PRIMARY MOTOR CORTEX INFLUENCES ON THE DESCENDING AND ASCENDING SYSTEMS

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures go verning eye, head, neck, trunk and limb musculature. Pyramidal tract a xons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory asc endant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres th at originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, th e great majority of the corticospinal neurones that emit multiple intr acraneal collateral branches terminate at the cervical spinal cord lev el. The pyramidal tract fibres directed to the dorsal column nuclei th at send collateral branches to supraspinal structures also show a clea r tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons a t both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor respo nses involving eye-neck-forelimb muscle synergies. The review makes sp ecial emphasis in the available evidence pointing to the role of the m otor cortex in co-ordinating the activities of both descending and asc ending pathways related to somatomotor integration and control. The mo tor cortex may function to co-operatively select a unique motor comman d by selectively filler sensory information and by co-ordinating the a ctivities of the descending systems related to the control of distal a nd proximal muscles. (C) 1997 Elsevier Science Ltd.