Forward and Backward Propagation of Dendritic Impulses and Their Synaptic Control in Mitral Cells

The site of impulse initiation is crucial for the integrative actions of mammalian central neurons, but this question is currently controversial. Some recent studies support classical evidence that the impulse always arises in the soma-axon hillock region, with back-propagation through excitable dendrites, whereas others indicate that the dendrites are sufficiently excitable to initiate impulses that propagate forward along the dendrite to the soma-axon hillock. This issue has been addressed in the olfactory mitral cell, in which excitatory synaptic input is restricted to the distal tuft of a single primary dendrite. In rat olfactory bulb slices, dual whole cell recordings were made at or near the soma and from distal sites on the primary dendrite. The results show that the impulse can be initiated in either the soma-axon hillock or in the distal primary dendrite, and that the initiation site is controlled physiologically by the excitatory synaptic inputs to the distal tuft and inhibitory synaptic inputs near the soma.