Interlimb reflex activity after spinal cord injury in man: strengthening response patterns are consistent with ongoing synaptic plasticity

• Published in: Clinical neurophysiology Vol. 116; no. 1; pp. 75 - 86
• Main Authors: Calancie, Blair, Alexeeva, Natalia, Broton, James G., Molano, Maria R.
• Format: Journal Article
• Language: English
• Published: Shannon Elsevier Ireland Ltd 2005; Elsevier Science
• Subjects: Adolescent; Adult; Aged; Biological and medical sciences; Cerebrospinal fluid. Meninges. Spinal cord; Development. Senescence. Regeneration. Transplantation; Electric Stimulation - methods; Electromyography - methods; Evoked Potentials, Motor - physiology; Evoked Potentials, Motor - radiation effects; Female; Follow-Up Studies; Functional Laterality; Fundamental and applied biological sciences. Psychology; Human; Humans; Interlimb reflex; Lower Extremity - physiopathology; Male; Medical sciences; Middle Aged; Muscle, Skeletal - physiopathology; Muscle, Skeletal - radiation effects; Nervous system (semeiology, syndromes); Neurology; Neuronal Plasticity - physiology; Neuronal Plasticity - radiation effects; Plasticity; Probability; Reaction Time - physiology; Reaction Time - radiation effects; Reflex - physiology; Reflex - radiation effects; Regenerative sprouting; Retrospective Studies; Spinal Cord Injuries - classification; Spinal Cord Injuries - physiopathology; Spinal cord injury; Tibial Nerve - physiopathology; Tibial Nerve - radiation effects; Time Factors; Vertebrates: nervous system and sense organs; Human; Interlimb reflex; Spinal cord injury; Regenerative sprouting; Plasticity; Reflex; Nervous system diseases; Lower limb; Motor neuron; Muscle contraction; Trauma; Cervical spinal cord; Regeneration; Interconnection; Central nervous system disease; Synaptic plasticity; Upper limb; Spinal cord disease; Spinal cord iqjury
• ISSN: 1388-2457; 1872-8952
• DOI: 10.1016/j.clinph.2004.07.018

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 enlargement. In this study, we asked if once formed, the strength of these synaptic connections increased over time, a finding that would be consistent with the above hypothesis.
We studied persons with sub-acute and/or chronic cervical SCI. ILR were elicited by brief trains of electrical pulses applied to the skin overlying the tibial nerve at the 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 unit response latency, probability, and peak duration. Comparisons of these parameters were made in subjects at sub-acute versus chronic stages post-injury.
In persons with sub-acute SCI, the probability of seeing ILR in a given muscle of the forearm or hand was 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 chronic (i.e. stable) SCI.
Our findings demonstrate that interlimb reflex activity, once established after SCI, shows signs of strengthening synaptic contacts between afferent and efferent components, consistent with ongoing synaptic plasticity.
Neurons within the adult human spinal cord caudal to a lesion site are not static, but appear to be capable of developing novel—yet highly efficacious—synaptic contacts following trauma-induced partial denervation. In this case, such contacts between ascending afferents and cervical motoneurons do not appear to provide any functional benefit to the subject. In fact their presence may limit the regenerative effort of supraspinal pathways which originally innervated these motoneurons, should effort in animal models to promote regeneration across the lesion epicenter be successfully translated to humans with chronic SCI.