Microanatomy Of Eye And Ear

Question 1

Describe the components of the external ear and relate these to their principal functions

Answer 1

• The external ear is essentially a funnel for conducting airborne vibrations to the tympanic membrane.

Auricle (pinna)

• Skin-covered elastic cartilage with some adipose.
• Collects and directs sound into the auditory canal.

External auditory meatus/canal
- Supported by fibrocartilage at its opening and by the temporal bone for the rest of it length.
The outer end of the canal is protected by guard hairs.
- Lined by skin with ceruminous and sebaceous glands whose secretions mix with dead skin cells to form cerumen (earwax).
- The cerumen is sticky and coats the guard hairs, making them more effective in blocking foreign particles from the auditory canal.

Tympanic membrane (eardrum)

• Shaped like an irregular cone.
• A collagenous bilayer with a radial and circular arrangement, covered by very thin skin externally, and simple cuboidal epithelium internally.
• Transmits vibrations to the middle ear.

Question 2

Describe the components of the middle ear and relate these to their principal functions

Answer 2

Tympanic cavity

• Air-filled space within the temporal bone which is lined with simple cuboidal epithelium.
• The eustachian tube connects the tympanic cavity with the nasopharynx. This equilibrates air pressures either side of the eardrum.

Auditory ossicles

• There are three: malleus (hammer), incus (anvil), and stapes (stirrup).
• They are composed of compact bone and articulated by synovial joints.
• They work like a lever system increasing the force transmitted. This converts sounds waves to mechanical vibrations in tissues and fluid-filled chambers.
• Stapedius and tensor tympani are skeletal muscles that dampen effects of loud noise.
• The main role of the ossicles is to transmit vibrations from the tympanic membrane to the oval window.

Question 3

Describe the components of the inner ear and relate these to their principal functions

Answer 3

• The inner ear is composed of two fluid-filled labyrinths known as the bony labyrinth and the membranous labyrinth.
• The bony labyrinth is a series of spaces within the petrous part of the temporal bone.
• The membranous labyrinth is a continuous epithelial-lined set of cavities within the bony labyrinth.

Vestibular apparatus

• Divided into two parts: three semicircular canals and the vestibule.
• The three semicircular canals respond to rotational movements of the head and body.
• The vestibule is composed of the utricle and saccule which respond to linear acceleration and perceive the direction of gravity.
• Movements are perceived by specialised hair cells in the endolymph which is similar to intracellular fluid.

Cochlea

• The cochlea is a spiral bony canal (bony labyrinth) which winds around a central bony axis known as the modiolus which contains the spiral ganglion and cochlear nerve.
• The cochlear duct (scala media) is an endolymph-filled part of membranous labyrinth which runs up the middle of the bony spiral.
• The cochlear duct divides the cochlea into three parallel chambers.
• The middle chamber is the triangular cochlear duct which is separated from the scala vestibuli above by a thin vestibular membrane and from the scala tympani below by a much thicker basilar membrane.
• The scala media and scala tympani are filled with perilymph unlike the cochlear duct which is filled with endolymph.
• Within the cochlear duct, supported on the basilar membrane, is the spiral organ, which has an epithelium composed of hair cells and supporting cells. The hair cells have long, stiff microvilli called stereocilia on their apical surfaces. Resting on top of the stereocilia is a gelatinous tectorial membrane.
• The cochlear duct is responsible for converting vibrations into nerve impulses.

Question 4

Describe the histological features of the main parts of the eye (cornea, iris, ciliary body, lens, retina, and relate these to their principal functions).

Answer 4

Cornea

• Five layers (from anterior surface to aqueous humor): non-keratinising stratified squamous epithelium, bowman’s layer, stroma, descemet’s membrane, and corneal endothelium.
• Sensitive due to numerous free nerve endings in the surface epithelium.
• Corneal epithelium is continuous with the epithelium of the conjunctiva at the limbus (corneosclera junction).
• Epithelium has a potent wound healing capability however damage to the bowman’s layer leads to scar formation and loss of visual acuity.
• Transparent due to arrangement of collagen into aligned lamellae.
• Protects the eye from germs and dust, and carries out the majority of light refraction.

Iris

• Delineates anterior and posterior chambers.
• Stromal melanocytes determine eye colour.
• Its heavily pigmented nature blocks light except at the aperture (pupil).
• Controls the size of the iris and therefore controls the amount of light that enters the eye.

Ciliary body

• A thickened extension of choroid.
• Forms a muscular ring around the lens.
• Supports the iris and lens and secretes a fluid called aqueous humor.

Lens

• Transparent and avascular.
• Biconvex in shape.
• Outer capsule composed of type 4 collagen.
• Layer of cuboidal epithelium on the anterior face only.
• A centre of tightly packed, anucleate cells packed with transparent proteins (crystallins) to form lens fibres.

Retina
- Two layers.
- Outer pigmented layer and inner retinal layer.
- Posterior 2/3 is light sensitive and contains the photoreceptors, rods and cones.
- Anterior 1/3 is not light sensitive and is separated from the posterior 2/3 by the ora serrata.
- Four cell groups found here: photoreceptor neurones (rods and cones), conducting neurones (bipolar and ganglion cells), association neurones (horizontal and amacrine cells), and neuroglial cells (Muller cells).
- Layers of the retina (anterior to posterior):
Internal limiting membrane
Nerve fibre layer
Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
Outer limiting membrane
Photoreceptors

Question 5

List the main distinctive features of the sclera and choroid

Answer 5

Sclera

• Covers most of the eye surface.
• Consists of dense collagenous connective tissue perforated by blood vessels and nerves.
• Serves as a tough fibrous protective cover for the eye and provides for attachment of the extrinsic muscles that move it.

Choroid
- Highly vascular, deeply pigmented layer of tissue behind the retina.