BIOL 1131 Lecture Notes - Chapter 15: Special Senses PDF

BIOL 1131 Lecture Professor: Rebecca Polich, PhD Pronouns: She/Hers/Her Email: [email protected] Office: R224 Office Hours: by appointment, Zoom and in- person available Outline My philosophy on teaching (and life) Why the rules I harp on have...

BIOL 1131 Lecture Professor: Rebecca Polich, PhD Pronouns: She/Hers/Her Email: [email protected] Office: R224 Office Hours: by appointment, Zoom and in- person available Outline My philosophy on teaching (and life) Why the rules I harp on have been put in place Anatomy terms and roots (and why we don’t make fun of other for “mispronouncing” words) How the class is organized and why Nursing Department Review of Canvas pages Class vs. lab Resources available in each Review of Mastering A&P homework Syllabus review Share out/let’s get to know each other Lecture: the special senses: vision Syllabus agreement activity  3 Canvas participation activities to complete:  Syllabus quiz  Cheating rules acknowledgement  Missed exam and lab acknowledgement  Disability and chronic illness communication acknowledgement  Found in the Week 1 module  Must complete in order to open the later modules Who I amo I am  B.S. from UC Davis, 2012  PhD from Iowa State University 2017  Former Professor at Rocky Mountain College Taught A&P!  Additionally I am a...  Daughter  Sister  Wife  Cat mom Share out  Find a partner or two in the class  Talk to them about your background  What college you are a student in  What year in school and your major  Your career path and why you chose it  Share out! We will spend a few minutes after time is up sharing our backgrounds. Chapter 15: The Special Senses Chapter 15 Learning Objectives  Vision Explain how light is directed to the fovea centralis of the retina. Describe the process by which images are focused on the retina. Describe the structure and function of the retina’s layers of cells. Explain the distribution of rods and cones and their relation to visual acuity. Describe the structure of the photoreceptors and how we are able to distinguish color. Chapter 15 Learning Objectives  Vision Explain photoreception and how visual pigments are activated. Explain how the visual pathways distribute information to their destinations in the brain. Describe disorders of vision and their possible treatments. Chapter 15 Learning Objectives  Equilibrium and Hearing Describe the structures and functions of the bony labyrinth and membranous labyrinth. Describe the functions of hair cells in the semicircular ducts, utricle, and saccule. Describe the structures and functions of the spiral organ. Explain the anatomical and physiological basis for pitch and volume sensations for hearing. Trace the pathways for the sensations of equilibrium and hearing to their respective destinations in the brain. Describe age-related disorders of olfaction, gustation, vision, equilibrium, and hearing. Chapter 15 Learning Objectives  Olfaction and Gustation Explain the roles of generation potentials and depolarization in sensory neurons and receptor cells. Trace the olfactory pathways to their destinations in the cerebrum and explain how olfactory perception occurs. Describe gustatory reception. Be able to answer questions about the physiological processes involved in taste. Trace the gustatory pathways. Eye layers and cavities  Three layers (tunics) of the eye  Fibrous layer Outermost layer of eye Consists of cornea (clear) and sclera (white) – Joined at corneoscleral junction Functions 1. Supports and protects the eye 2. Attachment site for extrinsic eye muscles 3. Curvature of the cornea aids in the focusing process (light first enters through cornea) Eye layers and cavities Three layers (tunics) of the eye (continued) 2. V ​ ascular layer (uvea)—contains many blood vessels, lymphatic vessels, and intrinsic (smooth) muscles of eye Includes iris, ciliary body, and choroid Functions 1. Provides route for blood vessels/lymphatics to eye tissues 2. Regulates amount of light entering eye (iris) 3. Secretes/reabsorbs aqueous humor (fluid) circulating in eye chambers 4. Controls shape of lens (ciliary body); essential for focusing Eye layers and cavities Three layers (tunics) of the eye (continued)  Iris = colored part of eye Blood vessels, pigment cells (melanocytes) Two layers of smooth muscle—contraction changes diameter of pupil to control amount of light entering  Ciliary body = thickened region bulging into interior of eye Ring of fibers connects ciliary body and lens  Choroid = vascular layer underlying sclera; has