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Anatomy and Physiology, Seventh Edition

Rod R. SeeleyIdaho State UniversityTrent D. StephensIdaho State UniversityPhilip TatePhoenix College

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes.

Chapter 15Chapter 15

Lecture OutlineLecture Outline**

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Chapter 15The Special Senses

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Special Senses

• Olfaction

• Taste

• Visual system

• Hearing and balance

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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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Odorant Binding to Membrane of Olfactory Hair

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Olfactory Neuronal Pathway

• Olfactory sensory pathway: olfactory neurons (bipolar) in the olfactory epithelium pass through cribiform plate to olfactory bulbs and synapse with tufted cells or mitral cells. These extend to the olfactory tract and synapse with association neurons.

• Association neurons also receive input from brain, so information can be modified before it reaches the brain.

• Information goes to olfactory cortex of the frontal lobe without going through thalamus (only major sense that does not go through thalamus).

• Three regions in frontal lobe affect conscious perception of smell and interact with limbic system.– Lateral olfactory area: conscious

perception of smell– Medial olfactory area: visceral and

emotional reactions to odors – Intermediate olfactory area: effect

modification of incoming information.

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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 the sides of the tongue. Contain most sensitive taste buds. Decrease in number with age.

– Filiform. Filament-shaped. Most numerous. No taste buds.

• Taste bud: supporting cells surrounding taste (gustatory) cells.– Taste cells have microvilli (gustatory

hairs) extending into taste pores– Replaced about every 10 days

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Taste Types• Sour. Most sensitive receptors on lateral aspects of the tongue.

• Salty. Most sensitive receptors on tip of tongue. Shares lowest sensitivity with sweet. Anything with Na+ causes depolarization plus other metal ions. Craved by humans.

• Bitter. Most sensitive receptors on posterior aspect. Highest sensitivity. Produced by alkaloids which are toxic.

• Sweet. Most sensitive receptors on tip of tongue. Shares lowest sensitivity with salty. sugars, some carbohydrates, and some proteins (NutraSweet: aspartame). Craved by humans.

• Umami (Glutamate). Scattered sensitivity. Caused by amino acids. Craved by humans.

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Taste

• Texture affects the perception of taste• Temperature affects taste perception• Very rapid adaptation, both at level of taste bud

and within the CNS• Taste influenced by olfaction• Different tastes have different thresholds with

bitter being the taste to which we are most sensitive. Many alkaloids (bitter) are poisonous.

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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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Actions of the Major Tastants

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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 hairs. Ciliary glands (modified sweat glands) empty into hair follicles. Meibomian glands 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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Sagittal Section Through Eye

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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 coats or tunics:• Fibrous: sclera and cornea• Vascular: choroid, ciliary body, iris• Nervous: retina

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Anatomy of the Eye: Fibrous Tunic• 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 Tunic• 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 anterior chamber

• 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 Tunic

• 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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Light and Dark Adaptation:Adjustment of Eyes to Changes in

Light Intensity

• Rods: in bright light, more rhodopsin broken down into Vitamin A, protecting the eye and making it less sensitive to light. In darker conditions, more rhodopsin produced so eye is more sensitive to light. Takes eyes a while to accommodate when going from dark to light and vice versa because of these chemical changes that must occur.

• Pupils: constriction in bright light; dilation in dim light.

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Opthalmoscopic View of Retina• Lens focuses light on macula

lutea and fovea centralis– Macula lutea: small

yellow spot– Fovea centralis: area of

greatest visual acuity; photoreceptor cells tightly packed

• Optic disc: blind spot. Area through which blood vessels enter eye, where nerve processes from sensory retina meet and exit from eye

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Compartments 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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The Lens

• Held by suspensory ligaments attached to ciliary muscles. Changes shape as ciliary muscles contract and relax. Made of long columnar epithelial cells that enucleate and produce proteins called crystallines.

