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Chapter 15



Eye and Associated Structuresy 70% of all sensory receptors are in the eye

y Most of the eye is protected by a cushion of fat and the

bony orbity Accessory structures include eyebrows, eyelids,

conjunctiva, lacrimal apparatus, and extrinsic eyemuscles



Eyebrowsy Coarse hairs that overlie the supraorbital margins

y Functions include;

y Shading the eyey Preventing perspiration from reaching the eye

y Orbicularis muscledepresses the eyebrows

y Corrugator musclemove the eyebrows medially



Palpebrae (Eyelids)y Protect the eye anteriorly

y Palpebral fissureseparates eyelids

y Canthimedial and lateral angles (commissures)y Lacrimal carunclecontains glands that secrete a

whitish, oily secretion (Sandmans eye sand)

y Tarsal plates of connective tissue support the eyelids

internally

y Levator palpebrae superiorisgives the upper eyelidmobility



Palpebrae (Eyelids)y E yelashes

y Project from the free margin of each eyelid

y Initiate reflex blinkingy Lubricating glands associated with the eyelids

y Meibomian glands and sebaceous glands

y Ciliary glands lie between the hair follicles



Palpebrae (Eyelids)



Conjunctivay Transparent membrane that:

y Lines the eyelids as the palpebral conjunctiva

y Covers the whites of the eyes as the ocular conjunctivay Lubricates and protects the eye



Lacrimal Apparatusy Consists of the lacrimal gland and associated ducts

y Lacrimal glands secrete tears



Tearsy Contain mucus, antibodies, and lysozyme

y Enter the eye via superolateral excretory ducts

y Exit the eye medially via the lacrimal punctumy Drain into the nasolacrimal duct



Lacrimal Apparatus



Extrinsic Eye Musclesy Six straplike extrinsic eye muscles

y Enable the eye to follow moving objects

y Maintain the shape of the eyebally Four rectus muscles originate from the annular ring

y Two oblique muscles move the eye in the vertical plane

y Superiordownward laterally

y Inferiorupward laterally



Extrinsic Eye Muscles



Summary of Cranial Nerves and

Muscle Actionsy Names, actions, and cranial nerve innervation of the

extrinsic eye muscles



Structure of the Eyebally A slightly irregular hollow sphere with anterior and

posterior poles

yThe wall is composed of three tunicsy Fibrous

y Vascular

y Sensory

y The internal cavity is filled with fluids called humorsy The lens separates the internal cavity into anterior and

posterior segments



Structure of the Eyeball



Fibrous Tunicy Forms the outermost coat of the eye and is composed

of:

y

Opaque sclera (posteriorly)y Clear cornea (anteriorly)

y The sclera protects the eye and anchors extrinsicmuscles

y The cornea lets light enter the eye



Vascular Tunic: Ciliary Bodyy A thickened ring of tissue surrounding the lens

y Composed of smooth muscle bundles (ciliary muscles)

y Anchors the suspensory ligament that holds the lensin place



Vascular Tunic: Irisy The colored part of the eye

y Pupilcentral opening of the iris

y Regulates the amount of light entering the eye



Pupily Close vision and bright lightpupils constrict

y Distant vision and dim lightpupils dilate

y Changes in emotional statepupils dilate when thesubject matter is appealing or requires problem-solving skills



Pupil Dilation and Constriction



Sensory Tunic: Retinay A delicate 2-layered membrane

y Pigmented layerthe outer layer that absorbs light

and prevents its scatteringy Neural layer which contains:

y Photoreceptors that transduce light energy

y Bipolar cells and ganglion cells

y Amacrine and horizontal cells



Sensory Tunic: Retina



The Retina: Ganglion Cells and the

Optic Disc



The Retina: Photoreceptorsy Rods:

y Respond to dim light

y

Are used for peripheral visiony Cones:

y Respond to bright light

y Have high-acuity color vision

y Are found in the macula luteay Are concentrated in the fovea centralis



Blood Supply to the Retinay The neural retinal receives its blood supply from two

sources:

y

The outer third receives its blood from the choroidy The inner two-thirds is served by the central artery and

vein

y Small vessels radiate out from the optic disc and can be

seen with an ophthalmoscope



Lensy A biconvex, transparent, flexible, avascular structure

that:

y

Allows precise focusing of light onto the retinay Is composed of epithelium and lens fibers

y Lens epitheliumanterior cells that differentiate intolens fibers

y Lens fiberscells filled with the transparent proteincrystallin

y With age, the lens becomes more compact and dense,and it loses its elasticity



Chemical Sensesy Gustation (taste) and olfaction (smell)

