Short-latency afferent inhibition in chronic spinal cord injury

BACKGROUND: Short-latency afferent inhibition (SAI) results when somatosensory afferent input inhibits the corticospinal output from primary motor cortex (M1). The present study examined SAI in the flexor carpi radialis (FCR) muscle in individuals with spinal cord injury (SCI) and uninjured controls. METHODS: Short-latency afferent inhibition (SAI) was evoked by stimulating the median nerve at the elbow at intervals of 15, 20 and 25 ms in advance of a transcranial magnetic stimulation (TMS) pulse over M1. SAI was tested with the FCR at rest and also during ~20% of maximum voluntary contraction. Corticospinal output was assessed through measuring both motor thresholds and motor evoked potential (MEP) recruitment curves. The afferent volley was assessed via the N20-P25 amplitude of the somatosensory evoked potential (SEP) and the amplitude of sensory nerve action potentials (SNAP) recorded over the median nerve at the elbow. RESULTS: SAI is reduced in SCI in both the contracted and non-contracted FCR muscle. MEP recruitment curves and thresholds were decreased in SCI only in the active state and not the resting state. N20-P25 amplitude was similar between groups in both the resting and active states although SNAP was significantly reduced in SCI at rest. CONCLUSIONS: We conclude that reduced SAI in SCI is likely attributed to neuroplasticity altering the intrinsic M1 circuitry mediating SAI and/or reduced afferent input traversing a direct thalamocortical route to M1. These data provide a new avenue of research aimed at identifying therapeutic approaches to alter SAI to improve upper limb function in individuals with SCI.