• Surrounded by a highly elastic, transparent capsule

• Transparent, biconvex

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Light

• Visible light: portion of electromagnetic spectrum detected by human eye

• Refraction: bending of light• Convergence: light striking a convex

surface• Focal point: point where light rays

converge and cross• Focusing: causing light to converge• Lens changes shape causing adjustment of

focal point on the retina

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Electromagnetic Spectrum

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Focusing

• Emmetropia: normal resting condition of lens. Ciliary muscle is relaxed. Lens is flat.

• Far point of vision: point at which lens does not have to thicken to focus. 20 feet or more from eye.

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Near Point of Vision• Closer than 20 feet. Changes occur in lens, size of

pupil, and distance between pupils– Accommodation: ciliary muscles contract due

to parasympathetic input via cranial nerve III. Pulls choroid toward lens reducing tension on suspensory ligaments. Lens becomes more spherical, greater refraction of light

– Pupil constriction: varies depth of focus– Convergence: as objects move close to the eye,

eyes are rotated medially. Reflex contraction of the medial rectus muscles

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Structure and Function of the Retina

• Sensory retina: three layers of neurons: photoreceptor, bipolar, and ganglionic– Cell bodies form nuclear layers separated by

plexiform layers, where neurons of adjacent layers synapse with each other

• Pigmented retina: single layer of cells; filled with melanin. With choroid, enhances visual acuity by isolating individual photoreceptors, reducing light scattering

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TheSensory Retina:

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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Rhodopsin Cycle

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Rod Cell Hyperpolarization

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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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Sensory Receptor Cells of the Retina

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Inner Layers of the Retina

• Rods and cones synapse with bipolar cells that synapse with ganglion cells in all areas except the fovea.

• Except in fovea centralis, ganglion cell axons converge at optic disc, then exit via optic nerve then impulses travel to visual cortex

• Fovea centralis: highest visual acuity• Rods: spatial summation. One bipolar cell receives

input from numerous rods, one ganglion cell receives input from several bipolar cells.

• Cones exhibit little or no convergence.

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Receptive Fields• Area from which a ganglion cell receives input• Roughly circular with receptive field center• Those in fovea centralis smaller than in other parts of

retina• Two types of receptive fields

– On-center ganglion cells: generate more action potentials when light is directed onto the receptive field. Respond to intensity of light

– Off-center neurons: more action potentials when light is off or when light does not hit center of field. Respond to contrasts in light

• Interneurons present in inner layers and modify signal before signal leaves retina. Enhance borders and contours, increasing intensity at borders

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Neuronal Pathways

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Visual Fields• Binocular vision: visual fields partially overlap

yielding depth perception

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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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The Ear

• 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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External and Middle Ear

• External ear– Auricle or pinna: elastic

cartilage covered with skin

– External auditory meatus: lined with hairs and ceruminous glands. Produce cerumen

– Tympanic membrane

• Thin membrane of two layers of epithelium with connective tissue between

• Sound waves cause it to vibrate

• Border between external and middle ear

• Middle ear– Separated from the inner air by

the oval and round windows– Two passages for air

• Auditory or eustachian tube: opens into pharynx, equalizes pressure

• Passage to mastoid air cells in mastoid process

– Ossicles: malleus, incus, stapes: transmit vibrations from eardrum to oval window

– Oval window: connection between middle and inner ear. Foot of the stapes rests here and is held in place by annular ligament

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Inner Ear• Labyrinths

– Bony: chambers in the temporal bone

• Cochlea: hearing• Vestibule: balance• Semicircular canals:

balance

– Membranous: membranous tunnels and chambers in the bony labyrinth

• Lymphs– Endolymph: in membranous

labyrinth– Perilymph: space between

membranous labyrinth and periosteum of bony labyrinth

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Inner Ear, Cont.• Oval window communicates

with vestibule which communicates with the scala vestibuli of the cochlea

• Scala vestibuli extends from oval window to helicotrema at cochlear apex

• Second cochlea chamber (scala tympani) from helicotrema to round window

• Scala vestibuli and tympani filled with perilymph

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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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Inner Ear• Spiral organ (organ of

Corti): cells in cochlear duct• Contain hair cells (sensory

cells) with hair-like projections at the apical ends. These are microvilli called stereocilia.

