Responses in small hand muscles from magnetic stimulation of the human brain.

Abstract

SUMMARY1. The magnetic field generated by a brief current in a 9 cm diameter flat circular coil varies rapidly with time and when applied over the human scalp it is capable of exciting motor structures subserving the small hand muscles. With a peak magnetic field at the centre of the coil of 09-1-6 Tesla, single scalp stimuli produced twitches in the right abductor digiti minimi (a.d.m.), first dorsal interosseous (f.d.i.) and adductor pollicis muscles. Compound muscle action potentials (c.m.a.p.s) and single motor units from these muscles were recorded.2. The twitch force and corresponding c.m.a.p. were greatly enhanced by voluntary background contraction of the muscle and depended on the direction and intensity of the magnetic field. Response amplitude was maximal with the stimulating coil centred near the vertex but precise coil position was not critical.3. When the orthodromic volley set up in the peripheral nerve by magnetic stimulation of the brain collided with a maximal antidromic volley set up by stimulation at the wrist, remaining electromyographic (e.m.g.) activity gave evidence of multiple firing of some spinal motoneurones, provided that the muscle exerted a slight voluntary background contraction and the stimulus intensity was above threshold for relaxed muscle.4. When the muscle changed from total relaxation to a slightly contracted state the onset latency of the c.m.a.p. was shortened by about 3 ms without further change when the background contraction increased. In slightly contracted muscle, c.m.a.p. onset latency was little affected over a wide range of stimulus intensities and was unaffected by the position of the stimulating coil within an area of 6 x 6 cm over the vertex. 7. It is likely that the enhancement of responses by voluntary background contraction is caused by additional recruitment of higher-threshold motor units in the motoneurone pool and by multiple firing of some motor units.8. There may be two mechanisms of enhancement of responses to cortical stimulation, one related to a rise in excitability of homologous spinal motoneurone pools on the two sides, and a second operating at a higher level when the subject focuses his attention on the motor performance of a particular hand. The latter mechanism can be seen more easily with magnetic than with electrical brain stimulation, probably because the latter in part bypasses intracortical neuronal elements.

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