Eye and Ear

EYE

Organs of sight in man are a pair of eyes located in the eye orbits of the skull.

The exposed part of the eye is protected by a upper and a lower eyelid which are provided with eye lashes.

Each eye is represented in the form of a spherical eye ball which is moved in the eye orbit by the help of six eye muscles namely superior oblique, inferior oblique, superior rectus, inferior rectus, external rectus and internal rectus.

Eyeball measures about 2.5 cm. in diameter and is hollow.

Its wall is formed of three layers or coats-the outermost is called fibrous coat, the middle one as vascular coat and the inner one as retina.

(i) Fibrous coat : The outer coat of the eyeball is thick and tough. It provides form and shape to the eyeball. Fibrous coat consists of two parts, the sclera and cornea.

Sclera constitutes about five-sixth of the outer coat. It is white (made up of tough but elastic sheath of fibrous connective tissue containing collagen fibres) and opaque, and popularly called white of the eye. Most part of sclera is concealed in the orbit.

Cornea is the anterior transparent part of sclera and constitutes about 1/6 th of the fibrous coat. It is non-vascular and convex anteriorly. The cornea is covered by a thin and transparent membrane called conjunctiva composed of stratified epithelium and continued over the inner surface of the lids.

(ii) Vascular coat:

The middle coat of the eyeball is differentiated into three regions namely choroid, ciliary body and iris.

(a) Choroid is delicate, highly vascular and pigmented part which lies in contact with the sclera. It provides dark colour to the interior of the eyeball, it is black in colour. It prevents internally reflected light within the eye. The blood vessels of choroid nourish the retina.

(b) Ciliary body. It is the part of vascular coat immediately behind the peripheral margin of the iris. Ciliary body is thicker and less vascular than choroid. Its inner surface is folded to form ciliary processes. Present within the ciliary body are ciliary muscles.

(c) Iris is the anterior part of vascular coat which lies behind the cornea. It is centrally perforated by pupil, the size of which is regulated by the iridial muscles arranged in radial and circular manner. The iris, being pigmented, provides colour to eye.

Concept Builder

Mirror like tapetum layer of carnivores like cats, dogs increases sensitivity by reflecting unabsorbed light back through photoreceptor layer to shine in dark.

(iii) Retina (Nervous Tunic):

The third and inner coat of the eyeball, the retina (nervous tunic), lines the posterior three-quarters of the eyeball and is the beginning of the visual pathway.

The optic disc is the site where the optic nerve exits the eyeball.

Bundled together with the optic nerve are the central retinal artery, a branch of the ophthalmic artery, and central retinal vein.

Branches of the central retinal artery fan out to nourish the anterior surface of the retina.

The central retinal vein drains blood from the retina through the optic disc.

The retina consists of a pigment epithelium (nonvisual portion) and a neural portion (visual portion).

The pigment epithelium is a sheet of melanin-containing epithelial cells that lies between the choroid and the neural portion of the retina some histologists classify it as part of the choroid rather than the retina.

Melanin in the choroid and the pigment epithelium absorbs stray light rays, which prevents reflection and scattering of light within the eyeball.

This enables that the image cast on the retina by the cornea and lens remains sharp and clear.

The pigmented layer is continuous over choroid, ciliary body and Iris while the nervous layer terminates just before ciliary body.

This point is called Orra serrata.

Albinos lack melanin pigment in all parts of the body, including the eye.

The neural portion of the retina is a multilayered out-growth of the brain.

It processes visual data extensively before transmitting nerve impulses to the thalamus, which then relays nerve impulses to the primary visual cortex.

Three distinct layers of retinal neurons, are separated by two zones where synaptic contacts are made, the inner and outer synaptic layers.

The three layer of retinal neurons, in the order in which they process visual input, are the photoreceptor layer, bipolar cell layer, and ganglion cell layer.

Note that light passes through the ganglion and bipolar cell layers before reaching the photoreceptor layer.

Two other types of cells present in the retina are called horizontal cells and amacrine cells. These cells form laterally directed pathways that modify the signals being transmitted along the pathway from photoreceptors to bipolar cells to ganglion cells.

Schematic diagram to show the layer of the
retina and main structures therein.

Photo receptors are specialized to transduce light rays into receptor potentials.

The two types of photoreceptors are rods and cones.

Each retina has about 6 million cones and 120 million rods.

Rods are most important for seeing shades of gray in dim light.

