eye & ear spr 2011. eye layers of the eye fibrous layer: anterior – cornea posterior –...

Eye & Ear

SPR 2011

Eye

Layers of the Eye

Fibrous layer: Anterior – cornea Posterior – sclera

Middle vascular layer: Choroid Ciliary body Iris

Neural layer: Retina – outer pigment &

inner neural layer

Cornea Anterior transparent layer of the eye ( outer fibrous layer) Refractive structure of the eye. Avascular - but can heal by

diffusion of nutrients from the limbus. Has numerous free nerve endings – Very sensitive!

Structure of the CorneaAnterior corneal epithelium

Stratified squamous non keratinizedEpithelium capable of regeneration if damageddrying leads to ulceration

Bowman’s anterior limiting membraneBarrier for infection

Substantia proprialamellae of collagen fibers (lymphocytes & fibroblasts are interspersed in between the

fibers) in a ground substance.Regular arrangement, Regular spacing gives cornea the transparency

Descemet’s membraneIncreases in thickness with age

Endothelial layerActive pump located in the cells that maintains the dehydrated state of the cornea

(substantia propria)

Sclera Posterior 5/6th of outer fibrous layer Connective tissue with collagen fibers Relatively avascular Yellow in elderly because of accumulation of Lipofuscin

pigments

Also called as the Limbus Corneal epithelium becomes the bulbar conjunctiva

Contains 2 very important structures:1. Trabecular meshwork2. Canal of Schlemm

Sclerocorneal Junction

LimbusCanal of Schlemm:

Canal lined with endothelium situated at the inner aspect of the corneal stroma close to the iris

Trabecular Meshwork: Fine collagenous

trabeculae enclosing endothelium lined spaces

Merge & open into canal of Schlemm

Chambers of the Eye

Anterior chamber – between iris & cornea

Posterior chamber – between lens & iris

Vitreous chamber – lens & retina

Aqueous Humor Clear watery Non- Refractile fluid

secreted by the ciliary body into the posterior chamber.

Similar to plasma with just 0.1% protein (as compared to 7% plasma protein in blood)

Provides nutrition to the lens & cornea. Replaced every 2-3 hrs. Determines intra-ocular pressure ( 15-

20mm of Hg)

Disturbances in production or flow – Glaucoma!

Glaucoma

Rise in intra-ocular pressure due to increased production or impaired drainage of aqueous humor.

Needs to be treated as can affect vision.

Vitreous Chamber

Gelatinous mass with some fluid 99% water with small amount of collagen &

Hyaluronic acid Few cells called Hyalocytes synthesize the

contents of the vitreous. Contains the hyaloid canal

Uveal Tract

Middle vascular layer

1. Choroid

2. Ciliary body

3. Iris

Choroid Loose vascularized connective tissue present between the retina &

sclera in the posterior 5/6th of the eyeball.

Contains a choriocapillary layer for nutrition & oxygenation of the underlying retina.

Choroid & retina separated by Bruchs membrane - a hyaline glassy membrane, extends from the optic disc to the ora serrata.

Contains pigmented melanocytes which absorb light that passes through the retina.

Ciliary Body Extends from the iris to the ora

serrata. Attached to the suspensory

ligaments of the lens. Contains ciliary muscle which

is made up of meridoneal, circular & radial fibers.

When it contracts, tension on the suspensory ligaments is reduced, thus permitting the lens to acquire a more convex shape for accommodation.

Has parasympathetic innervation.

Stroma made up of elastic fibers, blood vessels & melanocytes.

Lining epithelium is double layered:

Deep pigmented layer continuous with the retina.

Superficial non pigmented layer continuous with the neural layer of retina. Involved in transport of ions/water. Have features of fluid transporting cells.

Ciliary Processes

Finger like extensions. Have a connective

tissue core rich in fenestrated capillaries.

Produce aqueous humor.

Iris

Forms a diaphragm anterior to the lens.

Circular in shape with a central opening called Pupil.

