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. 2016 Apr;115(4):2076-82.
doi: 10.1152/jn.00885.2015. Epub 2016 Feb 10.

Arm Posture-Dependent Changes in Corticospinal Excitability Are Largely Spinal in Origin

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Free PMC article

Arm Posture-Dependent Changes in Corticospinal Excitability Are Largely Spinal in Origin

James L Nuzzo et al. J Neurophysiol. .
Free PMC article

Abstract

Biceps brachii motor evoked potentials (MEPs) from cortical stimulation are influenced by arm posture. We used subcortical stimulation of corticospinal axons to determine whether this postural effect is spinal in origin. While seated at rest, 12 subjects assumed several static arm postures, which varied in upper-arm (shoulder flexed, shoulder abducted, arm hanging to side) and forearm orientation (pronated, neutral, supinated). Transcranial magnetic stimulation over the contralateral motor cortex elicited MEPs in resting biceps and triceps brachii, and electrical stimulation of corticospinal tract axons at the cervicomedullary junction elicited cervicomedullary motor evoked potentials (CMEPs). MEPs and CMEPs were normalized to the maximal compound muscle action potential (Mmax). Responses in biceps were influenced by upper-arm and forearm orientation. For upper-arm orientation, biceps CMEPs were 68% smaller (P= 0.001), and biceps MEPs 31% smaller (P= 0.012), with the arm hanging to the side compared with when the shoulder was flexed. For forearm orientation, both biceps CMEPs and MEPs were 34% smaller (both P< 0.046) in pronation compared with supination. Responses in triceps were influenced by upper-arm, but not forearm, orientation. Triceps CMEPs were 46% smaller (P= 0.007) with the arm hanging to the side compared with when the shoulder was flexed. Triceps MEPs and biceps and triceps MEP/CMEP ratios were unaffected by arm posture. The novel finding is that arm posture-dependent changes in corticospinal excitability in humans are largely spinal in origin. An interplay of multiple reflex inputs to motoneurons likely explains the results.

Keywords: biceps brachii; cervicomedullary motor evoked potential; motoneuron; motor cortex; spinal cord.

Figures

Fig. 1.
Fig. 1.
Schematic representation of the setup (A) and experimental protocol (B). A: depiction of Hanging Neutral, Shoulder Flexed Neutral, and Shoulder Abducted Supinated arm postures. B: sets of evoked responses were obtained in 7 different arm postures. Each set (indicated by black arrows) consisted of 5 cervicomedullary motor evoked potentials (CMEPs), 10 motor evoked potentials (MEPs), and 2 maximal compound muscle action potentials (Mmax). The 7 postures were randomized in block 1 and then randomized again for block 2. Thus each posture was assumed twice, resulting in 10 CMEPs, 20 MEPs, and 4 Mmax per posture.
Fig. 2.
Fig. 2.
Raw traces of biceps brachii motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal compound muscle action potential (Mmax) in 1 subject during different orientations of the upper arm (Hanging, Shoulder Flexed, and Shoulder Abducted) and forearm (Pronated, P; Neutral, N; and Supinated, S). Each waveform represents the average of the multiple potentials acquired in block 1. Arrows indicate the stimulus artifact, and dashed lines indicate the amplitudes of the potentials in the Hanging Neutral posture. For this subject, biceps brachii MEPs were largest in the Shoulder Abducted Neutral and Supinated postures. CMEPs were largest in the Shoulder Flexed Supinated and Shoulder Abducted Supinated postures, and they were almost completely abolished in the Hanging Neutral posture. Mmax was greatest in the Shoulder Flexed Neutral posture.
Fig. 3.
Fig. 3.
Group data (means ± SD) of biceps brachii motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal compound muscle action potential (Mmax) during different orientations of the upper arm (Hanging, Shoulder Flexed, and Shoulder Abducted) and forearm (Pronated, P; Neutral, N; and Supinated, S). A: biceps brachii MEP area. B: biceps brachii CMEP area. C: biceps brachii Mmax area. MEPs and CMEPs are normalized to Mmax. Statistically significant comparisons are noted in the text.
Fig. 4.
Fig. 4.
Raw traces of triceps brachii motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal compound muscle action potential (Mmax) in 1 subject during different orientations of the upper arm (Hanging, Shoulder Flexed, and Shoulder Abducted) and forearm (Pronated, P; Neutral, N; and Supinated, S). Each waveform represents the average of the multiple potentials acquired in block 1. Arrows indicate the stimulus artifact, and dashed lines indicate the amplitudes of the potentials in the Hanging Neutral posture. For this subject, triceps brachii MEPs and CMEPs were greatest in the Shoulder Flexed Supinated posture. Mmax was greatest in the Hanging Neutral posture.
Fig. 5.
Fig. 5.
Group data (means ± SD) of triceps brachii motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) during different orientations of the upper arm (Hanging, Shoulder Flexed, and Shoulder Abducted) and forearm (Pronated, P; Neutral, N; and Supinated, S). A: triceps brachii MEP area. B: triceps brachii CMEP area. C: triceps brachii Mmax area. MEPs and CMEPs are normalized to Mmax. Statistically significant comparisons are noted in the text.

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