Plasticity of synapses innervating spinal motor neurons after spinal cord injury in rhesus monkeys

Patients with incomplete spinal cord injuries can experience partial recovery of motor function, a phenomenon believed to be linked to neural plasticity, as evidenced by numerous studies in rodent models. However, postinjury synaptic plasticity in non-human primates, which better mirror human neuroanatomy, is less well understood. This study investigated the effects of T10 thoracic spinal cord hemisection or over-hemisection (transection beyond the posterior median sulcus) injuries in 7 rhesus monkeys. Analysis of lumbar motor neurons (MNs) innervating gluteal and crural muscles showed increased inhibitory projections and a shift in the excitatory-inhibitory balance. In the hemisection group, spontaneous recovery of motor function occurred, with lumbar MNs on the injured side receiving increased proprioceptive afferent inputs compared to the intact side, a phenomenon absent in the over-hemisection group. Additionally, the injured side of hemisected monkeys retained more descending monoaminergic fibers, possibly due to sprouting from contralateral intact fibers. These changes, including enhanced proprioceptive inputs and monoaminergic fiber sprouting, may contribute to spontaneous recovery after hemisection and represent potential targets for therapeutic strategies to improve recovery following spinal cord injury.