On the site of impulse initiation in a neurone.

In the preceding paper (Moore & Westerfield, 1983) the effects of changes in membrane properties and non-uniform geometry on impulse propagation and threshold parameters were investigated. In this paper the contributions of these and other parameters to the site of initiation of an impulse were determined by computer simulations using the Hodgkin-Huxley membrane description, the cable equations, and geometry appropriate for a simplified motoneurone with a non-myelinated axon. Antidromic invasion of action potentials into the soma was found to depend upon (a) the ionic channel rate constants (determined by the temperature), (b) the abruptness of the transition from the small-diameter axon to the larger diameter (and increased load) of the soma-dendrite, (c) extensions of active properties into the dendrite, and (d) density of ion channels. The location of the apparent site of initiation of impulses was not necessarily at the site of synaptic input nor the nearest active membrane. Its position depended upon (a) the fraction of the dendritic tree with excitable membrane, and secondarily on (b) the stimulus strength. Even with uniform excitability in the active membrane, the apparent site of initiation could be moved a considerable distance from the soma and the site of stimulation by appropriate choice of the various parameters noted above.