Two types of electronic EPSP evoked in amphibian motoneurons by ventral root stimulation

Motoneurons of hemisected perfused frog spinal cord were used for intracellular recording of EPSP produced by ventral root stimulation. Two varieties of electrotonic VR-EPSP resistant to Ca-lack and to addition of 10 mM Mg2+ and 2 mM Mn2+ have been encountered. The early VR-EPSP started before the full somadendritic invasion and the time constant of their decay varied between 11 and 25 msec, mean 15.4 +/- 1.8 msec, i.e., it was significantly longer than the time constant of the motoneuronal membrane (2.6--6.9 msec, mean 4.4 +/- 0.5 msec). The late VR-EPSPs appeared after a latent period which was significantly longer than that of an antidromic spike. They started after the full somadendritic invasion and had very rapid time course, the time constant of their decay approximated that of the motoneuronal membrane. The shape of the late VR-EPSP both before and after treatment with 4-AP of TEA which markedly facilitated its appearance resembled the shape of an antidromic spike, whereas this correlation was not found in the case of the early VR-EPSP. When the temperature was elevated above 18--19 degree C the late VR-EPSPs disappeared but the early VR-EPSPs still could be recorded. The available evidence suggests that the early VR-EPSPs are produced by direct synaptic interconnections between motoneurons and recurrent collaterals of the motor axons, and the VR-EPSPs are transmitted across dendrodendritic junctions.