Stroma has pigment cells. Color of iris (eye) depends

on the pigment in the stroma.

Blue eyes have less pigment, brown eyes have more pigment.

Conjunctiva

Bulbar conjunctiva – covers exposed part of sclera

Palpebral conjunctiva – covers inner surface of eyelids.

Stratified columnar epithelium with goblet cells which secrete mucus.

Lacrimal Gland

Compound tubuloacinar gland.

Acini with a single layer of columnar to pyramidal cells (like parotid)

Secrete tears which is a watery secretion containing lysozyme & antibodies Ig A.

Eyelid

Hairy skin – thin & highly folded with a loose supporting tissue.

Skeletal muscle – orbicularis oculi & levator palpebrae.

Submuscular connective tissue – extremely lax, continuous with the loose areolar tissue layer of scalp.

Tarsal plate – thick fibroelastic plate with Meibomian glands (modified sebaceous glands) - CHALAZION

Conjunctiva

Lid margin has eye lashes associated with glands of Zeiss (sebaceous) & glands of Moll (modified sweat) - STYE

Retina Consists of an outer pigment layer & inner neural layer. Inner neural layer composed

Photoreceptors – rods & cones Conducting neurons – bipolar cells & ganglion cells Association neurons – horizontal cells & amacrine cells Supporting cells –Muller cells & neuroglial cells

Intraretinal space is cleavage plane.

Rods outnumber cones (120million to 7 million)

Rods more sensitive to light than cones.

Cones for color vision & high visual acuity.

RetinaComposed of 9+1 layers:

1. Outer pigment layer

2. Photoreceptor layer – rods & cones processes

3. Outer (external limiting membrane) – apical boundary of Muller cells, site of tight junction with photoreceptors.

4. Outer nuclear layer – nuclei of cell bodies of rods & cones

5. Outer plexiform layer – axodendritic synapses between axons of photoreceptors & dendrites of bipolar & horizontal cells.

6. Inner nuclear layer – cell bodies of horizontal, amacrine & bipolar cells

7. Inner plexiform layer – synapses between axons of bipolar cells & dendrites of ganglion cells

8. Ganglion cell layer – cell bodies of multipolar ganglion cells9. Optic nerve fiber layer – axons of ganglion cells 10. Inner limiting membrane – basal lamina of Muller cells – separates

retina from vitreous.

Rods & cones Have inner & outer segments,

a nuclear region & synaptic region.

Outer segment – contains flattened membranous discs which contain Rods - Rhodopsin. Cones- Iodopsin

- Eventually shed their discs which are phagocytosed by the pigment epithelium.

Inner segment – has organelles for rhodopsin & iodopsin production & numerous mitochondria.

Cones recognize – RED, GREEN, BLUE

Rods Rhodopsin or visual purple has two absorption

maxims: 350 and 500 nm. The spectral extinction curve for rods corresponds to that of rhodopsin, suggesting that rhodopsin is the chemopigment in rods.

Rhodopsin consists of a glycoprotein (opsin) and a chromophore group (11-cis-retinal).

Retinal is the aldehyde of vitamin A1 (retinol).

Photoisomerization in Rods Inside the rod a special amplification takes place.

Light absorption by a single rhodopsin molecule activates thousands of G-protein molecules (transducin), which then activate large quantities of cGMP phosphodiesterase in the discs.

With absorption, specifically, it is the 11-cis form of retinene, which, isomerizes to the all-trans form.

This isomerization converts the rhodopsin to its active form, metarhodopsin II.

This reconfiguration of the retinene molecule thus produces the same effect as if a neurotransmitter had suddenly bound to a receptor.

