Differential contributions of spinal and cortical motoneurons to input-output properties of human small hand muscle

Objective: It is not fully understood how the stimulus/response curves obtained by transcranial magnetic stimulation (TMS) reflect the function of the cortico-motoneuronal (CM) and spinal motoneuronal (SM) systems in healthy subjects. To understand these response functions, we studied patients with amyotrophic lateral sclerosis (ALS) whose upper but not lower motor neurons were affected. Methods: First, we determined the effects of voluntary muscle contraction and intensity of TMS on the motor evoked potentials (MEPs) of the first dorsal interossei (FDI) muscle in ten healthy control subjects (mean age: 35.1 ± 6.7 years) and two older female subjects (60 and 64 years old). Second, we investigated whether this relationship was altered in two ALS patients (60-year-old woman and 69-year-old man). The MEPs were recorded at different degrees of voluntary contraction by threshold stimulus intensities (TSIs) or at different levels of TMS with the muscle at rest. Results: In the controls, the MEP amplitudes of the FDI muscle elicited by TSI increased linearly with muscle contraction (right: Y=0.068X + 0.754; left: Y=0.0670X + 0.807), whereas the MEP amplitudes elicited by different TMS intensities increased sigmoidally with a flexion point at 1.10-1.15 TSI: right: Amp=2.21/{1 + exp[(S₅₀-Stim)/K]}, S₅₀=10.73, K=4.70; left: Amp=2.90/{1 + exp[(S₅₀-Stim)/K]}, S₅₀=13.64, K=5.98. The latter was abnormal only on one side of each ALS patient. Discussion: From these results, we suggest that the sigmoidal TMS intensity-size curves reflect mainly CM activities, while the linear contraction-size curves reflect SM activities.