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Brain Advance Access published online on June 30, 2004
Brain, doi:10.1093/brain/awh212
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Received January 9, 2003
Revised April 7, 2004
Accepted April 9, 2004

Article

Cortico-motoneuronal excitation of three hand muscles determined by a novel penta-stimulation technique

Ulf Ziemann 1*, Tihomir V. Ilic 1, Henrik Alle 1, Frank Meintzschel 1

1 Motor Cortex Laboratory, Department of Neurology, Johann Wolfgang Goethe-University of Frankfurt, Frankfurt am Main, Germany

* To whom correspondence should be addressed. E-mail: u.ziemann{at}em.uni-frankfurt.de.


  Abstract

Summary The cortico-motoneuronal system (CMS), i.e. the mono-synaptic projection from primary motor cortex to motoneurons in lamina IX of the spinal cord is, among all mammals, best developed in humans. Increasing evidence suggests that the CMS is crucially important for skilled individuated finger movements. Little is known about to what extent the strength of the CMS differs between hand muscles. Here we measured CMS excitation to the first dorsal interosseus (FDI), abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles in healthy subjects by using a novel penta-stimulation technique (PST) and single motor unit (SMU) recordings. The PST is an extension of the triple-stimulation technique. It applies two additional supramaximal electrical stimuli at the wrist to the 'peripheral nerve of no interest' (in the case of the FDI and ADM the median nerve, in the case of the APB the ulnar nerve) to collide with the descending volleys in that nerve elicited by transcranial magnetic stimulation of motor cortex and electrical stimulation of Erb's point. This eliminates volume conduction from neighbouring muscles innervated by the nerve of no interest and, therefore, allows accurate determination of the PST response. The PST response was significantly larger in the FDI compared with the ADM and APB. This was validated by the SMU recordings, which showed a higher estimated amplitude of the mean compound excitatory postsynaptic potential in spinal motoneurons of the FDI than in those of the APB and ADM. Finally, as a possible functional correlate, the maximum rate of repetitive voluntary finger movements was higher for index finger abduction (prime mover, FDI) than for little finger abduction (prime mover, ADM) and thumb abduction (prime mover, APB), and individual differences in maximum rate between the different movements correlated with individual differences in the corresponding PST responses. In conclusion, PST is a valuable novel method for accurate quantification of CMS excitation. The findings strongly suggest that CMS excitation differs between hand muscles and that these differences directly link to capability differences in individuated finger movements.

Keywords: human cortico-motoneuronal system; intrinsic hand muscles; penta-stimulation technique; transcranial magnetic stimulation; single motor unit recording.
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