They also allow us to see shapes and movement.

Cones provide color vision in bright light.

The visual pigments for colour vision are: erythropsin (sensitive to red), chloropsin (sensitive to green) and cyanopsin (sensitive to blue).

In moonlight we cannot see colors because only the rods are functioning.

Due to the low light level cones are not functioning.

The macula lutea is in the exact center of the posterior portion of the retina, at the visual axis of the eye.

The central fovea, a small depression in the center of the macula lutea, contains only cone photoreceptors.

In addition, the layers of bipolar and ganglion cells, which scatter light to some extent, do not cover the cones here; these layers are displaced to the periphery of the fovea.

As a result, the central fovea is the area of highest visual acuity or resolution (sharpness of vision).

Rods are absent from the fovea and macula and increase in number towards the periphery of the retina.

From photoreceptors, information flows to bipolar cells through the outer synaptic, layer and then from bipolar cells through the inner synaptic layer to ganglion cells.

The axons of ganglion cells extend posteriorly to the optic disc and exit they eyeball as the optic nerve.

The optic disc is also called the blind spot.

Since it contains no rods or cones.

Accomodation

Accommodation (focussing) is the reflex mechanism by which light rays from objects at various locations in the near visual field are brought to focus on the retina.

Altering the shape of the lens does this. In bright light the circular muscle of the iris contracts, the radial muscle relaxes, the pupil becomes smaller and less light enters the eye, preventing damage to the retina.

In dim light, the opposite muscular contractions and relaxations occur.

In the dark of night", your pupil may become up to 16 times bigger.

The added advantage of reducing the pupil size is that it increases the depth of focus of the eye, so that any displacement of the photosensors in the retina will not impair the focus.

Light rays from distant objects (>6 metres) are parallel when they strike the eye.

Light rays from near objects (<6 metres) are diverging when they reach the eye.

In both cases, the light rays must be refracted or bent to focus on the retina and refraction must be greater for light from near objects.

The normal eye is able to accommodate light from objects from about 25 cm to infinity.

With the involuntary ciliary muscles at rest, the flatter lens has the correct optical properties to focus distant images on the retina, but not close images.

The state of contraction of the ciliary muscles changes the tension of suspensory ligaments.

This acts on the natural elasticity of the lens, which causes it to change its radius of curvature, and thus, the degree of refraction.

As the radius of curvature of the lens decreases it becomes thicker, round up and amount of refraction increases.

It is the tension of the suspensory ligaments applied to the lens which determines the shape of the lens.

When the circular ciliary muscles are relaxed and the suspensory ligament becomes tout, the lens is pulled into a flattened shape suitable for focussing distant objects, decreasing the refraction.

When the tension is decreased, the circular ciliary muscles are contracted and the suspensory ligaments slack, consequently the lens becomes a more spherical shape suitable for focussing objects.

The image produced by the lens of eye on the retina is inverted and reversed.

However, objects are perceived the right way up because of the way in which the brain interprets the images.

The region of the environment from which each eye collects light is called the visual field.

Since both our eyes are frontally placed, there is an overlap between the visual fields of each eye. This is called binocular vision.

Concept Builder

Image formation is refractive process, maximum refraction takes place on cornea.

Extra ocular muscle of eye

Eye is rotated in the orbit by six strap shaped muscles inserted on the sclera. These are arranged in two groups, rectus and oblique.

Rectus: Superior rectus – Oculomotor nerve

Inferior rectus – Oculmotor nerve

Internal (medial) rectus – Oculomotor nerve

External (lateral) rectus – Abducens nerve

Oblique: Superior oblique – Trochlear nerve

Inferior oblique – Oculomotor nerve

CHAMBERS OF EYEBALL

The lens and suspensory ligament divide the interior of eyeball into two chambers, the anterior small aqueous chamber containing a watery fluid, the aqueous humour; and the posterior larger vitreous chamber containing viscous fluid, the vitreous humour.

Aqueous humour maintain intra ocular pressure mainly where as vitreous humour responsible for shaping of eye ball.

Mechanism of Vision

Light rays in visible wavelength focussed on the retina

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Activation of photopigment of rods and cones

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Light induces dissociation of the retinal form opsin resulting in the changes in the structure of the opsin.

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Membrane permeability of rods and cones changes.

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Potential difference generated in the photoreceptor cell.

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If generate action potential in the ganglionic cell s through the bipolar cells.

