Bipolar cell of the retina

As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells.

Bipolar cells are so-named as they have a central body from which two sets of processes arise. At one end, they form synapses with either a single cone cell, or a number of rod cells. The bipolar cells also make synapses with the Horizontal Cells. At the other end, they form synapses with ganglion cells, which fire action potentials along the optic nerve (cranial nerve II). Bipolar cells effectively transfer information from rods and cones to ganglion cells.

They can be categorized into two different groups, ON and OFF, according to how they react to glutamate produced by photoreceptor cells. When light hits a photoreceptor cell, the photoreceptor hyperpolarises, and its glutamate production decreases. An ON-cell will react to this change by depolarizing. An OFF-cell will depolarize when glutamate production increases(when light power decreases). The interactions between bipolar cells and the horizontal cells create an extra layer of complexity - ON bipolar cells will depolarise to a stimulus that is in the centre of their receptive field, and hyperpolarise to a stimulus that is in the periphery of the receptive field. OFF bipolar cells show exactly the opposite behaviour. This mutually inhibitory centre-surround relationship increases the contrast of the retinal image by enhancing the edges of the object that is viewed.

The mechanism for producing the center of a bipolar cell's receptive field is well known: direct innervation of the photoreceptor above it, either through a metabotropic (ON) or ionotropic (OFF) receptor. However, the mechanism for producing the monochromatic surround of the same receptive field is under investigation. While it is known that an important cell in the process is the horizontal cell, the exact sequence of receptors and molecules is as of yet unknown.