Transmission of the cortical command for human voluntary movement through cervical propriospinal premotoneurons

In man there is now strong evidence for the existence of a system of cervical propriospinal premotoneurons interposed in the corticospinal pathway to upper limb motoneurons. The relevant neurons are located rostral to upper limb motoneurons. They receive both weak excitation and strong inhibition from low threshold afferents in the upper limb. Ongoing EMG activity recorded in wrist and elbow extensors during voluntary movements of different types (tonic contraction, reaching) is suppressed by a cutaneous volley evoked by stimulation of the superficial radial nerve. There is evidence that this cutaneous-induced suppression is due to inhibition of transmission of the cortical command in the system of cervical propriospinal premotoneurons, thus indicating that part of the descending command for movement reaches the relevant motoneurons via these premotoneurons. In patients with hemiplegia, the cutaneous-induced depression of ongoing EMG activity is significantly larger on the hemiplegic side. This suggests that the mechanisms underlying residual motricity after a lesion of the corticospinal tract involve the system of propriospinal premotoneurons.