Chapter 16 The Special Senses Smell taste vision

Chapter 16 The Special Senses • • Smell, taste, vision, hearing and equilibrium Housed in complex sensory organs Ophthalmology is science of the eye Otolaryngology is science of the ear Tortora & Grabowski 9/e 2000 JWS 1

Chemical Senses • Interaction of molecules with receptor cells • Olfaction (smell) and gustation (taste) • Both project to cerebral cortex & limbic system – evokes strong emotional reactions Tortora & Grabowski 9/e 2000 JWS 2

Olfactory Epithelium • 1 square inch of membrane holding 10100 million receptors • Covers superior nasal cavity and cribriform plate • 3 types of receptor cells Tortora & Grabowski 9/e 2000 JWS 3

Cells of the Olfactory Membrane • Olfactory receptors – bipolar neurons with cilia or olfactory hairs • Supporting cells – columnar epithelium • Basal cells = stem cells – replace receptors monthly • Olfactory glands – produce mucus • Both epithelium & glands innervated cranial nerve VII. Tortora & Grabowski 9/e 2000 JWS 4

Olfaction: Sense of Smell • Odorants bind to receptors • Na+ channels open • Depolarization occurs • Nerve impulse is triggered Tortora & Grabowski 9/e 2000 JWS 5

Adaptation & Odor Thresholds • Adaptation = decreasing sensitivity • Olfactory adaptation is rapid – 50% in 1 second – complete in 1 minute • Low threshold – only a few molecules need to be present – methyl mercaptan added to natural gas as warning Tortora & Grabowski 9/e 2000 JWS 6

Olfactory Pathway • Axons from olfactory receptors form the olfactory nerves (Cranial nerve I) that synapse in the olfactory bulb – pass through 40 foramina in cribriform plate • Second-order neurons within the olfactory bulb form the olfactory tract that synapses on primary olfactory area of temporal lobe – conscious awareness of smell begins • Other pathways lead to the frontal lobe (Brodmann area 11) where identification of the odor occurs Tortora & Grabowski 9/e 2000 JWS 7

Gustatory Sensation: Taste • Taste requires dissolving of substances • Four classes of stimuli--sour, bitter, sweet, and salty • 10, 000 taste buds found on tongue, soft palate & larynx • Found on sides of circumvallate & fungiform papillae • 3 cell types: supporting, receptor & basal cells Tortora & Grabowski 9/e 2000 JWS 8

Anatomy of Taste Buds • An oval body consisting of 50 receptor cells surrounded by supporting cells • A single gustatory hair projects upward through the taste pore • Basal cells develop into new receptor cells every 10 days. Tortora & Grabowski 9/e 2000 JWS 9

Physiology of Taste • Complete adaptation in 1 to 5 minutes • Thresholds for tastes vary among the 4 primary tastes – most sensitive to bitter (poisons) – least sensitive to salty and sweet • Mechanism – dissolved substance contacts gustatory hairs – receptor potential results in neurotransmitter release – nerve impulse formed in 1 st-order neuron Tortora & Grabowski 9/e 2000 JWS 10

Gustatory Pathway • First-order gustatory fibers found in cranial nerves – VII (facial) serves anterior 2/3 of tongue – IX (glossopharyngeal) serves posterior 1/3 of tongue – X (vagus) serves palate & epiglottis • Signals travel to thalamus or limbic system & hypothalamus • Taste fibers extend from the thalamus to the primary gustatory area on parietal lobe of the cerebral cortex – providing conscious perception of taste Tortora & Grabowski 9/e 2000 JWS 11

Accessory Structures of Eye • Eyelids or palpebrae – protect & lubricate – epidermis, CT, orbicularis oculi m. , tarsal plate, tarsal glands & conjunctiva • Tarsal glands – oily secretions keep lids from sticking together • Conjunctiva Tortora & Grabowski 9/e 2000 JWS – palpebral & bulbar – stops at corneal edge – dilated BV--bloodshot 12

