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C H A P T E R

The Special Senses

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Accessory Structures of the Eye

• Lacrimal apparatus—keeps the surface of the eye moist• Lacrimal gland—

produces lacrimal

Lacrimal glandExcretory ducts of lacrimal glands

Lacrimal punctumLacrimal canaliculus

Lacrimal sac

produces lacrimal fluid

• Lacrimal sac—fluid empties into nasal cavity

Figure 16.5

Nasolacrimal duct

Inferior meatusof nasal cavityNostril

Extrinsic Eye Muscles

Superior oblique muscle

Trochlea

Superior oblique tendon

Superior rectus muscle

Lateral rectus muscle

Trochlea

Superioroblique

Lateralrectus

Superiorrectus

Medialrectus

Figure 16.6a, b

Inferiorrectusmuscle

Commontendinousring

Inferiorobliquemuscle

Lateral rectus muscle

(a) Lateral view of the right eye (b) Anterior view of the right eye

Inferiorrectus

Inferioroblique

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Anatomy of the Eyeball

• External wall consists of three layers (tunics)• Fibrous layer• Vascular layer• Sensory layer

I l i i fl id (h )• Internal cavity contains fluids (humors)• Anterior segment- aqueous humor• Posterior segment- vitreous humor

Medial View of the Eye

Ora serrata

Ciliary body

Ciliary zonule(suspensoryligament)CorneaIrisP il Posterior pole

Fovea centralisMacula lutea

Retina

Choroid

Sclera

Figure 16.7a

(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.

Anteriorpole

Pupil

Anteriorsegment(containsaqueous humor)LensScleral venoussinusPosterior segment (contains vitreous humor)

Optic nervePosterior pole

Central arteryand vein of the retinaOptic disc(blind spot)

Fibrous Layer

• Sclera- white of the eye (dura mater of brain); provides anchoring for extrinsic eye muscles

• Cornea- light enters here, transparentCornea light enters here, transparent• Junction of sclera and cornea- limbus

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Vascular Layer• Choroid-forms posterior 5/6 of vascular tunic,

continuous with ciliary body; prevents light scatter

• Ciliary body- a thickened ring of tissue that encircles the lens- consists of smooth muscle (ciliary muscle) – focuses the lens.

• Ciliary processes-generate suspensory ligament• Accomodation• Makes aqueous humor

• Iris- visible, colored part of eye. Allows light to enter. Has smooth muscle fibers

• Pupil- opening of iris (pupillary light reflex)-opening for light

The Vascular Layer

Ora serrata

Ciliary body

Ciliary zonule(suspensoryligament)CorneaIrisP il Posterior pole

Fovea centralisMacula lutea

Retina

Choroid

Sclera

Figure 16.7a

(a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.

Anteriorpole

Pupil

Anteriorsegment(containsaqueous humor)LensScleral venoussinusPosterior segment (contains vitreous humor)

Optic nervePosterior pole

Central arteryand vein of the retinaOptic disc(blind spot)

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Sensory Layer (Retina)

• Consists of two layers• Pigmented layer • Neural layer (external to internal)

• Photoreceptors (cones and rods)• Bipolar neurons• Ganglion cells

Posterior Aspect of the Eyeball

Neural layer of retina

Pigmentedlayer ofretina

ScleraChoroid

Optic disc

Pathway of light

Figure 16.8a

(a) Posterior aspect of the eyeball

Central arteryand vein of retina Optic

nerve

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Microscopic Anatomy of the Retina

RodPhotoreceptors

ConeBipolarcellsGanglion

cells

ChoroidOuter segmentsof rods and conesNuclei of

ganglioncells

Figure 16.8b, c

Pigmentedlayer of retinaPathway of light

Pathway of signal output

(b) Cells of the neural layer of the retina

Amacrine cellHorizontal cell

Pigmentedlayer of retina

Axons ofganglion cells

Nuclei ofrods andcones

Nucleiof bipolarcells

(c) Photomicrograph of retina

PhotoreceptorsProcessof bipolarcell

Outerfiber

Innerfibers

Rodcellbody

Conecellbody

Synapticterminals

Rod cellbody

Nuclei

Mitochondria

Connectingcilia

ent

Figure 16.9

Apicalmicrovillus

Discscontainingvisual pigments

Melaningranules

Discs beingphagocytized

Pigmentcellnucleus

Basal lamina(border with choroid)

Inne

r se

gme

Out

er s

egm

ent

Pig

men

ted

laye

r

Blood Supply of the Retina

• Retina receives blood from two sources• Outer third of the

retina—supplied Maculalutea

Central arteryand veinemergingfrom theoptic disc

Optic disc

Figure 16.10

by capillaries in the choroid

• Inner two-thirds of the retina—supplied by central artery and vein of the retina

Retina

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Internal Chambers, Fluids and the Lens

• Posterior cavity- vitreous humor• Anterior cavity- anterior chamber (aqueous humor) and

posterior chamber• Aqueous humor- is renewed continuously and is in constant

motion- formed as filtrate of the blood from capillaries in ciliary processes, flows through pupil into anterior chamber, drains into veins via canal of Schlemm and returns to blood.

