Bipolar cell of the retina
|Retinal Bipolar Cells|
|Location||Retina (Inner Nuclear Layer)|
|Function||Convey gradients between photoreceptor cells to retinal ganglion cells|
|Presynaptic connections||Rods , cones and Horizontal Cells|
|Postsynaptic connections||Retinal ganglion cells and Amacrine cells|
As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. They act to, directly or indirectly, transmit signals from the photoreceptors to the ganglion cells.
Bipolar cells are so-named as they have a central body from which two sets of processes arise. They can synapse with either rods or cones (but not both), and they also accept synapses from horizontal cells. The bipolar cells then transmit the signals from the photoreceptors or the horizontal cells, and pass it on to the ganglion cells through Localized Graded Potentials.
Bipolar cells accept synapses from either rods or cones, but not both, and they are designated rod bipolar or cone bipolar cells respectively. There are roughly 10 distinct forms of cone bipolar cells, however, only one rod bipolar cell, due to the rod receptor arriving later in the evolutionary history than the cone receptor.
Furthermore, they can be categorized into two different groups, ON and OFF, based on how they react to glutamate released by photoreceptor cells. When light hits a photoreceptor cell, the photoreceptor hyperpolarizes, and releases less glutamate. An ON bipolar cell will react to this change by depolarizing. An OFF bipolar cell will react to this change by hyperpolarizing.
Bipolar cells effectively transfer information from rods and cones to ganglion cells. The horizontal cells and the amacrine cells complicate matters somewhat. The horizontal cells introduce lateral inhibition and give rise to the center-surround inhibition which is apparent in retinal receptive fields. The amacrine cells also introduce lateral inhibition, however, its role is not yet well understood.
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.
- Nicholls, John G. (2001). From Neuron to Brain. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-439-1. Unknown parameter
- Masland RH (2001). "The fundamental plan of the retina". Nat. Neurosci. 4 (9): 877–86. doi:10.1038/nn0901-877. PMID 11528418.
- Retinal+bipolar+cells at the US National Library of Medicine Medical Subject Headings (MeSH)
- Diagram at mcgill.ca