Recent evidence for plateau potentials in human motoneurones.

Motoneurones in reduced animal preparations can exhibit plateau potentials that amplify their response to synaptic inputs and can persist for prolonged periods in the absence of synaptic drive. There is mounting evidence that a similar mechanism may be an integral part of the normal activation of motoneurones. Some of the work describing plateau potentials in reduced animal preparations is reviewed and then evidence that similar properties contribute to the normal activation of motoneurones in rats and humans is presented. Recent data have shown that during high-frequency electrical stimulation over human muscle, large contractions can develop which originate within the central nervous system and are present in addition to the contraction due to the direct activation of motor axons. These "extra" contractions may in part be due to plateau potentials in spinal motoneurones. It is becoming clear that intrinsic properties of human motoneurones may make a large contribution to muscle contractions during normal movements. The extent to which the nervous system uses this as a gain control mechanism to tailor motor output for a given task needs to be further explored.