Motoneuronal excitability during wakefulness and non-REM sleep: H-reflex recovery function in man

Comparisons were made in 13 normal young adults of alpha motoneuronal excitability during non-rapid-eye-movement (NREM) sleep (stages 2--4) and wakefulness based on a study of the recovery cycle of the H-reflex after a conditioning stimulus. During sleep, excitability was generally decreased and a peak of secondary facilitation, which characteristically occurs 100--300 msec after the conditioning stimulus during wakefulness, was reduced or absent. A variety of mechanisms, including long-loop reflexes and afferent discharges from reflex muscle contraction and cutaneous fibers, has been proposed to account for this phase of facilitation in the waking recovery curve. However, since studies to date indicate the absence of tonic presynaptic or postsynpatic inhibitory influences acting on the monosynatpic reflex pathway during NREM sleep, but report tonic reduction in fusimotor activity at this time, it is suggested that a primary factor underlying the absence of facilitation during sleep is a reduction in central excitability deriving from depressed fusimotor activity.