The independence of corticomotoneuronal and fusimotor pathways in the production of muscle contraction by motor cortex stimulation.

1. The response of thirty spindle afferents was studied during cortical stimulation of the ;best point' for eliciting a contraction of tibialis anticus in baboons anaesthetized with N(2)O/O(2) mixtures supplemented by small doses of barbiturates.2. No evidence of fusimotor activity before a contraction of the muscle was found. Evidence of fusimotor activity during the contraction and after the cortical stimulus was found. The various problems associated with interpretation of this activity are discussed.3. Depression of the spindle afferent response by tetanic cortical stimulation, which produced no detectable tension change, was found for four spindle afferents. This depression was particularly well documented for one spindle afferent. The depression could be explained by a decrease of static fusimotor activity.4. The spindle afferent response to muscle stretch was studied during light anaesthesia. The effect of barbiturates on this response suggested a depression of static fusimotor activity.