A quantitative assessment of the maturation of phasic motor inhibition during REM sleep

During rapid eye movement (REM) sleep, phasic and further motor inhibition occurs during clusters of REMs besides tonic motor inhibition. We describe the age-related quantitative change of the activity of this REM-related phasic motor inhibition. For this purpose, we introduced the phasic inhibition index (PII). PII is the rate of simultaneous occurrence of bursts of horizontal REMs and phasic mentalis muscle activity during REM sleep. We examined these phasic REM sleep parameters in 87 healthy children from premature babies to preadolescents. The incidence of bursts of REMs showed no age-related change, while that of the phasic mentalis muscle activity increased with age. The simple ratio between the incidence of bursts of REMs and that of phasic mentalis muscle activity showed no significant age-related change, whereas PII decreased rapidly during infancy and reached low constant values thereafter. We concluded that this age-related PII decline reflected the maturation of REM-related phasic motor inhibition. This is the first quantitative description on the development of human motor inhibition. Taken with the neuronal basis underlying REM-related phasic motor inhibition, we hypothesize that a PII value is within the normal low range as far as both the rostral pontine tegmentum and the brainstem inhibitory pathways are functionally intact.