Eyelashes & Eyebrows Eyeball = 1 inch diameter 5/6 of Eyeball inside orbit & protected • Eyelashes & eyebrows help protect from foreign objects, perspiration & sunlight • Sebaceous glands are found at base of eyelashes (sty) • Palpebral fissure is gap between the eyelids 13 Tortora & Grabowski 9/e 2000 JWS

Lacrimal Apparatus • About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called lysozyme. 14 Tortora & Grabowski 9/e 2000 JWS

Extraocular Muscles • Six muscles that insert on the exterior surface of the eyeball • Innervated by CN III, IV or VI. • 4 rectus muscles -superior, inferior, lateral and medial • 2 oblique muscles -inferior and superior Tortora & Grabowski 9/e 2000 JWS 15

Tunics (Layers) of Eyeball • Fibrous Tunic (outer layer) • Vascular Tunic (middle layer) • Nervous Tunic (inner layer) Tortora & Grabowski 9/e 2000 JWS 16

Fibrous Tunic -- Description of Cornea • Transparent • Helps focus light(refraction) – astigmatism • 3 layers – nonkeratinized stratified squamous – collagen fibers & fibroblasts – simple squamous epithelium • Transplants – common & successful – no blood vessels so no antibodies to cause rejection • Nourished by tears & aqueous humor Tortora & Grabowski 9/e 2000 JWS 17

Fibrous Tunic -- Description of Sclera • “White” of the eye • Dense irregular connective tissue layer -- collagen & fibroblasts • Provides shape & support • At the junction of the sclera and cornea is an opening (scleral venous sinus) • Posteriorly pierced by Optic Nerve (CNII) Tortora & Grabowski 9/e 2000 JWS 18

Vascular Tunic -- Choroid & Ciliary Body • Choroid – pigmented epithilial cells (melanocytes) & blood vessels – provides nutrients to retina – black pigment in melanocytes absorb scattered light • Ciliary body – ciliary processes • folds on ciliary body • secrete aqueous humor – ciliary muscle Tortora & Grabowski 9/e 2000 JWS • smooth muscle that alters shape 19 of lens

Vascular Tunic -- Iris & Pupil • Colored portion of eye • Shape of flat donut suspended between cornea & lens • Hole in center is pupil • Function is to regulate amount of light entering eye • Autonomic reflexes Tortora & Grabowski 9/e 2000 JWS – circular muscle fibers contract in bright light to shrink pupil – radial muscle fibers contract in dim light to enlarge pupil 20

Vascular Tunic -- Muscles of the Iris • Constrictor pupillae (circular) are innervated by parasympathetic fibers while Dilator pupillae (radial) are innervated by sympathetic fibers. • Response varies with different levels of light Tortora & Grabowski 9/e 2000 JWS 21

Vascular Tunic -- Description of lens • Avascular • Crystallin proteins arranged like layers in onion • Clear capsule & perfectly transparent • Lens held in place by suspensory ligaments • Focuses light on fovea Tortora & Grabowski 9/e 2000 JWS 22

Vascular Tunic -- Suspensory ligament • Suspensory ligaments attach lens to ciliary process 23 • Ciliary muscle controls tension on ligaments & lens Tortora & Grabowski 9/e 2000 JWS

Nervous Tunic -- Retina • Posterior 3/4 of eyeball • Optic disc – optic nerve exiting back of eyeball • Central retina BV – fan out to supply nourishment to retina – visible for inspection • hypertension & diabetes • Detached retina View with Ophthalmoscope Tortora & Grabowski 9/e 2000 JWS – trauma (boxing) • fluid between layers • distortion or blindness 24

Layers of Retina • Pigmented epithelium – nonvisual portion – absorbs stray light & helps keep image clear • 3 layers of neurons (outgrowth of brain) – photoreceptor layer – bipolar neuron layer – ganglion neuron layer • 2 other cell types (modify the signal) Tortora & Grabowski 9/e 2000 JWS – horizontal cells – amacrine cells 25

