جهد الفعل
Action potentials initiated in the axon normally travel down the axon away from the soma. However, it is also possible for an action potential to travel in the opposite direction, invade the soma, and then travel down the dendrite as a dendritic spike. This retrograde signal provides information to the synapse that the neuron has fired an output. The efficacy of the signal varies among different neuronal types. For example, backward propagation of action potentials is very limited in cerebellar Purkinje cells but is quite prevalent in interneurons of the medium ganglionic layer of the cerebellum-like lobe of some fish.