extensive capillary network supplying oxygen/ nutrients to neural layer Eye layers and cavities Three layers (tunics) of the eye (continued) 3. ​Inner layer, or retina = innermost layer of eye Outer pigmented layer absorbs light Thick, inner neural layer contains photoreceptors (the cells sensitive to light) The three layers of the eye and associated structures Eye layers and cavities Aqueous humor  Functions 1. Transports nutrients and wastes 2. Forms fluid cushion 3. Helps retain eye shape 4. Stabilizes position of the retina Eye layers and cavities Iris  Body of iris is highly vascular, pigmented loose connective tissue Anterior surface—incomplete layer of fibroblasts/melanocytes Posterior surface—lined by pigmented epithelium of neural layer The structures of the eye direct light along a visual axis to the fovea centralis of the retina Cornea  Allows light to enter eye—transparent and clear  Dense matrix of multiple layers of collagen fibers  Avascular; receives oxygen and nutrients from tears Eye structures Lens  Posterior to cornea; anchored by ciliary zonule of ciliary body,  Primary function: changes shape to focus image on photoreceptors Eye structures  Choroid = middle layer; blood vessels nourish all eye structures  Sclera (“white of the eye”)—dense fibrous connective tissue with collagen and elastic fibers Stabilizes eye shape during movement Insertion for extrinsic eye muscles  Optic nerve (II)—conveys visual information to brain Eye structures  Ciliary body—supports lens, controls its shape; tension in ciliary zonule resists tendency of lens to ball up  Retina—contains photoreceptors, pigment cells, supporting cells, neurons Eye structures Pupil = opening in iris through which light passes  Two pupillary muscles of iris regulate amount of light entering 1. D ​ ilator pupillae muscles—extend radially from pupil – Enlarge pupil; supplied by sympathetic nervous system Eye structures 2. ​Sphincter pupillae muscles—encircle pupil like a ring – Make pupil smaller; supplied by parasympathetic nervous system Eye structures Visual axis = imaginary line drawn from center of object you are looking at through the center of the cornea and lens to retina Eye structures Retina—photoreceptors, pigment cells, supporting cells, neurons  Photoreceptors in inner neural portion; type/density vary by area Macula—patch of retina with high density of photoreceptors Fovea centralis—central part of macula – Has highest concentration of photoreceptors – Point of sharpest vision Review of structures of the eye Focusing of light produces a sharp image on the retina Focusing process is a two-step process 1. Light is refracted (bent) when it passes from air into cornea Bending occurs because of the change in density Amount of refraction at cornea is constant 2. More refraction when light passes from aqueous humor into lens Bends light rays toward focal point—specific point on retina Focusing on the retina Focal distance of a lens  Distance between center of lens and its focal point  Determined by: 1. Distance from object to lens 2. Shape of lens  Distance from lens to retina cannot change Focus by changing shape of the lens = accommodation Focusing on the retina Accommodation = change in lens shape to keep focal distance constant and provide clear vision  For close vision: Ciliary muscle contracts—ciliary body moves toward lens, reduces tension in ciliary zonule Lens pulled into more spherical shape; increases refraction Light from near objects is focused on retina Focusing on the retina Accommodation (continued)  For distant vision: Ciliary muscle relaxes—ciliary zonule pulls on lens Lens flattens; brings image of distant object into focus on retina Focusing on the retina Image formation  Images are not single point, but rather consist of large numbers of individual points (like pixels on computer screen)—each focused on retina  Image is inverted and reversed; brain compensates —learned from experience Focusing on the retina Near point of vision = inner limit of clear vision  Determined by lens elasticity  Increases with age as lens becomes less elastic In children 7–9 cm (3–4 in.) Young adult 15–20 cm (6–8 in.) Age 60, about 83 cm (33 in.) Retinal layers and cells Photoreceptors of the retina  Rods Highly sensitive; allow vision in very dim light No color discrimination—provide black-and-white vision only  Cones Color vision Sharper, clearer images Require more intense light  Rods and cones synapse with bipolar cells; bipolar cells synapse on ganglion cells Photoreceptors of the retina Retinal layers