Activation properties of trigeminal motoneurons in participants with and without bruxism

• Published in: Journal of neurophysiology 2013-12, Vol.110 (12), p.2863-2872
• Main Authors: D'Amico, Jessica M, Yavuz, Ş Utku, Saraçoglu, Ahmet, Atiş, Elif Sibel, Gorassini, Monica A, Türker, Kemal S
• Format: Article
• Language: English
• Subjects: Adult; Bruxism - physiopathology; Case-Control Studies; Female; Humans; Male; Masseter Muscle - innervation; Masseter Muscle - physiopathology; Motor Neurons - metabolism; Motor Neurons - physiology; Muscle Contraction; Norepinephrine - metabolism; Recruitment, Neurophysiological; Serotonin - metabolism; Synapses - metabolism; Synapses - physiology; Trigeminal Nuclei - physiopathology
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00536.2013

In animals, sodium- and calcium-mediated persistent inward currents (PICs), which produce long-lasting periods of depolarization under conditions of low synaptic drive, can be activated in trigeminal motoneurons following the application of the monoamine serotonin. Here we examined if PICs are activated in human trigeminal motoneurons during voluntary contractions and under physiological levels of monoaminergic drive (e.g., serotonin and norepinephrine) using a paired motor unit analysis technique. We also examined if PICs activated during voluntary contractions are larger in participants who demonstrate involuntary chewing during sleep (bruxism), which is accompanied by periods of high monoaminergic drive. In control participants, during a slowly increasing and then decreasing isometric contraction, the firing rate of an earlier-recruited masseter motor unit, which served as a measure of synaptic input to a later-recruited test unit, was consistently lower during derecruitment of the test unit compared with at recruitment (ΔF = 4.6 ± 1.5 imp/s). The ΔF, therefore, is a measure of the reduction in synaptic input needed to counteract the depolarization from the PIC to provide an indirect estimate of PIC amplitude. The range of ΔF values measured in the bruxer participants during similar voluntary contractions was the same as in controls, suggesting that abnormally high levels of monoaminergic drive are not continually present in the absence of involuntary motor activity. We also observed a consistent "onion skin effect" during the moderately sized contractions (