Inactivation of interposed nuclei in the cat: classically conditioned withdrawal reflexes, voluntary limb movements and the action primitive hypothesis

The cerebellar interposed nuclei are considered critical components of circuits controlling the classical conditioning of eyeblink responses in several mammalian species. The main purpose of the present experiments was to examine whether the interposed nuclei are also involved in the control of classically conditioned withdrawal responses in other skeletomuscular effector systems. To achieve this objective, a unique learning paradigm was developed to examine classically conditioned withdrawal responses in three effector systems (the eyelid, forelimb and hindlimb) in individual cats. Trained animals were injected with muscimol in the cerebellar interposed nuclei, and the effects on the three conditioned responses (CRs) were examined. Although the effects of muscimol were less dramatic than previously reported in the rabbit eyeblink preparation, the inactivation of the cerebellar nuclei affected the performance of CRs in all three effector systems. In additional experiments, animals were injected with muscimol at the sites affecting classically conditioned withdrawal responses to determine the effects of these injections on reaching and locomotion behaviors. These tests demonstrated that the same regions of the cerebellar interposed nuclei which control withdrawal reflexes are also involved in the control of limb flexion and precision placement of the paw during both locomotion and reaching tasks. The obtained data indicate that the interposed nuclei are involved in the control of ipsilateral action primitives and that inactivating the interposed nuclei affects several modes of action of these functional units.