and cells Photoreceptor distribution  Cones: ~6 million per eye Most dense at fovea centralis of macula—no rods there – Cone density directly correlates with visual acuity (sharpness)  Rods: ~125 million per eye Maximum density at periphery; few cones there Photoreception occurs in the outer segment of rod and cone cells Photoreceptors (rods and cones) detect photons (basic units of light)  Light energy—type of radiant energy that travels in waves  Our visible spectrum: 400–700 nm Photoreception Visual pigments transduce light  Derived from rhodopsin (visual purple); pigment in rods  Have opsin (protein; determines wavelength absorbed) and retinal (pigment synthesized from vitamin A) Photoreception Photoreceptor structure  Pigmented epithelium adjacent to photoreceptors Absorbs photons not absorbed by visual pigments Phagocytizes old discs shed from tip of outer segment Photoreception Photoreceptor structure (continued)  Outer segment Flattened, membranous discs containing visual pigment – Cones: plasma membrane infoldings; outer segment tapers to blunt point – Rods: discs are separate structures; outer segment forms elongated cylinder Photoreception Photoreceptor structure (continued)  Inner segment Contains major organelles—responsible for all cell functions other than photoreception Each photoreceptor synapses with a bipolar cell Photoreception Color vision  Rods all contain same opsin; responds to blue- green wavelengths  Three types of cones 1. B​ lue cones (16% of cones) 2. ​Green cones (10%) 3. ​Red cones (74%) Photoreception  Three cone types have different opsins; sensitive to different wavelength ranges, with overlap  If all three types are stimulated equally, we see white Photoreception Color blindness—nonfunctional cones  Inability to distinguish certain colors  One or more types of cones are nonfunctional— absent or do not make necessary visual pigment  Most common type is red–green colorblindness—no red cones; cannot distinguish between red and green  Often inherited 10 percent of males 0.67 percent of females 1 in 300,000—no pigment Photoreception involves activation, bleaching, and reassembly of visual pigments 1. Resting state (in the dark)  Chemically gated sodium ion channels of outer segment stay open if cGMP present  Inner segment continuously pumps sodium ions out of cytosol Photoreception process 1. Resting state (in the dark) (continued)  This movement of ions = dark current  Keeps resting membrane potential about –40 mV  Photoreceptor continually releases neurotransmitters to bipolar cells Photoreception process 2. Retinal molecule in rhodopsin changes shape (activation) from a bent 11-cis form to more linear 11-trans form Photoreception process 3. Opsin activates transducin, a G protein bound to disc membrane  Transducin activates phosphodiesterase (PDE) Photoreception process 4. Phosphodiesterase breaks down cGMP, inactivating gated sodium channels  Sodium entry decreases Photoreception process Active state  Decreased sodium entry reduces dark current  Membrane potential drops to –70 mV (hyperpolarized) Photoreception process Active state (continued)  Hyperpolarization decreases neurotransmitter release  Decreased neurotransmitter signals bipolar cell that the photoreceptor has absorbed a photon The process of photoreception Photoreception process Rhodopsin cannot respond to another photon until original shape of retinal is regained  Three step process 1. ​Bleaching – Entire rhodopsin molecule first broken into retinal and opsin 2. Retinal converted back to cis shape—requires ATP 3. Opsin and retinal are reassembled as rhodopsin The visual pathways distribute visual information from each eye to both cerebral hemispheres Visual pathways  Photoreceptors to bipolar cells to ganglion cells  ~1 million axons from ganglion cell converge at optic disc; head toward diencephalon as the optic nerve (II) Visual pathways Visual pathways (continued)  Two optic nerves (one from each eye) reach diencephalon at the optic chiasm  From optic chiasm, continue along optic tracts About half of fibers go to lateral geniculate nucleus on same side of brain; other half go to opposite side Visual pathways Visual pathways (continued)  Optic radiation = bundle of projection fibers linking each lateral geniculate body with visual cortex, in occipital cortex on same side  Collaterals from fibers synapsing in lateral geniculate bodies go to subconscious processing centers in diencephalon and brainstem Visual pathways Visual pathways (continued)  Pupillary reflexes and others are triggered by