REPETITIVE FIRING TO CURRENT IN CAT MOTONEURONS AS A FUNCTION OF MUSCLE UNIT TWITCH TYPE

Spinal cord motoneurons innervating fast and slow twitch muscle units in the lateral and medial gastrocnemius, soleus, and plantaris muscles of the cat were investigated with regard to repetitive firing behavior with intracellular stimulation. Approximately two-thirds of the motor units had fast twitch muscle units and one-third had slow twitch units. Slightly more than half the fast twitch and all the slow twitch motoneurons fired repetitively to the intracellular injection of a current step. In the fast twitch group, the cells with antidromic action potentials greater than 72 mv that did and did not repetitively fire could not be separated on the basis of twitch time to peak or peak amplitude of the single twitch. There was, however, a significantly higher rheobase in the fast twitch cells that did not fire repetitively. © 1969.