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 disappear ance 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 recr uited at birth and studied 3 monthly until 12 months of age. Reciprocal inh ibitory and excitatory reflexes were recorded in the surface EMG of contrac ting biceps brachii (Bi), evoked by taps applied to the tendon of triceps b rachii (Tri). Reciprocal excitatory reflexes were recorded in all but one n eonate. 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 demonstrat ed in all subjects after 9 months of age. In four neonates there was depres sion 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 pre sent at 11 weeks post-conceptional age (PCA) and by 35 weeks PCA had mature morphological characteristics. In four neonates reciprocal inhibitory resp onses in Bi disappeared when the tap to Tri evoked its own homonymous phasi c stretch reflex, providing neurophysiological evidence for Renshaw inhibit ion of Group Ia inhibitory interneurones. (C) 2001 Elsevier Science B.V. Al l rights reserved.