Separate cell systems in the motor cortex of the monkey for the control of joint movement and of joint stiffness.

A series of new observations is described from experiments with alert monkeys, who have been trained to control the position of the wrist in the presence of perturbing forces. The results suggest that the wrist control area of the precentral motor cortex contains separate neuronal populations for the control of 2 major forms of motor output. The first activates particular groups of synergists independently, or the members of a set of antagonist muscles reciprocally; this form of activation provides, under natural conditions, for smooth movement of the joint from one position to another. The second population of cells evokes co-activation of antagonist muscles, at one or more joints in the arm. This population provides for the cortical control of the postural supporting reactions which must occur in the arm when one or more of its joints are moved. In addition, it appears to provide for a separate control of joint stiffness, thus allowing for a control of joint position in the presence of very rapid or unpredictable external force perturbations. A possible role for this system as a means of joint position control during the early phases of motor learning is also discussed.