collaterals going to superior colliculi Visual pathways Perception of visual image reflects integration of information arriving at visual cortex  Depth perception = ability to judge depth or distance by interpreting 3-D relationships Perceived by comparing relative positions of objects within images received by both eyes – Visual images from left and right eyes overlap – Each eye receives slightly different image due to: o Foveae centrales are 5–7.5 cm (2–3 in.) apart o The nose and eye socket block view of opposite side The visual pathway Internal ear sensory receptors  Equilibrium and hearing receptors—isolated and protected from external environment  Located in internal ear  Information integrated and organized locally; forwarded to CNS Internal ear sensory receptors Hair cells = sensory receptors in internal ear  Free surfaces covered with specialized nonmotile processes Stereocilia—resemble long microvilli; 80–100 per hair cell Kinocilium = single large cilium Internal ear sensory receptors Hair cells are mechanoreceptors—sensitive to contact/movement  External force pushing on hair cell processes distorts plasma membrane; alters neurotransmitter release Provides information about direction/strength of stimulus Monitored by dendrites of sensory neurons Internal ear sensory receptors Complex 3-D structure in internal ear determines what stimuli can reach hair cells in each region  Hair cells in one region respond only to gravity or acceleration  Hair cells in other regions respond only to rotation or to sound The ear is divided into the external ear, the middle ear, and the internal ear External ear—collects/directs sound waves toward middle ear  Auricle—elastic cartilage  External acoustic meatus—passageway in temporal bone Ceruminous glands— secrete waxy cerumen (earwax); keeps foreign objects out; slows growth of microorganisms Hairs—trap debris Anatomy of the ear Middle ear (tympanic cavity) = air-filled chamber from tympanic membrane to auditory ossicles; connects to pharynx by auditory tube  Tympanic membrane (tympanum, eardrum) = thin, semitransparent sheet that separates external ear and middle ear  Auditory ossicles = three tiny bones; connect tympanic membrane and inner ear Anatomy of the ear Internal ear  Contains sensory organs for hearing and equilibrium  Receives amplified sound waves from middle ear  Superficial contours established by layer of dense bone = bony labyrinth The anatomy of the ear Anatomy of the ear Middle ear  Auditory tube (pharyngotympanic tube, eustachian tube) Connects middle ear to nasopharynx Allows pressure equalization across tympanic membrane Can allow microorganisms into middle ear, causing infection (otitis media)—can impair hearing, may invade internal ear Anatomy of the ear Auditory ossicles  Malleus (malleus, hammer)—attaches to tympanic membrane  Incus (incus, anvil)—attaches malleus to stapes  Stapes (stapes, stirrup)—attached to oval window Structures of the middle ear In the internal ear, the bony labyrinth protects the membranous labyrinth and its receptors Bony labyrinth = shell of dense bone surrounding/protecting membranous labyrinth  Filled with perilymph = liquid similar to CSF; between bony labyrinth and membranous labyrinth  Three parts 1. S​ emicircular canals 2. ​Vestibule 3. ​Cochlea Labyrinths of the internal ear Membranous labyrinth = collection of fluid-filled tubes/chambers  Houses receptors for hearing and equilibrium  Contains fluid called endolymph Labyrinths of the internal ear Three parts (semicircular canals, utricle, and saccule are part of the vestibular complex, which maintains equilibrium) 1. ​Semicircular ducts (within semicircular canals) Receptors stimulated by rotation of head 2. Within the vestibule—utricle and saccule Provide sensations of gravity and linear acceleration 3. ​Cochlear duct (within cochlea) Sandwiched between pair of perilymph-filled chambers Resembles snail shell Receptors stimulated by sound The anatomy of the internal ear Hair cells in the semicircular ducts respond to rotation; hair cells in the utricle and saccule respond to gravity and linear acceleration Semicircular ducts  Three ducts (anterior, posterior, lateral)— continuous with utricle and filled with endolymph  Ampulla = enlarged part of duct that houses receptors Receptors for equilibrium Semicircular ducts (continued)  Ampullary crest = region in wall of ampulla with receptors  Ampullary cupula = gelatinous structure extending through ampulla with kinocilia and stereocilia of hair cells embedded in it Receptors for equilibrium Head rotating