MODELING BACK PROPAGATING ACTION-POTENTIAL IN WEAKLY EXCITABLE DENDRITES OF NEOCORTICAL PYRAMIDAL CELLS

Simultaneous recordings from the soma and apical dendrite of layer V n eocortical pyramidal cells of young rats show that, for any location o f current input, an evoked action potential (AP) always starts at the axon and then propagates actively, but decrementally, backward into th e dendrites. This back-propagating AP is supported by a low density (< (g)over bar g(Na)> = approximate to 4 mS/cm(2)) of rapidly inactivatin g voltage-dependent Na+ channels in the soma and the apical dendrite. Investigation of detailed, biophysically constrained, models of recons tructed pyramidal cells shows the following. (i) The initiation of the AP first in the axon cannot be explained solely by morphological cons iderations; the axon must be more excitable than the soma and dendrite s. (ii) The minimal Na+ channel density in the axon that fully account s for the experimental results is about 20-times that of the soma. If g(Na), in the axon hillock and initial segment is the same as in the s oma {as recently suggested by Colbert and Johnston [Colbert, C. M. & J ohnston, D. (1995) Sec. Neurosci. Abstr. 21, 684.2]}, then <(g)over ba r (Na)>, in the more distal axonal regions is required to be about 40- times that of the soma. (iii) A backward propagating AP in weakly exci table dendrites can be modulated in a graded manner by background syna ptic activity. The functional role of weakly excitable dendrites and a more excitable axon for forward synaptic integration and for backward , global, communication between the axon and the dendrites is discusse d.