The tonic glycinergic drive present at the trigeminal motor pool in NREM sleep is abruptly switched off in REM sleep. A , Typical EMG and EEG traces showing how left masseter muscle (LM) activity changes during the transitions into and out of REM sleep during baseline (i.e., aCSF at the trigeminal nucleus). B , Another trace from the same rat showing that strychnine perfusion increases LM activity during the NREM and waking periods preceding and after REM onset (compared with traces in A ), but that masseter atonia persists even though glycine receptors are blocked. C , A typical raw trace showing the abrupt loss of masseter tone on entrance into REM sleep, despite strychnine perfusion at the trigeminal nucleus. D , Group data showing that compared with baseline, strychnine perfusion significantly increased LM activity in NREM and post-REM waking (QW), but not during REM sleep. A.U., Arbitrary units. All values are mean ± SEM.