Specificity of intramuscular activation during rhythms produced by 4 spinal patterning systems in the in vitro neonatal rat with hindlimb 5 attached preparation

30 In intact adult vertebrates, muscles can be activated with a high degree of specificity, so that even 31 within a single traditionally defined muscle, groups of motor units can be differentially activated. Such 32 differential activation might reflect detailed control by descending systems, potentially resulting from 33 postnatal experience such that activation of motor units is precisely tailored to their mechanical actions. 34 Here we examine the degree to which such specific activation can be seen in the rhythmic patterns 35 produced by isolated spinal motor systems in neonates. We examined motor output produced by the in 36 vitro neonatal rat spinal cord with hindlimb attached. We recorded the activity of different regions 37 within the posterior portion of biceps femoris (BFp; i.e. excluding the anterior/vertebral head). We 38 found that in the rhythms evoked by bath application of 5-HT/NMDA, all regions of BFp were active 39 during extension. However, the regions of BFp were activated in a specific sequence, with the activation 40 of rostral regions consistently preceding those of more caudal regions in both afferented and 41 deafferented preparations. In the rhythms evoked by cauda equina (CE) stimulation, rostral and middle 42 regions of BFp remained active in extension, but the caudal region of BFp was usually active in flexion. 43 Stimulation of L5 and S2 dorsal roots typically evoked rhythms with all regions of BFp active during 44 extension and although the same rostral to caudal sequence of activation observed in 5-HT/NMDA 45 evoked rhythms could also be observed in these rhythms, we also observed cases with reversed 46 sequences, with activity proceeding from caudal to rostral. S2 dorsal root stimulation occasionally 47 evoked rhythms with flexor activity in caudal BFp, similar to CE evoked rhythms. Taken together, these 48 results suggest a high degree of individuated control of muscles by spinal pattern generating networks, 49 even at birth. 50