cqvip:Objective: Previous reports from our laboratory have described short-latency contractions in muscles of the distal upper limb following stimulation of lower limb nerves or skin in persons with injury to the cervical spinal cord. It takes 6 or more months for interlimb reflexes (ILR) to appear following acute spinal cord injury (SCI), suggesting they might be due to new synaptic interconnections between lower limb sensory afferents and motoneurons in the cervical enlargemen t. In this study, we asked if once formed, the strength of these synaptic connec tions increased over time, a finding that would be consistent with the above hyp othesis. Methods: We studied persons with sub-acute and/or chronic cervical SCI . ILR were elicited by brief trains of electrical pulses applied to the skin ove rlying the tibial nerve atthe back of the knee. Responses were quantified based on their presence or absence in different upper limb muscles. We also generated peri-stimulus time histograms for single motor unitresponse latency, probabilit y, and peak duration. Comparisons of these parameters were made in subjects at s ub-acute versus chronic stages post-injury. Results: In persons with sub-acut e SCI, the probability of seeing ILR in a given muscle of the forearm or hand wa s low at first, but increased substantially over the next 1-2 years. Motor unit responses at this sub-acute stage had a prolonged and variable latency, with a lower absolute response probability, compared to findings from subjects with ch ronic (i.e. stable) SCI. Conclusions: Our findings demonstrate that interlimb re flex activity, once established after SCI,shows signs of strengthening synaptic contacts between afferentand efferent components, consistent with ongoing synapt ic plasticity. Significance: Neurons within the adult human spinal cord caudal t o a lesion site are not static, but appear to be capable of developing novel-ye t highly efficacious-synaptic contacts following trauma-induced partial denerv ation. In this case, such contacts between ascending afferents and cervical moto neurons do not appear to provide any functional benefit to the subject. In fact their presence may limit the regenerative effort of supraspinal pathways which o riginally innervated these motoneurons, should effort in animal models to promot e regeneration across the lesion epicenter be successfully translated to humans with chronic SCI.
展开▼
