Difference between revisions of "Amacrine Cell"

From Eyewire
Jump to: navigation, search
(Open questions)
(Connections)
Line 32: Line 32:
  
 
===Connections===
 
===Connections===
Amacrine cells are postsynaptic targets of [[Bipolar Cell|bipolar cells]]; these bipolar-to-amacrine cell synapses occur in the inner plexiform layer. Amacrine cells have their postsynaptic targets in the inner plexiform layer as well. Amacrine cell dendrites are known to synapse onto [[Ganglion Cell |ganglion cell]] neurites in the IPL, mediating "antagonistic inputs from bipolar cells in the ganglion cell's surround." <ref>Tessier-Lavigne, M. "Visual Processing by the Retina." ''Principles of Neural Science''. New York: McGraw-Hill Medical, 2000. 507-522.</ref> Amacrine cells are also known to form "reciprocal synapses" onto the [[Bipolar Cell |bipolar cells]] that synapse onto them<ref name="Dowling 1996">{{cite journal  | author=J. E. Dowling; B. B. Boycott |title=Organization of the Primate Retina: Electron Microscopy |journal=Proc. R. Soc. A |volume=166 |issue=1002 |pages=80–111 |year=1966 |url=http://links.jstor.org/sici?sici=0080-4649%2819661115%29166%3A1002%3C80%3AOOTPRE%3E2.0.CO%3B2-N }}</ref>; this suggests that amacrine cells serve to regulate the output of [[Bipolar Cell|bipolar cells]] in a negative-feedback loop fashion. Further, "amacrine processes are also seen to contact other amacrine processes" in the IPL.<ref name="Dowling 1996">{{cite journal  | author=J. E. Dowling; B. B. Boycott |title=Organization of the Primate Retina: Electron Microscopy |journal=Proc. R. Soc. A |volume=166 |issue=1002 |pages=80–111 |year=1966 |url=http://links.jstor.org/sici?sici=0080-4649%2819661115%29166%3A1002%3C80%3AOOTPRE%3E2.0.CO%3B2-N }}</ref> Thus, amacrine cells form synapses with [[Bipolar Cell|bipolar cells]], [[Ganglion Cell|ganglion cells]], and other amacrine cells, all in the inner plexiform layer.
+
Amacrine cells are postsynaptic targets of [[Bipolar Cell|bipolar cells]]; these bipolar-to-amacrine cell synapses occur in the inner plexiform layer. Amacrine cells have their postsynaptic targets in the inner plexiform layer as well. Amacrine cell dendrites are known to synapse onto [[Ganglion Cell |ganglion cell]] neurites in the IPL, mediating "antagonistic inputs from bipolar cells in the ganglion cell's surround." <ref>Tessier-Lavigne, M. "Visual Processing by the Retina." ''Principles of Neural Science''. New York: McGraw-Hill Medical, 2000. 507-522.</ref> Amacrine cells are also known to form "reciprocal synapses" onto the [[Bipolar Cell |bipolar cells]] that synapse onto them<ref name="Dowling 1996">{{cite journal  | author=J. E. Dowling; B. B. Boycott |title=Organization of the Primate Retina: Electron Microscopy |journal=Proc. R. Soc. A |volume=166 |issue=1002 |pages=80–111 |year=1966 |url=http://links.jstor.org/sici?sici=0080-4649%2819661115%29166%3A1002%3C80%3AOOTPRE%3E2.0.CO%3B2-N }}</ref>; this suggests that amacrine cells serve to regulate the output of [[Bipolar Cell|bipolar cells]] in a negative-feedback loop fashion. Further, "amacrine processes are also seen to contact other amacrine processes" in the IPL.<ref name="Dowling 1996">{{cite journal  | author=J. E. Dowling; B. B. Boycott |title=Organization of the Primate Retina: Electron Microscopy |journal=Proc. R. Soc. A |volume=166 |issue=1002 |pages=80–111 |year=1966 |url=http://links.jstor.org/sici?sici=0080-4649%2819661115%29166%3A1002%3C80%3AOOTPRE%3E2.0.CO%3B2-N }}</ref> Thus, amacrine cells form synapses onto [[Bipolar Cell|bipolar cells]], [[Ganglion Cell|ganglion cells]], and other amacrine cells, all in the inner plexiform layer.
  
 
==Molecules==
 
==Molecules==

Revision as of 22:28, 3 April 2012

File:A2.png
AII amacrine cell, reconstructed in EyeWire.

Amacrine cells are interneurons in the retina. Amacrine cells are responsible for 70% of input to retinal ganglion cells. Bipolar cells, which are responsible for the other 30% of input to retinal ganglia, are regulated by amacrine cells.

Amacrine cells operate at the inner plexiform layer (IPL), the second synaptic retinal layer where bipolar cells and ganglion cells form synapses. There are about 40 different types of amacrine cells and are classified by the width of their field of connection, which layer(s) of the stratum in the IPL they are in, and by neurotransmitter type. No single type of amacrine cell predominates; the type with most frequency is observed only 13% of total population, and the remainders are distributed among many types of cell, each making up 5% or less of the total amacrine cell population. The average diameter of dendritic field for each type varies over 34 to 400 microns, and their overall shapes alone are enough to serve as criterion of classification.

There is no clear distinction between dendrites and axons in the processes of most of the amacrine cells, though they are often referred to as dendrites in general.

Like horizontal cells, amacrine cells work laterally affecting the output from bipolar cells, however, their tasks are often more specialized. Each type of amacrine cell connects with a particular type of bipolar cell, and generally has a particular type of neurotransmitter. One such population, AII, 'piggybacks' rod bipolar cells onto the cone bipolar circuitry. It connects rod bipolar cell output with cone bipolar cell input, and from there the signal can travel to the respective ganglion cells.

