ANTIDROMIC AND SYNAPTIC ACTIVATION OF FROG MOTOR NEURONS

Abstract

The intracellular potentials of frog motor neurons activated by antidromic impulses, by stimulation of dorsal roots and by excitation of the lateral column have been recorded in the isolated spinal cord. Spikes generated over the 3 routes of activation exhibit a notch on their rising phase, indicating the presence of 2 spike components. With double antidromic stimulation the 1st component alone could be obtained. Antidromic spikes were followed either by hyperpolarization and subsequent hypopolarization or by hypopolarization exhibiting a dip. The hyperpolarization or dip was diminished by an increment of the K concentration in the outside medium. Spontaneous excitatory postsynaptic potentials with a uniform time course have been recorded. Dorsal root stimulation with stimuli which were subthreshold for spike discharge elicited temporally dispersed excitatory postsynaptic potentials. On the contrary, the response evoked by lateral column exhibited a fairly high degree of synchronization of excitatory postsynaptic potentials. The threshold level of depolarization of the 1st spike component was higher (9.5 mv.) for responses elicited by dorsal root than by lateral column (8.4 mv.), but the average rate of rise from the origin of the 1st to the origin of the 2d component was practically identical for the 2 routes of excitation. The apparent difference in threshold may be attributed to extrinsic potential changes.