Thresholds of cortical activation of baboon alpha and gamma-motoneurones during halothane anaesthesia.

1. In the halothane anaesthetized baboon the threshold for cortically elicited primary and secondary spindle afferent discharge was compared to the threshold at the ;best point' for a cortically elicited tibialis anticus tension change.2. The cortical threshold for eliciting a tension change was greater than the cortical threshold for eliciting spindle afferent discharge for some afferents and less for other afferents.3. The pattern of cortically elicited spindle afferent discharge during muscle stretch suggested static gamma-motoneurone activation. No convincing evidence of dynamic gamma-motoneurone activation was seen.4. Inhibition of spindle afferent discharge was occasionally found and the ;best point' was the same as the cortical ;best point' for eliciting monosynaptic alpha-motoneurone activity. This result suggested that the cortical projections which inhibit the pre-existent activity of static gamma-motoneurones are in a close anatomical relation to those cortical projections which elicit activity of alpha- and static gamma-motoneurones.