Brain Research 899 (2001) 66–81 www.elsevier.com / locate / bres Research report Reciprocal and Renshaw (recurrent) inhibition are functional in man at birth * S.M. Mc Donough, G.J. Clowry, S. Miller, J.A. Eyre Developmental Neuroscience, Department of Child Health, University of Newcastle upon Tyne, Newcastle upon Tyne NE14HH, UK Accepted 2 January 2001 Abstract 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.  2001 Elsevier Science B.V. All rights reserved. Theme: Development Topic: Motor systems Keywords: Reciprocal inhibition; Renshaw inhibition; Recurrent inhibition; Neonate; Development; Phasic stretch reflex; Human 1. Introduction The degree of co-contraction expressed by agonist / antago- nist muscle pairs during limb movement depends upon During spontaneous movement human neonates show a many interrelated factors, in particular the descending predominant pattern of co-contraction of agonist / antago- commands to each motoneuronal pool and the balance nist muscle pairs in the limbs, with distinct alternating between reciprocal excitation [32,36,37] and reciprocal patterns of activation of agonist / antagonist pairs not being inhibition [4,30]. The magnitude of reciprocal inhibition is achieved until several years later in childhood [17–19]. fine-tuned by recurrent inhibition [4]. Descending com- mands and segmental afferents modulate the excitability of Group Ia inhibitory interneurones and Renshaw cells [26] Abbreviations: Bi, biceps brachii; C, condition; CMCD, central motor and there is also a pattern of mutual agonist / antagonist conduction delay; MVC, maximum voluntary contraction; PCA, post inhibition such that the Group Ia inhibitory interneurones conceptional age; PRBS, pseudo-random binary sequence; PMCD, peripheral motor conduction delay; R, cross-correlation function; t, test; inhibit the Group Ia inhibitory interneurones of the antago- T, threshold; Tri, triceps brachii; TMCD total conduction delay; V, nist muscle [4,22] and Renshaw cells excited by agonists cross-covariance inhibit those excited by antagonists and vice versa [40]. *Corresponding author. Department of Child Health, The Royal Thus a-motoneurones, Group Ia inhibitory interneurones Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NE1 4LP, and Renshaw cells together with Group Ia primary spindle UK. Tel.: 144-191-2023-013; fax: 144-191-2023-022. E-mail address: j.a.eyre@ncl.ac.uk (J.A. Eyre). afferent input form part of an ‘output stage’ spinal 0006-8993 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0006-8993(01)02151-5