Spikes Initiate in the Axon
Figure 4. This figure illustrates action potentials recorded from the cell body of spinal motoneurons. Below each trace is the first derivative of the action potential. Note the presence of two prominent components. The first component comes from the initial segment of the axon and is known as the "IS" spike. The second component comes from the soma and is known as the "SD" spike. These investigators demonstrated that spikes originate in the axon initial segment and this back-propagates into the soma and activates the local generation of a spike - giving rise to the two components of the spike and its derivative. From "The Physiology of Synapses", by Sir John Eccles, 1964. Modified from Coombs, Curtis and Eccles, J. Physiol. 139: 232 (1957).