Journal of Neuroscience, Vol 16, 2585-2591, Copyright © 1996 by Society for Neuroscience
Action potential initiation site depends on neuronal excitation
R Melinek and KJ Muller
Department of Physiology and Biophysics, University of Miami Medical School, Florida 33136, USA.
The initiation site in neurons is where the excitatory and inhibitory
inputs sum to generate action potentials. It is generally considered to be
at a fixed location, typically at the axon hillock or initial segment,
although action potentials, or impulses, could in theory arise at a site
that shifts dynamically. The data reported here show that the initiation
site can shift in a graded manner, by as much as 175 microm, depending on
the level of neuronal excitation. Laser axotomy reveals that the Anterior
Pagoda (AP) neuron of the leech is excitable within the synaptic neuropil
before its axon bifurcates. Using an electrophysiological technique to
measure relative delays in impulses arriving at different sites, we have
found that depolarization, either by applied current or by synaptic input,
can shift the site of impulse initiation in the cell proximally toward the
soma and neurites receiving synaptic input. Impulse initiation in this
region should enhance the efficacy of inputs synapsing there. Conversely,
hyperpolarization can shift the initiation site distally. A shifting
initiation site, therefore, may be a mechanism by which synaptic inputs can
rapidly enhance or suppress the active response of the AP neuron to other
synaptic inputs.
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