in plane of a duct moves endolymph; pushes ampullary cupula to side, distorting receptor processes  Movement in one direction causes stimulation; opposite direction causes inhibition  Ampullary cupula rebounds to normal position when endolymph stops moving Receptors for equilibrium Even complex angular movements can be analyzed by movement of the three rotational planes  Horizontal rotation (“no”) stimulates lateral duct receptors  Nodding (“yes”) stimulates anterior duct receptors  Tilting head to side stimulates posterior duct receptors Receptors for equilibrium Utricle and saccule  Provide equilibrium sensations, whether body is stationary or moving Receptors for equilibrium Utricle and saccule (continued)  Utricle and saccule contain hair cells clustered in maculae Macula of utricle senses horizontal movement Macula of saccule senses vertical movement Hair cell processes embedded in gelatinous otolithic membrane Receptors for equilibrium Utricle and saccule (continued)  Change in head position causes distortion of hair cell processes in the maculae, sending signals to the brain Head in upright position—otoliths sit on top of otolithic membrane in utricle Head in tilted position or with linear movement—gravity pulls on otoliths, shifts them to side Movement distorts hair cell processes; stimulates macular receptors The cochlear duct contains the hair cells of the spiral organ that function in hearing Cochlear duct (scala media)  Filled with endolymph Between two chambers with perilymph  Scala vestibuli (vestibular duct)  Scala tympani (tympanic duct) Encased by bony labyrinth except at oval/round windows  Interconnect at tip of cochlear, forming single long chamber from oval window to round window Cross-sections of the cochlea Receptors for hearing Spiral organ  Hair cells lack kinocilia  Stereocilia are in contact with overlying tectorial membrane  Bulk of hair cell embedded in basilar membrane Receptors for hearing Spiral organ (continued)  Sound waves create pressure waves in perilymph  Pressure waves cause basilar membrane to vibrate up and down  Vibrations of basilar membrane press stereocilia into tectorial membrane, distorting them Receptors for hearing Spiral organ (continued)  Distortion triggers nerve impulse  Sensory neurons relay signal through spiral ganglion to cochlear branch of vestibulocochlear nerve (VIII) External and cross-sectional views of the cochlea Anatomy of the spiral organ A pressure wave in the perilymph causes movement of the hair cells and basilar membrane Sound waves lead to movement of the basilar membrane in the process of hearing Hearing = perception of sound; sound = waves of pressure  In air, pressure wave causes alternating areas of compressed/separated molecules  Wavelength of sound = distance between adjacent wave crests (peaks) or adjacent troughs Physiology of hearing Frequency = number of waves (cycles) passing fixed point in given time  Pitch = our perception of frequency High frequency (short wavelength) = high pitch Physiology of hearing Intensity (loudness) = amount of energy in sound waves  Amplitude of soundwave reflects amount of energy (intensity) Greater energy = larger amplitude = louder sound  Measured in decibels (dB) Physiology of hearing For hearing:  Stapes pushes on the oval window Inward movement causes distortion of basilar membrane toward the round window Opposite action when stapes moves outward Physiology of hearing For hearing: (continued)  Flexibility of basilar membrane varies along its length Different sound frequencies affect different parts of the membrane – Location of vibration interpreted as pitch – Number of stimulated hair cells interpreted as volume Events involved in hearing Neural pathways for the sense of equilibrium Vestibulocochlear nerve function Hearing  Nerve signals for hearing are carried on the cochlear nerve, which is part of the vestibulocochlear nerve Vestibulocochlear nerve function Hearing  Nerve signals for hearing are carried on the cochlear nerve, which is part of the vestibulocochlear nerve Neural pathways for the sense of hearing Vestibulocochlear nerve function Hearing (continued)  Most auditory information from one cochlea is projected to the auditory cortex on opposite side  Some information from cochlea reaches auditory cortex on its same side Aids in localizing sounds (left/right) Reduces functional impact of damage to one cochlea or ascending pathway A generator potential is a depolarization of the membrane  Five special senses 1. Olfaction (smell) 2. Gustation (taste) 3. Vision 