A tonic glycinergic/GABAergic drive at the trigeminal motor pool functions to suppress masseter tone in waking and NREM sleep, but not during tonic REM sleep. A , Typical EMG and EEG traces illustrating that strychnine/bicuculline perfusion into the left trigeminal motor pool increases left masseter muscle (LM) tone during waking and NREM sleep; however, it has no effect on masseter atonia in tonic REM sleep despite provoking masseter muscle twitch activity in phasic REM. B , Group data (n = 11) showing that strychnine/bicuculline perfusion increases LM tone in AW and QW and in NREM sleep, without changing levels of motor tone during tonic REM sleep. C , Group data showing strychnine/bicuculline perfusion into the left trigeminal nucleus increases masseter muscle twitch duration in phasic REM sleep, without significantly affecting either twitch amplitude or frequency. *p < 0.05. A.U., Arbitrary units; #/s, number muscle twitches per second. All values are mean ± SEM.