Patterns of cortical projection to hindlimb muscle motoneurone pools.

This study was undertaken to examine the organization of the hindlimb area of the motor cortex. Two specific questions were posed. The first was: are the cortical neurones which control the excitability of a given motoneurone pool localized in a small zone of corjex or are they diffuse? The second question was: does microstimulation in the hindlimb area of the motor cortex activate spinal motoneurones in a reciprocal fashion, i.e., is cortically elicited facilitation of a muscle accompanied by inhibition of the antagonist? Intracortical microstimulation was used to condition monosynaptic reflexes of the cat hindlimb to study the organization of a cortical projection to lumbar motoneurone pools. Cortical neurones which produced facilitation or inhibition of a given monosynaptic reflex were localized within a small zone of the cortex. Facilitatory and inhibitory effective zones were found to have similar shape and size. Intracortical microstimulation elicited facilitation or inhibition of individual monosynaptic reflexes without eliciting reciprocal effects on the antagonists. The pyramidal tract was shown to play an important role in the mediation of cortically elicited facilitation as well as inhibition of the monosynaptic reflexes in the lumbar cord.