Membrane potential, synaptic activity, and excitability of hindlimb motoneurons during wakefulness and sleep.

1. The membrane potential and excitability of hindlimb motoneurons was recorded in chronically implanted, restrained cats during natural sleep and wakefulness. 2. The potential of all motoneurons varied in a systematic and predictable manner during the sleep-wake cycle, suggesting that all motoneurons contributed to variations in muscle tension across the cycle. 3. Motoneuron excitability and level of polarization did not change at sleep onset. 4. During REM sleep, motoneurons had a sustained hyperpolarization, a diminished probability of antidromic and monosynaptic activation, and a reduced responsiveness to depolarizing current pulses. Motoneuron excitability is thus reduced in this phase of sleep. 5. Synaptic activity, but not necessarily synaptic noise, was higher during REM sleep. Hindlimb motoneuron hyperpolarization in REM sleep in concluded to arise from augmented asynchronous inhibitory synaptic activity distributed on the soma.