The Special Senses – Eye & EarWS O

School of Biomedical Sciences

University of Hong Kong

• Olfaction

• Taste

• Visual system - Eye

• Hearing and balance - Ear

Anatomy of the Eye

Components of the eye:

• Eyelids and lacrimal glands – provide physical protection and

lubrication of the anterior, exposed ocular surface.

• The corneosclera coat - the outermost of three layers forming the

globe of the eye, with the anterior portion forming the transparent

cornea.

• The uveal tract forms the middle layer of the eye ball, composed

anteriorly of iris and ciliary body, extending posteriorly to the

vascular chorioid.

• The lens is a biconvex elastic structure, anterior to which is a

chamber of aqueous humor and posterior to which the vitreous

body, occupying the inner cavity of the eye.

• The retina forms the innermost layer consists of nerve cells in

multiple layers.

• The optic nerve is a tract of the central nervous system that

receives fibers from the retina and supplies vascular elements to the

superficial strata of the retina.

Eyelids and

Associated

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 platethat 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

Sagittal Section Through Eye

Eyelids and associated structures:

• The eyelids are lined externally by skin and internally by

the conjunctival epithelium, a thin mucous membrane

that moistens the ocular surface. Skeletal muscle fibers

and sweat glands are present in the subcutaneous

tissue, reinforced posterior by the tarsus, a plate of

dense connective tissue.

• Modified sebaceous glands contribute to an oily film over

the tears. The tears are produced by the lacrimal

gland, located in the upper lateral margin of the orbit.

The tear film is about 40 µm deep (the oily film is about

0.1µm deep) containing immunoglobulin A, lysosome

and anti-inflammatory proteins

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

Anatomy of the Eye

• Three coats or tunics:

• Fibrous: sclera and cornea

• Vascular: choroid, ciliary body, iris

• Nervous: retina

The Corneoscleral coat

• Cornea: It makes up about 1/6 of the corneoscleralenvelope and is the transparent tissue in the body, consisting of 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.

• 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.

The Uveal tract – the middle coat of the eyeball

• 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.

The Lens

• Transparent, biconvex

• Surrounded by a highly elastic, transparent capsule (cuboidal epithelium). At the equator the epithelial cells proliferate and greatly elongate (hence the term lens fibers), losing their nuclei but retaining a very high concentration of proteins termed crystallins. New cells are arranged in layers on top of each other, and are produced throughout life.

• Held by suspensory ligaments attached to ciliary muscles. Changes shape as ciliary muscles contract and relax.

The Lens

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.

Anatomy of the Eye

• Three coats or tunics:

• Fibrous: sclera and cornea

• Vascular: choroid, ciliary body, iris

• Nervous: retina

Structure and Function of the RetinaThe retina is a thin layer neural tissue lining the inner eye. Histologically, it is stratified having 10 layers of neurons or cell bodies, synapses, glia cell, the photoreceptive rods and cones and an outermost pigmented epithelium.

• 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

The

Sensory 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 Afrom 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.

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.

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

Histological section of a retina

Nerve Fiber Layer

Ganglion Cells

Inner Plexiform layer

Inner Nuclear Layer

Outer Plexiform Layer

Outer Nuclear

Layer

Layer of

Rods & Cones

Schematic diagram of EM of

a rod and a cone cell

EMgraph of portions of the inner and outer

segments of a rod (a) and a cone (b) cell.

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

Inner Layers of the Retina

• Rods and cones synapse with bipolar cellsthat 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.

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

The Ear

Anatomically the ear is subdivided into external,

middle and inner regions. The inner comprises of

the organ of hearing and the organs that detect

linear and rotatory accelerations.

• The main objectives is to discuss the functional

histology of the inner ear, which, as the organ

of hearing and balance, is clinically important in

relation to deafness.

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

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

Inner earThe inner ear cavity is made up of

• the sensory apparatus for hearing, the cochlea (containing the organ

of Corti);

• the system of membranous sacs and tubes that detect changes in

equilibrium (the vestibular apparatus).

Both sensory organs are encased in dense bone in order to

protect and insulate the delicate vibrations of the fluids that

they enclose. The cochlea is a spiral canal lying within bone;

the vestibular apparatus (utricle, saccule and semicircular

canals) is also located within bone, but the canals ampullae

and sacs are membranous ducts or tubes lying close to the

surrounding osseous labyrinth, suspended in fluid

(perilymph). All the membranous labyrinth contains

endolymph fluid. Hair cells occur in the inner epithelial lining

of the cochlea and vestibular apparatus.

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

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.

Crista ampullaris

The Cochlea• The cochlea has a snail shell-like appearance with 2½ turns and it

spiral around a core of spongy bone that contains the spiral gnaglion

where nerve fibers from the sensory (auditory) hair cells arrive.

• The hair cells in the cochlea are basically mechano-receptors

sensitive to extremely delicate vibrations or displacement of the

lymph-filled canals within the cochlea. The spiral turns of the

cochlear tube contain 3 longitudinal compartments: 2 outer canals

(filled with perilymph) that meet in continuity at the end of the spiral,

and a central cochlear compartment or duct filled with endolymph.

Each compartment is kept separate by a membrane. At the tympanic

cavity, the outer canals are enclosed by the stapes displaces the

lymph contents of the cochlea’s three internal compartments, which

causes one of the membrane partitions, the basilar membrane

(Reissner’s membrane), to move up and down. On this membrane

are found the hair cells of the organ of Corti, which are simulated by

the shearing motion between the hairs (stereocilia) and an overlying

hanging-roof type structure called the tectorial membrane.

Schematic diagram to show

sound waves on cocchlear

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

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

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

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

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)

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

Spiral ganglia

SV - Scala

vestibuleST – Scala

tympani

CD – cocchleal

duct

Organ of Corti and vestibule

Cochlea

V - vestibule

Ampullary crista

CU - cupula

References

• Basic Histology Junqueira L.C.. Carneiro J, Kelley R.O.

pp.470-494; Lange Medical Book,Prentice-Hall

International.

• Atlas of Functional Histology Kerr J.B.pp.361-363;

Mosby, London.