Repetitive Transcranial Magnetic Stimulation in Individuals With Spinal Cord Injury: A Pilot Study

Experience and injury-induced neuroplasticity occur in sensory and motor pathways and cortices. Sensory and motor cortices are interconnected and collectively allow for the execution of fine, skilled hand control. Of particular interest is the role of somatosensory cortex in modulating the neural activity within primary motor cortex and ultimately influencing the control of hand movement. For somatosensory cortex to be effective in guiding hand movement it must receive an uninterrupted stream of somatosensory afferent input from skin, muscle and tendons that are active during hand movement. Damage to afferent input pathways compromise the integrity of necessary somatosensory input and contribute to impaired motor control of the hand. Following an incomplete spinal cord injury (SCI), damage to the somatosensory and motor pathways are widespread thereby compromise the integrity of afferent input that reaches somatosensory cortex [1,2]. Reducing the afferent input to somatosensory cortex leads to local changes in the concentration of GABA and promotes sensorimotor reorganization [3].

Transcranial magnetic stimulation has been used in SCI to measure residual corticospinal function [4], determine changes in the excitability of neural circuitry within the primary motor cortex [5] and induce short-lasting changes in the excitability of spinal motor neurons [6]. To date, TMS research in SCI has primarily focused on lower limb function, likely fueled by the desire to promote functional recovery of locomotion and balance control. However, one of the main determinants in promoting independent living is the ability to feed and groom oneself, relying primarily on the control of the upper limb and the dexterous use of the hand. The proposed research will investigate the ability to increase motor output to muscle of the hand and upper limb through inducing short-lasting changes within SI and investigate whether this has functional implication in force production, motor control and touch perception.

TMS may be delivered as a single, pair or a train of repetitive pulses over a particular body representation within the primary motor cortex or primary somatosensory cortex. One type of repetitive TMS is called continuous theta-burst stimulation (cTBS). The present study will deliver cTBS over the primary motor and primary somatosensory cortices in individual with chronic SCI. Measurements will be made before and following the cTBS protocol. These measures include 1) the amplitude of the motor evoked potential (MEP) that is evoked by single pulse TMS and measured in the forearm muscle, 2) force production whereby participants will be asked to perform a grip strength task where maximum force production will be measured and ability to dynamically control force production will be assessed, and 3) touch sensation whereby participants will partake in a temporal order judgment psychophysical task such that they must identify which fingertip received the tactile stimulus.