Arm posture-dependent changes in corticospinal excitability are largely spinal in origin

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Motor evoked potentials (MEPs) elicited by cortical stimulation are known to be modulated by arm posture.
• The precise origin of this postural influence on MEPs, whether central (brain) or peripheral (spinal), remains unclear.

Purpose of the Study:

• To investigate whether the observed effects of arm posture on biceps brachii motor evoked potentials (MEPs) are spinal in origin.
• To differentiate between cortical and spinal contributions to posture-dependent modulation of corticospinal excitability.

Main Methods:

• Twelve subjects performed static arm postures varying in shoulder and forearm orientation.
• Motor evoked potentials (MEPs) were elicited via transcranial magnetic stimulation (TMS) of the motor cortex.
• Cervicomedullary motor evoked potentials (CMEPs) were elicited via electrical stimulation of corticospinal axons at the cervicomedullary junction.

Main Results:

• Biceps MEPs and CMEPs were significantly smaller when the arm hung to the side compared to shoulder flexion, and also smaller in pronation versus supination.
• Triceps CMEPs were smaller with the arm hanging to the side compared to shoulder flexion, but triceps MEPs were unaffected by forearm posture.
• MEP/CMEP ratios for both biceps and triceps were not influenced by arm posture, indicating a spinal locus for the observed effects.

Conclusions:

• Posture-dependent modulation of corticospinal excitability in humans is predominantly of spinal origin.
• The findings suggest an interplay of multiple reflex inputs to motoneurons underlies these spinal adaptations to arm posture.