Corti *16,000 hair cells have 30-100 stereocilia/microvilli *microvilli make contact with tectorial membrane (gelatinous membrane that overlaps the spiral organ of Corti) *basal sides of inner hair cells synapse with 1 st order sensory neurons whose cell body is in spiral ganglion --Tubular Structures of the Cochlea *stapes pushes on fluid of scala vestibuli at oval window *at helicotrema, vibration moves into scala tympani *fluid vibration dissipated at round window which bulges *the central structure is vibrated (cochlear duct) --Physiology of Hearing *Auricle collects sound waves *Eardrum vibrates a. slow vibration in response to low pitched sounds b. rapid vibration in response to high pitched sounds *Ossicles vibrate since malleus attached to eardrum; ossicles are bone amplifier
*Stapes pushes on oval window producing fluid pressure waves in scala vestibuli and tympani a. oval window vibration 20x more vigorous than eardrum *Pressure fluctuations inside cochlear duct move the hair cells against the tectorial membrane *Microvilli are bent producing receptor potentials **High pitch sound (high frequency wavelengths) is detected near the oval window (near the base of the cochlea) **Low pitch sound is detected near the helicotrema --Hair Cell Physiology *hair cells convert mechanical deformation into electrical signals *As microvilli are bent, mechanically-gated channels in the membrane let in K+ ions *This depolarization spreads and causes voltage-gated Ca++ channels at the base of the cell to open *Triggering the relase of NT onto the 1 st order neuron (more NT means more nerve impulses) --Otoacoustic Emissions *Cochlea can produce inaudible sounds a. caused by shortening and lengthening of outer hair cells in response to signals from motor neurons b. vibration travels backwards toward the eardrum c. can be recorded by sensitive microphone next to the eardrum *Purpose a. as outer hair cells shorten, they stiffen the tectorial membrane b. amplifies the responses of the inner hair cells c. increasing our auditory sensitivity --Auditory Pathway *Cochlear branch of CN VIII sends signals to cochlear nucleus in the medulla (on the same side) and from there to the superior olivary nuclei (of both sides) in the pons (differences in the arrival time of impulses from both ears and difference in the wave phase allow us to locate the source of a sound) *Fibers ascend to the a. inferior colliculus b. thalamus (medial geniculate body) c. primary auditory cortex in the temporal lobe (areas 41 and 42) --Deafness a. Nerve deafness *damage to hair cells from antibiotics, high pitched sounds, anticancer drugs (the louder the sound the quicker the hearing loss, cumulative effects), fail to notice until dfficulty with speech b. Conduction deafness: perforated eardrum, otosclerosis c. Weber and Rinne tests Vestibular Apparatus --Two Components a. dynamic: detection of linear and rotational acceleration/deceleration b. Static: detection of head orientation in space relative to gravity --Two purposes of the vestibular system
a. Control eye reflexes to achieve fixation at a point b. Control reflexes for postural control --Physiology of Equilibrium (Balance) a. Static Equilibrium *maintain the position of the body (head) relative to the force of gravity *macular receptors within saccule and utricle (in the vestibule) b. Dynamic Equilibrium *maintain body position (head) during sudden movement of any type rotaitonal or linear deceleration or acceleration *Crista receptors within ampulla of semicircular ducts for rotational movement *Maculae for linear movement --Detection of angular/rotational acceleration/deceleration a. 3 semicircular canals *orientation towards one another *orientation relative to head *filled with endolymph b. Receptor: *Crista ampullaris (cupula with sensory hair cells embedded in gelatinous membrane) c. Detection sensitivity: 1 degree/second 2 d. Note: hair cells are not stimulated when there is no movement, movement at constant speed --Semicircular Canals a. Posterior canal: head tilt left to right b. horizontal canal: shake head for no c. Superior canal: shake head for yes --Crista: Ampulla of Semicircular Ducts *Small elevation within each of three semicircular ducts *Hair cells covered with cupula of gelatinous material *When you move, fluid in canal bends cupula stimulating hair cells that release NT (glutamate) --Detection of Rotational Movement *when head