A MODEL OF SPIKE INITIATION IN NEOCORTICAL PYRAMIDAL NEURONS

Neocortical pyramidal cells possess voltage-dependent dendritic sodium channels that promote propagation of action potentials into the dendr itic tree but paradoxically may fail to originate dendritic spikes. A biophysical model was constructed to reconcile these observations with known anatomical and physiological properties. When dendritic and som atic sodium channel densities compatible with electrophysiological mea surements were combined with much higher densities in the axon initial segment then, regardless of the site of stimulation, spikes initiated at the initial segment and subsequently invaded the dendrites. The lo wer initial segment threshold arose from high current density and elec trical isolation from the soma. Failure of dendritic channels to initi ate spikes was due to inactivation and source-load considerations, whi ch were more favorable for conduction of back-propagated spikes.