Corticorubrospinal, corticoreticulospinal, and corticovestibulospinal projections contribute to limb and trunk muscle contractions, especially for sustained contractions. Such contractions of muscle are important for stabilizing the trunk and proximal muscles during actions. The reticular and vestibular descending pathways project bilaterally in the ventral and ventrolateral funiculi of the spinal cord, reaching the ven-tromedial zone of the anterior horns to contribute to postural and orienting movements of the head and body and synergistic movements of the trunk and limbs. Kuypers suggested that the interneurons of the ventromedial intermediate zone of spinal gray matter represent a system of widespread connections among a variety of motor neurons, whereas the dorsal and lateral zones, which receive direct corticospinal inputs, are a focused system with a limited number of connections.106
Other corticomotoneurons project directly and by collaterals to the upper medullary medial reticular formation. Their spinal projections overlap the descending reticulospinal pathway to the same spinal cord gray matter in the intermediate zone. Some reticulospinal fibers run from the ventrolateral pons and accompany the corticospinal pathway in the dor-solateral column. Within these pathways, then, potential redundancy exists that could, after an injury, allow partial sparing or reorganization, especially for axial and proximal movements. Indeed, a patient with severe hemiparesis who cajoles minimal voluntary flexion in the affected leg or extension of the elbow, wrist, and fingers may rely on corticorubrospinal and cor-ticoreticulospinal pathways for functional hip and knee flexion during walking or extension for reaching to an object and on corti-covestibulospinal projections for leg extension and postural control.
Corticospinal fibers from M1, SMA, and BA
24 and 2/5 project to the parvicellular nucleus of the red nucleus, which sends most of its output to the olivary nucleus. This half of the red nucleus apparently has only modest connections to its lower half, the somatotopically arranged magnocellular nucleus. The magno-cellular division of the red nucleus also receives cortical sensory inputs. Magnocellular neurons create the rubrospinal tract, which crosses and intermingles with corticospinal fibers. The rubrospinal fibers terminate on interneurons and directly on some motoneurons in the dor-solateral intermediate zone of the ventral horn, where they contribute to motor control of the limbs. Similar movement-related cell discharges occur in M1 and in magnocellular cells, as well as in the basal ganglia and cerebellum, which points to their close functional relationship. The neurons are tuned to particular directions of movement, best worked out for hand reaching. These cells respond to skin touch and joint movements. The red nuclei help control the extremities and digits for skilled steering and fractionated movements. These midbrain neurons, which also receive cerebel-lar projections, may independently subserve some aspects of the motor control for the distal arm after a hemispheric injury.109
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