Definitions

bipolar cell

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. They act to, directly or indirectly, 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 through Localized Graded Potentials.

Specification

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.

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 change by hyperpolarizing.

Under dark conditions a photoreceptor cell will release glutamate, which inhibits the ON bipolar cell. 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 bipolar is activated. In an OFF bipolar cell, light strikes a photoreceptor causing it to be inhibited and thus the bipolar cell will be inhibited as well. In darkness, the photoreceptor cell will be excited by darkness, give off glutamate, and will in effect excite the OFF bipolar cell. OFF bipolar cells are sign-conserving(+ and +, - and -) whereas ON bipolar cells are sign-inverting (+ and -, - and +).

However rod bipolar cells are neither ON or OFF, but through the AII amacrine cell can selectively excite cone ON bipolar cells (via gap junctions) and inhibit cone OFF bipolar cells (via Gly inhibitory synapses.)

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

  • Nicholls, John G.; A. Robert Martin, Bruce G. Wallace, Paul A. Fuchs (2001). From Neuron to Brain. Sunderland, Mass: Sinauer Associates.
  • Masland RH (2001). "The fundamental plan of the retina". Nat. Neurosci. 4 (9): 877–86.

External links

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