• Lens- a thick transparent, biconvex disc that changes shape to allow focusing of the light on the retina.

Internal Chambers and Fluids

Anterior chamberAnteriorsegment

Cornea

Cornea

Corneal epithelium

Corneal endothelium

Aqueous humor

Iris

LensLens epithelium

Lens

Posterior segment(contains vitreoushumor)

Ciliary zonule(suspensory

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Figure 16.11

Aqueous humor is formed by filtration from the capillaries in the ciliary processes.

Sclera

Bulbarconjunctiva

Scleral venous sinus

Posterior chambersegment(containsaqueoushumor)

Corneoscleraljunction

(suspensoryligament)

Ciliaryprocesses

Ciliarymuscle

Ciliary body

Aqueous humor flows from the posterior chamber through the pupil into the anterior chamber. Some also flows through the vitreous humor (not shown).

Aqueous humor is reabsorbed into the venous blood by the scleral venous sinus.

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Visual Pathways

• Most visual information travels to the cerebral cortex

• Responsible for conscious “seeing”• Other pathways travel to nuclei in the midbrain

d di h land diencephalon

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Visual Pathways to the Cerebral Cortex• Pathway begins at the retina

• Light activates photoreceptors • Photoreceptors signal bipolar cells• Bipolar cells signal ganglion cells • Axons of ganglion cells exit eye as the opticAxons of ganglion cells exit eye as the optic

nerve

Visual Pathways to the Cerebral Cortex

• Optic tracts send axons to:• Lateral geniculate nucleus of the thalamus

• Synapse with thalamic neurons • Fibers of the optic radiation reach the

primary visual cortex

Right eye Left eye

Fixation point

Optic nerveSupra-chiasmatic

Both eyes

Visual Pathways to the Brain and Visual Fields

Figure 16.15a

Pretectalnucleus

Opticradiation

Optic tractOptic chiasma

Uncrossed(ipsilateral) fiberCrossed(contralateral) fiber Lateral

geniculatenucleus ofthalamus

Superiorcolliculus

Occipitallobe(primary visualcortex)

(a) The visual fields of the two eyes overlap considerably.Note that fibers from the lateral portion of each retinal fielddo not cross at the optic chiasma.

chiasmaticnucleus

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Visual Pathways to Other Parts of the Brain

• Some axons from the optic tracts• Branch to midbrain

• Superior colliculi• Pretectal nuclei

• Other branches from the optic tractsOther branches from the optic tracts• Branch to the suprachiasmatic nucleus

The Ear and Equilibrium

• The ear is divided into the outer, middle and inner ear.

• Outer and middle ear participate in hearing only• Inner ear functions for both hearing and

ilib iequilibrium

Structure of the Ear

Auricle(pinna)

External ear Middleear

Internal ear(labyrinth)

Figure 16.17a

Externalacousticmeatus

(a) The three regions of the ear

Helix

Lobule

Pharyngotympanic(auditory) tube

Tympanicmembrane

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Tympanic Membrane

• Separates outer from middle ear.• Shaped like a flattened cone• Transmits air vibrations to auditory ossicles

Structures of the Middle Ear

Auditory

Entrance to mastoid antrum in the epitympanic recess

Semicircularcanals

Vestibularnerve

Oval window(deep to stapes)

Incus(anvil)

Malleus(hammer)

Vestibule

Figure 16.17b

Pharyngotympanic(auditory) tube

ossicles

Tympanicmembrane

Cochlea

Cochlearnerve

Round window

(anvil)Stapes(stirrup)

(b) Middle and internal ear

Middle Ear

• Tympanic cavity• Ossicles- malleus, incus, stapes (merge onto oval

window)- transmit sound from external ear to internal ear. Increase force but not the amplitude of vibrations transmitted by tympanic membrane

• eustachian tube/ auditory tube- equalizes air pressure

• Oval window (stapes)• Round window – dissipates left-over energy in

cochlea

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Middle Ear

• Round window – dissipates left-over energy in cochlea

• Reflexive muscles that protect from loud sounds (tympanic reflex)

S di ( )• Stapedius (stapes)• Tensor tympani (malleus)

Malleus

View

Superior

Anterior

Lateral

Incus Epitympanic recess

The Middle Ear

• Ear ossicles—smallest bones in the body• Malleus—attaches to

the eardrum • Incus—between the

malleus and stapes• Stapes vibrates

Pharyngotym-panic tube

Tensortympanimuscle

Tympanicmembrane(medial view)

Stapes Stapediusmuscle

Figure 16.18

• Stapes—vibrates against the oval window

• Tensor tympani and stapedius• Two tiny skeletal

muscles in the middle ear cavity

Inner Ear

• Vestibule

• Semicircular canals

• Utricle and saccule- balance and position of head; movement

• Semicircular ducts-concerned with movement; ampullae are swellings near utricle

• Cochlea• Cochlear duct- contains spiral

organ of Corti, converts mechanical sound to nerve impulses

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Structure of the Ear

Auricle(pinna)

External ear Middleear

Internal ear(labyrinth)

