Pain, Function, and Parafunction of the Jaw System in Relation to Neuroplasticity

Pain and parafunctional behaviour may influence how the jaw motor system functions and adapts. The jaw system provides a relevant model for studying these interactions because of its high sensorimotor demands and its susceptibility to temporomandibular disorders (TMD) and bruxism. This thesis investigated how pain and parafunction influence training-induced neuroplasticity and sensorimotor performance in the jaw system, with a focus on corticomotor excitability assessed via transcranial magnetic stimulation. Three studies were conducted. Study I was a systematic review of 17 studies examining the effects of acute and chronic pain on training-induced corticomotor plasticity and functional outcomes. Study II was a cross-sectional observational study of 70 adults with and without TMD pain, combining ecological momentary assessment of awake bruxism with simultaneous masseter surface electromyography (sEMG) to assess jaw muscle overload in daily life. Study III was a case-control intervention study with 29 participants examining the effects of a seven-day jaw exercise programme on jaw sensorimotor performance and masseter corticomotor excitability. Study I showed that both acute and chronic pain may perturb training-induced corticomotor neuroplasticity, with more consistent effects in acute pain. Study II showed that self-reported awake bruxism correlated with sEMG-assessed muscle overload, and that individuals with TMD pain exhibited more awake bruxism, higher stress levels, and longer periods of low-intensity muscle overload. Study III showed that short-term jaw training improved sensorimotor performance in both the pain-free and TMD pain groups, although the pattern differed between groups. Higher bruxism scores were associated with higher active motor threshold and smaller training-related increases in motor evoked potential amplitude. Taken together the results suggest that pain and parafunctional behaviour can influence the loading, capacity, and adaptive potential of the jaw motor system. These findings support a model in which TMD pain and bruxism interact to shape corticomotor neuroplasticity and jaw sensorimotor adaptation.