Excitability of corticospinal neurons during tonic muscle contractions in man.

A magnetic stimulus applied to the human scalp over the motor cortex causes a short latency contraction of contralateral limb muscles. This is presumed to result from the indirect excitation of corticospinal neurons with monosynaptic connections to motoneurons. The excitability of these cortical neurons can be estimated from the magnitude of the postsynaptic potentials produced in spinal motoneurons by a given magnetic stimulus. In man the characteristics of these postsynaptic potentials can be derived from changes in the firing probability of single motor units. When a subject increases the level of a sustained voluntary contraction the excitability of the corticospinal neurons estimated in this way becomes less. We conclude that the additional synaptic input to motoneurons required to maintain a stronger muscle contraction comes from fiber systems other than the population of fast corticospinal neurons activated by magnetic stimulation.