• Basilar region of hair cells covered by synaptic terminals of sensory neurons

• Cell bodies of afferent neurons grouped into cochlear (spiral) ganglion

• Afferent fibers form the cochlear nerve

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Inner Ear• Hair cells arranged in rows.

Of inner hair cells (responsible for hearing) and outer hair cells (regulate tension on basilar membrane)

• Hair bundle: stereocilia of one inner hair cell

• Tip link (gating spring) attaches tip of each stereocilium in a hair bundle to the side of the next longer stereocilium.

• As stereocilia bend, they open K+ gates (mechanically gated ion channel)

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Opening of K+ Channels

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Effect of Sound Waves on Cochlear Structures (1)

1. Sound waves strike the tympanic membrane and cause it to vibrate.

2. Vibration of the tympanic membrane causes the three bones of the middle ear to vibrate.

3. The foot plate of the stapes vibrates in the oval window.

4. Vibration of the foot plate causes the perilymph in the scala vestibuli to vibrate.

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Effect of Sound Waves on Cochlear Structures (2)

5. Vibration of the perilymph causes displacement of the basilar membrane. Short waves (high pitch) cause displacement of the basilar membrane near the oval window, and longer waves (low pitch) cause displacement of the basilar membrane some distance from the oval window. Movement of the basilar membrane is detected in the hair cells of the spiral organ, which are attached to the basilar membrane.

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Effect of Sound Waves on Cochlear Structures (3)

6. Vibrations of the perilymph in the scala vestibuli and of the endolymph in the cochlear duct are transferred to the perilymph of the scala tympani.

7. Vibrations in the perilymph of the scala tympani are transferred to the round window, where they are dampened.

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Auditory Function

• Vibrations produce sound waves– Volume or loudness : function of wave amplitude– Pitch: function of wave frequency– Timbre: resonance quality or overtones of sound

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Muscles of the Middle Ear

• Tensor tympani: inserts on malleus; innervated by cranial nerve V

• Stapedius: inserts on stapes and innervated by cranial nerve VII

• Attenuation reflex: muscles contract during loud noises and prevent damaging vibrations

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Effect of Sound Waves on Points Along the Basilar Membrane

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Sensitivity of Hearing• Fine-tuning tension on basilar membrane

– More than 90% of afferent axons of the cochlear ganglion synapse with inner hair cells, 10-30/hair cell.

– A few small-diameter afferent axons synapse with rows of outer hair cells.

– Outer hair cells receive efferent input causing them to shorten

• Tuning hair cells to specific frequencies

– Actin filaments in hair cells attach to K+ gated channels and can move them along the cell membrane and tighten or loosen the spring. Hair cells tuned to very specific frequencies.

• Localization of pitch along the cochlea

– Afferent cochlear nerve fibers send action potentials to superior olivary nucleus in medulla oblongata. These are compared to one another and strongest is taken as standard.

– Efferent action potentials inhibit other action potentials. Action potentials from maximum vibration go to cortex and are perceived

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CNS Pathways for Hearing

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Balance

• Static labyrinth: utricle and saccule of the vestibule– Evaluates position of head relative to gravity– Detects linear acceleration and deceleration (as in a car)

• Kinetic labyrinth: semicircular canals– Evaluates movement of the head in three dimensional

space

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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 bending hair cells and initiating action potentials

– Otoliths stimulate hair cells with varying frequencies

– Patterns of stimulation translated by brain into specific information about head position or acceleration

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Function of Vestibule in Maintaining Balance

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Kinetic Labyrinth

• Three semicircular canals filled with endolymph: transverse plane, coronal plane, sagittal plane

• Base of each expanded into ampulla with 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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CNS 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