Rods & Cones--Photoreceptors • Rods----rod shaped – shades of gray in dim light – 120 million rod cells – discriminates shapes & movements – distributed along periphery • Cones----cone shaped – sharp, color vision – 6 million – fovea of macula lutea Tortora & Grabowski 9/e 2000 JWS • • densely packed region at exact visual axis of eye 2 nd cells do not cover cones 26 sharpest resolution or acuity

Pathway of Nerve Signal in Retina Tortora & Grabowski 9/e 2000 JWS • Light penetrates retina • Rods & cones transduce light into action potentials • Rods & cones excite bipolar cells • Bipolars excite ganglion cells • Axons of ganglion cells form optic nerve leaving the eyeball (blind spot) • To thalamus & then the primary visual cortex 27

Cavities of the Interior of Eyeball • Anterior cavity (anterior to lens) – filled with aqueous humor • produced by ciliary body • continually drained • replaced every 90 minutes – 2 chambers • anterior chamber between cornea and iris • posterior chamber between iris and lens • Posterior cavity (posterior to lens) – filled with vitreous body (jellylike) – formed once during embryonic life – floaters are debris in vitreous of older individuals Tortora & Grabowski 9/e 2000 JWS 28

Aqueous Humor • Continuously produced by ciliary body • Flows from posterior chamber into anterior through the pupil • Scleral venous sinus – canal of Schlemm – opening in white of eye at junction of cornea & sclera – drainage of aqueous humor from eye to bloodstream • Glaucoma – increased intraocular pressure that could produce blindness – problem with drainage of aqueous humor Tortora & Grabowski 9/e 2000 JWS 29

Major Processes of Image Formation • Refraction of light – by cornea & lens – light rays must fall upon the retina • Accommodation of the lens – changing shape of lens so that light is focused • Constriction of the pupil – less light enters the eye Tortora & Grabowski 9/e 2000 JWS 30

Definition of Refraction • Bending of light as it passes from one substance (air) into a 2 nd substance with a different density(cornea) • In the eye, light is refracted by the anterior & posterior surfaces of the cornea and the lens Tortora & Grabowski 9/e 2000 JWS 31

Refraction by the Cornea & Lens • Image focused on retina is inverted & reversed from left to right • Brain learns to work with that information • 75% of Refraction is done by cornea -- rest is done by the lens • Light rays from > 20’ are nearly parallel and only need to be bent enough to focus on retina • Light rays from < 6’ are more divergent & need more refraction – extra process needed to get additional bending of light is called accommodation Tortora & Grabowski 9/e 2000 JWS 32

Accommodation & the Lens • Convex lens refract light rays towards each other • Lens of eye is convex on both surfaces • View a distant object – lens is nearly flat by pulling of suspensory ligaments • View a close object – ciliary muscle is contracted & decreases the pull of the suspensory ligaments on the lens – elastic lens thickens as the tension is removed from it – increase in curvature of lens is called accommodation Tortora & Grabowski 9/e 2000 JWS 33

Near Point of Vision and Presbyopia • Near point is the closest distance from the eye an object can be & still be in clear focus – 4 inches in a young adult – 8 inches in a 40 year old • lens has become less elastic – 31 inches in a 60 to 80 year old • Reading glasses may be needed by age 40 – presbyopia – glasses replace refraction previously provided by increased curvature of the relaxed, youthful lens Tortora & Grabowski 9/e 2000 JWS 34

Correction for Refraction Problems • Emmetropic eye (normal) – can refract light from 20 ft away • Myopia (nearsighted) – eyeball is too long from front to back – glasses concave • Hypermetropic (farsighted) – eyeball is too short – glasses convex (coke-bottle) • Astigmatism Tortora & Grabowski 9/e 2000 JWS – corneal surface wavy – parts of image out of focus 35

