Reciprocal and Renshaw (recurrent) inhibition are functional in man at birth.

The aims were (1) to determine when in human postnatal development Group Ia reciprocal and Renshaw inhibition can be demonstrated; (2) to explore the relationship between the expression reciprocal inhibition and the disappearance of Group Ia excitatory reflexes between agonist and antagonist muscles. Studies were performed on 99 subjects, aged 1 day to 31 years, of whom 53 were neonates. A longitudinal study was also performed on 29 subjects recruited at birth and studied 3 monthly until 12 months of age. Reciprocal inhibitory and excitatory reflexes were recorded in the surface EMG of contracting biceps brachii (Bi), evoked by taps applied to the tendon of triceps brachii (Tri). Reciprocal excitatory reflexes were recorded in all but one neonate. Reciprocal inhibition was observed in 25% of neonates; evidence is provided that it was likely to have been masked by low threshold reciprocal excitation in the remaining neonates. Reciprocal inhibition was demonstrated in all subjects after 9 months of age. In four neonates there was depression of inhibition of Bi during co-contraction of Bi and Tri implying that Group Ia interneurones may be under segmental and suprasegmental control at birth. Renshaw cells, identified in human postmortem cervical spinal cord by their morphology, location and calbindin D28K immunoreactivity, were present at 11 weeks post-conceptional age (PCA) and by 35 weeks PCA had mature morphological characteristics. In four neonates reciprocal inhibitory responses in Bi disappeared when the tap to Tri evoked its own homonymous phasic stretch reflex, providing neurophysiological evidence for Renshaw inhibition of Group Ia inhibitory interneurones.