THE EYE

The globe is spherical in shape . The eyeball consists of 3 concentric layers

of tissue .

1 -The outermost is a fibrous and protective. It is formed of transparent cornea through which light enters the eye. The sclera to which is attached the extraocular muscles. It

is an opaque white coat .

2 - Middle vascular and muscular coat

Anteriorly:

The iris ( smooth muscle ), some of the muscle fibers of it are arranged in a circular fashion and innervated by parasympathetic neurons which act to constrict the pupil ( central aperture ) thus reduce amount of light falling on the retina. The other fibers are radial and innervated by sympathetic neurons

to dilate the pupil .

In the middle:

The ciliary body and its ciliary muscle. This muscle is innervated by parasympathetic neurons and its contraction alters the shape and focal length ( power) of the lens ( accommodation ).

The central aperture of the ciliary body is occupied by the transparent and biconvex lens which focus

light upon the retina .

The lens is held by a suspensory ligament . Relaxation of suspensory ligament increase the convex of lens. The lens and suspensory ligament divide the lumen of the eyeball into anterior and

posterior part .

Posteriorly:

The choroid and its cells which contain dark pigments that absorb light and thus reduces

reflection within the eye .

3 .Nervous coat : Retina.

Retina

It consists of a non-neural and a neural portion. The non-neural part is the pigment epithelium. It is a single layer of light

absorbing cells adjacent to the choroid .

The neural part contains photoreceptors; neurons and neuroglia and a rich capillary network.These photoreceptors transduce light energy into electrical energy. They are Rods and Cones. The rods are 20 times more than

cones .

Rods are sensitive to light . They are important for vision in dim lighting conditions. They are predominate in the peripheral parts but their numbers decrease towards the macula lutea ( the surrounding 1cm to fovea centralis ) ,

where Cons are more .

Cons are responsible for colour vision and due to their arrangement and neuronal

connections , they confer high visual activity .

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At the fovea centralis only cons are present Also, the neurons and capillaries, through which light has pass are displaced. So, cons are directly exposed to light to provide

for maximal visual acuity .

The retina contains the 1st neuron of the central visual pathway which is formed of the bipolar cells and also the 2nd neuron which is formed of the ganglion cells. The axons of the ganglion cells form the optic

nerve.

The retina also contains interneurons as horizontal cells and amacrine cells . These modulate transmission between the photoreceptors and bipolar cells and

between the bipolar and ganglion cells .

Medial to macula lutea is a region where retinal axons accumulate to leave the eye in the optic nerve. This area known as the optic disc ( blind spot ) . Photoreceptors are

absent from this region .

Refracting media

1 -lens

2 -Aqueous humour :

It is present in the anterior part in front of the lens . It is a thin watery fluid . It is secreted from the ciliary body . It is reabsorbed into the ciliary body through canal of Schlemm where it is returned to venous blood .

3 -Vitreous humour ( body ):

It lies behind the ciliary body . It is a gelatinous material

N.B .

The image is centered near the posterior pole of the eye along the line of the visual axis in the fovea centralis and the surrounding 1 cm ( macula lutea ).

The formed image is inverted in both lateral and

vertical dimensions .

The objects that lie in the left half of the visual field form an image upon the nasal ( right ) half of the left retina ( of the same side ) and the temporal ( right ) half of the right

retina and vice versa.

Central visual pathway

The optic nerve enters the cranial cavity through the optic canal . The 2 optic nerves converge to form the optic chiasma on the base of the brain . It lies immediately rostral to the tuber cinereum of the hypothalamus and between the terminating internal carotid

arteries .

In the chiasma , the axons derived from the nasal portions of the 2 retinae decussate and pass into the contralateral optic tract , while those from the temporal hemiretinae remain

ipsilateral.

The optic tracts pass round the cerebral peduncle to terminate mainly in the lateral

geniculate nucleus of the metathalamus .

From the lateral geniculate nucleus , 3rd thalamocortical neurons project through the retrolenticular part of the internal capsule and form the optic radiation , which terminate in the primary visual cortex on the medial surface of the hemisphere above and below

the calcarine sulcus.

As the thalamocortical fibers leave the lateral geniculate nucleus they pass around the lateral ventricle.

Those representing the lower part of the visual field coursing superiorly to terminate in the visual cortex above the calcarine sulcus.

Those which represent the upper part of the visual field sweep into the temporal lobe ( Meyer’s loop ) then below the calcarine sulcus.

Surrounding the primary visual cortex , the rest of the occipital lobe constitutes the visual association cortex . It concerned with Interpretation of visual images ,

recognetion and depth perception ( stereoscopic vision ) and colour vision .

Each optic nerve carries information concerning both halves of visual field.

Due to decussation of fibers from the nasal hemiretinae at the optic chiasma , each optic tract , lateral geniculate nucleus and visual cortex receives information relating only to the contralateral half of the

visual field .

This combination of images from both eyes is necessary for stereoscopic vision.

The upper half of the visual field forms images upon the lower halves of the retinae. And the lower visual field upon the upper hemiretinae

So, There is both lateral and vertical inversion in the projection of the visual field upon the visual cortex such as , the upper left quadrant of the visual field is represented in the lower right quadrant of

the visual cortex.

Pupillary light reflex

The small number of fibers leave the optic tract, before reaching the lateral geniculate nucleus ,they pass to superior colliculus to terminate in the

pretectal nucleus in the midbrain .

The axons of the cells of the pretectal nucleus synapse around the cells of Edinger Westephal portion of the oculomotor nucleus of the same side and opposite side . The axons of these cells ( parasympathetic ) pass in both oculomotor nerves to relay in the ciliary

ganglion in the orbit .

Through the Short ciliary nerves constriction of the pupil of both eyes occur . The constriction of the pupil of the non- illuminated eye is called the Consensual light reflex. The constriction of the pupil of the illuminated eye is

called Direct light reflex .

Accommodation Reflex

Fixation upon a nearby object from a distance lead to convergence of the optic axes ( eyes ); contraction of the ciliary muscles and relaxation of the suspensory ligaments to increase the covexity of the lens, thus focusing the image. The pupil

is constricted to increase the depth of the focus .

The pathways of it comprise the optic nerve; optic tract; lateral geniculate body; optic radiation and visual area of the cerebral cortex which is connected by the superior longitudinal fasciculus to the eye field of the frontal cortex. The fibers

descend through the internal capsule to the nuclei of oculomotor nerve .

From the accessory oculomotor nuclei fibers pass to ciliaris and sphincter pupillae ( relaying in the ciliary ganglion) and from the ventral part of the oculomotor nucleus fibers supply the medial recti for the action of convergence of

the eyes .

Visual field deficits Disease of the eyeball , cataract , intraocular

haemorrhage ,retinal detachment .

Disease of the optic nerve , multiple sclerosis and tumours lead to loss of vision in the

affected eye Monocular blindness.

Compression of the optic chiasma by pituitary tumour lead to bitemporal

hemianopia .

Vascular and neoplastic lesions of the optic tract , optic radiation or occipital cortex , produce a contralateral homonymous hemianopia.

Retinitis pigmentosa is an inherited metabolic disorder of the photoreceptor and retinal pigment epithelial cells. There is progressive night blindness , peripheral visual field constriction and pigmentation of

the retina.