Small depolarizing wavelets evoked in extraocular motoneurons by intracellular stimulation

Repetitive, small, depolarizing wavelets are evoked by intracellular constant current stimulation of antidromically identified extraocular motoneurons in the anesthetized and paralyzed cat. These small depolarizing wavelets vary in amplitude (1-4 mV) and duration (1.5-2.5 msec) and are most prominent in motoneurons with resting membrane potentials greater than 65 mV which also discharge repetitively to sustained depolarizing currents. They may attain threshold for a full size spike discharge. However, the previous occurrence of a full size spike is not essential for their production. These wavelets are not seen during the passage of hyperpolarizing currents through the intracellular electrode, nor are they seen when no current is being applied. These observations, coupled with the known absence of recurrent collaterals of extracular motoneurons indicate that the extraocular are not post synaptic potentials. The small depolarizing wavelets do not collide with antidromic spikes, indicating that they are not attenuated initial segment spikes nor are they axon spikes. It is suggested that these wavelets are generated in the dendrites of extraocular motoneurons. Such dendritic spikes might constitute an essential step in the production of sustained high frequency discharge of extraocular motoneurons.