Figure 16.17a

Externalacousticmeatus

(a) The three regions of the ear

Helix

Lobule

Pharyngotympanic(auditory) tube

Tympanicmembrane

The Internal Ear

Anterior

Semicircular ducts insemicircular canals

PosteriorL t l

Temporalbone Facial nerve

Vestibular nerveSuperior vestibularganglionInferior vestibularganglion

Figure 16.19

Lateral

Cristae ampullaresin the membranousampullaeUtricle investibuleSaccule investibule

Stapes inoval window

ganglionCochlear nerveMaculaeSpiral organ (of Corti)Cochlear duct incochlea

Round window

Utricle and Saccule-vestibular apparatus

• Concerned with balance and position of head while stationary

• Acceleration (linear)• Endolymph • Has special sensory epithelium called Macula

• Receptor hair cells

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The Maculae in the Internal Ear

Macula ofsaccule

Otoliths

Hair bundle

Kinocilium

Stereocilia

Otolithicmembrane

Macula ofutricle

Figure 16.22a

Hair bundle

Vestibularnerve fibers

Hair cellsSupportingcells

(a)

The Maculae in the Internal EarHaircell

OtolithsOtolithicmembrane

Force ofgravity

Figure 16.22b(b)

Head upright Head tilted

Semicircular Ducts &Vestibular apparatus

• Rotational movement• Anterior, posterior, and lateral semicircular duct.• Opens onto utricle• Ampulla

• Crista ampullaris- (crest with hair cells)• Cupula- jelly-like pointed cap

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Structure and Function of the Crista Ampullaris

Hair bundle (kinociliumplus stereocilia)

Crista ampullaris

Endolymph

Cupula

Figure 16.23a, b

Fibers of vestibular nerve

plus stereocilia)

Hair cell

Supporting cell

Membranous labyrinth

Cristaampullaris

(a) Anatomy of a crista ampullaris in a semicircular canal

(b) Scanning electron micrographof a crista ampullaris (45X)

Structure and Function of the Crista Ampullaris

Figure 16.23c

Fibers ofvestibular

nerve

At rest, the cupula stands upright.

Section ofampulla,filled withendolymph

(c) Movement of the cupula during rotational acceleration and deceleration

Cupula Flow of endolymph

During rotational acceleration, endolymph moves inside the semicircular canals in the direction opposite the rotation (it lags behind because of inertia). Endolymph flow bends the cupula and excites the hair cells.

As rotational movement slows, endolymph keeps moving in the direction of the rotation, bending the cupula in the opposite direction from acceleration and inhibiting the hair cells.

Cochlear Duct• A spiral blind tube • Perilymph-filled chambers- scala vestibuli (starts

from oval window) and scala tympani Merge onto round window• Vestibular membrane• Basilar membrane

• EndolymphEndolymph• Organ of Corti

• Hair cells- stereocilia• Supporting cells

• Tectorial membrane

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The Cochlea

(a)Helicotrema

ModiolusCochlear nerve,division of thevestibulocochlearnerve (VIII)

Cochlear duct(scala media)

Spiral ganglionOsseous spirallaminaVestibularmembrane

Figure 16.20a, b(b)

Cochlear duct(scala media;containsendolymph)

Tectorial membraneVestibular membrane

Scala vestibuli(containsperilymph)

Scala tympani(containsperilymph)

Basilarmembrane

Spiral organ(of Corti)

Striavascularis

Spiralganglion

Osseous spiral lamina

The Cochlea

Cochlear duct(scala media;containsendolymph)

Tectorial membrane

Vestibular membrane

Scala vestibuli(containsperilymph)

Scala tympani(containsperilymph)

Basilarmembrane

Spiral organ(of Corti)

Striavascularis

Spiralganglion

Osseous spiral lamina

Tectorial membrane Inner hair cell

Figure 16.20b, c

(b)

(c)

Tectorial membrane Inner hair cell

Outer hair cells

Hairs (stereocilia) Afferent nervefibers

Basilarmembrane

Fibers ofcochlearnerve

Supporting cells

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Hearing Pathway• Sound vibrations• Ossicles- stapes and oval window• Perilymph of scala vestibuli and tympani• Endolymph of scala media• Tectorial membrane stays still, hair cells on basilar

membrane (stereocilia) have mechanically gated ion h l (K+) l d tchannels (K+) leads to

• Hair cells stimulated (excess released via round window)

• AP in CNVIII to thalamus (via medulla and pons)• Thalamus• Primary auditory cortex in temporal lobe

The Role of the Cochlea in Hearing

Sound waves vibrate the tympanic membrane.

Malleus Incus

Auditory ossicles

Stapes

Ovalwindow

Scala vestibuli

Helicotrema

Cochlear nerve

Scala tympani

Cochlear duct

Basilarmembrane

Auditory ossicles vibrate. Pressure is amplified.

Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli.

Sounds with frequencies below hearing travel through the

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4a

2 3

4a

4b

Figure 16.21

Roundwindow

Tympanicmembrane

ea g t a e t oug t ehelicotrema and do not excite hair cells.

Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

4b

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