Bipolar Cell

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File:Sensoryneuron.GIF
Image of a monopolar and bipolar neuron


Bipolar cells are a class of neurons that are primarily utilized as sensory afferents, carrying signal from the various sense organs towards higher level processing areas. Within the retina, bipolar cells act as the signal couriers between the photoreceptors that react to light stimuli and the ganglion cells that carry these signals out of the eye and into the cortex. Bipolar cells are so-called because they have two polar extensions that protrude from opposite ends of the soma. One of these extensions extends to a single photoreceptor (either a rod or cone) while the other delivers the processed signal to the dendritic arbors of the ganglion cells.

There are at least nine morphological types of cone bipolar (CB) and one type of rod bipolar (RB) cells in the mammalian retina. They have distinctive morphology from amacrine cells and ganglion cells, characterized by varicose axon terminals in the IPL.

Physiology

Processing Activity

The ON and OFF center circuits within the retina are a product of the either sign-conserving or sign-reversing synapse that the bipolar cell shares with its paired photoreceptor. Sign-conserving synapses result in an OFF center while sign-reversing synapses produce an ON center. The surround portion of the center/surround functionality is dependent upon the aggregate signals from surrounding horizontal and amacrine cells.

Unlike other neurons, bipolar cells do not transmit signals by way of action potentials. They instead make use of a potential gradient that can be modulated by the connecting horizontal and amacrine cells.

Electrophysiology

  • Sign-conserving vs. Sign-reversing details


Anatomy

File:Bp types.png
Bipolar cell types of the mouse retina and their corresponding genetic markers and transgenic mouse lines [1].

Bipolar cells have distinctive morphology from ACs and GCs in the IPL, characterized by the varicose axon terminals. The different types of bipolar cells differ in their dendritic branching pattern, the number of cones contacted, and the stratification level of their axons in the IPL. A type of bipolar cells tile up the entire space of the strata, with little overlapping region to each other.

Location

Shape

Connections

Molecules

History

File:Cajal-fig1.png
Santiago Ramón y Cajal's 1894 diagram of retinal neurons. Layer E consists of bipolar cells.

Bipolar cells have been known since at least 1894 by Santiago Ramón y Cajal,[2] and possibly back to 1887, as he says of Ferruccio Tartuferi's Sulla anatomia della retina (Archivio per le science mediche, Vol. XI. No. 16. p. 335. 1887): "[Tartuferi] succeeded, above all, to detect the true morphology of bipolar cells in the inner nuclear layer."

Open questions / status / relevance to eyewire

References

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