The photoisomerization of 11-cis retinal to all-trans retinal in photoreceptors is the first step in vision – occurs in the OUTER SEGMENT

in the LIGHT  rhodopsin changes conformation (TRANS), which activated enzyme TRANSDUCIN;     

Transducin activates a PHOSPHODIESTRASE, converts cyclic-GMP to GMP;                      

GMP closes the Na channels, and HYPERPOLARIZES rod cells (RECEPTOR POTENTIAL);                

hyperpolarized rod cell stops releasing GLUTAMATE, allowing bipolar cells repsond*   

Vitamin A & Rods

The Vitamin A that our bodies produce from the beta carotene in many of the foods we eat (including, most famously, carrots) is needed to synthesize the retinene bound to the centre of the rhodopsin molecule.

Indeed, a severe Vitamin A deficiency impairs night vision, because of the smaller amounts of retinene being produced.

During the daytime, however, there is generally enough light to allow relatively normal vision despite low levels of visual pigments

Cones In the cones, the photosensitive pigment is opsin,

a transmembrane protein that is very similar to rhodopsin

The fovea only contains cones.

Cones function in the daytime with maximal visual acuity and colour vision.

The human eye possesses three types of cones, each with a specific pigment related to the three basic colours: red (erythrolab), green (chlorolab) and blue (cyanolab).

The cones in the fovea do not contain cyanolab.

Colour blindness

Classic red/green colour blindness is the result of a lack of red cones in the retina.

Forms of colour blindness are usually classified according to the type of cone affected.

Thus there are three kinds of colour blindness, corresponding to the three kinds of cones.

Blindness to green, due to deficiency of the green pigment, is called deuteranopia, and is the most common form

Macula: Yellow pigment zone

(xanthophyll) surrounding the fovea

Retinal vessels are absent here

Ganglion cells heaped to the sides so that light can pass unimpeded to the fovea.

Fovea Depression in the inner layer of

retina. Located 2.5mm temporal to the optic

disc. High concentration of photoreceptors

(only cones) for maximal visual acuity.

Ophthalmoscopic examination reveals brownish choroid pigments.

Optic Disc:• Site where optic nerve

exits the retina ( Blind Spot as no photoreceptors there)

• Central artery of the retina pierces it.

Optic Nerve

Forms innermost strata of the retina.

Unmyelinated intra-retinally. Converge at optic disc

(blind spot) & then acquires myelin.

Exits through lamina cribrosa of the sclera.

Extraocular part is surrounded by meninges.

Lens It is an encapsulated, elastic,

biconvex transparent structure.

Suspensory ligament connects it to the ciliary body.

Consists of anucleated fibers derived from the anterior epithelium. The synthetic rate decreases with age.

Contains the protein crystallin. Subcapsular cuboidal

epithelium anteriorly. Opacity causes cataract

Ear

Ear

External Ear: Receives & transmits the

sound waves to the middle ear.

Middle Ear: Sound waves converted into

mechanical movement of the ossicles & this is transmitted to the inner ear

Inner Ear: Sound waves transduced into

nerve signals. Vestibular organ, responsible

for equilibrium, located in the inner ear.

External Ear

Pinna – Has plates of elastic cartilage covered by skin (except lobule). Has hair follicles & sebaceous glands.

External auditory meatus - outer 1/3rd is cartilage & inner 2/3rd is bony. Canal is lined by skin which is closely bound to the underlying

cartilage or bone by a dense collagenous tissue. Hair follicles have sebaceous glands associated with. Ceruminous glands (modified sweat glands) secrete earwax

(cerumen) into the lumen along with sebaceous glands. Meatal hair – protection from foreign bodies Cerumen – protects against moisture & infection

Middle Ear Cavity – Tympanic Membrane

It is a thin fibrous membrane the separates the external ear from the middle ear.

Pars flacida – loose transparent part in the superior quadrant with a thin connective tissue membrane called Sharpnell’s membrane.

Pars tensa – thick non-transparent membrane

Normal

perforated

Middle Ear Cavity – Tympanic Membrane

Has 3 layers:1. Outer cuticular layer – thin hairless skin with no epidermal

ridges, dermis contains a fine vascular network.

