Difference between revisions of "Trigeminal Nervous System Segmentation"

The afferents from group II in the division of the maxillary and mandibular trigeminal nerve apply a powerful inhibition of the motoneurons of the mastication muscles through synaptic and polysynaptic reflexes. A remarkable feature of the mandibular reflexes, however, is their bilateral symmetry. In some patients with multiple sclerosis (MS) the latency is prolonged, whereas it lacks in others: this reflex can sometimes be essential for diagnosing the lesions of the brainstem in MS. It might alsi be more effective for therapy (<translate>Figure</translate> 6).

====Masseteric Mechanical Silent Period====

[[File:PSM_-_Masseter_mechanical_Silent_Period.jpg|left|thumb|'''<translate>Figure</translate> 7:''' Representation of a typical masseteric inhibitory reflex (MSP) ]]

The jaw jerk is a short-latency excitatory reflection that can be evoked by a stretch of the mandibular elevators through a percussion produced by a triggered neurological hammer. The excitation on motoneurons <code>α</code> from the neuromuscular spindles is the only generally accepted explanation. When this type of mechanical stimulus is applied during voluntary activation EMG, (by pressing the teeth,as an example) the jaw overlaps with the Interference  EMG activity, and is followed by a period of absence or depression of the electromyographic activity, the so-called '''Masseteric Silent Period''' (MSP)<ref>{{cita libro  

The MSP has sparked particular interest as it has been shown that the duration of the silent period is higher in patients with TMDs<ref>{{cita libro  

  }}</ref>. This period of depression or absence of EMG activity can be caused by various active inhibitory influences, but it can also be caused by synchronization of the a motoneurons after the initial efferent volley.

The criteria for clinical evaluation remain unclear: there is still a certain confusion about both measurement and reported values, which vary considerably in different studies. Klineberg and collaborators provoked a percussion on the chin through the use of a standardized solenoid in 10 healthy subjects during mass isometric activity: it generated a vertical movement that was parallel to the surface fibres of the superficial muscle of the masseter muscle, and they after analysed the mechanical silent period. The reflection evoked the classic jaw jerk in short latency (mean, 8.3 ms) followed by an SP (average latency, 15.9 ms, average duration, 42.6 ms). The duration of the SP greatly varies not only between subjects, but also within individuals<ref>{{cita libro  

====Recovery Cycle of Masseteric Inhibitory Reflex====

[[File:CR_MIR_masseter_inhibitory_recovery_cycle_reflex.jpg|left|thumb|'''<translate>Figure</translate> 8:''' Representation of the masseteric inhibitory reflex recovery cycle. Note the pair of electrical stimuli (S1 and S2) and the corresponding silent periods (ES1 and ES2)]]

Headache is oftes associated with a ‘sensitization’ of the nociceptive trigeminal system with the involvement of anti-nocicective mesenchephalic structures such as periaqueductal substance, locus coeruleus, and the nuclei of the raphe, which have a modulator effect on trigeminal sensitive nuclei<ref>{{cita libro  

The descending modulator effect from these nuclei has been shown through experimental studies on animals, and the temporary trigeminal ‘sensitization’ effect during hemicranic attack has been shown to be clinically demonstrated through electrophysiological studies<ref>{{cita libro  

  }}</ref>. The most credited hypothesis considers the dysfunction of anti-nocicadjective nuclei as the cause of a disinhibition of the trigeminal nociceptive system<ref>{{cita libro  

Orofacial pain, including algies from TMDs, appears to have a modulation effect on the mandibular reflexes. Electrophysiological studies have shown that the experimentally induced pain by infusion of saline hypertonic solution to 5% in the masseter results in an increase in the peak-to-peak size of the jaw spider. This facilitatory effect seems to be related to the increased sensitivity of the fusimotor system, which causes muscle stiffness<ref>{{cita libro  

Other animal studies have been carried out in the experimentally induced muscle pain, demonstrating that the activation of muscle nociceptors  markedly influences the proprioceptive properties of neuromuscular fusis through a central neural pathway.<ref>{{cita libro  

  }}</ref> However, the washing of the local algogena substance leads to a return to normality of the tendon reflexes. In pain from TMDs, there is a dichotomy between schools of thought: while one school proposes an ‘hyperactivity’ effect of the central nervous system (CNS: Central Nervous System)<ref>{{cita libro  

  }}</ref>; the other suggests a normal excitability of the trigeminal nervous system<ref>{{cita libro