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

RETINA

RETINA (part of CNS)	Has 5 cell types, Photoreceptor cells has two types RODS, CONES. BIPOLAR CELLS with INTERNEURONS – Connects rods and cones with ganglion cells by synapsing. GANGLION CELLS : Ganglion cell layer-single layer of multipolar ganglion cells with scattered neuroglia cells with some blood vessels. Transparent and Unmyelinated carry impulses to brain via optic nerve. AMACRINE CELLS and HORIZONTAL CELLS : are interneurons lie between the bipolar and the ganglion cells. The light has to pass all the other layers like the nerve fibre layer, ganglion cell layer, etc., to reach the layer of rods and cones.
FOVEA	Area present in the back of the retina – Light rays are not distorted in this area, devoid of ganglion and bipolar cells. Light rays directly fall on retinal photoreceptors. Provides sharp visual image. FOVEA IS ABSENT IN DOMESTIC ANIMALS.
CONES	They are present in dense in the central portion of the retina - ( in primates) cones are best in day light – PHOTOPIC VISION. They are color sensitive. Similarly to fovea more no. of cones are present in MACULA or CENTRALIS in other animals.
RODS	Extremly sensitive to light, present in the periphery of retina. Responsible for NIGHT/SCOTOPIC vision, Sensitive to blue – green light.
BLIND SPOT	No photoreceptors at the origin of optic disk which is viz Blind spot.(Free from Rods and cones).