Phasic changes in motoneuron membrane potential during REM periods of active sleep.

Most of the phasically occurring periods of rapid eye movements (REMs) of active sleep are accompanied by enhanced suppression of somatomotor activity; however, during some of the REM episodes there are muscular twitches and jerks. The membrane potential changes underlying these motor processes were examined by recording intracellularly from lumbar motoneurons in cats that were undrugged, unanesthetized and normally respiring. Summated hyperpolarizing potentials were evident during REM episodes in conjunction with a decrease in motoneuron excitability. During other episodes of REMs there occurred summated depolarizing potentials which occasionally produced action potentials. These depolarizing events were in most cases preceded by a brief period of hyperpolarization. Thus, it appears that there is inhibitory input to lumbar motoneurons during all REM periods of active sleep; in some episodes the simultaneous coactivation of excitatory input leads to depolarization of the membrane and action potential generation.