Control of Movement

Four major pathways descend from the brain to the spinal cord to control muscles that move the skeleton. These pathways arise in the vestibular nuclei (vestibulospinal), in the brainstem reticular formation (reticulospinal), in the red nucleus (rubrospinal), and in the cerebral cortex (corticospinal). While these four descending pathways work together to provide seamless control of movements from postural reflexes to delicate manipulation, they can be separated broadly into two main systems. Vestibulospinal and reticulospinal pathways can be grouped together as a medial system: their axons descend through the brainstem and spinal cord close to the midline, and chiefly innervate axial and proximal limb musculature in which motoneurons lie medially in the ventral horn. The corticospinal and rubrospinal pathways together can be considered a lateral system: their axons descend in the lateral column of the spinal cord and chiefly innervate limb musculature, particularly distal musculature, in which motoneurons lie laterally in the ventral horn. More than just an anatomical distinction, the medial and lateral systems have different principal functions. The medial system provides postural control. Monkeys with medial system lesions fall over when they attempt to ambulate and climb, but when supported, they can use their hands and fingers adeptly in retrieving food pieces from narrow holes (Lawrence & Kuypers, 1968b). The lateral system provides fine control of voluntary movement. Monkeys with lateral system lesions rapidly recover the use of all four extremities in activities such as ambulation and climbing, but they are unable to make the fine finger movements needed to extract small pieces of food from narrow holes (Lawrence & Kuypers, 1968a). This chapter examines the contributions of first the medial and then the lateral systems to control of movement.