Retina

Photoreceptors of the Retina
The retina contains rods and cones – photoreceptor neurons that synapse with bipolar cells. Rods are responsible for colorless vision in relatively dim light. Cones provide color vision and function in day light. A light sensitive pigment in rods (Rhodopsin) decomposes in the presence of light and triggers a complex series of reactions that initiate nerve impulses on the optic nerve. 3 sets of cones (Red, blue, green), provide color vision. each set contains a different light–sensitive pigment , and each set is sensitive to a different wave length of light. The color perceived depends on which set or sets of cones are stimulated. 
 

Physiology of vision
When light strikes the rods it causes photodissociation of Rhodopsin into retinene and opsin .This bleaching reaction occurs maximally with a light wave length of 5oo nm.
Photo dissociation is caused by the conversion of the ll-cis form of retinene to the all – trans form that can not bind to opsin .
In the dark, more rhodopsin can be produced and increased rhodopsin in the rods makes the eye more sensitive to light.
The rods provide black and white vision under conditions of low light intensity . At higher light intensity, the rods are bleached out and the cones provide color vision. 3. In the dark , a constant movement of Na+ into the rods produces what is know as a "dark current". In the dark rhodopsin (R) molecules are stable signaling Na+ channels to stay open. Inflow of Na+ depolarizes the rods and their synapses. This activates the inhibitory bipolar cells , which exert inhibition over the G- cells (Ganglion cells) , thus G- cells are quiet in the dark.

 

Neural processing of visual information
Photoreceptor neural (Rods & cones) , transmit impulses to bipolar cells , which in turn send signals to Ganglion cells (some bipolar cells are inhibitory and have no axons).
Visual fields: The right half of the visual field is projected to the left half of the retina of each eye .
The left half of the left retina sends fibers to the left lateral Geniculate body of the thalamus. The left half of the Right retina also sends fibers to the left lateral geniculate body. This is because these fibers decussate in the optic chiasma.

 

by the end of this lesson, students should know about

1. photoreceptors of retina

2. layers of retina

3. Function and structure of retina

4. photochemical of vision