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.

Axonal Propagation of Simple and Complex Spikes in Cerebellar Purkinje Neurons

“…These results provide evidence that the axonal recording sites were distal to the spike initiation zone, which has been estimated to be 30 -60 m from the soma in unmyelinated axons (Colbert and Johnston, 1996) but may be farther out in myelinated axons (Colbert and Pan, 2002). We reasoned that if our axonal recording sites had been upstream from the spike initiation zone, every somatic spike would have corresponded to an axonal success as the action potential back-propagated toward the soma.…”

“…Because propagation failures were evident, however, the axonal recording electrode was more likely to have been beyond the spike initiation zone. As an additional test, we examined the axonal noise measurements for instances of spike-like events occurring in the absence of somatic action potentials, because axonal successes with somatic failures have been documented for axonal recordings between the soma and the spike initiation zone (Colbert and Johnston, 1996;Williams and Stuart, 1999). In our recordings, however, no axonal spikes were ever observed in the absence of a somatic action potential.…”

“…In cerebellar Purkinje neurons, as in other neurons, initiation of action potentials occurs in the axon (Stuart and Häusser, 1994;Stuart and Sakmann, 1994;Colbert and Johnston, 1996), producing two types of somatic spikes. Simple spikes, which occur spontaneously, exceed 100 spikes/sec in response to excitation from parallel fibers (Thach, 1968;Gilbert and Thach, 1977;Kitazawa et al, 1998).…”

Reemerging role of cable properties in action potential initiation

“…This is a complicated experimental issue for which direct evidence has been elusive. In what is arguably the most direct approach, voltage clamp of outside-out (11,12) and conventional cell-attached (13) membrane patches has been used to assess Na current density along the AIS. However, the fidelity of these approaches may be limited by localized geometric factors, such as Na channel anchoring by cytoskeletal elements (14) that may limit channel density in recorded membrane patches.…”

“…Conflicting results have been reported. For example, a hyperpolarizing shift in voltage-dependence of Na channel activation in the distal AIS has been reported that would contribute to the AP initiation in distal AIS (11,12,17), but this difference in gating has not always been consistently observed (13).…”