y Their chemoreceptors respond to chemicals in

aqueous solutiony Tasteresponse to substances dissolved in saliva

y Smellresponse to substances dissolved in f luids of thenasal membranes



Sense of Smelly The organ of smell is the olfactory epithelium, which

covers the superior nasal conchae

y

Olfactory receptor cells are bipolar neurons withradiating olfactory cilia

y Olfactory receptors are surrounded and cushioned by supporting cells

y Basal cells lie at the base of the epithelium



Olfactory Receptors



Taste Budsy Most of the 10,000 or so taste buds are found on the

tongue

y

Taste buds are found in papillae of the tongue mucosay Papillae come in three types: filiform, fungiform, and

circumvallate

y Fungiform and circumvallate papillae contain taste

buds



Taste Buds



Structure of a Taste Budy Gourd-shaped

y Consists of 3 major cell types:

y Supporting cellsinsulate the receptory Basal cellsdynamic stem cells

y Gustatory cellstaste cells



Taste Sensationsy Sweetsugars, saccharin, alcohol, and some amino

acids

y

Saltmetal ionsy Sourhydrogen ions

y Bitteralkaloids such as quinine and nicotine

y Umamielicited by the amino acid glutamate



Physiology of Tastey In order to be tasted, a chemical:

y Must be dissolved in saliva

y

Must contact gustatory hairs



Influence of Other Sensations on Tastey Taste is 80% smell

y Thermoreceptors, mechanoreceptors, and nociceptors

also influence tastesy Temperature and texture enhance or detract from taste



Hearing and Balance



The Eary Three parts of the ear:

y Inner ear

y

Outer eary Middle ear

y Outer and middle ear are involved with hearing

y Inner ear functions in both hearing and equilibrium

y Receptors for hearing and balance:y Respond to separate stimuli

y Are activated independently



Outer Eary The auricle (pinna) is composed of:

y The helix (rim)

y

The lobule (earlobe)y External auditory canal

y Short, curved tube filled with ceruminous glands



Outer Eary Tympanic membrane (eardrum)

y Thin connective tissue membrane that vibrates inresponse to sound

y Transfers sound energy to the middle ear ossicles

y Boundary between outer and middle ears



Middle Ear (Tympanic Cavity)y A small, air-filled, mucosa-lined cavity

y Flanked laterally by the eardrum

y

Flanked medially by the oval and round windowsy Epitympanic recesssuperior portion of the middle

ear

y Pharyngotympanic tubeconnects the middle ear to

the nasopharynxy Equalizes pressure in the middle ear cavity with the

external ear pressure



Ear Ossicles



Inner Eary Bony labyrinth

y Tortuous channels worming their way through thetemporal bone

y Contains the vestibule, the cochlea, and the semi-circular canals

y Filled with perilymph

y

Membranous labyrinthy Series of membranous sacs within the bony labyrinth

y Filled with a potassium-rich fluid



Inner Ear



The Vestibule



The Semicircular Canalsy Three canals that each define two-thirds of a circle and

lie in the three planes of space

y

Membranous semicircular ducts line each canal andcommunicate with the utricle

y The ampulla is the swollen end of each canal andhouses equilibrium receptors in a region called the

crista ampullarisy These receptors respond to angular movements of the

head



The Semicircular Canals



The Cochleay A spiral, conical, bony chamber that:

y Extends from the anterior vestibule

y

Coils around a bony pillar called the modiolusy Contains the cochlear duct, which ends at the cochlear

apex

y Contains the organ of Corti (hearing receptor)



The Cochleay The cochlea is divided into three chambers:

y Scala vestibulioval window

y

Scala mediacochlear ducty Scala tympaniround window



Sound and Mechanisms of Hearingy Sound vibrations beat against the eardrum

y The eardrum pushes against the ossicles, which

presses fluid in the inner ear against the oval andround windows

y This movement sets up shearing forces that pull onhair cells

y

Moving hair cells stimulates the cochlear nerve thatsends impulses to the brain



Deafnessy Conduction deafnesssomething hampers sound

conduction to the fluids of the inner ear

y E.g., impacted earwax, perforated eardrum,osteosclerosis of the ossicles

y Sensorineural deafnessresults from damage to theneural structures at any point from the cochlear haircells to the auditory cortical cells



Deafnessy Tinnitusringing or clicking sound in the ears in the

absence of auditory stimuli

y

Menieres syndromelabyrinth disorder that affectsthe cochlea and the semicircular canals

y Causes vertigo, nausea, and vomiting



Developmental Aspectsy All special senses are functional at birth

y Chemical sensesfew problems occur until the fourthdecade when these senses begin to decline

y Vision is not fully functional at birthy Babies are hyperopic, see only gray tones, and eye

movements are uncoordinated

y Depth perception and color vision is well developed by

age fivey With age the lens loses clarity, dilator muscles are less

efficient, and visual acuity is drastically decreased by age70

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