Retracing your footsteps: developmental insights to spinal network plasticity following injury

C. Jean-Xavier, S. A. Sharples, K. A. Mayr, A. P. Lognon, P. J. Whelan

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the second postnatal week and is heralded by the onset of weight-bearing locomotion on the 8th-9th postnatal day. Concomitantly there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns such as spasticity and clonus that emerge following central injury.