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VISUAL REFLEXES

LIGHT REFLEX:

In a normal human, exposure of light on to one eye will cause the constriction of both eye pupils. For example: if light is made to fall on the left eye and left eye pupil constricts, we say the DIRECT LIGHT REFLEX for left eye is POSITIVE. The right eye pupil will also constrict, even though the left eye is stimulated, we say the INDIRECT OR CONSENSUAL LIGHT REFLEX for the left eye is also positive.  Same is true if the right eye is stimulated.

 

The red line indicates the 10% fibers from the optic tract entering the midbrain. They terminate at the pretectal nucleus (anterior to the superior colliculus). When light stimulation from one eye is accepted by that eyes retina. Fibers from the retina move via the optic chiasma to both right and left optic tracts. 10% of these tracts enter the midbrain and synapse at the pretactal nucleus. The pretactal nucleus gives axons to the rostral (upper) part of the EDINGER WESTIPHAL NUCLEUS: (cranial nucleus for parasympathetic system) of the same side. Also both right and left pretactal nucleus are connected with eachother by the POSTERIOR COMMISURE, via which some fibers go to the opposite pretactal nucleus.  On the medial sides of the edinger westiphal nucleus lies the right and left occulomoter nerve nucleus. From the

 

 

 

occulomoter nerve nucleus the occulomoter the nerve arises. THE AXONS FROM THE WESTIPHAL NUCLEUS ALSO JOINS THE AXONS FROM THE OCCULOMOTER NERVE NUCLEUS TO FORM THE OCCULOMOTER NERVE. The occulomoter nerve synapses at the ciliary ganglion. From the ciliary ganglion short ciliary nerves arise, penetrate the sclera, move between the sclera and choroid, and pierce the cilliary muscle to reach the sphinter pupillae (circular smooth muscles that contracts pupil).

 

ACH is released at the ciliary ganglion as well as the sphinter pupillae. {on the sphinter pupillae there are M3 receptors which when stimulated by ACH, inturn stimulate Gq protein, which in turn stimulates phospholipase C. Phospholipase C breaks  phosphatidyl-inositol  diphosphate in the cell membrane to IP3 and diacylglycerol. IP3 acts on endoplasmic reticulum and pumps out Ca from it. Presence of Ca brings about contraction}

 

ACCOMODATION REFLEX:

 

Accommodation reflex is performed by the eye when a far object is brought near. It includes changes in lens, and constriction of the pupil. The pupil is constricted and the lens are made more rounded and converging by the contraction of the ciliary muscle {receive short ciliary fibers from the ciliary body. The whole pathway mentioned earlier}

Additional to this the medial rectus muscles also contracts to reduce ocular axis. When the neurons in the optic radiation terminate at the calcarine sulcus, some next order neurons move backwards from the medial side of cerebral hemisphere to its lateral side and terminates in the frontal eye fields situated in the middle frontal gyrus anterior to the precentral gyrus.

Axons from the frontal eyefields move upwards and downwards medially to synapse with the edhinger westphal nucleus and the occulomoter nerve nucleus, which again give axons that move along the occulomoter nerve. These fibers leave the occulomoter nerve to synapse with the the medial rectus muscles.

{The ciliary muscle innervates the zonules that innervate the lens, while the sphinter pupillae innervates the anterior pupil. When the sphinter pupilae contracts the pupil constricts and when the ciliary muscle contracts, the zonules contract and the lens becomes rounded.}