Bilateral cortical control of the human anterior digastric muscles |
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Authors: | B R Gooden M C Ridding T S Miles M A Nordstrom P D Thompson |
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Institution: | (1) Departments of Physiology and Medicine, The University of Adelaide, Adelaide, SA 5005, Australia e-mail: timothy.miles@adelaide.edu.au Tel.: +61-8-83035108, Fax: +61-8-83033356, AU |
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Abstract: | Transcranial magnetic stimulation (TCMS) was used to determine the organisation of cortical motor projections to the anterior
digastric muscles in 12 normal human subjects. Two distinct types of potentials were evoked in anterior digastric with a figure-of-eight
coil. A short-latency (3 ms) response appeared bilaterally on the surface electromyogram (EMG), but only ipsilaterally on
intramuscular recordings: this was the result of direct stimulation of the ipsilateral trigeminal motor root. Motor evoked
potentials (MEPs) were elicited in the anterior digastric muscles at variable onset latencies of around 10 ms by stimulation
of scalp areas antero-lateral to the area for the first dorsal interosseous muscle of the hand. These were evoked bilaterally
in relaxed anterior digastric muscles in six of the seven subjects. In the other subject, the responses in the relaxed muscle
were exclusively ipsilateral. However, when the anterior digastric muscles were contracted, the responses were bilateral in
all subjects. TCMS and spike-triggered averaging revealed that the bilateral responses were not due to the branching of axons
from individual digastric motoneurones to muscles on each side. Because the digastric motor nucleus may contain separate populations
of ipsi- and contralateral projecting motoneurones, it was necessary to study single motor-unit responses to TCMS to demonstrate
a bilateral corticobulbar projection. The responses of 17 single motor units in the anterior digastric muscle to TCMS were
recorded. All were activated by contralateral stimulation. Approximately 80% were also activated by ipsilateral TCMS, although
one well-characterised motor unit was inhibited by ipsilateral TCMS. When bilateral activation was present, the ipsilateral
responses were more secure than the contralateral responses, which may indicate an additional interneurone in the pathway
to the contralateral motoneurone. The major conclusions from this study are that (1) the cortical representation of the anterior
digastric muscle is antero-lateral to hand muscles; (2) the cortical projection to the anterior digastric muscles is bilateral;
(3) the corticobulbar projection is stronger contralaterally than ipsilaterally but may involve at least one additional synapse;
and (4) anterior digastric motoneurones do not branch to innervate the muscles bilaterally.
Received: 8 March 1999 / Accepted: 16 June 1999 |
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Keywords: | Corticomotoneuronal Trigeminal Transcranial magnetic stimulation Motor cortex Map Ipsilateral projection Cross-talk Single motor units |
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