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 very excessively bright light Tortora & Grabowski 9/e 2000 JWS 36

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 – required so that light rays from the object will strike both retinas at the same relative point – extrinsic eye muscles must coordinate this action Tortora & Grabowski 9/e 2000 JWS 37

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” & replaced at a very rapid rate • Photopigments = opsin (protein) + retinal (derivative of vitamin A) – rods contain rhodopsin – cone photopigments contain 3 different opsin proteins permitting the absorption of 38 3 different wavelengths (colors) of light Tortora & Grabowski 9/e 2000 JWS

Color Blindness & Night Blindness • Color blindness – inability to distinguish between certain colors – absence of certain cone photopigments – red-green color blind person can not tell red from green • Night blindness (nyctalopia) – difficulty seeing in low light – inability to make normal amount of rhodopsin – possibly due to deficiency of vitamin A Tortora & Grabowski 9/e 2000 JWS 39

Photopigments • Isomerization – light cause cis-retinal to straighten & become transretinal shape • Bleaching – enzymes separate the transretinal from the opsin – colorless final products • Regeneration – in darkness, an enzyme converts trans-retinal back to cis-retinal (resynthesis of a photopigment) Tortora & Grabowski 9/e 2000 JWS 40

Regeneration of Photopigments • Pigment epithelium near the photoreceptors contains large amounts of vitamin A and helps the regeneration process • After complete bleaching, it takes 5 minutes to regenerate 1/2 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 41 they are bleached as fast as they regenerate. Tortora & Grabowski 9/e 2000 JWS

Light and Dark Adaptation • Light adaptation – adjustments when emerge from the dark into the light • Dark adaptation – adjustments when enter the dark from a bright situation – light sensitivity increases as photopigments regenerate • during first 8 minutes 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 Tortora & Grabowski 9/e 2000 JWS 42

Formation of Receptor Potentials • In darkness – Na+ channels are held open and photoreceptor is always partially depolarized (-30 m. V) – continuous release of inhibitory neurotransmitter onto bipolar cells • In light – enzymes cause the closing of Na+ channels producing a hyperpolarized receptor potential (-70 m. V) – release of inhibitory neurotransmitter is stopped – bipolar cells become excited and a nerve impulse will travel towards the brain 43 Tortora & Grabowski 9/e 2000 JWS

Release of Neurotransmitters Tortora & Grabowski 9/e 2000 JWS 44

Retinal Processing of Visual Information • Convergence – one 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 – 126 million photoreceptors converge on 1 million ganglion cells • Horizontal and amacrine cells – horizontal cells enhance contrasts in visual scene because laterally inhibit bipolar cells in the area – amacrine cells excited bipolar cells if levels of illumination change Tortora & Grabowski 9/e 2000 JWS 45

Brain Pathways of Vision synapse in thalamus & visual cortex Tortora & Grabowski 9/e 2000 JWS 46

Processing of Image Data in the Brain • Visual information in optic nerve travels to – occipital lobe for vision – midbrain for controlling pupil size & coordination of head and eye movements – hypothalamus to establish sleep patterns based upon circadian rhythms of light and darkness Tortora & Grabowski 9/e 2000 JWS 47

Visual fields Tortora & Grabowski 9/e 2000 JWS • Left occipital lobe receives visual images from right side of an object through impulses from nasal 1/2 of the right eye and temporal 1/2 of the left eye • Left occipital lobe sees right 1/2 of the world • Fibers from nasal 1/2 of each retina cross in optic 48 chiasm

Anatomy of the Ear Region Tortora & Grabowski 9/e 2000 JWS 49

External Ear • Function = collect sounds • Structures – auricle or pinna • elastic cartilage covered with skin – external auditory canal • curved 1” tube of cartilage & bone leading into temporal bone • ceruminous glands produce cerumen = ear wax – tympanic membrane or eardrum • epidermis, collagen & elastic fibers, simple cuboidal epith. • Perforated eardrum (hole is present) – at time of injury (pain, ringing, hearing loss, dizziness) – caused by explosion, scuba diving, or ear infection Tortora & Grabowski 9/e 2000 JWS 50

