title: Axonal action-potential initiation and Na+ channel densities in the soma and axon initial segment of subicular pyramidal neurons.
abstract: A long-standing hypothesis is that action potentials initiate first in the axon hillock/initial segment (AH-IS) region because of a locally high density of Na+ channels. We tested this idea in subicular pyramidal neurons by using patch-clamp recordings in hippocampal slices. Simultaneous recordings from the soma and IS confirmed that orthodromic action potentials initiated in the axon and then invaded the soma. However, blocking Na+ channels in the AH-IS with locally applied tetrodotoxin (TTX) did not raise the somatic threshold membrane potential for orthodromic spikes. TTX applied to the axon beyond the AH-IS (30-60 microm from the soma) raised the apparent somatic threshold by approximately 8 mV. We estimated the Na+ current density in the AH-IS and somatic membranes by using cell-attached patch-clamp recordings and found similar magnitudes (3-4 pA/microm2). Thus, the present results suggest that orthodromic action potentials initiate in the axon beyond the AH-IS and that the minimum threshold for spike initiation of the neuron is not determined by a high density of Na+ channels in the AH-IS region.
authors.affiliation: Division of Neuroscience, Baylor College of Medicine, Houston, Texas 77030, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Colbert
authors.forename: D
authors.initials: D
authors.lastname: Johnston
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D001369:Axons / Q000502:physiology* / Q000648:ultrastructure; D005260:Female; D006624:Hippocampus / Q000166:cytology; D008297:Male; D008857:Microscopy, Interference; D018408:Patch-Clamp Techniques; D017966:Pyramidal Cells / Q000737:chemistry / Q000502:physiology* / Q000648:ultrastructure; D051381:Rats; D017207:Rats, Sprague-Dawley; D026941:Sodium Channel Blockers; D015222:Sodium Channels / Q000032:analysis / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D026941:Sodium Channel Blockers; D015222:Sodium Channels; D013779:Tetrodotoxin