Is the human masticatory system devoid of recurrent inhibition?

• Published in: Experimental brain research Vol. 179; no. 1; pp. 131 - 144
• Main Authors: TÜRKER, K. S, SCHMIED, A, ROSSI, A, MAZZOCCHIO, R, SOWMAN, P. F, VEDEL, J. P
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01-05-2007; Springer Nature B.V
• Subjects: Acetylcarnitine - pharmacology; Action Potentials - drug effects; Action Potentials - physiology; Adult; Biological and medical sciences; Bite Force; Central nervous system; Central neurotransmission. Neuromudulation. Pathways and receptors; Electromyography; Feedback - drug effects; Feedback - physiology; Female; Fundamental and applied biological sciences. Psychology; Humans; Hypotheses; Isometric Contraction - drug effects; Isometric Contraction - physiology; Male; Mastication - drug effects; Mastication - physiology; Masticatory Muscles - innervation; Masticatory Muscles - physiology; Middle Aged; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor Neurons - drug effects; Motor Neurons - physiology; Neural Inhibition - drug effects; Neural Inhibition - physiology; Nootropic Agents - pharmacology; Physiology; Sodium Chloride - pharmacology; Synaptic Transmission - drug effects; Synaptic Transmission - physiology; Trigeminal Nerve - drug effects; Trigeminal Nerve - physiology; Trigeminal Nuclei - drug effects; Trigeminal Nuclei - physiology; Vertebrates: nervous system and sense organs; Human; Masseter muscle; Masseter; Isometric muscular contraction; Motor neuron; Recurrent inhibition; Recurrent; Single motor units; Synchronization; Muscle contraction; Renshaw neuron; Motor unit; Trigeminal; Masticatory muscle; Coherence; Abductor muscle
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-006-0774-2

The aim of the present study was to investigate the existence or otherwise of a functional recurrent inhibitory system (Renshaw cell system) in the motoneurons that innervate human masticatory muscles. In a previous study, L: -acetylcarnitine (L: -Ac), a substance known to potentiate recurrent inhibition in humans was found to alter, in a specific way, the discharge variability, and the synchronous activity of motor units depending on the presence or absence of recurrent inhibition in the corresponding motoneuron pool. Using a similar paradigm, we have recorded the tonic discharge activity of motor unit pairs from the masseter muscle during voluntary isometric contraction while subjects were undergoing continuous intravenous saline (SAL, NaCl 0.9%) perfusion. Following a brief baseline-recording period, the subjects were given a test injection of either L: -Ac or isotonic saline (SAL) in a double blind manner. The variability, synchronization, and coherence between the motor unit discharges were analysed during three successive periods: pre-injection, during injection, and post-injection, each lasting 2-3 min. Neither L: -Ac nor SAL injection induced a significant change in the inter-spike interval (ISI) or the coefficient of variation of the ISIs in the motor units tested. There were also no significant changes in the pattern of synchronous activity or in the coherence, which reflects the common frequency content of the unit discharges. Reminiscent of what had been observed previously with motoneurons without recurrent inhibition in the Abductor Digitorum Minimi muscle, the lack of effects of L: -Ac injection on the firing behaviour of masseter motoneurons may suggest that classical Renshaw cell inhibition is lacking in this motoneuron pool.