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Action potentials are transmitted by the optic nerve to the visual cortex of brain.

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Image formed on the retina is recognised based on earlier memory and experience.

Protective devices of Eye

(1) The Eyebrows: Two arched eminences of skin having numerous hairs project obliquely from the surface of the skin. The function of the eyebrows is to protect the anterior aspect of the eyeball from sweat, dust and other foreign bodies.

(2) The Eyelids (Palpebrae) and Eyelashes: The eyelids are two movable folds and have on their free edges, hairs -the eyelashes. The third eyelid is vestigial and is called plica semilunaris (nictitating membrane). The inner surface of each eyelid and parts of the eyeball are covered with mucous membrane, called the conjunctiva.

(3) Glands of Zeis: These are modified sebaceous glands which are associated with the follicles of eye lashes. They open into the follicles of eye lashes. Meibomian or tarsal glands are also modified sebaceous glands (oil glands) which are present along the edges of the eyelids. They produce an oily secretion which serves to lubricate the corneal surface and hold a thin layer of tears over the cornea. Glands of Moll are modified sweat glands at the edge of the eye lid.

(4) Conjunctiva: The palpebral conjunctiva is very vascular and has numerous papillae. Over the sclera the ocular conjunctiva is loosely connected to the eye ball; here it is thin, transparent, without papillae and slightly vascular. Reaching the cornea it continues as the corneal epithelium. The epithelium of the palpebral conjunctiva near the margin of the lids is non-keratinized squamous stratified epithelium. The conjunctiva helps to protect the eye ball and keeps it moist. It is this membrane that becomes inflamed in conjunctivitis or "pink eye".

(5) The Lacrimal Apparatus: The lacrimal apparatus of each eye consists of a lacrimal gland and its numerous ducts, the superior and inferior canaliculi, a lacrimal sac and nasolacrimal duct. The lacrimal gland secretes tears which are composed of water, salts and bactericidal protein called lysozyme. Lysozyme destroys microorganisms present on the front of the eyeball.

(6) Adipose Tissue (fat):A layer of adipose tissue surrounds the eyeball in the orbit. It serves as soft, shockproof pad.

Disorders of Eye:

(1) Myopia or Nearsightedness : In this case, the eyeball is antero-posteriorly elongated so that the image of distant objects is formed in front of yellow spot. The defect can be removed by using concave glasses.

(2) Hypermetropia or Long sightedness : The person can see distant objects clearly, but not those which are closer. This is due to antero-posterior shortening of the eyeball, so that the image is formed behind the yellow spot. The defect can be overcome by using convex lens.

(3) Presbyopia : A common defect in old age people due to the loss of elasticity of lens and reduced power of accomodation. The disorder can be corrected by convex lenses.

(4) Astigmatism : The disorder due to rough curvature of cornea or lens which can be corrected by the use of cylindrical glasses.

(5) Cataract: The sight is impaired due to the lens becoming opaque (Safaid Motia). The defect can be cured by surgical removal of the defective lens.

(6) Glaucoma: It occurs due to increase in intra-occular pressure as may develop due to blockage of canal of schlemn. It exerts pressure on optic nerve causing its damage. It leads to permanent blindness (Kala Motia)

(THE EARS)

The organs (Phonoreceptors) in man are a pair of ears situated on the head. Apart from their auditory function, the ears are also the organs of balancing.

Each ear has three portions-the external ear, the middle ear and internal ear.

(1) External Ear:

It consists of pinna and external auditory canal.

The latter is a curved passage which is lined by profusion of hair and about 4,000 ceruminous glands.

The glands secrete cerumen, a waxy material which entraps dust and also lubricates tympanum.

Tympanum or ear drum is a circular membrane present on the inner end the external auditory canal and partitions it from the tympanic cavity.

(2) Middle Ear:

The middle ear is represented by air-filled tympanic cavity which communicates with the pharynx by a passage called eustachian canal.

Present in the inner wall of the tympanic cavity are two openings, the upper fenestra ovalis and the lower, fenestra rotunda, each covered by a membrane.

The tympanic cavity contains three small bones, the ear ossicles which from outside to inside include malleus, incus and stapes.

Malleus is hammer shaped; incus is anvil shaped and the stapes is stirrup shaped.

The outer arm of malleus is in contact with inner surface of tympanum, while the inner end of stapes forms contact with the membrane on fenestra ovalis.

