Sensory afferent inhibition within and between limbs in humans.

Objective: To examine the distribution and inter-limb interaction of short-latency afferent inhibition (SAI) in the arm and leg.

Methods: Motor evoked potentials (MEPs) in distal and proximal arm, shoulder and leg muscles induced with ranscranial magnetic stimulation (TMS) were conditioned by painless electrical stimuli applied to the index finger (D2) and great toe (T1) at interstimulus intervals (ISIs) of 15, 25-35, 80 ms (D2) and 35, 45, 55, 65 and 100 ms (T1) in 27 healthy human subjects. TMS was delivered over primary motor cortex (M1) arm and leg areas. Electrical stimulus intensities were varied between 1 and 3 times the sensory perception thresholds. We also tested effects of posterior cutaneous brachial nerve (PCBN) stimulation on MEPs in arm muscles at ISIs of 18 and 28 ms.

Results: D2 but not PCBN electrical conditioning reduced MEP amplitudes in upper limb muscles at ISIs of 25 and 35 ms. SAI was more pronounced in distal as compared to proximal arm muscles. Also, SAI following D2 stimulation increased with higher conditioning intensities. D2 stimulation did not change lower limb muscles MEPs. In contrast, T1 stimulation did not induce SAI in any muscles but caused MEP facilitation in a foot muscle at an ISI of 55 ms and in upper limb muscles at ISIs of 35 and 55 ms. Short interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were not affected by electrical T1 conditioning.

Conclusion: D2 stimulation causes segmental SAI in upper limb muscles with a distal to proximal attenuation without affecting leg muscles. In contrast, toe stimulation facilitates motor output both in foot and upper arm muscles.

Significance: Our data suggest that cutaneo-motor pathways in arms and legs are functionally organized in a different way with cutaneo-motor interactions induced by toe stimulation probably relayed at a thalamic level. Abnormal cutaneo-motor interactions following electrical toe stimulation may serve as an electrophysiological marker of thalamic dysfunction, e.g. in neurodegenerative diseases.