Subthreshold transcranial magnetic stimulation during the long latency component of the cutaneomotor reflex

Modulation of ongoing electromyographic (EMG) activity in the small hand muscles can be induced by electrical stimulation of the digital nerves or by stimulation of the skin of the fingers. Several groups have attempted to establish a role for the motor cortex in the generation of the facilitatory component of the cutaneomotor reflex. Our aim was to establish if the facilitatory component of the reflex could be diminished by a procedure known to inhibit the motor cortex, namely, subthreshold transcranial magnetic stimulation. During sustained small contractions of the first dorsal interosseus muscle transcranial magnetic stimuli (TMS), which were subthreshold for the generation of a motor evoked potential, were delivered via a figure-of-eight coil. Inhibition of ongoing EMG was observed in all subjects. In two separate series of trials, TMS was timed so that the resultant inhibition occurred coincident with either the short or long latency stretch reflex or with the initial or later part of the facilitatory component of the cutaneomotor reflex. The short latency stretch reflex is known to involve a largely monosynaptic loop via the spinal cord, whereas the long latency response involves a transcortical loop. The long latency response was reduced in size following subthreshold TMS, whereas the short latency response was unchanged. This provides evidence of the effectiveness of subthreshold TMS in inhibiting a transcortical reflex. When the TMS was timed so that the inhibition occurred coincident with the facilitatory component of the cutaneomotor response, neither the early nor later changes were inhibited. Thus, the pathway of the long-latency cutaneomotor reflex is not similar to the transcortical pathway of the stretch reflex. Either the response does not travel via the cortex or it involves different cortical neurones.