How many inputs are needed to excite/saturate a neuron

mvcy at vax5.cit.cornell.edu mvcy at vax5.cit.cornell.edu
Wed Mar 3 17:02:52 EST 1993

> On the one hand, anatomical evidence suggests that a neuron has, on
> average, on the order of THOUSANDS of neurons impinging on it.  On the
> other hand, physiological evidence shows that only a few epsps are
> needed to generate an action potential - which would imply that ONE
> cell is sufficient for exciting the postsynaptic neuron.
> Can anyone explain this disparity between anatomy and physiology?

Actually I take issue with the assumption that only a few epsps are 
necessary.  This might be true at the vertebrate neuromuscular junction or
giant fiber escape systems, but not generally true.

The thousands of synapses may require simultaneous activation (like in the
cerebellar purkinje cells) to be effective.  Alternatively, the distal
dendritic portions may be just involved in dendro-dendritic interactions
and may not have any appreciable effect on the parent SIZ.  (But such a
cell may connect rather close to a neighbor's SIZ and recurrenct connections
or dendritic propagation of APs might have a large effect on the parent cell

In response to the original question, I don't think it's easy to answer 
what the typical 'saturation' characteristics of a neuron is because of the
variability.  The cells I'm working on reaches a maximum response of 1 spike
every 100 stimuli or so.  (yet, it always receives psps)

In another post, someone mentioned that electrotonic characteristics of EPSPs
and IPSPs are different.  That is not true.  They may travel differently, but
there is nothing inherently different about their electrotonic spread.

Sat Amagai
mvcy at cornella.cit.cornell.edu

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