Effects of inactivation of the anterior interpositus nucleus on the kinematic and dynamic control of multijoint movement

We previously showed that inactivating the anterior interpositus nucleus in cats disrupts prehension; paw paths, normally straight and accurate, becom e curved, hypometric, and more variable. In the present study, we determine d the joint kinematic and dynamic origins of this impairment. Animals were restrained in a hammock and trained to reach and grasp a cube of meat from a narrow food well at varied heights; movements were monitored using the Ma cReflex analysis system. The anterior interpositus nucleus was inactivated by microinjection of the GABA agonist muscimol (0.25-0.5 mg in 0.5 mu L sal ine). For each joint, we computed the torque due to gravity, inertial resis tance (termed self torque), interjoint interactions (termed interaction tor que), and the combined effects of active muscle contraction and passive sof t tissue stretch (termed generalized muscle torque). Inactivation produced significant reductions in the amplitude, velocity, and acceleration of elbo w flexion. However, these movements continued to scale normally with target height. Shoulder extension was reduced by inactivation but wrist angular d isplacement and velocity were not. Inactivation also produced changes in th e temporal coordination between elbow, shoulder, and wrist kinematics. Dyna mic analysis showed that elbow flexion both before and during inactivation was produced by the combined action of muscle and interaction torque, but t hat the timing depended on muscle torque. Elbow interaction and muscle torq ues were scaled to target height both before and during inactivation. Inact ivation produced significant reductions in elbow flexor interaction and mus cle torques. The duration of elbow flexor muscle torque was prolonged to co mpensate for the reduction in flexor interaction torque. Shoulder extension was produced by extensor interaction and muscle torques both before and du ring inactivation. Inactivation produced a reduction in shoulder extension, primarily by reduced interaction torque, but without compensation. Wrist p lantarflexion, which occurred during elbow flexion, was driven by plantarfl exor interaction and gravitational torques both before and during inactivat ion. Muscle torque acted in the opposite direction with a phase lead to res train the plantarflexor interaction torque. During inactivation, there was a reduction in plantarflexor interaction torque and a loss of the phase lea d of the muscle torque. Our findings implicate the C1/C3 anterior interposi tus zone of the cerebellum in the anticipatory control of intersegmental dy namics during reaching, which zone is required for coordinating the motions of the shoulder and wrist with those of the elbow. In contrast, this cereb ellar zone does not play a role in scaling the movement to match a target.