Revisiting motor unit recruitment to TMS in amyotrophic lateral sclerosis: cortical inhibition is retained during voluntary contractions

Transcranial magnetic stimulation (TMS) has been used for many years to study the pathophysiology of amyotrophic lateral sclerosis (ALS). Based on single- or dual-pulse TMS and EMG and/or single motor unit (MU) recordings, many groups have described a loss of central inhibition as an early marker of ALS dysfunction, reflecting a state of cortical 'hyperexcitability'. This conclusion is not without its detractors, however, leading us to reexamine this issue using 4-pulse TMS, shown previously to be more effective for testing central motor pathway functional integrity. A total of 221 motor units were tested in 13 subjects (6 controls; 7 with ALS) across a total of 798 unique TMS conditions. MUs were studied from hand muscles (usually first dorsal interosseus) and from tibialis anterior (TA). Subjects were required to recruit a MU to fire rhythmically, during which time 4-pulse trains of TMS were delivered. A given motor unit's recruitment was examined for different stimulus intensities and interpulse intervals (IPI). All motor units from control subjects showed short latency excitation to TMS, and short latency inhibition for TMS pulses of slightly weaker intensity (i.e. the threshold for inhibition was lower than that for excitation). The same was largely true for MUs studied in subjects with ALS, with the primary difference between control and ALS subjects being the need for stronger stimulus intensities to effect recruitment in subjects with ALS. We saw no evidence for a loss or reduction of inhibition of central motor output in persons with ALS, at least when tested during voluntary contractions.