Amacrine cell

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Amacrine cells are neurons found in the retina.

Illustrations of various rabbit amacrine cells; shows the diversity in amacrine morphologies. [1]
Dendritic impulse collisions and shifting sites of action potential initiation contract and extend the receptive field of an amacrine cell. Vis Neurosci. 2007 Jul-Aug;24(4):619-34.

Amacrine Cells

Amacrine cells are interneurons found in the retina [1]. They are part of the inner plexiform layer, a section of the retina in which bipolar neurons communicate with amacrine cells and ganglion cells [2]. Amacrine cells serve to integrate for ganglion cells the signals coming from bipolar neurons [1].

Overview

Amacrine cells perform where the second synaptic retianl layer where ganglion cells and bipolar cells form a synapse. There are close to 50 types of amacrine cells, many of which do not have axons.An amacrine cell is a horizontal cell and it works laterally to interact with the output of the bipolar cell. Usually, their job is more specialized. A specific amacrine cell interacts with a specific bipolar cell, and usually have a specific neurotransmitter. In one such example, the amacrine cell connects between a rod bipolar neuron's output and a cone bipolar neuron's input which guides the signal to the determined ganglion cell.

Classification

Amacrine cells are classified by the width of their connection, which layers of the inner plexiform layer they are in, and by the neurotransmitter they use.

Anatomy

“Amacrine” means “long fiber” [3]; amacrine cells bear this name because it is believed that they do not have true axons, although some of the larger cells have projections that function as axons. Unlike the axons of ganglion cells in the inner plexiform layer, these projections do not continue out of the retina via the optic nerve [1].

There are currently about 40 known subtypes of amacrine cells [1]; they are classified based on the dendritic properties of tree size, branching style, and location within the strata of the inner plexiform layer [2]. Amacrine dendrites are both presynaptic and postsynaptic sites [6].

Cells of the same type will have different dendritic tree sizes depending on distance from the fovea; cells that are farther from the fovea will have a larger tree spread than those that are closer [2].

Molecular profile

Neurotransmitter: Varies. Usually GABA or glycine [1], but sometimes serotonin (rabbit A17) [4], acetylcholine (starburst cells), or dopamine (A18) [5].

Physiology

Synaptic Connections

  • Synaptic Inputs

Amacrine cells receive information from bipolar neurons in the inner plexiform layer of the retina [1].

  • Synaptic Outputs

Amacrine cells send information to ganglion cells in the inner plexiform layer of the retina [1].

Spiking properties

Amacrine cells do not have axons, but their dendrites function as such in that they exist as presynaptic sites and they propagate action potentials; the mechanism for this propagation is as yet unknown. It has been shown in vitro that GABA can inhibit action potential propagation in one dendrite while allowing it in others, leading scientists to think that dendritic action potentials may be sensitive to external environmental factors, allowing for nonuniform dendritic propagation in vivo [6].

Function

Not much is known about the amacrine cell’s functional roles. Amacrine cells with long, large dendrites are said to promote to the inhibitory process from the feedback received hat comes form the bipolar cells and ganglion cells. In some instances, amacrine cells give more input to certain types of ganglion cells than to others. Some other amacrine cells have a modulatory role between rod bipolar cells and cone bipolar cells. This allows the amacrine cell to mediate and adjust some sensitivity between the rod bipolar cell and cone bipolar cell for vision.

Amacrine cells are part of the visual system and do not directly affect animal behavior.

They are generally involved in regulation of ganglion cell response, although some amacrine cells are known to have very specific functions. For examples, dopaminergic amacrines adjust retinal response in accordance with environmental light; starburst cells synapse with certain ganglion cells, and, using both GABA and acetylcholine, allow for sensitivity to moving stimuli [7].

References

  1. Kolb, Helga, Eduardo Fernandez, Ralph Nelson. http://webvision.med.utah.edu/amacrines1.html
  2. Kolb, Helga, Eduardo Fernandez, Ralph Nelson. http://retina.umh.es/webvision/IPL.html#Interactions
  3. Chudler, Eric H. http://faculty.washington.edu/chudler/neuroroot.html
  4. Kolb, Helga, Eduardo Fernandez, Ralph Nelson. http://webvision.med.utah.edu/amacrines2.html
  5. Kolb, Helga, Eduardo Fernandez, Ralph Nelson. http://webvision.med.utah.edu/amacrines3.html
  6. Yamada Y, Koizumi A, Iwasaki E, Watanabe S, and Kaneko A. (2002) Propagation of Action Potentials From the Soma to Individual Dendrite of Cultured Rat Amacrine Cells Is Regulated by Local GABA Input. The Journal of Neurophysiology, Vol. 87: 2858-2866. PMID: 12037189.
  7. Masland, RH. (2001) The Fundamental Plan of the Retina. Nature Neuroscience, Vol. 4: 877-887. PMID: 11528418.
  8. Hain-Ann Hsueh, Alyosha Molnar and Frank S. Werblin. “Amacrine-to-Amacrine Cell Inhibition in the Rabbit Retina.” 31 March 2008. 27 July 2008 < jn.physiology.org/cgi/content/abstract/100/4/2077>.
  9. G Maguire, P Lukasiewicz and F Werblin. “Amacrine cell interactions underlying the response to change in the tiger salamander retina.” The Journal of Neuroscience. 1989 <www.jneurosci.org/cgi/content/abstract/9/2/726>
  10. Godinho L, Mumm JS, Williams PR, Schroeter EH, Koerber A, Park SW, Leach SD, Wong RO. “Targeting of amacrine cell neurites to appropriate synaptic laminae in the developing zebrafish retina.” Pubmed. Nov. 2005 < www.ncbi.nlm.nih.gov/pubmed/16258076>
  11. Bear, Mark. Neuroscience: Exploring the brain. 3rd. Baltimore, MD;Philadelphia, PA: Lippincott Williams & Wilkins, 2007. Print.
  12. Royer AS, Miller RF. “Dendritic impulse collisions and shifting sites of action potential initiation contract and extend the receptive field of an amacrine cell.” Photo. Vis Neurosci Aug. 2007 <www.neuroscience.umn.edu/research/Aug07.jpg>


Additional information

Webvision: Amacrine cells