Abstract
Previously we have reported a biphasic increase in excitatory drive of the masseter muscle during natural chewing in young adults. We now hypothesize that sensory inputs from the periodontal mechanoreceptors (PMRs) are responsible for the late increase in excitatory drive during this biphasic movement. 13 participants with implant-supported bridges in both jaws, and thus lacking PMRs, and 13 participants with natural dentition chewed and swallowed model food of different hardness. Electromyographic (EMG) activity of the masseter muscle was recorded, along with the position of the mandible, and the muscle activity and jaw kinematics during the different phases of the chewing cycle were analyzed. Throughout the entire masticatory sequence, the excitatory drive of the masseter muscle during the jaw closing increased in a biphasic manner for the dentate participants; whereas biphasic elevation was observed only during the middle and last segments in the implant participants. Dentate participants exhibited significantly greater boosting of the EMG activity during late jaw closing than the implant participants, irrespective of food hardness and segment of the masticatory sequence. Sensory information from PMRs are required for boosting the enhancement of masseter muscle activity during the late jaw closing, during tooth-food contact.
Highlights
During mastication, the central pattern generators in the brain stem continuously senses inputs, primarily from the muscle spindles in the jaw-closing muscles and from the periodontal mechanoreceptors (PMRs)[1,2]
In an earlier examination of variations in the masseter muscle activation with time during natural chewing cycles in young adults, we found that the excitatory drive of the masseter muscle during the jaw-closing phase increases in a biphasic manner showing an early and a late component[17]
As the food was particularized, the masseter EMG activity gradually declined as the masticatory sequence progressed (F2:48 = 30.5; p < 0.001), this modulation of the muscle activity was significantly smaller in the implant group than in the dentate group (F2:48 = 6.0, P < 0.01 for a two-way interaction between the segment of the masticatory sequence and group). (For details regarding changes in integrated EMG activity during the different phases of the chewing cycle, see Grigoriadis et al.3)
Summary
The central pattern generators in the brain stem continuously senses inputs, primarily from the muscle spindles in the jaw-closing muscles and from the periodontal mechanoreceptors (PMRs)[1,2]. Since the transition between these components occurs approximately at the time of tooth–food contact, we proposed that signals from PMRs during the initial tooth-food contact contribute to initiation of the late component We tested this hypothesis by analyzing the EMG activity of the masseter muscle during the jaw-closing phase in individuals with dental implants chewing food items of different hardness. Since the PMRs are suited to convey detailed information about contact between the food and dentition during biting and chewing[8,18,19,20,21,22,23,24] we predicted that modulation during the early component of the masticatory sequence in response to changes in the properties of food might be impaired in individuals with implants If so, this would suggest that the PMRs are critical for obtaining information concerning the current properties of the food for use in subsequent chewing cycles
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