[Synaptic interaction of individual motor neurons of the isolated frog spinal cord]

The mode of synaptic transmission between single lumbar motoneurons in isolated spinal cord of frog was investigated by means of parallel penetration of two separate microelectrodes in two neighbouring motoneurons. The synaptic transmission between them was electrically mediated in 82 of 89 cases studied, which was indicated by the lack or very short latency of unitary intermotoneuronal EPSPs and by its amplitude persistence in Ca2+-free, Mn2+-containing solution. Effective electrotonic spread in either direction was demonstrated: both depo- and hyperpolarizing currents passed through an electrode in one motoneuron produced corresponding potentials in coupled motoneuron. The rise and decay of these potentials have much longer time constant as compared to time constant of both coupled motoneurons. The blockade of SD-component of an action potential in "trigger" motoneuron produced a decrease in the EPSP amplitude in the coupled motoneuron. Electrotonic synapses between motoneurons revealed no rectification. In four cases unitary intermotoneuronal EPSPs were chemically mediated as was indicated by their latencies (1.3-3.3 ms) and by their full blockade in CA2+-free, Mn2+-containing solution. The amplitude of this group of EPSPs fluctuated in accordance with binomial or Poisson statistics. In three cases double-component unitary intermotoneuronal EPSPs were recorded, first and second components of which were electrically and chemically mediated, respectively. Morphological structures which could be responsible for the generation of these three groups of unitary EPSPs are considered.