Short-latency afferent inhibition modulation during finger movement

Abstract

When somatosensory input via electrical stimulation of a peripheral nerve precedes a transcranial magnetic stimulation (TMS) pulse over the primary motor cortex (M1) the corticospinal output is substantially reduced, a phenomenon known as short-latency afferent inhibition (SAI). The present study investigated SAI during rest and during pre-movement, phasic and tonic components of movement. Participants were required to perform an index finger flexion reaction time task in response to an auditory cue. In a series of experiments, SAI was evoked from the mixed, median nerve at the wrist or the cutaneous, digital nerve stimulation of the index finger. To assess the spinal versus cortical origin of movement-related modulation of SAI, F-wave amplitudes were measured during rest and the three movement components. Results indicated that SAI was reduced during all movement components compared to rest, an effect that occurred for both nerves stimulated. Pre-movement SAI reduction was primarily attributed to reduced cortical inhibition, while increased spinal excitability additionally contributed to reduced SAI during tonic and phasic components of movement. SAI was differentially modulated across movement components with mixed but not cutaneous nerve stimulation. These findings reveal that SAI is reduced during movement and this reduction begins as early as the preparation to move. Further, these data suggest that the degree of SAI reduction during movement may be specific to the volume and/or composition of afferent input carried by each nerve.

Figure 1. Task conditions.

The timeline of the rest, pre-movement, phasic, tonic, and no stimulation conditions. The first and second speaker icons represent the ‘warning’ and ‘go’ cue, respectively. The timing of the nerve stimulation and TMS pulse are shown schematically.

Figure 2. SAI induced by mixed nerve stimulation during different components of movement with TMS normalized to ∼1 mV.

Left: Group-averaged data (with standard error of the mean) for rest and each component of movement. Right: individual trial EMG traces from one participant demonstrating changes in SAI across task conditions. An asterisk over a single component of movement indicates it was significantly different than all other components of the movement. An asterisk over a bar connecting two components of movement indicates those phases are significantly different. Significant differences were tested at p≤0.05.

Figure 3. SAI induced by cutaneous nerve stimulation during different components of movement with TMS normalized to ∼1 mV.

Left: Group-averaged data (with standard error of the mean) for rest and each component of movement. Right: individual trial EMG traces from one participant demonstrating changes in SAI across task conditions. An asterisk over a single component of movement indicates it was significantly different than all other components of the movement. Significant differences were tested at p≤0.05.

Figure 4. F-wave amplitude during different movement components in the index finger flexion task.

Group-averaged data (with standard error of the mean) for each component of movement. The asterisk over the bar connecting the movement phases to rest and pre-movement conditions indicates that F-wave amplitude in both movement phases were significantly different than rest and pre-movement. Significant differences were tested at p≤0.05.

Figure 5. SAI induced by mixed nerve stimulation during different components of movement with TMS intensity at 1.2 RMT.

Left: Group-averaged data (with standard error of the mean) for rest and each component of movement. Right: individual trial EMG traces from one participant demonstrating changes in SAI across task conditions. An asterisk over a single component of movement indicates it was significantly different than all other components of the movement. An asterisk over a bar connecting two components of movement indicates those components are significantly different. Significant differences were tested at p≤0.05.