2. Intermediate fibrous layer – outer layer of radiating collagen fibers, inner layer of circumferentially arranged fibers

3. Inner mucous layer – single layer of cuboidal cells with no cilia & goblet cells, thin lamina propria with its own blood supply.

Auditory Tube Connects middle ear with Nasopharynx & allows for

equalization of pressure in the middle ear. 2 parts – bony & cartilaginous (towards pharynx). Lined with simple columnar near tympanic cavity & respiratory

epithelium near the pharynx.

Ear Ossicles

Malleus – attaches to tympanic membrane

Incus – middle ear ossicle

Stapes – fixed to the oval (vestibular) window & is in contact with the perilymph of the inner ear.

Inner Ear

Vestibule Semicircular canals

CRISTA AMPULLARIS– ROTATIONAL MOVEMENT

Utricle & saccule MACULA – LINEAR

MOVEMENT

Cochlea

Vestibule

Structure: central, between cochlea and semicircular canals

Function: detect static equilibrium, linear acceleration

Maculae (inside utricle and saccule) contain receptor cells with hairs imbedded in membrane with otoliths (“ear stones” - CaCO3), position detected by pulling hairs

Inner Ear - vestibule

Bony Labyrinth: Intercommunicating periosteum lined

canals - semicircular canals, vestibule & cochlea.

Contains perilymph which is similar to CSF in composition.

Membranous Labyrinth: Closed sacs in the bony labyrinth.

Consists of the cochlear duct, saccule & utricle, semicircular ducts & endolymphatic sac.

Contains endolymph which is similar to intracellular fluid & is absorbed by the cells of endolymphatic sac.

Membranous Labyrinth

Saccule & Utricle – specialized receptors called Macula

Semicircular ducts 2 vertical & 1 horizontal, oriented at

right angles, communicate with utricle at both ends

one end of each duct dilated to form ampulla

Crista ampullaris specialized part of ampulla for balance & equilibrium

Cochlea – specialized structures called Organ of Corti for hearing.

Separated laterally from the middle ear by a thin bony plate with 2 openings in them

Oval window – closed by footplate of stapes

Round window – closed by a thin membrane called as secondary tympanic membrane that serves to dissipate spent sound waves from the internal ear.

Bony Labyrinth

Crista Ampullaris - ampulla of the semicircular canal

Receptor for angular (rotational) movement.

Rotation of the head causes movement of the semicircular canal & the duct contained in it.

2 types of cells: Hair cells – sensory

Type I – goblet like with basal nuclei, surrounded by basket like nerve endings at the base.

Type II – columnar cells with a central nucleus surrounded by many small dendrites.

Supporting cells – tall columnar cells with apical microvilli, nucleus at base of cell.

Crista Ampullaris Both types have long microvilli

called stereocilia & a single non motile kinocilium.

Surface is covered with a gelatinous glycoprotein called cupula (no otoliths – Ca deposits)

Hair processes are embedded in the cupula.

Macula – in saccule and utricle Detect linear movement Structurally resembles

crista ampullaris BUT has a gelatinous otolithic membrane with calcium carbonate crystals (otoliths or otoconia) on the surface of the membrane.

Function of Macula

Senses static position of head in space, especially when eyes closed or in water.

Hair cells in macula are oriented in different directions, thus in different positions of the head, different groups of hair cells are stimulated.

Vertigo – sense of rotation of head at equilibrium. Due to excessive stimulation of the crista ampullaris or compression of the vestibule by a growth (tumor)

Motion Sickness – excessive stimulation of the macula. ( going in a merry go round)

Defection towards kinocilium – depolarization

Defection away from kinocilium - hyperpolarization

Cochlea Central axis formed by a bony core called modiolus

which is perforated by nerve fibers.

Cochlea

Cochlear duct – a diverticulum of the membranous saccule within the cochlea. Follows the course of the bony cochlea.Divides the bony cochlea into:

1. Upper scala vestibuli 2. Lower scala tympani

In between the 2 is the scala media which contains the cochlear duct containing the endolymph.

