The Kugelberg lecture. Brain mechanisms of voluntary motor commands--a review

At a time when both electrical and magnetic stimulation of the human brain are being used to assess cortical motor outflow in man, it is important to re-examine the organization of the motor areas of the cerebral cortex, the evidence which exists about the structures that are activated by epicortical stimulation, the nature of the projections from the cerebral cortex to lower motor centers, the physiological influences that are exerted on motoneurons and interneurons by these descending pathways and the relevance of all of these for the control of voluntary movement. This review examines the relationships of neuronal activities in some motor regions of the cerebral cortex to movement performance as revealed by recording of neuronal discharges during self-paced movements performed by conscious monkeys, and the relevance of these observations to the understanding of mechanisms of voluntary control of skilled movement performance in man. It documents detailed information about the precise connections made by descending cortico-motoneuronal fibers in the monkey and proposes a relationship between the analogous structural arrangements in man and the voluntary control of skilled movement. Comparison of some of the general propositions which derive from studies of cortico-motoneuronal commands in the monkey with the EMG results from magnetic stimulation in man leads to the suggestion that direct monosynaptic excitatory effects may be relevant to the control of both proximal and distal muscles in man. Electrical recordings of the neuronal activities of individual cells in strategic cytoarchitectonic areas of the monkey's cerebral cortex also provide a neurophysiological correlate of observations on regional cerebral blood flow and regional metabolism as these are studied in human subjects performing movement tasks. Hence it is possible to explain the involvement of supplementary motor areas of both hemispheres in the organization of the time-ordered commands for manual tasks. A deficit in bimanual coordination is evident in monkeys with a unilateral lesion of the supplementary motor area and this deficit mimics the disorder of voluntary control of bimanual manipulation reported in man.