The unique inhibitory potentials in motoneurons that occur during active sleep are comprised of minimal unitary potentials

Loss of muscle tone during active (rapid-eye-movement, REM) sleep is due to the inhibition of motoneurons. This inhibition is manifest in high-gain intracellular electrophysiological records as hyperpolarizing synaptic noise, which includes large amplitude active sleep-specific inhibitory postsynaptic potentials (IPSPs). We report here evidence that the large active sleep-specific IPSPs are comprised of a small number of minimal unitary potentials that are characterized by fast rise-times (10-90% rise-times < or = 0.75 ms); they are present in high-gain records during quiet sleep or during active sleep where they are intermingled with larger active sleep-specific IPSPs with 10-90% rise-times > or = 1.00 ms and amplitudes that are integer multiples of the minimal unitary potentials. In hypoglossal motoneurons, the amplitude of these minimal unitary potentials averaged 0.33 +/- 0.04 mV (mean +/- S.D., n = 6). It is concluded that the large IPSPs with slow rise-times that are observed in motoneurons during active sleep are due to the nearly simultaneous arrival of multiple (< or = 5) minimal unitary potentials. We hypothesize that the same inhibitory interneurons that produce small IPSPs with fast rise-times during quiet sleep are also responsible for the large amplitude active sleep-specific IPSPs.