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Chapter 15The Special Senses

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Olfaction• Seven primary odors now recognized, but

average person can recognize 4000 different odors. Perceived by olfactory epithelium

• Dendrites of olfactory neurons have enlarged ends (olfactory vesicles).

• Cilia (olfactory hairs) of olfactory neuron embedded in mucus. Odorants dissolve in mucus. Somehow (mechanism unknown) odorants attach to receptors, cilia depolarize and initiate action potentials in olfactory neurons. One receptor may respond to more than one type of odor.

• Olfactory epithelium is replaced as it wears down. Olfactory neurons are replaced by basal cells every two months. Unique: most neurons are permanent cells (aren't replaced if they die).

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Olfactory Neuronal Pathways and Cortex

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Taste• Taste buds located, primarily, on papillae• Types of papillae

– Vallate. Largest, least numerous. 8-12 in V along border between anterior and posterior parts of the tongue. Have taste buds.

– Fungiform. Mushroom-shaped. Scattered irregularly over the superior surface of tongue. Look like small red dots interspersed among the filiform. Have taste buds.

– Foliate. Leaf-shaped. In folds on thesides of the tongue. Contain mostsensitive taste buds. Decrease innumber with age.

– Filiform. Filament-shaped. Mostnumerous. No taste buds.

• Taste bud: supporting cells surroundingtaste (gustatory) cells.

– Taste cells have microvilli(gustatory hairs) extending into tastepores

– Replaced about every 10 days

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Neuronal Pathways for Taste• Chorda tympani (part of VII): carry sensations from anterior

one-third of tongue (except from circumvallate papillae• Cranial nerve IX and X carry information from posterior one-

third tongue, circumvallate papillae, superior pharynx, epiglottis.

• Information goes to medulla oblongata where decussation takes place and information projects from there to the thalamus. Then projects to taste area of cortex (extreme inferior end of the postcentral gyrus)

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Visual System: Accessory Structures• Eyebrows: shade; inhibit sweat• Eyelids (palpebrae) with conjunctiva.

– Palpebral fissure: space between eyelids. – Canthi: lateral and medial, eyelids meet. – Medial canthus has caruncle with modified sweat and sebaceous glands– Five layers of tissues including a dense connective tissue tarsal plate that

helps maintain shape of lid

•Eyelashes: double/triple row of hairsCiliary glands (modified sweat glands)empty into hair follicles. Meibomianglands at inner margins produce sebum.•Conjunctiva: thin transparent mucous membrane

–Palpebral conjunctiva: inner surface eyelids–Bulbar conjunctiva: anterior surface of eye except over pupil

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Lacrimal Apparatus• Lacrimal gland: produces

tears to moisten, lubricate, wash. Tears pass through ducts and then over eye.

• Lacrimal canaliculi: collect excess tears through openings called puncta.

• Lacrimal sac leads to nasolacrimal duct: opens into nasal cavity beneath the inferior nasal conchae.

• Nasolacrimal duct: opens into nasal cavity

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Extrinsic Eye Muscles

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Anatomy of the Eye

• Three layers:• Fibrous: sclera and cornea• Vascular: choroid, ciliary body, iris• Nervous: retina

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Anatomy of the Eye: Fibrous Layer• Sclera: white outer layer. Maintains shape,

protects internal structures, provides muscle attachment point, continuous with cornea. Dense collagenous connective tissue with elastic fibers. Collagen fibers are large and opaque.

• Cornea: connective tissue matrix containing collagen, elastic fibers and proteoglycans. Layer of stratified squamous epithelium on the outer surface. Collagen fibers are small, thus transparent. More proteoglycans than sclera, low water content (water would scatter light). Avascular, transparent, allows light to enter eye; bends and refracts light.

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Anatomy of the Eye: Vascular Layer• Middle layer. Contains most of the

blood vessels of the eye: branches off the internal carotid arteries. Contains melanin.– Iris: colored part of the eye.

Controls light entering the pupil. Smooth muscle determines size of pupil.

• Sphincter pupillae: parasympathetic

• Dilator pupillae: sympathetic

– Ciliary body: produces aqueous humor that fills anteriorchamber

• Ciliary muscles: control lens shape; smooth muscle. Ciliary processes attached to suspensory ligaments of lens

– Choroid: associated with sclera. Very thin, pigmented.

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Anatomy of the Eye: Nervous Layer

• Two layers– Pigmented retina: outer, pigmented

layer; pigmented simple cuboidal epithelium. Pigment of this layer and choroid help to separate sensory cells and reduce light scattering.

