Action potential initiation site depends on neuronal excitation

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.