Voluntary Activity, Readiness Potential, and Motor Program

Abstract

Preceding all kinds of voluntary movement in humans there is a surface negative potential, called Bereitschaftspotential (BP), starting about 800 msec before the onset of a simple finger movement. Its maximum is over the anterior midline of the head. This midline localization does not result from bilateral summation but from a paralimbic midline source, the supplementary motor area (SMA). The BP ends about 90 msec before a simple fast finger movement; this end is called premotion positivity (PMP). About 55 msec before the onset of the forearm muscle activity a well-localized potential begins over the precentral motor cortex hand area, called motor potential (MP), which reflects the firing onset of the pyramidal tract neurons. Between PMP and MP subcortical mechanisms precede the motor cortex activity. This subcortical activity (mainly of the basal ganglia, the cerebellum, and some brain stem nuclei) seems necessary to provide the spatiotemporal functions and programs for self-generated activity (in contrast to stimulus-dependent movements). The precentral motor cortex is probably unable to generate voluntary movements without cooperation from subcortical ganglia. The function of the motor cortex is likely to be advanced tactile and proprioceptive adjustment of the finger, hand, toe, lip, and tongue movements. The same tongue movement, however, which during chewing is regulated by the motor cortex, is guided during speech production by Wernicke’s cortex of the temporal lobe. Although eye movements had an evolution comparable to finger movements, they did not enter the motor cortex, but stayed in the midbrain tectum, with help from visual and motivational mechanisms of the forebrain. Thus, the system of voluntary movement is a highly distributed system with several alternative pathways. Despite this decentralization, however, the maximum Bereitschaftspotential preceding all kinds of voluntary movement including speech and eye movement is over the SMA. This led in 1980 to the idea that the SMA is one way that channel drives and will into action.