VPB 121: Veterinary Physiology - II (2+1)

PHOTO TRANSDUCTION

• The light falls on the retina by the adjusting the refractive media of the eye.
• As the light falls on the retina, the rods and cones absorb energy and convert it into electrical signal.
• The rods and cones are located at the back of the retina synapse with bipolar cells which in turn synapse with the ganglion cells.
• Many rods cells synapse with single bipolar cell and many bipolar cells synapse with single ganglion cell.
• The rods provide hazy images, but the visual field is much larger.
• Each cone synapse with single bipolar cell and single bipolar cell synapse with single ganglion cell.  Thus cone provide very fine detailed image of the object, but the receptive field of a cone photoreceptor is very small.
• The receptive fields of ganglion cells contain both ‘on’ and ‘off’ regions.  In both regions, the 11-cis retinal is converted to all-trans retinal when the light activates the photoreceptors. 
• The all-trans retinal in turn activates transducin, a G-protein which decreases the cGMP found inside the photoreceptor cell.  As the cGMP is decreased it closes the Na+ channel, thereby hyperpolarizing the cell.
• The hyperpolarized cell releases neurotransmitter, the glutamate.  The activity of the glutamate determines the ‘on’ and ‘off’ region of the receptive field.
• The glutamate depolarizes the bipolar cells, which synapse with the ‘on’ region photoreceptor and increases neurotransmitter release from the bipolar cell which in turn depolarizes the ganglionic cell.
• The glutamate hyperpolarizes the bipolar cells, which synapse with ‘off’ region photoreceptor, and decreases the neurotransmitter release from bipolar cell, which causes hyperpolarization in the ganglion cell.
• The complex receptive fields of ganglion cells improve the ability to detect the difference between light and dark.  When both the receptive fields of the ganglion cell are exposed to light, both ‘on’ and ‘off’ regions are activated and result in weak stimulation.
• The horizontal cells enhance the contrast through lateral inhibition of the bipolar cells. The lateral inhibition increases contrast, sharpen borders and edges as it is excited strongly when the light is focussed on the ‘on’ region photoreceptors. Another step in increasing the integration of the image is by the synapsing of the bipolar cells with amacrine cells via gap junctions. 
• It modifies the inputs from many bipolar cells and alters the neurotransmitter release in to the ganglion cell.