RECURRENT FACILITATION OF FROG MOTONEURONS

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Abstract
The central events produced by activation of recurrent collaterals of motor axons in the isolated frog spinal cord have been tested by intracellular recordings (15-18 C). Antidromic impulses in either the ventral root of a given cell or in its neighbor induced transient graded depolarization, having a latency of 2.2 [plus or minus] 0.3, a rise time of 1.4 [plus or minus] 0.4 and a decay time-constant of 3.5 [plus or minus] 1.0 msec. This depolarization was encountered in 73% of tested motor neurons and resulted in spike generation in 45% of responsive cells. It was accompanied by a reduction in threshold to direct excitation having a parallel time-course. It has been identified as an EPSP [excitatory post-synaptic potential] of ventral root origin. The VR-EPSP was not electrically influenced by electrically induced alterations of resting membrane potential. The size of the spike of antidromic invasion was not reduced during the EPSP. Membrane potential changes were poorly sustained during repetitive activation. It was concluded that the recurrent VR-EPSP is initiated in dendritic membrane of the motor neuron, probably by monosynaptic connections. No recurrent inhibitory action has been observed.