retina

Molecular Biology of Vision Different types of ganglion cells respond differentially to different types of stimuli, such as onset of light, onset of darkness, motion, direction of motion, color, contrast, and others.

When the bipolar cell depolarizes, it releases more glutamate onto the terminal of the amacrine cell. These ganglion cells are thus phasically active, firing a burst of action potentials immediately after the onset of a stimulus but fewer as the stimulus continues.

Some bipolar cells respond to glutamate by hyperpolarization, or inhibition. In the dark, the photoreceptors maximally release glutamate and these bipolar cells are maximally hyperpolarized. In the light, the photoreceptors are hyperpolarized and they release less glutamate.

Curiously, in order to reach the photoreceptors, incoming light must first pass through all the other layers of cells in the retina (see sidebar). The first of these is the ganglion cell layer, composed of the bodies of ganglion cells. The distribution of the rods and cones in the retina is not uniform.

The visual pathway in the retina consists of a chain of different nerve cells. Light first travels through all the layers until it reaches the photoreceptor layer, the rod and cone layer. Rods and cones use photopigments, which contain opsin and a chromophore, to help them convert light into energy.

The primary light-sensing cells in the retina are the photoreceptor cells, which are of two types: rods and cones….