Motoneuron properties during electromyogram pauses in sleep.

Electrophysiological antecedents to electromyogram pauses of NREM sleep were studied intracellularly in hindlimb motoneurons. Phasic-event-related hyperpolarizing potentials (PRHP) were 0.2-7.1 mV in amplitude and 9-90 ms in duration, and coincided with dorsal neck EMG pauses and PGO waves. Motoneurons, if spontaneously discharging, were silenced during PRHPs. The discharge rate of spike trains elicited by depolarizing current reduced during PRHPs. Membrane conductance measurements indicated that PRHPs did not arise simply from soma inhibition but from either presynaptic inhibition or conductance changes in electrically-remote dendrites. EMG pauses, which normally occur spontaneously, could be evoked by stimulation of low threshold fibers in peripheral nerves. PRHPs were most prevalent in REM sleep. We concluded that EMG pauses are generated by the same cholinergic brainstem mechanism that generates spino-bulbo-spinal presynaptic inhibition of group Ia afferents. The pauses resemble the inhibitory phase of startle responses, providing evidence for the hypothesis that startle responses are spontaneously elicited in NREM and REM sleep.