Modeling back propagating action potential in weakly excitable dendrites of neocortical pyramidal cells (compartmental modelyaction potential initiation site)

Simultaneousrecordingsfromthesomaand apicaldendriteoflayerVneocorticalpyramidalcellsofyoung rats show that, for any location of current input, an evoked action potential (AP) always starts at the axon and then propagates actively, but decrementally, backward into the dendrites. This back-propagating AP is supported by a low density (gNa 5 ' 4m S ycm 2 ) of rapidly inactivating voltage- dependent Na 1 channels in the soma and the apical dendrite. Investigationofdetailed,biophysicallyconstrained,modelsof reconstructed pyramidal cells shows the following. (i) The initiationoftheAPfirstintheaxoncannotbeexplainedsolely by morphological considerations; the axon must be more excitable than the soma and dendrites. (ii) The minimal Na 1 channel density in the axon that fully accounts for the experimental results is about 20-times that of the soma. IfgNa in the axon hillock and initial segment is the same as in the soma {as recently suggested by Colbert and Johnston (Col- bert, C. M. & Johnston, D. (1995) Soc. Neurosci. Abstr. 21, 684.2)},thengNainthemoredistalaxonalregionsisrequired to be about 40-times that of the soma. (iii) A backward propagating AP in weakly excitable dendrites can be modu- latedinagradedmannerbybackgroundsynapticactivity.The functional role of weakly excitable dendrites and a more excitable axon for forward synaptic integration and for back- ward, global, communication between the axon and the den- drites is discussed. Dendritesarefine,highlybranchedprocessesthatserveasthe major receptive area for synaptic connections. Most of the input-output functions of the neuron are carried out in the dendritesandthesefunctionsdependcriticallyontheelectrical properties of the dendritic membrane (1, 2). Application of