Most are inhibitory using either GABA or glycine as neurotransmitters.

Physiology

Visual Response Properties

A few types of amacrine cells are associated with their respective functions and with corresponding ganglion cells. For example, starburst amacrine cells (SACs) are known to make synapses onto on/off direction-selective ganglion cells (On/Off DSGCs) [1], and wide-field (WF) amacrine cells, also known as polyaxonal amacrine cells, are considered to be associated with object motion sensitive ganglion cells either directly or indirectly via bipolar cells [2].

Cellular Biophysics

Amacrine cells are interesting biophysically in that they operate using both sodium-mediated action potentials and sodium-independent graded potential changes.[3] This has been shown in inhibitory (i.e., GABAergic/glycinergic) amacrine cells[3], which make up the majority of amacrine cells, though it is unclear if this holds for all amacrine cells.

Anatomy

Location

File:Amacrine cells.png
Narrow-field amacrine cells

Amacrine cells have their cell bodies located in the inner nuclear layer of the retina and have projections in the inner plexiform layer. Different subtypes of amacrine cells project differently in the inner plexiform layer[4], as shown in the figure to the right depicting different types of narrow-field amacrine cells.

Shape

File:Ac types.png
Two amacrine cells with distinctive shapes. They can be easily mapped to their respective class in the catalog.

Amacrine cells send projections from their cell bodies into the inner plexiform layer. These projections arborize differently for different subtypes of amacrine cells. Amacrine cells have these projections distributed roughly circularly in the inner plexiform layer, though some subtypes arborize asymmetrically. Most amacrine cells can be classified according to the diameter of their projection arborization: "narrow-field" cells have arbors less than 125 µm in diameter, "medium-field" cell arbors range from 125 to 400 µm in diameter, and "wide-field" cell arbors are larger than 400 µm.[4]

Their overall shapes alone are enough to serve as criterion for the classification.

Connections

Amacrine cells are postsynaptic targets of bipolar cells; these bipolar-to-amacrine cell synapses occur in the inner plexiform layer. Amacrine cells have their postsynaptic targets in the inner plexiform layer as well. Amacrine cell dendrites are known to synapse onto ganglion cell neurites in the IPL, mediating "antagonistic inputs from bipolar cells in the ganglion cell's surround." [5] Amacrine cells are also known to form "reciprocal synapses" onto the bipolar cells that synapse onto them[6]; this suggests that amacrine cells serve to regulate the output of bipolar cells in a negative-feedback loop fashion. Further, "amacrine processes are also seen to contact other amacrine processes" in the IPL.[6] Thus, amacrine cells form synapses onto bipolar cells, ganglion cells, and other amacrine cells, all in the inner plexiform layer.

Molecules

Molecular markers

There exist several molecular markers for amacrine cells, including Pax6, Tcfap2b, Gad1, and GlyT1.[7] However, no markers exclusively expressed in amacrine cells are known to exist, and there exist "far fewer molecular markers [for amacrine cells] than known morphological types" of amacrine cells.[7]

Neurotransmitters

Most amacrine cells are inhibitory and secrete GABA or glycine, though in total, amacrine cells as a class use eight different neurotransmitters.[8] A particular class of amacrine cells—the starburst amacrine cell—has been found to be both cholinergic and GABAergic.[9]

History

The first characterization of amacrine cells is often attributed to Santiago Ramón y Cajal. Using the Golgi method of staining neurons, he first saw these cells in the avian retina in the late 1880s, naming them "amacrine" cells ("amacrine" meaning "without axon" in Greek).[10] Though he was the first to call them amacrine cells, he built on the earlier work of J. Müller, who had previously described "spongioblasts" in the retina that were likely the very same cells Ramón y Cajal later named "amacrine."[10]

Open questions/status/relevance to Eyewire

Open questions

Though it is more or less well-established how inhibitory amacrine cells function, it is less clear what functions non-GABAergic/glycinergic amacrine cells have in the retina. In particular, it is not well understood for which functions starburst amacrine cells require acetylcholine secretion or how starburst amacrine cells might use both GABA and acetylcholine in concert to accomplish certain fucntions. Starburst amacrine cells also exhibit very curious biophysics in that any given individual SAC dendrite is selectively activated by visual stimuli centrifugal with respect to that particular dendrite.[11] The mechanism for this selectivity remains unknown.

Also, as stated above, no amacrine cell-exclusive molecular markers are known to exist[7]; the discovery of such a marker would be incredibly beneficial to further amacrine cell research.

Status/relevance to Eyewire

Thus far, one starburst amacrine cell has been reconstructed through Eyewire, as well as at least one AII amacrine cell.

File:Figeyewire.jpg
Starburst amacrine cell reconstructed in Eyewire

References

  1. Script error: No such module "Citation/CS1".
  2. Script error: No such module "Citation/CS1".
  3. 3.0 3.1 Script error: No such module "Citation/CS1".
  4. 4.0 4.1 Script error: No such module "Citation/CS1".
  5. Tessier-Lavigne, M. "Visual Processing by the Retina." Principles of Neural Science. New York: McGraw-Hill Medical, 2000. 507-522.
  6. 6.0 6.1 Script error: No such module "Citation/CS1".
  7. 7.0 7.1 7.2 Script error: No such module "Citation/CS1".
  8. Tessier-Lavigne, M. "Visual Processing by the Retina." Principles of Neural Science. New York: McGraw-Hill Medical, 2000. 507-522.
  9. Script error: No such module "Citation/CS1".
  10. 10.0 10.1 Script error: No such module "Citation/CS1".
  11. Script error: No such module "Citation/CS1".