4. Equilibrium (balance) 5. Hearing  All originate with one of two types of sensory receptor cells 1. Dendrites of specialized neurons – Example: olfactory receptors 2. Specialized cells that synapse with sensory neurons  Depolarization of sensory neuron = a generator potential Generator potential 1. Olfactory receptors are dendrites of specialized neurons  Dissolved odorants bind to olfactory receptors  Triggers depolarization = generator potential  With strong enough stimulus, generator potential triggers action potentials that go to CNS Generator potential 2. Receptors for taste, vision, equilibrium, hearing are specialized cells with inexcitable membranes  Synapse with sensory neurons  Stimulation—triggers graded depolarization Generator potential  Graded depolarization of the receptor cell triggers neurotransmitter release  Neurotransmitter depolarizes sensory neurons, causing generator potential that can trigger action potential  Action potentials are propagated to CNS Olfaction involves specialized chemoreceptive neurons and delivers sensations directly to the cerebrum  Olfaction = sense of smell  Paired olfactory organs in nasal cavity, each side of nasal septum Contain olfactory receptor cells Distributed along cribriform plate, superior portion of the perpendicular plate, superior nasal conchae  Olfactory organs have two layers 1. ​Olfactory epithelium 2. ​Lamina propria An olfactory organ, showing the olfactory epithelium and lamina propria Olfaction  Odorants = dissolved chemicals that stimulate olfactory neurons  Bind membrane receptors (odorant-binding proteins) on dendrites of olfactory receptor cells  Generally small organic molecules  As few as four odorant molecules can activate receptor cell Olfaction  ​Olfactory reception process 1. O​ dorant binds to receptor protein; activates adenylate cyclase (enzyme that converts ATP to cyclic AMP) 2. cAMP opens sodium channels in plasma membrane; starts depolarization 3. ​If enough depolarization occurs, action potential is triggered, and information is relayed to CNS The process of olfaction The olfactory pathway Gustation involves epithelial chemoreceptor cells located in taste buds  Gustation = taste  Taste receptors (gustatory receptor cells) Most important receptors on superior surface of tongue Also in adjacent parts of pharynx, larynx, epiglottis – Numbers decrease with age Gustation involves epithelial chemoreceptor cells located in taste buds  Gustation = taste (continued)  Lingual papillae = epithelial projections on tongue surface Contain taste buds = sensory structures with taste receptors that respond to various chemical stimuli, and specialized epithelial cells. The tongue Gustation  Four primary taste sensations: sweet, salty, sour, bitter  No difference in taste bud structure  Taste buds in all portions of tongue provide all four sensations  Umami = pleasant, savory taste characteristic of broths, cheese Binds receptors for amino acids, small peptides, nucleotides Gustation  Four primary taste sensations: sweet, salty, sour, bitter (continued)  Water receptors Concentrated in pharynx Output goes to hypothalamus; affects water balance and regulation of blood volume Prevent overingesting H2O Gustation  Taste buds  Recessed into epithelium  Gustatory receptor cell (taste receptor cell) Extends slender microvilli (taste hairs) into surrounding fluids through a taste pore Typically lives about 10 days before it is replaced About 40–100 gustatory cells in each taste bud  Basal cells = stem cells that divide and mature to produce transitional cells that mature into new gustatory cells Taste bud structure overview Gustation  Taste receptor sensitivity  Threshold varies for the four primary sensations  More sensitive to unpleasant stimuli 100,000 times more sensitive to bitter; 1000 times more sensitive to sour (acids) than to sweet or salty Survival value—acids burn tissues; many toxins are bitter Overall sensitivity declines with age—leads to changing taste sensations with age Gustatory reception relies on membrane receptors and ion channels, and sensations are carried by facial, glossopharyngeal, and vagus nerves  Gustatory reception stimulated by dissolved chemicals (like smell)  Chemicals contacting taste hairs may: Diffuse through plasma membrane leak channels Bind to receptor proteins of gustatory receptor cell  ~ 90 percent of gustatory receptor cells respond to at least two taste stimuli  Different tastes involve different receptor mechanisms Gustatory reception  Two types of gustatory reception 1. Salt and sour receptors