Middle ear is responsible for amplification of signal due to leverage system of ossicle (10 times) by ear ossicles and 2.2 times by smaller size of membrane covering fenestra ova lis. The oval window is the door to internal ear.

(3) Internal Ear:

It is also called membranous labyrinth and is surrounded by bony labyrinth of almost similar shape.

The space between the membranous labyrinth and bony labyrinth is filled by a watery fluid, the perilymph.

The membranous labyrinth contains endolymph.

The internal ear is a delicate organ and differentiated into vestibule, semicircular canals and cochlear duct.

The vestibule is the central body and is formed of two chambers, the upper utriculus and the lower sacculus.

The parts of ear

Semicircular canals are three arched structures which emerge from utriculus and open back into it.

They include anterior and posterior vertical canals and a horizontal canal.

The vertical canals join to form a common passage crus commune, before they open into utriculus.

Each semicircular canal is dilated at the base to form ampulla which contains sensory spot called crista formed of receptor cells and supporting cells.

The receptor cells bear sensory hair, which are embedded into a gelatinous cupule above.

The vestibule also contains two sensory spots called maculae, one in sacculus and another in utriculus.

Ear Ossicles
 Membranous labyrinth

Maculae are similar to cristae, but there is no cupule.

The sensory hair are embedded in otolith membrane containing calcareous bodies called otoliths.

The cristae and maculae are the receptors of balance.

The auditory region of intern al ear is represented by a spirally coiled structure called cochlea.

It consists of cochlear duct arising from the sacculus, which is surrounded by similarly shaped cochlear canal, a part of bony labyrinth.

The cochlear duct is fused with cochlear canal on lateral sides, but is free laterally therefore, in T.S., the cochlea shows three chambers, the upper scala vestibuli, the middle scala media and the scala tympani.

The scala media is partitioned from the scala vestibuli by Reissner's membrane and from the scala tympani by basilar membrane.

Scala vestibuli and scala tympani contain perilymph while scala media is filled with endolymph.

The upper and lower chambers communicate through helicotrema, a narrow opening present at the distal end of cochlea.

The basilar membrane, sensory hair cells and tectorial membrane make up the smallest unit of the ear, called the organ of Corti, first described by Italian microscopist, Alfonso Corti (1822-1888).

Sensory hair cells inside the ear resemble tracts left in the sand by truck tires.

The cochlea contains 16,000 to 24,000 hair cells arranged in four rows.

In three of the rows, the hairs form V-shaped patterns. In the fourth row, the hair stand in a straight line.

Each hair cell has up to 100 hairs.

When sound vibrations pass through the oval window, they create waves in the lymph fluid of the cochlea, like sea wave in a tidal current.

The waves cause the basilar membrane to ripple.

This movement bends the hair cells, pressing against the tectorial membrane and setting off nerve impulses in their associated afferent neurons.

T.S. Cochlea

More than 30,000 neurons and nerve fibres emerging from these, convey the electrical Signals to the brain, just 2 cm away via auditory (vestibulocohlear) nerve.

The basal ends of hair cells synapse with fibres of cochlear branch.

When the waves reach the round windows of the cochlea, they die away.

Mechanism of Hearing

External ear receives sound waves

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Directed towards the ear drum

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When the waves strike the tympanicmembrane the alternate compression and

decompression of the air causes the membrane to vibrate

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Vibrations are transmitted through the ear ossicles (M -7 I -7 S) to oval window

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The movement of the oval window set up wave in the perilymph of scala vestibuli

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Vibration of endolymph of scala media

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The waves in the endolymph induced a ripple in the basilar membrane

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Basilar movements bend the hair

cells pressing them against the tectorial membrane

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Nerve impulse generated in the associated afferent neurons

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Impulse transmitted to auditory region of brain via auditory nerve

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Impulse get analysed and sound is recognised

Concept Builder

The high frequency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea while low frequency resonance occurs near the apex mainly because of stiffness of fibres of basilar membrane.

The three internal ossicles of ear are malleus incus and stapes.

In case of non-mammals (amphians, reptiles, birds) there is just one bone called columella auris.

Diagram showing the conduction of sound vibrations in the ear

Physiology of Equilibrium

There are two kinds of equilibrium (balance).

One, called static equilibrium, refers to the maintenance of the position of the body (mainly the head) relative to the force of gravity.

The second king, dynamic equilibrium, is the maintenance of body position (mainly the head) in response to sudden movements such as rotation, acceleration, and deceleration.

