Synaptic inputs limit the propagation of action potentials (APs) into the dendrites of neocortical pyramidal neurons in vivo.
Denis Paré, Eric Lang and Alain Destexhe

Society for Neuroscience Abstracts 22: 494, 1996.

To determine if the backpropagation of APs in dendrites occurs in vivo, we performed intracellular recordings of layer V pyramidal neurons located within 2mm of an electrode array that allowed the delivery of electrical stimuli at various depths. Recording pipettes contained K-ac or Cs-ac. The shape of APs varied depending on whether they were triggered synaptically or by current injection. In comparison to current-evoked spikes, orthodromic APs elicited by intracortical stimuli were reduced in magnitude and duration. The magnitude of this reduction was correlated to the stimulation depth with superficial and deep cortical stimuli producing the smallest and largest decrements, respectively. For instance, in Cs-filled cells, spike amplitude amplitude and duration could be reduced by as much as 30% and 90%, respectively. This phenomenon resulted from the fact that stimuli applied at different depth preferentially activated afferents ending at corresponding dendritic levels as demonstrated by shifts in IPSP reversals and related Rin decreases as a function of the stimulation depth. These results were replicated in a computational model based on recent data on the distribution of Na+ channels in the dendrites, soma and axon initial segment of layer V pyramidal cells. Simulated proximal IPSPs were the most effective in preventing the activation of dendritic Na+ channels by current-evoked spikes. Collectively, these findings suggest that synaptic inputs impose significant limitation on the retrograde propagation of action potentials in the dendritic tree.

Supported by MRC, FRSQ and NINDS.