Sodium ions (salt) or hydrogen ions (sour) diffuse through Na+ leak channels Membrane/cell depolarizes; neurotransmitter is released  Neurotransmitters are released at synapse with sensory neurons  Depolarization of sensory neurons leads to generator potential, which can produce action potentials along gustatory pathway to CNS Gustatory reception 2. Sweet, bitter, and umami receptors: Binding to these receptors activates G-protein complexes called gustducins (protein complexes that use second messengers to produce effects) Activated second messenger causes neurotransmitter release  Neurotransmitters are released at synapse with sensory neurons  Depolarization of sensory neurons leads to generator potential, which can produce action potentials along gustatory pathway to CNS Gustatory reception  Gustatory pathway (continued) 1. Gustatory receptor cells bind dissolved chemicals Generator potential occurs Triggers action potentials  Gustatory reception  Gustatory pathway (continued) 2. Information is relayed on cranial nerves Facial nerve (VII)—taste buds on anterior two- thirds of tongue, from the tip to the vallate papillae Glossopharyngeal nerve (IX)—vallate papillae, posterior one-third of tongue Vagus nerve (X)—epiglottis © 2018 Pearson Education, Inc. Gustatory reception  Gustatory pathway (continued) 3. Sensory afferents synapse in solitary nucleus of medulla oblongata Gustatory reception  Gustatory pathway (continued) 4. Axons of postsynaptic neurons cross over; enter medial lemniscus of medulla oblongata Gustatory reception  Gustatory pathway (continued) 5. Synapse in thalamus; then information is projected to appropriate portions of gustatory cortex of the insula Gustatory reception  Taste = conscious perception produced by processing at the primary somatosensory cortex  Information from taste buds is integrated with other sensory data Texture of food Taste-related sensations (“peppery” or “burning hot”) —carried by trigeminal nerve (V)  Taste receptors adapt slowly, but central adaptation reduces sensitivity to a new taste quickly  Level of olfactory stimulation plays important role Several thousand times more sensitive to “tastes” when sense of smell is functioning Aging is associated with many disorders of the special senses; trauma, infection, and abnormal stimuli may cause problems at any age Olfaction disorders  Head injury—damage to olfactory nerve (I)  Age-related changes Olfactory receptors are regularly replaced by stem cells, but number declines with age Remaining receptors become less sensitive Disorders of the special senses Gustation disorders  Problems with olfactory receptors—decreased smell dulls taste  Damaged taste buds—mouth infection, inflammation  Damaged cranial nerves (VII, IX, X)—trauma or compression  Natural age-related changes Disorders of the special senses Vision disorders  Senile cataract—lens loses transparency Natural consequence of aging; can be surgically corrected Progresses—person needs more light to read; acuity may decline to blindness  Presbyopia—age-related farsightedness due to loss of lens elasticity (less accommodation possible for close vision) Disorders of the special senses Equilibrium disorders  Vertigo—false perception of spinning From conditions altering function of: – Internal ear receptor complex – Vestibular nerve (of vestibulocochlear nerve VIII) – Sensory nuclei and CNS pathways Can be due to vision problems or drug use (including alcohol) Disorders of the special senses Vertigo (continued)  Stimulated by anything that sets endolymph in motion  Motion sickness is most common cause Symptoms—headache, sweating, flushing of face, nausea, vomiting Disorders of the special senses Hearing disorders  Partial hearing deficit affects ~37.5 million in United States  Two types: conductive and sensorineural Conductive hearing loss—problem conducting sound waves  Causes include impacted earwax, infection, perforated tympanic membrane Disorders of the special senses Sensorineural hearing loss—damage to cochlea or nerve pathways from internal ear to brain  Causes include exposure to loud noise, head trauma, and aging  Age changes Tympanic membrane loses flexibility Articulations between auditory ossicles stiffen Round window may start to ossify Participation 6/12  Find a partner or two in the class  Answer the questions below in the participation activity, “Participation 6/12”  Found in the Week 1 Module 1. Name the two photoreceptors of the eye 2. Where are the photoreceptive cells of the eye found? 3. Explain the significance of the fovea centralis