Middle Ear Cavity Tortora & Grabowski 9/e 2000 JWS 51

Middle Ear Cavity • Air filled cavity in the temporal bone • Separated from external ear by eardrum and from internal ear by oval & round window • 3 ear ossicles connected by synovial joints – malleus attached to eardrum, incus & stapes attached by foot plate to membrane of oval window – stapedius and tensor tympani muscles attach to ossicles • Auditory tube leads to nasopharynx – helps to equalize pressure on both sides of eardrum • Connection to mastoid bone =mastoiditis Tortora & Grabowski 9/e 2000 JWS 52

Muscles of the Ear • Stapedius m. inserts onto stapes – prevents very large vibrations of stapes from loud noises • Tensor tympani attaches to malleus – limits movements of malleus & stiffens eardrum to prevent damage 53 Tortora & Grabowski 9/e 2000 JWS

Inner Ear---Bony Labyrinth Vestibule canals ampulla • Bony labyrinth = set of tubelike cavities in temporal bone – semicircular canals, vestibule & cochlea lined with periosteum & filled with perilymph – surrounds & protects Membranous Labyrinth 54 Tortora & Grabowski 9/e 2000 JWS

Inner Ear---Membranous Labyrinth • Membranous labyrinth = set of membranous tubes containing sensory receptors for hearing & balance and filled with endolymph – utricle, saccule, ampulla, 3 semicircular ducts & cochlea Tortora & Grabowski 9/e 2000 JWS 55

Cranial nerves of the Ear Region • Vestibulocochlear nerve = CN VIII – ampullary, utricular & saccular brs. form vestibular branch – cochlear branch has spiral ganglion in bony modiolus 56 Tortora & Grabowski 9/e 2000 JWS

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 57 vibrations into the fluid of the scala vestibuli Tortora & Grabowski 9/e 2000 JWS

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) 58 Tortora & Grabowski 9/e 2000 JWS

Section thru one turn of Cochlea • Partitions that separate the channels are Y shaped – bony shelf of central modiolus – vestibular membrane above & basilar membrane below form the central fluid filled chamber (cochlear duct) • Fluid vibrations affect hair cells in cochlear duct Tortora & Grabowski 9/e 2000 JWS 59

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 60 neurons whose cell body is in spiral ganglion Tortora & Grabowski 9/e 2000 JWS

Sound Waves • Vibrating object causes compression of air around it = sound waves – audible range is 20 to 20, 000 Hz – hear best within 500 to 5000 cycles/sec or Hz – speech is 100 to 3000 Hz • Frequency of a sound vibration is pitch – higher frequency is higher pitch • Greater intensity (size) of vibration, the louder the sound measured in decibels (d. B) – Conversation is 60 d. B; pain above 140 d. B – OSA requires ear protection above 90 d. B Tortora & Grabowski 9/e 2000 JWS 61

Deafness • Nerve deafness – damage to hair cells from antibiotics, high pitched sounds, anticancer drugs • the louder the sound the quicker the hearing loss – fail to notice until difficulty with speech • Conduction deafness – perforated eardrum – otosclerosis Tortora & Grabowski 9/e 2000 JWS 62

Physiology of Hearing • Auricle collects sound waves • Eardrum vibrates – slow vibration in response to low-pitched sounds – rapid vibration in response to high-pitched sounds • Ossicles vibrate since malleus attached to eardrum • Stapes pushes on oval window producing fluid pressure waves in scala vestibuli & tympani – oval window vibration 20 X more vigorous than eardrum • Pressure fluctuations inside cochlear duct move the hair cells against the tectorial membrane 63 Tortora & Grabowski 9/e 2000 JWS • Microvilli are bent producing receptor potentials

