Voluntary contraction and responses to submaximal cervical nerve root stimulation

Area of Science:

• Neuroscience
• Motor Control
• Electrophysiology

Background:

• Voluntary muscle contraction is known to increase compound muscle action potential (CMAP) amplitudes.
• This phenomenon has been traditionally attributed to intraspinal, presynaptic mechanisms.
• The precise mechanism underlying this modulation remains a subject of investigation.

Purpose of the Study:

• To investigate the impact of voluntary hand muscle contraction on hypothenar CMAP amplitudes.
• To differentiate between peripheral and central mechanisms responsible for the observed increase in CMAP amplitudes.
• To explore the role of motor axon recruitment order in voluntary activation versus electrical/magnetic stimulation.

Main Methods:

• Assessed changes in hypothenar CMAP amplitudes in 5 volunteers.
• Utilized electrical peripheral nerve stimulation at the wrist and electrical percutaneous cervical stimulation (EPCS).
• Employed magnetic stimulation at the neck and tested conditioning subthreshold cortical magnetic stimulation with EPCS at rest.

Main Results:

• CMAP amplitudes significantly increased with voluntary contraction of the target muscle, irrespective of stimulation type or location (P < 0.001).
• No significant increase in CMAP amplitude was observed when a conditioning transcranial stimulus was applied with EPCS (P = 0.35).

Conclusions:

• The findings suggest a peripheral mechanism, not a central one, underlies the increase in CMAP amplitudes during voluntary contraction.
• The effect is likely associated with the recruitment order of motor axons during voluntary activation compared to electrical or magnetic stimulation.