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I was wondering if a neuron can make a synapse to itself? I suspect that it would be extremely unusual for a neuron to do this. Anyway, has anyone seen even a single instance of this?

Is the process by which a neuron 'knows' not to make a synapse onto itself is known?

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A synapse from a neuron unto itself is called an Autapse. Not a whole lot is known about them. Tamas et al. (1) give a summary:

Van der Loos and Glaser (1972)proposed the word “autapse” to describe a transmitter release site made by the axon of a neuron and its own somatodendritic domain. In addition to their original Golgi study in rabbit neocortex predicting the existence of autapses, possible autaptic contacts have been described in dog (Shkol’nik-Yarros, 1971) and rat (Peters and Proskauer, 1980; Preston et al., 1980) cerebral cortex, monkey neostriatum (DiFiglia et al., 1976), and cat spinal cord (Scheibel and Scheibel, 1971). (...) Although autapses formed in cell cultures have been used extensively to study the physiology of synaptic mechanisms (Segal, 1991; Pan et al., 1993;Shi and Rayport, 1994), few proposals have been made for the functional significance of inhibitory autaptic innervation in vivo(neostriatum, Park et al., 1980; Aplysia buccal ganglia,White and Gardner, 1981).

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Thanks for the references. I'll probably wait for few more days before accepting it as answer. Let's see if someone can comment on why autapse are rare. –  Dilawar Apr 26 at 7:32
    
@dilawar it would also be interesting to know what function,if any is done by such neurons. If they do not have a significant role they would most likely be rare. –  biogirl Apr 26 at 7:58

There isn't enough research out there to explain the role of autapses however having read a selection of the latest research I can perhaps explain some of the proposed theory. Autapses may self inhibit or self excite. In the latter, one of their roles is thought to be to make a rhythmic action potential. This allows the brain to have an action potential which fires rhythmically if needed. Autapses of this nature have been identified in Interneurones for this purpose of continual activity.

Then there is even less researched areas. Imagine you need a neurone to fire twice to certain neurones but once to others or other similar requirements. A mixture of autapses with self inhibition and self stimulation properties can achieve this elegantly in a short space.

Things like this however aren't required frequently. Usually interconnecting neurones is sufficient, however possibly in places of high rhythmic activity or fine control these autapses may be helpful.

http://www.sciencedirect.com/science/article/pii/S0960982203003634

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