A new cellular mechanism for coupling inputs arriving at different cortical layers

Pyramidal neurons in layer 5 of the neocortex of the brain extend their axo ns and dendrites into all layers. They are also unusual in having both an a xonal and a dendritic zone for the initiation of action potentials(1-6). Di stal dendritic inputs, which normally appear greatly attenuated at the axon , must cross a high threshold at the dendritic initiation zone to evoke cal cium action potentials(1,7) but can then generate bursts of axonal action p otentials. Here we show that a single back-propagating sodium action potent ial generated in the axons(8) facilitates the initiation of these calcium a ction potentials when it coincides with distal dendritic input within a tim e window of several milliseconds. Inhibitory dendritic input can selectivel y block the initiation of dendritic calcium action potentials, preventing b ursts of axonal action potentials. Thus, excitatory and inhibitory postsyna ptic potentials arising in the distal dendrites can exert significantly gre ater control over action potential initiation in the axon than would be exp ected from their electrotonically isolated locations. The coincidence of a single back-propagating action potential with a subthreshold distal excitat ory postsynaptic potential to evoke a burst of axonal action potentials rep resents a new mechanism by which the main cortical output neurons can assoc iate inputs arriving at different cortical layers.