Is the human masticatory system devoid of recurrent inhibition?

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• The Renshaw cell system is a recurrent inhibitory feedback pathway in the spinal cord.
• L-acetylcarnitine (L-Ac) is known to modulate recurrent inhibition in human motoneurons.
• Previous studies suggest L-Ac affects motor unit discharge variability and synchrony based on recurrent inhibition presence.

Purpose of the Study:

• To determine if a functional recurrent inhibitory system exists in human masticatory motoneurons.
• To investigate the effect of L-acetylcarnitine on the firing behavior of masseter muscle motor units.

Main Methods:

• Recorded tonic discharge activity of masseter muscle motor unit pairs during voluntary isometric contraction.
• Administered intravenous L-acetylcarnitine (L-Ac) or saline (SAL) in a double-blind manner.
• Analyzed motor unit discharge variability, synchrony, and coherence before, during, and after injection.

Main Results:

• Neither L-Ac nor SAL injection significantly altered inter-spike interval (ISI) or its coefficient of variation.
• No significant changes were observed in motor unit synchronous activity patterns or discharge coherence.
• The lack of L-Ac effect mirrors findings in motoneurons lacking recurrent inhibition.

Conclusions:

• The absence of L-Ac-induced changes in masseter motoneuron firing suggests a lack of functional classical Renshaw cell inhibition.
• This finding implies that the recurrent inhibitory system may be absent in the motoneuron pool innervating human masticatory muscles.