Retinal bipolar cells
Retinal bipolar cells are interneurons that form a part of the direct visual information from a light source to the brain. They are located in the outer and inner plexiform layer of the retina, which lie between the photoreceptor and ganglion cell layers. Although only photoreceptors can receive light and only ganglion cells can fire action potentials to the optic nerve, bipolar cells, along with amacrine and horizontal cells, control the detection of light by graded impulse responses from glutamate that is released by the photoreceptors.
Neuronal Type: local interneuron
Bipolar cells contain a cell body between two dendritic ends and two axons. The axons contain vesicles and mitochondria which allows better shaping of the structure. There are eleven types of bipolar cells, ten of which receive input from cones and one from rods.
There are two bipolar cell pathways through which visual information can travel to reach the ganglions cells: ON and OFF. The ON bipolar cell pathway uses a G-protein-coupled signal transduction pathway to make a change in the membrane resting potential while the OFF pathway uses a glutamate-gated cation channel to cause the membrane resting potential changes.
In the ON pathway, glutamate binds to the G-protein-coupled receptor and activates the G protein. The G protein then activates the effector enzyme cascades which affect membrane ion channels.
For the OFF pathway, the cell depolarizes when light is NOT present.
Photoreceptors and horizontal cells.
Amacrine cells and retinal ganglion cells
Bipolar neurons are non-spiking cells.
Kolb, H., Linberg, K.A. and Fisher, S.K. (1992) The neurons of the human retina: a Golgi study. J. Comp. Neurol. 318, 147-187.
Connaughton, V. P., Behar, T. N., Liu, W. L., and Massey, S. C. (1999). Immunocytochemical localization of excitatory and inhibitory neurotransmitters in the zebrafish retina. Vis Neurosci, 16, 483-490.
Connaughton, V. P., and Nelson, R. (2000). Axonal stratification patterns and glutamate-gated conductance mechanisms in zebrafish retinal bipolar cells. J Physiol (Lond), 524, 135-146.