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Under dark conditions, a photoreceptor cell will release glutamate, which inhibits the ON bipolar cells and excites (or activates) the OFF bipolar cells. In light, however, light strikes the photoreceptor which causes it to be inhibited, and thus no glutamate to be given off. In this scenario, the ON bipolar loses its inhibition and becomes active, while the OFF bipolar cell loses its excitation and becomes silent.<ref name="isbn0-87893-694-7">{{cite book |author=Kevin S. LaBar; Purves, Dale; Elizabeth M. Brannon; Cabeza, Roberto; Huettel, Scott A. |title=Principles of Cognitive Neuroscience |publisher=Sinauer Associates Inc |location=Sunderland, Mass |year=2007 |isbn=0-87893-694-7 |oclc= |doi= |accessdate= |page=253}}</ref>
Under dark conditions, a photoreceptor cell will release glutamate, which inhibits the ON bipolar cells and excites (or activates) the OFF bipolar cells. In light, however, light strikes the photoreceptor which causes it to be inhibited, and thus no glutamate to be given off. In this scenario, the ON bipolar loses its inhibition and becomes active, while the OFF bipolar cell loses its excitation and becomes silent.<ref name="isbn0-87893-694-7">{{cite book |author=Kevin S. LaBar; Purves, Dale; Elizabeth M. Brannon; Cabeza, Roberto; Huettel, Scott A. |title=Principles of Cognitive Neuroscience |publisher=Sinauer Associates Inc |location=Sunderland, Mass |year=2007 |isbn=0-87893-694-7 |oclc= |doi= |accessdate= |page=253}}</ref>


Rod bipolar cells, which receive inputs only from rod photoreceptors, always depolarize in response to photoreceptor hyperpolarization, thus they are all 'off' cells. They also do not synapse directly on to ganglion cells. Instead, rod bipolar cells synapse on to A II amacrine cells, which in turn excite cone ON bipolar cells (via gap junctions) and inhibit cone OFF bipolar cells (via [[glycine receptor|glycine]]-mediated inhibitory synapses) {{fact|date=April 2009}} thereby overtaking the cone pathway in order to send signals to ganglion cells at scotopic (low) ambient light conditions.
Rod bipolar cells do not synapse directly on to ganglion cells. Instead, rod bipolar cells synapse on to A II amacrine cells, which in turn excite cone ON bipolar cells (via gap junctions) and inhibit cone OFF bipolar cells (via [[glycine receptor|glycine]]-mediated inhibitory synapses) {{fact|date=April 2009}} thereby overtaking the cone pathway in order to send signals to ganglion cells at scotopic (low) ambient light conditions.


OFF bipolar cells synapse in the outer layer of the inner plexiform layer of the retina, and ON bipolar cells terminate in the inner layer of the inner plexiform layer.
OFF bipolar cells synapse in the outer layer of the inner plexiform layer of the retina, and ON bipolar cells terminate in the inner layer of the inner plexiform layer.

Revision as of 16:53, 18 September 2009

Retinal Bipolar Cells
Details
LocationRetina (Inner Nuclear Layer)
Shapebipolar
FunctionConvey gradients between photoreceptor cells to retinal ganglion cells
Presynaptic connectionsRods , cones and Horizontal Cells
Postsynaptic connectionsRetinal ganglion cells and Amacrine cells
Identifiers
MeSHD051245
NeuroLex IDnifext_31
Anatomical terms of neuroanatomy

As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.

Overview

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. Unlike many neurons, bipolar cells communicate via graded potentials, rather than action potentials.

Specification

Bipolar cells receive synaptic input 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.

Cone bipolar cells 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 decrease in glutamate by hyperpolarizing.

Under dark conditions, a photoreceptor cell will release glutamate, which inhibits the ON bipolar cells and excites (or activates) the OFF bipolar cells. In light, however, light strikes the photoreceptor which causes it to be inhibited, and thus no glutamate to be given off. In this scenario, the ON bipolar loses its inhibition and becomes active, while the OFF bipolar cell loses its excitation and becomes silent.[1]

Rod bipolar cells do not synapse directly on to ganglion cells. Instead, rod bipolar cells synapse on to A II amacrine cells, which in turn excite cone ON bipolar cells (via gap junctions) and inhibit cone OFF bipolar cells (via glycine-mediated inhibitory synapses) [citation needed] thereby overtaking the cone pathway in order to send signals to ganglion cells at scotopic (low) ambient light conditions.

OFF bipolar cells synapse in the outer layer of the inner plexiform layer of the retina, and ON bipolar cells terminate in the inner layer of the inner plexiform layer.

Functionality

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.

See also

References

  1. ^ Kevin S. LaBar; Purves, Dale; Elizabeth M. Brannon; Cabeza, Roberto; Huettel, Scott A. (2007). Principles of Cognitive Neuroscience. Sunderland, Mass: Sinauer Associates Inc. p. 253. ISBN 0-87893-694-7.{{cite book}}: CS1 maint: multiple names: authors list (link)
  • Nicholls, John G. (2001). From Neuron to Brain. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-439-1. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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