Reflex control of masticatory muscles in man

An understanding of the functional connections between the jaw muscles and various receptor systems is necessary to elucidate the process of mastication and its underlying mechanisms. Unless the details of the neural mechanisms that control the motor functions of the masticatory system in health and disease are thoroughly understood, the diagnosis and treatment of masticatory dysfunctions and denture technology will remain at the present "symptomatic" state. Presently, our knowledge of mastication and its control by various receptors is patchy. Most of the work in this area comes from different animal species, using widely varying methodologies. For human work, no standardized methodology exists for investigating the connections of various afferents to the motoneurons that innervate the masticatory muscles. This review will bring together what is known about the basic wiring of the masticatory system and put forward a method for standardized investigation

Jaw movement alters the reaction of human jaw muscles to incisor stimulation

The original publication can be found at www.springerlink.comThe changes in the minimum time to consciously react (reaction time) and the order of jaw muscle recruitment to precisely controlled axial stimulation of the incisors during controlled jaw movements are not known. To this end, ten subjects were recruited to investigate the reaction time of bilateral temporalis and masseter muscles and bite force. Stimuli were delivered axially to the upper central incisors during active jaw closing and opening, and under static conditions. The results showed that the reaction time was increased an average of 35% during both jaw opening and closing movements when compared with static jaw conditions. The left temporalis was recruited approximately 10 ms before the right temporalis, whereas no significant side differences were found between the masseter muscles. The masseter muscles were recruited an average of 20 ms before the temporalis muscles during jaw closing, but no difference existed during opening. Under static conditions the reaction time in the bite force was approximately 16 ms longer than the left temporalis, but was not significantly different from the reaction time of any of the other muscles, indicating that, under the static conditions tested, the left temporalis was more often responsible for initiation of the mechanical reactions in the jaw. Because of active compensation, no force measurements were made during jaw movement. This study is a prerequisite for investigations into the modulation of reflexes during jaw movement, because a response to a stimulus commencing after the minimum reaction time may not be entirely reflex in origin.Russell S. A. Brinkworth and Kemal S. Türke

Periodontal anaesthetisation decreases rhythmic synchrony between masseteric motor units at the frequency of jaw tremor

The original publication is available at www.springerlink.comThis study links the reduction in jaw physiological tremor around 8 Hz following periodontal mechanoreceptor (PMR) anaesthetisation to changes in coherence between masseteric motor unit discharges. We have recorded single motor unit activity from two separate sites in the right masseter muscle during a low level tonic contraction, both prior to and during anaesthetisation of the peri-incisal PMRs. Anaesthetisation of PMRs decreased coherent activity between motor units circa 8 Hz, and decreased synchrony between the same motor unit pairs. It is proposed that tremor-generating inputs that cause rhythmic synchronisation of masseteric motor units arise from, or are amplified by the PMRs.Paul F. Sowman, Kirstin M. Ogston and Kemal S. Türke

A study on synaptic coupling between single orofacial mechanoreceptors and human masseter muscle

The original publication can be found at www.springerlink.comThe connection between individual orofacial mechanoreceptive afferents and the motoneurones that innervate jaw muscles is not well established. For example, although electrical and mechanical stimulation of orofacial afferents in bulk evokes responses in the jaw closers, whether similar responses can be evoked in the jaw muscles from the discharge of type identified single orofacial mechanoreceptive afferents is not known. Using tungsten microelectrodes, we have recorded from 28 afferents in the inferior alveolar nerve and 21 afferents in the lingual nerve of human volunteers. We have used discharges of single orofacial afferents as the triggers and the electromyogram (EMG) of the masseter as the source to generate spike-triggered averaged records to illustrate time-based EMG modulation by the nerve discharge. We have then used cross correlation analysis to quantify the coupling. Furthermore, we have also used coherence analysis to study frequency-based relationship between the nerve spike trains and the EMG. The discharge patterns of the skin and mucosa receptors around the lip and the gingiva generated significant modulation in EMGs with a success rate of 40% for both cross correlation and coherence analyses. The discharge patterns of the periodontal mechanoreceptors (PMRs) generated more coupling with a success rate of 70% for cross correlation and about 35% for coherence analyses. Finally, the discharges of the tongue receptors displayed significant coupling with the jaw muscle motoneurones with a success rate of about 40% for both analyses. Significant modulation of the jaw muscles by single orofacial receptors suggests that they play important roles in controlling the jaw muscle activity so that mastication and speech functions are executed successfully.Kemal S. Türker, Skjalg E. Johnsen, Paul F. Sowman and Mats Trulsso