双极细胞

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This page is a translated version of the page Bipolar Cell and the translation is 12% complete.

A diagram of a bipolar cell[1]

在视网膜里,“双极细胞”作为光感受器和神经节细胞之间的信使,负责传送眼睛外的信号到皮质。之所以称为双极细胞是因为,它们在 soma (cell body)相反的两端有延伸。其中一端延伸连接着光感受器(多个视杆细胞或一个视锥细胞),另一端则将接收到的信号传送到神经节细胞的树突


在哺乳动物视网膜中有至少九种形态类型的锥双极(CB)和一种类型的杆双极(RB)细胞。 它们具有来自无长突细胞神经节细胞的特征性形态,其特征在于IPL中的静脉曲张轴突终端。

Physiology

Visual Response Function

Bipolar cells receive upstream innervations from the retina's photoreceptors. Although photoreceptors respond to light by hyperpolarizing, bipolar cells can translate this signal in either a sign-conserving or sign-inverting fashion.

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.

Anatomy

Bipolar cell types of the mouse retina and their corresponding genetic markers and transgenic mouse lines [2].

Bipolar cells have distinctive morphology from Amacrine Cell and Ganglion Cell 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

Within the retina, bipolar cell bodies are located within the the inner nuclear layer (INL). The cells' dendrites project to the outer plexiform layer, where they receive signals from the photoreceptors and horizontal cells. The cells' axons project to the inner plexiform layer, where they synapse with amacrine and ganglion cells.

Within rabbit retinas, bipolar cells were found to be 41% of all inner nuclear layer cells. Subsequent protein kinase C staining showing that rod and cone bipolar cells were 10% and 31% of the total INL cells, respectively.[3]

This ratio is not constant across all mammal species. In rabbits, the rod to cone bipolar cell ratio is approximately 50 to 1 but within monkeys the ratio is closer to 12.5 to 1.[3]

Shape

All bipolar cells share the same general morphological shape: a cell body with two projections that extend in opposite directions. The specific length and arborizations of the dendrites is a factor that is used in classifying the nine different cone bipolar cell and one rod bipolar cell subtypes (image at right).

Connections

Bipolar cells make synaptic connections with photoreceptors, as well as amacrine, horizontal, and ganglion cells.

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.

Molecules

Rod bipolar cells express protein kinase C, which is not the case with cone bipolar cells.[3] Antibodies against this molecule can be used to determine the ratio population of bipolar cells that synapse with rods or cones.

History

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,[4] 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

In EyeWire, the primary focus is to catalog the connections made between ganglion, bipolar, and amacrine cells. Understanding these synaptic inputs will allow a better comprehension of how retinal processing is able to create emergent ganglion cell functions, such as direction-selective cells, motion-selective cells, and even general center-surround inhibition circuits. Is it hypothesized that these properties do not only arise once the signals reach the ganglion cells, but rather begin to form within the upstream signals exchanged within the outer plexiform layer (OPL) where the bipolar cells synapse with horizontal cells and photoreceptors. Compiling information on the bipolar cells connected to a given direction-selective ganglion cells may give a clearer picture of how these computations are developed and transmitted.

References

  1. http://test.classconnection.s3.amazonaws.com/649/flashcards/343649/jpg/bipolar_neuron.jpg
  2. H. Wassle; C. Puller; F. Muller; S. Haverkamp. (2009) "Cone Contacts, Mosaics, and Territories of Bipolar Cells in the Mouse Retina" J. Neurosci. '29' (1):106-117 doi:10.1523/JNEUROSCI.4442-08.2009
  3. 3.0 3.1 3.2 http://hebb.mit.edu/courses/connectomics/Strettoi%20Masland%20Inner%20nuclear%20layer%20rabbit%20retina%2095.pdf
  4. Santiago Ramón y Cajal The Retina of the Vertebrates [Die Retina der Wirbelthiere] 1894