– Sensory retina: inner layer of rod and cone cells sensitive to light.

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Chambers of the Eye• Anterior compartment: anterior to lens; filled with aqueous humor

– Anterior chamber: between cornea and iris– Posterior chamber: between iris and lens– Helps maintain intraocular pressure; supplies nutrients to structures bathed

by it; contributes to refraction of light•Produced by ciliary process; returned to venous circulation through canal of Schlemm or scleral venous sinus•Glaucoma: abnormal increase in intraocular pressure

•Posterior compartment: posterior to lens. Filled with jelly-like vitreous humor. Helps maintain intraocular pressure, holds lens and retina in place, refracts light.

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Rods• Bipolar photoreceptor cells; black and white vision. • Found over most of retina, but not in fovea. More sensitive to

light than cones. – Protein rhodopsin changes shape when struck by light; and eventually

separates into its two components: opsin and retinal– Retinal can be converted to Vitamin A from which it was originally

derived. In absence of light, opsin and retinal recombine to form rhodopsin. –Rods are unusual sensory cells: when not stimulated they are hyperpolarized. Light causes them to depolarize.–Depolarization of rods causes depolarization of bipolar cells causing depolarization of ganglion cells–Light and dark adaptation: adjustment of eyes to changes in light. Happens because of changes in amount of available rhodopsin.

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Cones• Bipolar receptor cells. • Responsible for color vision and

visual acuity. – Numerous in fovea and

macula lutea; fewer over rest of retina.

– As light intensity decreases so does our ability to see color.

– Visual pigment is iodopsin: three types that respond to blue, red and green light

– Overlap in response to light, thus interpretations of gradation of color possible: several millions

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Neuronal Pathways

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Eye Disorders• Myopia: Nearsightedness

– Focal point too near lens, image focused in front of retina

• Hyperopia: Farsightedness– Image focused behind retina

• Presbyopia– Degeneration of

accommodation, corrected by reading glasses

• Astigmatism: Cornea or lens not uniformly curved

• Strabismus: Lack of parallelism of light paths through eyes

• Retinal detachment– Can result in complete

blindness• Glaucoma

– Increased intraocular pressure by aqueous humor buildup

• Cataract– Clouding of lens

• Macular degeneration– Common in older people,

loss in acute vision• Diabetes

– Dysfunction of peripheral circulation

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Hearing and Balance

• External ear: hearing. Terminates at eardrum (tympanic membrane). Includes auricle and external auditory meatus

• Middle ear: hearing. Air-filled space containing auditory ossicles• Inner ear: hearing and balance. Interconnecting fluid-filled tunnels

and chambers within the temporal bone

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Inner Ear• Wall of scala vestibuli is vestibular membrane• Wall of scala tympani is basilar membrane• Cochlear duct (scala media): space between vestibular and

basilar membranes. Filled with endolymph•Width of basilar membrane increases from 0.04 mm near oval window to 0.5 mm near helicotrema

–Near oval window basilar membrane responds to high-frequency vibrations–Near helicotrema responds to low-frequency vibrations

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Neuronal Pathways for Hearing

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Static Labyrinth• Utricle has macula oriented parallel to base of skull • Saccula has macula oriented perpendicular to base of skull• Macula: specialized epithelium of supporting columnar cells

and hair cells with numerous stereocilia (microvilli) and one cilium (kinocilium) embedded in gelatinous mass weighted by otoliths–Gelatinous mass moves in response to gravity bendinghair cells and initiating action potentials–Otoliths stimulate hair cells with varying frequencies–Patterns of stimulation translated by brain intospecific information abouthead position or acceleration

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Kinetic Labyrinth• Three semicircular canals filled with

endolymph: transverse plane, coronal plane, sagittal plane

• Base of each expanded into ampullawith sensory epithelium (crista ampullaris)

• Cupula suspended over crista hair cells. Acts as a float displaced by fluid movements within semicircular canals

• Displacement of the cupula is most intense when the rate of head movement changes, thus this system detects changes in the rate of movement rather than movement alone.

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Neuronal Pathways for Balance

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Effects of Aging on the Special Senses

• Slight loss in ability to detect odors• Decreased sense of taste• Lenses of eyes lose flexibility• Development of cataracts, macular

degeneration, glaucoma, diabetic retinopathy• Decline in visual acuity and color perception