How are motor primitives regulated? - origins of control signal over spinal circuitry -
It is an intriguing idea that modular basic sets, or a collection of ‘primitives’ are implemented in the central nervous system for motor control, since such 'motor building blocks' arguably deal with a long-standing problem on the degree of freedom arising from the redundancy in motor apparatus.
Recently it is proposed that the motor primitive may be expressed as force fields generated by multiple joints, and further, multiple muscle activation patterns, or 'muscle synergies'. These primitives or synergies were experimentally demonstrated to be built into the spinal cord in lower vertebrates such as frogs. In mammals, similar muscle synergies were shown to be extracted by means of statistical decomposition analyses of electromyography.
However, it still needs to be elucidated how such synergistic organization in the spinal cord is embodied in interneuronal circuitry, and further, what pathway drives (i.e. premotor drives) the circuitry to achieve complicated and flexible motor control for primates including humans. Of particular interest is the issue as to which descending tract among corticospinal, rubrospinal and reticulospinal tracts plays particular roles in commanding premotor drives for a variety of components of movement. To tackle this issue, we observe the interneuronal activity in relation to multiple muscle activities, and specify inputs to such neurons from multiple descending tracts using electrophysiological techniques in a behaving monkey.