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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 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 2 Olfactory Epithelium • 1 square inch of • • membrane holding 10100 million receptors Covers superior nasal cavity and cribriform plate 3 types of receptor cells 3 Olfaction: Sense of Smell • Odorants bind to • • • receptors Na+ channels open Depolarization occurs Nerve impulse is triggered 4 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 5 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 6 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. 7 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 8 Accessory Structures of Eye • Eyelids or palpebrae – protect & lubricate • Tarsal glands – oily secretions keep lids from sticking together • Conjunctiva – stops at corneal edge – dilated BV--bloodshot 9 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 10 Lacrimal Apparatus • About 1 ml of tears produced per day. Spread over eye by blinking. Contains bactericidal enzyme called11 Extraocular Muscles • Six muscles that • • • insert on the exterior surface of the eyeball . 4 rectus muscles -superior, inferior, lateral and medial 2 oblique muscles -inferior and superior 12 Tunics (Layers) of Eyeball • Fibrous Tunic • • (outer layer) Vascular Tunic (middle layer) Nervous Tunic (inner layer) 13 Fibrous Tunic -- Description of Cornea • Transparent • Helps focus light(refraction) – astigmatism • Transplants – common & successful – no blood vessels so no antibodies to cause rejection 14 Fibrous Tunic -- Description of Sclera • “White” of the eye • Dense irregular • connective tissue layer -collagen & fibroblasts Provides shape & support 15 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 16 • smooth muscle that alters 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 17 Vascular Tunic -- Description of lens • Avascular • Crystallin proteins • arranged like layers in onion Clear capsule & perfectly transparent 18 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 View with Ophthalmoscope • Detached retina – trauma (boxing) 19 • fluid between layers 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 • densely packed region • at exact visual axis of eye • 2nd cells do not cover 20 cones Pathway of Nerve Signal in Retina • 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) 21 Aqueous Humor • Continuously produced • • by ciliary body Flows from posterior chamber into anterior through the pupil Glaucoma – increased intraocular pressure that could produce blindness – problem with drainage of aqueous humor 22 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 23 Definition of Refraction • Bending of light as it passes from one substance • (air) into a 2nd substance with a different density(cornea) In the eye, light is refracted by the anterior & posterior surfaces of the cornea and the lens 24 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 25 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 26 – glasses replace refraction previously provided 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 – corneal surface wavy 27 – parts of image out of focus onstriction of the Pupil • Constrictor pupillae muscle contracts • 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 28 nvergence 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 29 Photoreceptors • 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 3 different wavelengths (colors) of light 30 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 31 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 32 seen as color Brain Pathways of Vision synapse in thalamus & visual cortex 33 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 34 Visual fields • 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 in35 Anatomy of the Ear Region 36 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) 37 Middle Ear Cavity 38 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 39 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 40 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 41 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 1st order 42 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 (dB) – Conversation is 60 dB; pain above 140dB – OSA requires ear protection above 90dB 43 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 44 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 20X more vigorous than eardrum • Pressure fluctuations inside cochlear duct 45 Overview of Physiology of Hearing 46 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 47 Physiology of Equilibrium (Balance) • Static equilibrium – maintain the position of the body (head) relative to the force of gravity • Dynamic equilibrium – maintain body position (head) during sudden movement of any type--rotation, deceleration or acceleration 48 Detection of Position of Head • Movement of stereocilia or kinocilium results in the release of neurotransmitter onto the vestibular branches of the vestibulocochler nerve 49 Detection of Rotational Movement • When head moves, the attached semicircular ducts • and hair cells move with it Nerve signals to the brain are generated indicating which direction the head has been rotated 50