Histological Structure of Cochlear Duct

Reissner’s membrane: 2 layers of flattened

squamous epithelium held together by tight junctions to maintain ionic gradients between the perilymph & endolymph.

Histological Structure of Cochlear Duct

Stria Vascularis: A vascular epithelium that

lines the lateral aspect of the cochlear duct.

Contains ion & water transporting marginal cells that maintain the endolymph.

Histological Structure of Cochlear Duct

Basilar membrane: Separates cochlear duct

from scala tympani.

Resting on it is the Spiral Organ of Corti.

Organ of CortiHair cells:

Neuroepithelial cells that respond to sound.

Outer hair cells (3-5rows) W shaped stereocilia

Inner hair cells – linear stereocilia.- Both types of cells are associated with nerve endings.

Supporting cells: Inner & outer pillar cells

Inner & outer phalangeal / deiter cells

Tectorial membrane: Makes contact with the processes

of the hair cells.

A 10-month-old male infant is brought to the emergency department on a sweltering day in June. He is listless, cranky, and floppy as a dishrag. He has no hair or teeth. Over the past 2 weeks, he has not smiled, babbled, rolled over, sat up, or attempted to stand. The mother says that both she and her son cannot tolerate heat and that she has always had thin hair and still has some baby teeth. Which of the following skin structures is most likely to be absent or greatly decreased in number in the mother and the child?

A) Langerhans cells

B) Merkel's cells

C) Pacinian corpuscle

D) Sebaceous glands

E) Sweat glands

Sebaceous glands Present in all areas of hairy skin. Simple acinar glands with a duct that opens into the hair follicle. Nucleus shrinks and the cell bursts delivering contents into the

duct. ( Holocrine) Sebum is a complex mixture of triglycerides, waxes, cholesterol &

its ester. Is antibacterial & moisturizing. Stimulated mainly at puberty by sex hormones. When the duct is blocked, infection of the gland ensues,

accumulation of pus causes Acne

Sweat Glands Simple coiled tubular glands

with duct opening on the epidermal surface. (unrelated to hair follicle)

Merocrine (eccrine) distributed over the entire body.

Has 2 parts: secretory portion & duct

Secretory portion has simple cuboidal epithelium that has mainly 3 types of cells

Excretory ducts of sweat glands : stratified cuboidal epithelium

Sweat Glands Apocrine Sweat Glands

Duct – stratified cuboidal, cells smaller & darker. Absorb NaCl & glycoproteins.

Secretion includes NaCl, ammonia, urea & uric acid.

It is HYPOTONIC! Functions include

thermoregulation & waste excretion.

Present in the axillary, anal & areolar regions.

Secrete sweat into the hair follicle. Larger in diameter. Viscous secretion containing

proteins, carbohydrates and lipids. Bacteria act on lipids and produce

odoriferous compounds. (Pheromones)

Glands become functional at puberty influenced by sex hormones.

Under Adrenergic Sympathetic control!

Respond to emotional & sensory stimuli and NOT to heat.

Eccrine sweat glands – sympathetic cholinergic

Apocrine sweat glands – sympathetic adrenergic

Cystic fibrosisAnti-cholinergic drugs

Nail

Lunula – crescent shaped white area near root; cells are partially keratinized. (visible part of nail matrix)

Eponychium – formed by str corneum, keratinized edge of nail fold over lunula.

Hyponychium – thickened epidermis below free edge of nail

Summary Free Nerve Ending - basic sensations like pain, touch, temp & pressure

Merkel’s disc - Sense pressure & touch (deep dermis)

Meissner’s corpuscles - ‘two-point’ or discriminative touch

Pacinian - pressure, touch & vibration

Ruffini’s - Pressure & touch (dermis, hypodermis & joint capsules)

Krause end bulbs - receptors for cold

Muscle spindle - change in muscle length

Golgi tendon organ - Responds to muscle tension

GO THORUGH PICTURES OF THESE RECEPTORS