Hr. Luscher et Me. Larkum, MODELING ACTION-POTENTIAL INITIATION AND BACKPROPAGATION IN DENDRITESOF CULTURED RAT MOTONEURONS, Journal of neurophysiology, 80(2), 1998, pp. 715-729
Regardless of the site of current injection, action potentials usually
originate at or near the soma and propagate decrementally back into t
he dendrites. This phenomenon has been observed in neocortical pyramid
al cells as well as in cultured motoneurons. Here we show that action
potentials in motoneurons can be initiated in the dendrite as well, re
sulting in a biphasic dendritic action potential. We present a model o
f spinal motoneurons that is consistent with observed physiological pr
operties of spike initiation in the initial segment/ axon hillock regi
on and action potential back-propagation into the dendritic tree. It a
ccurately reproduces the results presented by Larkum et al. on motoneu
rons in organotypic rat spinal cord slice cultures. A high Na+-channel
density of (g) over bar(Na) = 700 mS/cm(2) at the axon hillock/initia
l segment region was required to secure antidromic invasion of the som
ato-dendritic membrane, whereas for the orthodromic direction, a Na+-c
hannel density of (g) over bar(Na) = 1,200 mS/cm(2) was required. A ''
weakly'' excitable ((g) over bar(Na) = 3 mS/ cm(2)) dendritic membrane
most accurately describes the experimentally observed attenuation of
the back-propagated action potential. Careful analysis of the threshol
d conditions for action potential initiation at the initial segment or
the dendrites revealed that, despite the lower voltage threshold for
spike initiation in the initial segment, an action potential can be in
itiated in the dendrite before the initial segment fires a spike. Spik
e initiation in the dendrite depends on the passive cable properties o
f the dendritic membrane, its Nai-channel density, and local structura
l properties, mainly the diameter of the dendrites. Action potentials
are initiated more easily in distal than in proximal dendrites. Whethe
r or not such a dendritic action potential invades the soma with a sub
sequent initiation of a second action potential in the initial segment
depends on the actual current source-load relation between the action
potential approaching the soma and the electrical load of the soma to
gether with the attached dendrites.