Collectively, the receptor organs for equilibrium are called the vestibular apparatus, which includes the saccule, utricle, and semicircular ducts.

Static Equilibrium

The wall of both the utricule and saccule contains a small, thickened region called macula (Pleural maculae).

The maculae are the receptors for static equilibrium and also contribute to some aspects of dynamic equilibrium.

For static equilibrium they provide sensory informations on the position of head and essential for maintaining appropriate posture and balance.

For dynamic equilibrium they linear acceleration and deceleration.

For example the sensation you feel while in an elevator or a car that is speeding up or slowing down.

Dynamic Equilibrium

Vestibular apparatus contains three semicircular canals positioned at right angles to one another.

The dilated portion of each duct, ampulla, contains a small elevation called the crista.

Each crista is composed of a group of hair cells, supporting cells covered by a mass of gelatinous material called the cupula.

The cristae in the three semicircular canals maintain dynamic equilibrium.

Structure of Macula and Crista
Structure of Macula and Crista

Diseases of the Ear

1. Meniere's diseases: due to increased amount of the fluid of internal ear, loss of hearing.

2. Myringitis: Inflammation of tympanic membrane.

3. Otitis media: Acute infection in middle ear.

4. Vertigo: Type of dizziness where there is feeling of motion when one is stationary.

5. Cobyrinthine diseases: Improper functioning of internal ear.

Some Important Points

1. Most domestic mammals and sharks lack colour vision.

2. Tapetum lucidum. It is a part of choroid adjacent to the retina in the eyes of large number of elasmobranchs (cartilaginous fish). It possesses cells containing light-reflecting guanine crystals. It reflects light and causes the eyes to shine in dark. It also reflects additional light on the retinal cells to enable the fish to see in water where light is poor.

3. Accommodation : Fishes are able to see objects at different distances by changing the size of eyeball.

4. Pecten. It is a remarkable, highly vascular and pigmented structure projecting into the vitreous chamber from the blind spot normally. The pecten occurs in all birds except Kiwi (Apteryx). It is also found in some reptiles (e.g. Uromastix) but is absent in mammals. In Uromastix it is like cushion however, in pigeon it is comb-like and folded like a fan. The actual function of pecten is unknown but possibly it aids in the nutrition of the eye ball. In birds, it also helps in accommodation which is remarkably will developed in birds, by pressing the lens forward.

5. Phaco-emulsification technique in cataract surgery -"stitchless" technique. Foldable IOL (intraocular lens) is used.

6. Most birds have only day vision as their retina has mainly cones.

7. Owls have much better night vision as they contain a large number of rods and few cones in their retina.

8. Taste of chillies, is not true sensation. It is mainly sensation of burning pain produced by the stimulation of pain receptors of the tongue.

9. Hordeolum. Inflammation of sebaceous glands of eye lid.

10. Owls and cats see only with the help of available light from the stars or moon at night.

11. Frog is short sighted in air and long sighted in water.

12. Many insects like honeybees possess the gustatory receptors on their feet.

13. Largest cranial nerve-Trigeminal. .

14. Smallest/Thinnest cranial nerve-Pathetic/Trochlear.

15. Other names of various parts of brain

(i) Fore Brain = Prosencephalon

(ii) Mid Brain = Mesencephalon

(iii) Hind Brain = Rhombencephalon

(iv) Olfactory lobes = Rhinencephalon

(v) Cerebrum = Telencephalon

(vi) Diencephalon = Thalamencephalon

(vii) Cerebellum and Pons = Metencephalon

(viii) Medulla oblongata = Myelencephalon

(ix) Fourth ventricle = Metacoel

(x) Third ventricle = Diocoel

(xi) Iter = Mesocoel and aqueduct of sylvius.

(xii) Lateral ventricle = Paracoel

(xiii) Spinat canal = Myelocoel

(xiv) Cavity of olfactory lobe = Rhinocoel (absent in human)

16. Origin of CNS-develops from neural tube that is formed by infolding of ectoderm in early embryo.

17. Neopallium-Dorsal wall of cerebrum/cerebral cortex of brain.

18. Monosynaptic/simple reflex involves a single sensory fibre and a single motor fibre e.g., knee jerk. No interneuron. Polysynaptic/compound reflex involves one (or more) sensory and more than one motor nerve fibres. A number of interneurons are present. Polysynaptic reflexes are more common. All our visceral reflexes are polysynaptic