REM sleep atonia: from basic background to clinical application.

Muscle tone is profoundly suppressed during rapid-eye-movement sleep (REMS). Two indices that quantify this muscle activity suppression were introduced; the tonic inhibition index (TII) and the phasic inhibition index (PII). TII expresses the shortness of phasic chin muscle activity, and PII indicates the weakness of rapid eye movements-related phasic chin muscle activity loss. TII increased significantly with age, while PII decreased significantly. These chronological changes were concluded to indicate that the activity of the human nervous systems involved in both tonic and phasic inhibition in REMS increases with age. TII was found to reach the adult level at 12.3 years of age, while PII decreased to the adult value at 0.4 years. According to this difference in age between their maturation, the human nervous systems involved in muscle activity suppression during REMS are hypothesized to comprise at least 2 independent systems. TII and PII are also hypothesized to be affected by the activity of the brainstem inhibitory/facilitatory centers, which might be implicated in the control of muscle activity during wakefulness as well.