Abstract
Lesions of the ventrolateral nucleus (VLN) of the thalamus relieve limb hypertonia in humans through mechanisms that are essentially unknown. To test the hypothesis that a decrease of muscle spindle activity may be involved, experiments were performed to determine the effects of lesions in VLN on the responses to muscle extension of medial gastrocnemius spindle primary afferents in the cat. The effect of pulvinar lesions was also studied. The lesions were produced by a stereotaxically placed cryogenic probe. Ventrolateral nucleus lesions significantly depressed spindle afferent responses with probe tip temperatures of —20 C, but lesions of the pulvinar did not depress spindle afferent responses despite cooling to —60 C. It is concluded that the VLN has a net excitatory effect on hindlimb extensor fusimotor neurons. This finding is compatible with the view that a depression of fusimotor activity after VLN lesions is an important mechanism underlying the beneficial effects of such lesions in humans with limb hypertonia.
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