Possible role of backpropagating action potentials in corticospinal neurons in I-wave periodicity following a TMS pulse.

A single pulse of TMS or direct electric stimulation over M1 causes repetitive synchronized firing of corticospinal tract (CST) neurons. Two mechanisms for the repetitive firing have been proposed: a cascade of synaptic inputs to the pyramidal neurons and a single reverberating circuit of interneurons. Here, we propose another possibility in which bursting of CST neurons is produced by dendritic Ca-spikes. Backpropagation of the initial action potential (I1-wave) from the soma interacts with synaptic input in the dendrites to initiate a dendritic calcium spike. These Ca-spikes produce a burst of somatic action potentials that starts about 1.5 ms after the initial discharge of the neuron, which may produce the later I-waves.