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neuron triggering

Richard Norman rsnorman at mw.mediaone.net
Mon Feb 15 21:28:25 EST 1999

jamie morken wrote in message <36C7C994.6A599AB1 at uvic.ca>...
>Do neurons fire when the inputs sum to a certain voltage, or is it
>clear cut than this?  Thanks

The simple (and overly simplistic) answer is yes, neurons fire when
voltage at the spike initiation zone reaches threshold.

The facts are considerably less clear cut.  The "inputs" are synaptic
events which summate in a horribly non-linear way.  This is caused
in part by the notion of "reversal potential" -- the amplitude of
effect depends on the local membrane potential at the moment.  It is
also caused by the fact that synapses alter the membrane conductance,
and therefore influence the current loops that carry the potential
from the
site of the synapse to the spike initiation zone.  Even an excitatory
"short circuits" the membrane and reduces the effect of more distant

Furthermore, synapses are notoriously plastic.  The amplitude of any
specific "input" varies from one activation to another depending on
all sorts of events, some history dependent, other simply an
of metabolic state.

And then the excitability of the neuron can be plastic, so that
might vary.  Once a second messenger signalling mechanism cuts loose
and starts to phosphorylate all sorts of cell proteins, and possibly
activating gene control mechanisms, there is no telling how, how much,
or for how long, the cell is likely to change.

In short, the McCullough-Pitts threshold neuron model which dates back
now some 50 years is merely a first approximation of reality.

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