Proximo-distal gradient of spinal recurrent inhibition: implications for motor pool fractionation and dexterity.

Spinal recurrent inhibition (RI), mediated by Renshaw cells, exhibits a highly conserved proximal-to-distal gradient across mammals: it is robust in proximal motoneuron (MN) pools but absent in MNs innervating both extrinsic and intrinsic digit muscles. Although this organizational pattern has been recognised for more than six decades, its mechanistic basis has remained unexplained. The widely accepted view that the gradient reflects the evolutionary emergence of monosynaptic corticomotoneuronal (CM) projections is challenged by the cat, which lacks monosynaptic CM input to digit MNs yet exhibits the same RI gradient observed in primates. We performed a comparative quantitative analysis of 16 published MN pools from humans, macaque monkeys, and cats. Four nested nonlinear regression models and a Bayesian hierarchical errors-in-variables model were used to evaluate the relative contributions of CM input and recurrent collateral density to normalised RI strength. The robustness of the CM contribution was assessed through Bayesian prior sensitivity analyses using both HalfNormal and symmetric Normal priors. Recurrent collateral density provided the strongest explanation for the conserved RI gradient, accounting for 97.7% of the cross-species variance (NLS R