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How many inputs are needed to excite/saturate a neuron

Leslie Kay lmk2 at garnet.berkeley.edu
Fri Feb 26 12:05:23 EST 1993

In article <TAL.93Feb25224725 at copley.bu.edu> tal at copley.bu.edu (Doron Tal) writes:
>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?
>Some hypotheses:
>1. most of the thousands of synapses on a neuron are inactive
>2. a neuron that has thousands of inputs has an extremely large and
>finely-tuned range
>If we knew what the range of a neuron is, on average, then we'd be
>able to tell between the two above hypotheses.

In addition to the excitatory connections which a neuron can 
receive on its dendrites, it can also (and often does) receive
inhibitory synapses.  Inhibition has not only a net effect when
looking at the dynamics of neural masses, but it also has the
effect of modulating the efficacy of excitatory input.  There 
is an interesting review article in a recent issue of TINS by
Roland Jones (TINS, Vol 16, No 2, p58).  In it he discusses
the regulation of EC input to the Hippocampus (EC= Entorhinal 
Cortex) by large networks of inhibitory neurons in both layer
II of the EC and the Dentate Gyrus of the Hippocampus.  As
a result, he reports that it can take up to 400 axons in
the perforant path to excite one granule cell in the Dentate
Gyrus.  (This, I believe, is because of the inhibition in
the DG).  In addition it seems to be very hard to excite the
neurons in the EC (spiny stellate cells) which project through
the perforant path, because of the inhibitory network in the EC.

In answer to your question, I'd say "it depends".

Leslie Kay

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