The human eye vision

The human eye vision

Eye is the principal organ of vision.It is located in the eye ball with accessory visual apparatus.
Cornea:Cornea is the anterior modified part of the scleroses. It is the round,transparent and convexity in the anterior part of the eye ball
Pupil :Pupil is the central round aperture within the iris.The normal size of the pupil is 3-4 mm.contraction of the pupil miosis .The dilation of the pupil is called mydriasis.Pupil modifies the amount of the light entering the eye.pupil controls the depth of the focus. Acuti of vision is dependent upon pupillary size.
Retina : It development from the outer and inner layers of the optic cup, which is formed by the optic vesicle ( optic vesicle is form-
ed by the ectodermal outpocketing from diencephalon.
Lens of eye : It is the chif refracting media of the eye ball having the maximum refractive power.It is transparent ,elastic and biconvex, enclosed in a cpasule. Posteriorly, it is circular about 11mm in diameter. the thickness in the centre is circular is about 3.6-3.9 Refractive index-1.4 at the centre, less in the periphery.It is held in situ by suspensory ligament.

Nuteition:It has no vascular supply.It gets its nutrition almost entirely from the aqueos humour and party from the vitreous humour.
Function:It refract light from distant or near ojects intoa focus exactly on the retina and thus aids in vision. It absorbs all rays below 300 nm or above 2500nm.

Arteries, arterioles, and veins of ratina : The arteries, arterioles, and veins in the superficial layers of the retina near its vitreous surface can be seen through the opthalmoscope.Since this is the only place in the body where arterioles are readily visible. Ophthalmoscopic examination is of great value in the diagnosis and evaluation of diabetes mellitus hypertension and other diseases that affect blood vessels . 
i. The retinal vessels supply the bipolar and ganghion cells.
ii. The receptors are nourished for the most part by the capillary plexus in the cboroid. This is why retinal detachment is so amaging to the receptor cells.  

Dark adaptation :If a person spends a considerable length of time in brightly lighted surroundings and then moves to a dimly lighted environment, the retinas slowly become more sensitive to light as the individual becomes accustomed to the dark.This decline in visual threshold is known as dark  adaptation.It is nearly maximal in about 20 minutes,although there is some fuether decline over longer periods. The total change in threshold between the light-adapted and the fully dark-adapet eyes is very great.The time required for dark adaptation is determined in part by the time required to build up the rhodopsin stores.In bright light, much of the pigment is continuously being broken down and some time is required in dim light for accumulation of the amounts necessary for optimal rod function.

Light adaptation :On the other hand,when one passes suddenly from a dim to a brightly lighted environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination and the visual threshold rises.This adaptation occurs over a period of about 5 minutes and is called light adaptation,although,strictly speaking,it is merely the disappearance of dark adaptation.   

Direct and consequential light reflexes

If a light is shone inti one eye,the pupils of both ryes normally constrict.The constriction of the pupil upon which the light is shone is called the direct light reflex.The afferent impulses travel through the optic nerve,optic chiasma,and optic tract.Here a small number of fibers leave the optic tract and synapes on nerve cells in the pretectal nucleus, which lies close to the superior colliculus .The impulses are passed by axons of the pretectal nerve cells to the parasympathetic nuclei of the third cranial nerve on both sides.Here
the fibers synapse and the parasympathetic nerves travel through the third cranial nerve to the cilliary ganglion in the orbit. Finally, postganglionic parasympathetic fibers pass through the short clliary nerves to the eyeball and the constrictor pupillas muscle of the iris. Both pupils constrict in the consensual light reflex because the pretectal nucleus sends fibers to the parasympathetic nuclei on both sides of the midbrain. The fibers that cross to the cerebral aqueduct in the posterior commissure.

Accommodation reaction : When the eyes are directed from a distant to a near object, contraction of the medial recto brings about convergence of the ocular axes ; the lens thicken to increase its refractive power by contraction of the biliary muscle ; and the pupils constrict to restrict the light waves to the thickest central part of the lens. The afferent impulses travel through the optic nerve, the optic masochism, the optic tract, the lateral gesticulate body, and the optic radiation to the visual cortex. The visual cortex is connected to the eye field of the frontal cortex. Frontal cortex.Cortical fibers descend through the internal capsule to the locomotion nuclei in the mid brain . The locomotion nerve travels, to the medial recto muscles.Some of the descending cortical fibers synapse with the parasympathetic nuclei of the third cranial nerve on both sides. Here the fibers synapse and the parasympathetic nerves travel through the occulomotor nerve to the ciliary muscle of the ciliary body and the constrictor pupillae muscle of the iris.               
Corneal reflex : Light touching of the cornea or conjunctive results in blinking of the eyelids . Afferent impulses from the cornea or conjunctive travel through the ophthalmic division of the germinal nerve. Interracial  neurons connect with the motor nucleus of the facial nerve on both sides through the medial longitudinal fascicles . The facial nerve and its branches supply the orbicular is oculist muscle, which causes closure of the eyelids.

Neural pathways and primary visual cortex

Primary visual cortex : The primary visual receiving area is located principally on the sides of the calcine fissure. The axons of the ganglion cells pass caudally in the optic nerve and optic tract to end in the lateral geniculate body, a part of the thalamus. The fibers from each nasal hemiretina decussate in the optic chiasm. In the lateral geniculate body, the fibers from the nasal half of one retina and the temporal half of the other synapse on the cells whose axons from the geniculocalcarine tract. This tract passes to the occipital lobe of the cerebral cortex. Some ganglion cell axons pass from the optic tract to the pretectal region of the midbrain and thesuperior collusion, where they from connections that mediate papillary reflexes and eye movements. The frontal cortex is also concerned with eye movement, and especially its refinement. The bilateral frontal eye fields in this part of the cortex are concerned with control of cascades, and an area just anterior to these fields is concerned with vergence and the near response. The frontal areas concerned with vision probably project to the nucleus reticular is mentalists pontinus, and from there to the other brain stem nuclei mentioned above. Other axons pass directly from the optic chasm to the uncharismatic nuclei in the hypothalamus, where they from connections that synchronize a variety of endocrine and other circadian rhythms with the light-dark cycle. Activation occurs not only in the occipital lobe but also in parts of the inferior temporal cortex, the posterior parietal cortex, portions of the frontal lobe and the amygdala.              

Photoreceptor mechanism 

Ionic basis of photoreceptor potentials in rods and cones:Na+ channels in the outer segments of the rods and cones are open in the dark,so current flows from the inner to the outer segment. Current also flows to the synaptic ending of the photoreceptor.Na+ -K+ ATP ase in the inner segment maintains ionic equilibrium. Release of synaptic transmitter is steady in the dark. When light strikes the outer segment, the reactions that are initiated close some of the Na+ channels, and the result is a hyper polarizing receptor potential.The hyper polarization reduces the release of synaptic transmitter,and this generates a signal that ultimately leads to action potentials in ganglion cells. The action potentials are transmitted to the brain.

Optic disk and blind spot : The optic nerve leaves the eye and the retinal blood vessels enter it at a point 3 mm medial to the slight above the posterior pole of the globe. This region is visible through the ophthalmologist as the optic disk. There are no visual receptors overlying the disk, and consequently this spot is blind. A thinned-out, rod-free portion of the Macaulay lute. In it, the cones are densely packed, and each synapses to a single bipolar cell which in turn synapses on a single ganglion cell, providing a direct pathway to the brain. There are very few overlying cells and no blood vessels. Consequently, the forgave is the point where visual acuity is greatest. When attention is attracted to or fixed on an object, the eyes are normally moved so that light rays coming from the object fall on the fovea.