Excitatory and inhibitory effects on human spinal motoneurones from magnetic brain stimulation

Stimulation of baboon motor cortex causes in the motoneurones (MNs) of intrinsic hand muscles monosynaptic excitatory postsynaptic potentials (EPSPs) and a disynaptic inhibitory postsynaptic potential (IPSP). These phenomena have been investigated in human MNs by applying pulsed magnetic stimuli over the scalp at random times during the tonic discharge of single hand muscle motor units (MUs). Post-stimulus time histograms (PSTHs) demonstrated an increased firing probability at between 25 and 35 ms. This major firing peak showed a multimodal form with interpeak intervals of 1.4-1.8 ms. When MUs were not fired by the stimulus, they were nevertheless inhibited from firing spontaneously. There are thus short latency excitatory and inhibitory cortical inputs to human MNs.