Magnetic motor evoked potentials (MEPs) in masseter muscles

Area of Science:

• Neuroscience
• Electrophysiology
• Motor Control

Background:

• The neural pathways controlling facial muscles, particularly the masseter, are complex.
• Understanding these pathways is crucial for diagnosing neurological disorders affecting motor function.

Purpose of the Study:

• To investigate the origin and pathways of electromyographic responses in the masseter and orbicularis oris muscles using transcranial magnetic stimulation.
• To determine the laterality of corticomotoneuronal projections to the masseter muscle.

Main Methods:

• Surface and needle electrodes were used to record electromyographic responses (MEPs) from masseter and orbicularis oris muscles.
• Transcranial magnetic stimulation was applied to evoke MEPs and silent periods.
• Stimulation parameters and responses were analyzed in healthy individuals and patients with ischemic lesions.

Main Results:

• Masseter MEPs with a latency of 6.9 ms were evoked via contralateral cortico-nuclear connections, followed by bilateral silent periods.
• Ipsilateral responses with shorter latencies and higher amplitudes were observed, attributed to direct trigeminal nerve stimulation.
• Cortical MEPs were absent in patients with lesions affecting the stimulated hemisphere, indicating impaired corticomotoneuronal pathways.

Conclusions:

• Direct corticomotoneuronal projections to the masseter muscle are predominantly crossed (contralateral).
• Transcranial magnetic stimulation is a valuable tool for assessing the integrity of motor pathways in facial muscles.
• The findings contribute to understanding the neurophysiology of masticatory muscles and related neurological conditions.