Overview of Physiology of Hearing Tortora & Grabowski 9/e 2000 JWS 64

Distinguishing Different Sounds? • Sounds at different frequencies vibrate different portions of the basilar membrane – high pitched sounds vibrate the stiffer more basal portion of the cochlea – low pitched sounds vibrate the upper cochlea which is wider and more flexible • Loud sounds vibrate cause a greater vibration of the basilar membrane & stimulate more hair cells which our brain interprets as “louder” 65 Tortora & Grabowski 9/e 2000 JWS

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 & causes voltagegated Ca+2 channels at the base of the cell to open • Triggering the release of neurotransmitter onto the first order neuron 66 – more neurotransmitter means more nerve impulses Tortora & Grabowski 9/e 2000 JWS

Otoacoustic Emissions • Cochlea can produce inaudible sounds – caused by shortening & lengthening of outer hair cells in response to signals from motor neurons – vibration travels backwards toward the eardrum – can be recorded by sensitive microphone next to the eardrum • Purpose – as outer hair cells shorten, they stiffen the tectorial membrane – amplifies the responses of the inner hair cells – increasing our auditory sensitivity 67 Tortora & Grabowski 9/e 2000 JWS

Auditory Pathway • Cochlear branch of CN VIII sends signals to cochlear and superior olivary nuclei (of both sides) within medulla oblongata – differences in the arrival of impulses from both ears, allows us to locate the source of a sound • Fibers ascend to the – inferior colliculus – thalamus – primary auditory cortex in the temporal lobe (areas 41 & 42) Tortora & Grabowski 9/e 2000 JWS 68

Cochlear Implants • If deafness is due to destruction of hair cells • Microphone, microprocessor & electrodes translate sounds into electric stimulation of the vestibulocochlear nerve – artificially induced nerve signals follow normal pathways to brain • Provides only a crude representation of sounds Tortora & Grabowski 9/e 2000 JWS 69

Physiology of Equilibrium (Balance) • Static equilibrium – maintain the position of the body (head) relative to the force of gravity – macula receptors within saccule & utricle • Dynamic equilibrium – maintain body position (head) during sudden movement of any type--rotation, deceleration or acceleration – crista receptors within ampulla of semicircular ducts Tortora & Grabowski 9/e 2000 JWS 70

Vestibular Apparatus • Notice: semicircular ducts with ampulla, utricle & saccule Tortora & Grabowski 9/e 2000 JWS 71

Otolithic Organs: Saccule & Utricle • Thickened regions called macula within the saccule & utricle of the vestibular apparatus • Cell types in the macula region – hair cells with stereocilia (microvilli) & one cilia (kinocilium) – supporting cells that secrete gelatinous layer • Gelatinous otolithic membrane contains calcium carbonate 72 Tortora & Grabowski 2000 JWS that move when you tip your head crystals called 9/e otoliths

Detection of Position of Head • Movement of stereocilia or kinocilium results in the release of neurotransmitter onto the vestibular branches 73 of the vestibulocochler nerve Tortora & Grabowski 9/e 2000 JWS

Crista: Ampulla of Semicircular Ducts • Small elevation within each of three semicircular ducts – anterior, posterior & horizontal ducts detect different movements • Hair cells covered with cupula of gelatinous material • When you move, fluid in canal bends cupula stimulating 74 hair cells that release neurotransmitter Tortora & Grabowski 9/e 2000 JWS

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 75 head has been rotated Tortora & Grabowski 9/e 2000 JWS

Equilibrium Pathways in the CNS • Fibers from vestibulocochlear nerve (VIII) end in vestibular nuclei and the cerebellum • Fibers from these areas connect to: – cranial nerves that control eye and head and neck movements (III, IV, VI & XI) – vestibulospinal tract that adjusts postural skeletal muscle contractions in response to head movements • 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 76 Tortora & Grabowski 9/e 2000 JWS