moves, the attached semicircular ducts and hair cells move with it endolymph fluid does not and bends the cupula and enclosed hair cells *nerve signals to the brain are generated indicating which direction the head has been rotated *the kinocilium (attach to rest of hairs) is located always on one side of the hair cells and on the same side overall in the cupula --Frequency of action potential depends on magnitude and direction of force on hair cells *this is unlike many other neurons whose AP frequency depends only on stimulus magnitude --Detection of linear acceleration/deceleration and position in space relative to gravity *Organs: a. Utricle (horizontal plane) b. Sacculus (up and won) *Filled with endolymph *Sensory structure (macula) the maculae are perpendicular to one another, with hair cells embedded in gelatinous membrane containing otolyths *Hair cells are oriented in different directions so that some are stimulated when head bends forward, others when head bends backward, a different pattern of excitation occurs for each position of the head *Otolyths: calcium carbonate crystals with glycoproteins (otoconin) *Detection sensitivity (0.5 degree decreases with increase in tilt)
--Otolithic Organs: Saccule and Utricle *thickened regions called macula within the saccule and utricle of the vestibular apparatus *cell types in the macula region a. hair cells with stereocilia (microvilli) and one kinocilium (cilia) b. supporting cells that secrete gelatinous layer *gelatinous otolithic membrane contains calcium carbonate crystals called otoliths that move when you tip your head --Integrative pathways: *seonsory (hair) cells primary sensory neurons in vestibular nerve (VIII) vestibular nuclei of medulla (brain stem) and cerebellum reticular nuclei *Two pathways: a. Down into brain stem and spinal cord for controlling antigravity muscles b. upward into midbrain for controlling thalamus cerebrum c. vestibular nuclei motor neuron of eye movement and postural reflexes --Equilibrium Pathways in the CNS a. fibers from CN VIII end in vestibular nuclei in the medulla and the cerebellum b. Fibers from these areas connect to: *cranial nerves that controle eye, head, and neck movements (III, IV, VI, XI) *vestibulospinal tract that adjusts postural skeletal muscle contractions in response to head movements c. cerebellum receives constant updated sensory information which it sends to the motor areas of the cerebral cortex (motor cortex can adjust its signals to maintain balance) -Eye Reflexes a. Nystagmus: an oscillatory eye movement *Slow component: always in the direction of the flow of endolymph *Reflex arc: i. afferent: cristae to vestibular nuclei in the brain stem ii. efferent: vestibular center to extrinsic eye muscles *Fast component: the recover phase of eye rotation (this is the direction of nystagmus) in direction opposite to flow of endolymph *Reflex arc: i. afferent: cristae to vestibular nuclei in the brain stem to vestibular reticular formation in brain stem ii. efferent: reticular formation to eye muscles --Muscle Tone Reflexes 1. Falling reaction: change in tonus of antigravity muscles *increase tonus on ipsilateral side toward which endolymph is displaced *decrease tonus on contralateral side 2. past pointing 3. neck reflexes 4. exteroceptive information from other parts of body 5. importance of visual info in maintenance of equilibrium
--Menier s disease *characterized by hearing loss, tinnitus (ringing in ear) and attacks of vertigo *Caused by overproduction or underabsorption of endolymph *involves cochlea and semicircular canals *disease progresses over years and may result in total hearing loss *Treatments: a. Medical: reduce excess amount of endolymph by decreasing salt consumption and increasing intake of diuretics b. Surgical: attempt to preserve hearing, labyrinthectomy --Endolymph a. continuously secreted by epithelial cells b. continuous with cochlear endolymph c. drains into the venous sinus of dura mater of the brain d. rate of synthesis = rate of removal e. Summary of the functions of the endolymphatic sac (ES) 1. breakdown and removal of waste products from the endolyphatic proteins and otoconia 2. Secretory activity primarily in the form of acidic glycoproteins referred to as homogenous substance which normally fills the ES lumen 3. Volume regulation of the endolymph by cycles of secretion and degradation/resorption of the osmotically active HS