The Special Senses Chapter 16B Smell, taste, vision, hearing and equilibrium Housed in complex sensory organs The Special Senses 1 2 Chemical Senses Interaction of molecules with chemoreceptor cells Olfaction (smell) and gustation (taste) Both project to cerebral cortex & limbic system evoke strong emotional reactions Olfactory Epithelium The receptors for olfaction are in the nasal epithelium in the superior portion of the nasal cavity 3 4
Cells of the Olfactory Membrane Olfactory receptors bipolar neurons with cilia (olfactory hairs) combine to form olfactory (I) nerve Basal cells = stem cells replace receptors monthly Olfactory glands produce mucus Physiology of Olfaction - Overview Genetic evidence suggests there are hundreds of primary scents Odorant molecules bind to receptor proteins and nerve impulses are triggered 5 6 Odor Thresholds & Adaptation Low threshold only a few molecules need to be present methyl mercaptan added to natural gas as warning Adaptation = decreasing sensitivity Olfactory adaptation is rapid 50% in 1 second complete in 1 minute for some strong odors Gustatory Sensation: Taste Taste is a chemical sense To be detected, molecules must be dissolved 5 classes of stimuli: sour, sweet, bitter, salty & umami (savory/meaty/ glutamate) Other tastes are a combination of the 5 primary taste sensations plus olfaction Taste buds found on sides of papillae 7 8
An oval body consisting of 50 receptor cells plus basal cells and supporting cells Each receptor cell contains a single gustatory hair (microvillus) Anatomy of Taste Buds Physiology of Taste Mechanism dissolved substance contacts gustatory hairs receptor potential results in neurotransmitter release nerve impulse formed in first-order neuron Thresholds for tastes vary among the 4 primary tastes most sensitive to bitter (poisons) least sensitive to salty and sweet 9 10 VISION More than half the sensory receptors in the human body are located in the eyes. A large part of the cerebral cortex is devoted to processing visual information. Accessory Structures of Eye Eyelids Conjunctiva mucous membrane lines the inner eyelids folds back on surface of the eyeball Extrinsic eye muscles 11 12
Eyelids, Eyelashes & Eyebrows Lacrimal Apparatus Help protect from foreign objects, perspiration & sunlight Tarsal glands type of sebaceous (oil) gland keeps eyelids from sticking to each other slows evaporation of tears blocked gland can cause sty Lacrimal glands produce tears Spread over eyes by blinking Contains bactericidal enzyme lysozyme 13 14 Fibrous Tunic 3 Tunics (Layers) of the Eyeball Cornea Fibrous Tunic (outer layer) Vascular Tunic (middle layer) Nervous Tunic (inner layer) Transparent Helps focus light (refraction) Transplants common & successful no blood vessels so no antibodies to cause rejection nourished by tears & aqueous humor Sclera White of the eye 15 16
Vascular Tunic Choroid melanocytes & blood vessels provides nutrients to retina Ciliary body ciliary muscle alters shape of lens Iris colored portion of eye melanocytes and smooth muscle hole in center is pupil function is to regulate amount of light entering eye Vascular Tunic -- Muscles of the Iris 17 18 Nervous Tunic -- Retina Avascular Crystallin proteins arranged like layers in an onion Connected to ciliary body Focuses light on retina With aging the lens loses elasticity resulting in presbyopia 20 Lens View with Ophthalmoscope Lines posterior 3/4 of eyeball Optic disc (blind spot) optic (II) nerve exiting back of eyeball Blood vessels nourishment to retina visible for inspection hypertension, macular disease & diabetes Macula lutea central fovea Detached retina trauma (boxing) 19
Layers of Retina Pigmented epithelium absorbs stray light & helps keep image clear 3 layers of neurons photoreceptor layer bipolar neuron layer ganglion neuron layer Photoreceptors - Rods & Cones Rods -- rod shaped shades of gray in dim light 120 million rod cells shapes & movements distributed along periphery Cones -- cone shaped sharp, color vision in bright light 6 million cone cells central fovea of macula lutea high concentration of cones no rods at exact visual axis of eye bipolar & ganglion cells do not cover cones in this region sharpest resolution 21 22 Pathway of Nerve Signal in Retina Light penetrates retina Rods & cones change light into action potentials Rods & cones excite bipolar cells Bipolars excite ganglion cells Axons of ganglion cells form optic (II) nerve leaving the eyeball (blind spot) To thalamus & then the primary visual cortex Cavities of the Interior of Eyeball Anterior cavity filled with aqueous humor produced by ciliary body continually drained into canal of Schlemm Vitreous chamber filled with vitreous body (jellylike) formed once during embryonic life floaters are debris in vitreous body 23 24
Intraocular Pressure Intraocular pressure is controlled mainly by the aqueous humor The intraocular pressure and the vitreous body maintain the shape of the eyeball and keep the retina smoothly applied to the choroid Glaucoma increased intraocular pressure problem with drainage of aqueous humor may produce degeneration of the retina and blindness Major Processes of Image Formation Constriction/dilation of the pupil controls amount of light entering the eye Refraction of light by cornea & lens Accommodation of the lens changing shape of lens so that light is focused 25 26 Refraction by the Cornea & Lens Image focused on retina is inverted & reversed from left to right Brain learns to work with that information 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 Accommodation is an increase in the curvature of the lens, initiated by ciliary muscle contraction, which allows the lens to focus on near objects Correction for Refraction Problems Emmetropia (normal) Myopia (nearsighted) eyeball is too long from front to back glasses concave Hypermetropia (farsighted) eyeball is too short glasses convex Astigmatism corneal surface wavy parts of image out of focus 27 28
