CORTICAL CONTROL OF HUMAN MOTONEURON FIRING DURING ISOMETRIC CONTRACTION

We recorded whole scalp magnetoencephalographic (MEG) signals simultan eously with the surface electromyogram from upper and lower limb muscl es of six healthy right-handed adults during voluntary isometric contr action. The 15- to 33-Hz MEG signals, originating from the anterior ba nk of the central sulcus, i.e., the primary motor cortex, were coheren t with motor unit firing in all subjects and for all muscles. The cohe rent cortical rhythms originated in the hand motor area for upper limb muscles (1st dorsal interosseus, extensor indicis proprius, and bicep s brachii) and close to the foot area for lower limb muscles (flexor h allucis brevis). The sites of origin corresponding to different upper limb muscles did not differ significantly. The cortical signals preced ed motor unit firing by 12-53 ms. The lags were shortest for the bicep s brachii and increased systematically with increasing cortico-muscula r distance. We suggest that the motor cortex drives the spinal motoneu ronal pool during sustained contractions, with the observed cortical r hythmic activity influencing the timing of efferent commands. The cort ical rhythms could be related to motor binding, but the rhythmic outpu t may also serve to optimize motor cortex output during isometric cont ractions.