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neuron triggering

Matt Jones jonesmat at ohsu.edu
Mon Feb 22 14:48:28 EST 1999


In article <36C7C994.6A599AB1 at uvic.ca> jamie morken, foster at uvic.ca
writes:
>Hi all,
>
>Do neurons fire when the inputs sum to a certain voltage, or is it less
>clear cut than this?  Thanks

To a first approximation, yes reaching this threshold is the requirement
for an action potential. But in the details, no, it is definitely not so
clear cut. It DOES matter how rapidly the membrane crosses threshold,
because if you ramp the membrane potential up slowly, you will inactivate
sodium channels and there will not be enough available to generate the
required current density for spike firing. Also, if you ramp the
potential up slowly, you will turn on potassium channels that inactivate
very slowly, so the net balance of sodium versus potassium conductances
will favor the potassium conductance. This "shunts" the voltage (i.e.,
reduces the membrane resistance, and since Ohm's Law roughly applies, dV
= dIR, the change in voltage you can get during the spike is reduced when
the resistance gets smaller). Synaptic conductances also shunt the spike,
and slowly rising, large synaptic conductances like the NMDA EPSC are bad
at driving spikes compared to fast ones like AMPA EPSCs. 

Finally, action potentials are only all or none events to a first
approximation. When recorded at the soma, they are not all the same size,
or all the same duration. Their height and width depends on what other
conductances are also open as the membrane approaches threshold. For
example, if you give a square current step to a pyramidal neuron, you
often observe that the spikes get less frequent as the step is
maintained, but also that they get smaller and wider. This is because
other shunting conductances are opened by the sustained depolarization in
response to your current step.

Cheers,

Matt Jones



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