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Understanding the mechanisms that regulate muscle contraction is key to your question as to why muscles “light up”. Protein filaments of myosin tugging on protein filaments of actin shorten, or contract. These are the elemental entities within sarcomere which are within all the myofibrils which in turn are within muscle fibers which in turn make up muscle.
Muscle Physiology of Habitual Accommodating Rest Position: Hyper muscle activity occurs when cross bridging of the actin and myosin is either greatly over contracted or over stretched. Normal healthy levels of muscle tonus are no longer able to be maintained.
Neuro Physiology: Proprioceptive fibers (from the peripheral muscle motor units to the trigeminal nerve) go to the sensory and spinal nucleus of Trigeminal nerve. The main relay nuclei are the sensory and spinal nuclei. Most of the afferent nerve cell bodies form information from the head are located in the V (Semilunar) ganglion. The sensory root is the mandibular V3 which sends peripheral signalling via the mesencephalic trigeminal nucleaus which gives reflex control of the mandible/ the bite.
When normal muscle posture is no longer able to be maintained the nocioceptive feedback mechanism: A compensating“tonic stretch reflex” occurs to adjust muscular posture. A “reciprocal Inhibition” and “crossed extensor reflex” reaction occurs. Muscle length changes via the gamma feedback mechanism: A constant feedback loop to keep muscles shortened. Muscle spindles alter – proprioceptors between the muscle fibers that are sensitive to muscle length and rate of change occurs. Tendons and muscles are also involved via the golgi tendon apparatus which monitor tension rather than length. Tendon, joints, periosteum, fascia and subcutaneous tissue and skin have Pacini’s corpuscles that mediate pressure. Muscles fibers = Motor Unit – Neurons stimulating many muscle fibers. Periodontal membrane around each tooth have afferent pathways from dental pulp receptors.
This is the Peripheral Neural Feedback Mechanism that relates to the proprioceptive (periodontal mechanoreceptor mentioned above) which then contribute to hyper muscle activity when a bad bite is out of alignment or balance – muscles then “light up”.
The nerve cell body, the long axon running down the motor nerve, its terminal branches and all the associated muscle fibers supplied by these branches is what makes up a motor unit.
Surface electrodes is the preferred choice when monitoring the potentials of many motor units (motor unit pools) via surface skin electrodes which contribute substantially to the sEMG recordings that “light up” (displaying hyper muscle activity or high amplitude EMG readings when the muscles are at their habitual accommodated rest position.
This is one part of the explanation as to WHY muscles are hyperactive but also has a multitude of other physiologic factors and neuro – muscular mechanisms that must be considered, but unfortunately rarely discussed nor understood among a majority of clinicians today.
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