Transmission in the spinal reciprocal Ia inhibitory pathway preceding willed movements of the human wrist.

Transmission in the Ia inhibitory pathway from wrist extensor muscles onto flexor motoneurones was investigated in the 160 ms period prior to a willed movement in 6 human subjects. H-reflexes were evoked in flexor muscles at various times before the onset of electromyographic activity. Disynaptic inhibition of the H-reflex was produced by low intensity stimulation of the antagonist radial nerve. Prior to a wrist flexion movement, transmission in the inhibitory pathway was diminished. Prior to an extension movement it was facilitated. The findings are consistent with the notion that the command to the agonist motoneurones acts also on a specific population of Ia inhibitory interneurones.