Physiology of Vision The first step in vision transduction is the absorption of light by photopigments in rods and cones Photopigments undergo structural changes upon light absorption Retinal is the light absorbing part of photopigments Photopigments also contain a protein called opsin There are 4 different opsins Cones contains one of three different kinds permit the absorption of 3 different wavelengths (colors) of light Rods contain a single type (rhodopsin) Photoreceptors Named for shape of outer segment Receptors transduce light energy into a receptor potential Photopigments opsin (protein) + retinal (derivative of vitamin A) 29 30 Regeneration of Bleached Photopigments Bleaching and regeneration of the photopigments accounts for light and dark adaptation After complete bleaching, it takes 5 minutes to regenerate 1/2 of the rhodopsin 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 Only 90 seconds are required to regenerate the cone photopigments Color Blindness & Night Blindness Color blindness inability to distinguish between certain colors inherited deficiency in one of the three 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 32
External Ear Anatomy of the Ear Region The external (outer) ear collects sound waves and passes them inwards Structures auricle or pinna elastic cartilage covered with skin external auditory canal ceruminous glands produce cerumen = ear wax tympanic membrane or eardrum 33 34 Middle Ear Cavity Middle Ear Cavity Air filled cavity in the temporal bone Separated from external ear by eardrum and from internal ear by oval & round windows 3 ear ossicles malleus attached to eardrum incus stapes attached to oval window Auditory (Eustachian) tube leads to nasopharynx helps to equalize pressure on both sides of eardrum 35 36
Inner Ear Labyrinth Cranial nerves of the Ear Region Carved out of temporal bone Fluid-filled membranous ducts Contain sensory receptors for Hearing cochlea Balance vestibule (utricle & saccule), 3 semicircular ducts & ampulla Vestibulocochlear nerve = cranial nerve VIII 37 38 Overview of Physiology of Hearing Physiology of Hearing - Overview Auricle collects sound waves Eardrum vibrates Ossicles vibrate Stapes pushes on oval window producing fluid pressure waves in cochlea Pressure fluctuations inside cochlear duct move the hair cells against the tectorial membrane Microvilli are bent producing receptor potentials 39 40
Cochlear Anatomy Zoom Out Cochlear Anatomy Zoom In 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 Fluid vibrations affect hair cells in cochlear duct into the fluid of the scala vestibuli 41 42 Organ of Corti Projecting over and in contact with the microvilli of hair cells is the tectorial membrane, a delicate and flexible gelatinous membrane Pressure fluctuations move the hair cells against the tectorial membrane Bending of the microvilli produces receptor potentials Sound Waves Sound waves result from the alternate compression and decompression of air molecules Frequency of a sound vibration is percieved as pitch higher frequency is higher pitch speech is 100 to 3000 hertz (Hz) The volume of a sound is its intensity (amplitude) the greater the amplitude of the vibration, the louder the sound conversation is ~60 decibels (db) OSHA requires ear protection above 90 db Leads to the generation of nerve impulses in cochlear nerve 43 44
Pitch and Volume High-frequency (high-pitch) tone causes the membrane to vibrate near the base of the cochlea Low-frequency (low-pitch) tone causes the membrane to vibrate near the apex of the cochlea Sounds of the same pitch vibrate the same region of the membrane, and thus stimulate the same cells Louder sound causes a greater vibration amplitude which our brain interprets as louder Deafness Nerve deafness possibly nerve damage (CN VIII) usually damage to hair cells from antibiotics, loud sounds, anticancer drugs, meningitis or congenital Conduction deafness vibrations are not conducted to hair cells perforated eardrum otosclerosis: overgrowth of spongy bone over the oval window that immobilizes the stapes 45 46 Cochlear Implants Vestibular Apparatus 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 47 Semicircular ducts with ampulla Vestibule (utricle & saccule) 48
Physiology of Equilibrium (Balance) The Macula 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 The maculae of the utricle and saccule are the sense organs of static equilibrium Hair cells with stereocilia Gelatinous otolithic membrane contains calcium carbonate crystals called otoliths that move when you tip your head 49 50 Detection of Position of Head Membranous Semicircular Ducts The three semicircular ducts, along with the saccule and utricle maintain dynamic equilibrium anterior, posterior & horizontal ducts detect different movements (combined 3-D sensitivity) The cristae in the ampulla of the semicircular ducts are the primary sense organs of dynamic equilibrium Movement of stereocilia results in the release of neurotransmitter onto the vestibular branches of the vestibulocochler nerve (VIII) 51 52
Crista: Ampulla of Semicircular Ducts Detection of Rotational Movement Small elevation within each of three semicircular ducts Hair cells are covered with cupula (gelatinous material) When you move, the cupula moves, but the fluid in canal tends to stay in place, thus dragging on the cupula, bending the hair cells and causing the release of neurotransmitter Nerve signals to the brain are generated indicating which direction the head has been rotated 53 54 AGING AND THE SPECIAL SENSES After age 50 some individuals experience loss of olfactory and gustatory receptors Age related changes in the ears Presbycusis hearing loss due to damaged or loss of hair cells in the organ of Corti Tinnitus (ringing in the ears) becomes more common Age related changes in the eyes Presbyopia Cataracts (loss of transparency of the lens ) Diseases such as age related macular disease, detached retina, and glaucoma Decrease in tear production Sharpness of vision as well as depth and color perception are reduced DISORDERS: HOMEOSTATIC IMBALANCES Otitis media is an acute infection of the middle ear, primarily by bacteria Characterized by pain, malaise, fever, and reddening and outward bulging of the eardrum, which may rupture unless prompt treatment is given Children are more susceptible than adults 55 56