title: Arm posture-dependent changes in corticospinal excitability are largely spinal in origin.
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.
authors: Neuroscience Research Australia, Randwick, New South Wales, Australia; School of Medical Sciences, University of New South Wales, Kensington, New South Wales, Australia; and.
authors: James L
authors: JL
authors: Nuzzo
authors: Neuroscience Research Australia, Randwick, New South Wales, Australia;
authors: Gabriel S
authors: GS
authors: Trajano
authors: Neuroscience Research Australia, Randwick, New South Wales, Australia; School of Medical Sciences, University of New South Wales, Kensington, New South Wales, Australia; and.
authors: Benjamin K
authors: BK
authors: Barry
authors: Neuroscience Research Australia, Randwick, New South Wales, Australia; Prince of Wales Clinical School, University of New South Wales, Kensington, New South Wales, Australia.
authors: Simon C
authors: SC
authors: Gandevia
authors: Neuroscience Research Australia, Randwick, New South Wales, Australia; School of Medical Sciences, University of New South Wales, Kensington, New South Wales, Australia; and j.taylor@neura.edu.au.
authors: Janet L
authors: JL
authors: Taylor
mesh_terms: D000293:Adolescent; D000328:Adult; D001132:Arm / Q000294:innervation / Q000502:physiology*; D019054:Evoked Potentials, Motor; D005260:Female; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D011187:Posture; D011712:Pyramidal Tracts / Q000502:physiology*; D012018:Reflex; D050781:Transcranial Magnetic Stimulation
keywords: biceps brachii; cervicomedullary motor evoked potential; motoneuron; motor cortex; spinal cord