Difference between revisions of "Trigeminal Nervous System Segmentation"

Motoneuronal activation follows the principle of the section: the smaller motorneurons are activated first<ref name="roth" />. Masseteric motorneurons show normal excitability when they are triggered by the stimulation from the motor cortex. Schwartz and Lund (1995) have recently studied the effect of nociceptive pressure on the mandibular movement and the EMG activity of the masseter in decerebrated rabbits. The motoneurons stimulated the corticobulbar tract through the same path, but the two entrances differed. In our experiments the short-lived high frequency discharge of action potential is of a phasic type, even if in the works of Schwartz and Lund<ref>{{cita libro  

  }}</ref>, the potentials for action are tonic. In their experiments on tonic pain, the amplitude of mandibular movements and the recruitment of the masseteric motorneurons decrease. However, we can exclude a greater excitability all along the path from the motor cortex to the lower motorneurons.

===Trigeminal Peripheral Area===

[[File:Bilateral Root-MEPs.jpg|left|thumb|'''Figure 2:''' bilateral Root-MEPs]]

When we need to assess the integrity of the trigeminal roots, we should evoke a motor action potential of the peripheral trigeminal motor tract in a synchronous and symmetrical way. In this situation, a _m TCS is not suggested: it would rather be advisable to adopt the bilateral electric stimulation routines called _bilaterallyRoot-MEPs technology that we fine-tuned in our neurophysiology laboratories<ref>{{cita libro  

In order to achieve this target, synchronicity, symmetry, and the pulse maximum power are essential. This is why electromyographers must be used for evoked potential and they must be equipped with two independent and autonomous high voltage electrostimulators. The EMG that employ tools with a single multiplexated electrostimulator to deliver two or more stimuli are not appropriated: the second impulse would lose synchronicit, because the condensers need time to recharge (Fig. 2).

[[File:FEM.jpg|left|thumb|'''Figure 3:''' Electrophysiological procedure for trigeminal evoked potentials and intracranial diffusion of the induced electric field. ]]

Figure 3A shows the position of the electrodes (Figure 12A); the maximum current will spread under the cathodes (red in colour) in the parietal cortex (Figure 3B); in the cranial cortex region, near the trigeminal motor root, the current density is of low magnitude (Figure 3C, black arrows). Figure 3D shows the current density that spreads in the brain tissue. It underlines the least amount of electrical current within the brain tissue necessary to saturate the trigeminal motor (relative to the amount required to evoke a response of the trigeminal motor cortex under the cathode). This is one of the reasons that urged us to choose this kind of evoked response (absolutely peripheral) rather than the cortical one (with higher threshold, lower stability, and less focus).

[[File:Test trigeminali.jpg|left|thumb|'''Figure 5:''' Tests mainly used in trigeminal neurophysiopathology]]

The trigeminal brainstem area is the most complex area to study and interpret because of the complexity of its multi-synaptic connections. The following electro-physiological tests are objectively sufficient to understand the cryptic language of the SCNS. These will be treated in this section, but they will also be resumed in other editions. The following trigeminal reflexes will be considered: the ''jaw jerk'', the ''mechanical and electrical silent period'', the ''recovery cycle of the masseteric inhibitory reflex'' and the ''laser silent period'', as well as the ''masseteric laser-evoked potentials'' (Fig. 5).

[[File:Riflesso mandibolare.jpg|left|thumb|'''Figure 6:''' Mandibular reflex performed with Nihon Kohden instrumentation]]

  }}</ref>. Although in trigeminal neuropathy or multiple sclerosis the jaw jerk reflex may be retarded by several milliseconds, asymmetries of latency of only 0.8 ms were considered to be a higher limit of normality in neurological studies (Fig.6).

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 (Fig. 6).

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

  }}</ref> (Fig.7).  

[[File:CR_MIR_masseter_inhibitory_recovery_cycle_reflex.jpg|left|thumb|'''Figure 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)]]

The recovery cycle of the masseteric reflection reflex (_rc MIR: recovery cycle Masseteric Inhibitory Reflex) has been studied by generating pairs of stimuli with identical characteristics provided percutaneously by an electrical stimulator positioned on the patient’s face in the emergency area of the mental nerves (Fig. 8).

[[File:Laser test.jpg|thumb|'''Figure 9:''' Preparation of the subject to laser stimulation to evoke nociceptive evoked potentials and masseteric inhibitory reflexes.]]

To evoke motor responses from the masticatory muscles or potential laser evenings, we employ a laser stimulator CO₂ (Neurolas, Florence, Italy) capable of generating a radiant caloria (10.6 mm; intensity 1.5–15 W; duration of 10–15 ms; diameter of the 2.5 mm spot) in the area of the skin in upper and lower upper and lower region (trigeminal terrain V2 and V3). The subjects must sit comfortably in the dental chair and wear protective goggles (Fig. 9). To avoid the habit to the nociceptive stimuli and the overheating of the skin, the irradiated points have been shifted after each stimulation.   

[[File:Laser_Evoked_Potentials_-_Blink_reflex_-_Masseter_Silent_Period.jpg|left|thumb|'''Figure 9:'''  Laser stimulation that evoke a Blink reflex (BR), a Masseter Inhibitor Reflex (MIR) and Evoked Potentials Laser (LEPs)]]The perception threshold has been determined by the limits method in two sets of intensity by increasing or decreasing the trigger stimulation. The intensity of the laser beam is fixed at 1.5 the perceptual threshold. With regard to LEP (LEP: Laser Evoked Potentials), signals are recorded through the top-disk electrodes (Cz) with two references for each side on the ear lobes (A1, A2). Two blocks of eight to 12 tests each are mediated off-line. Signs are amplified, filtered (0.5–50 Hz), and stored by means of an analyser for biopotential (Premiere, Medelec, UK). For each block, we have measured the latency of the N and P components and the peak-to-peak width of the potential evoked.<ref>Romaniello, A., et al., ''[https://www.ncbi.nlm.nih.gov/pubmed/?term=Assessment+of+nociceptive+trigeminal+pathways+by+laser-evoked+potentials+and+laser+silent+periods+in+patients+with+painful+temporomandibular+disorders.+Pain%2C+2003 Assessment of nociceptive trigeminal pathways by laser-evoked potentials and laser silent periods in patients with painful temporomandibular disorders.]'' Pain, 2003. 103(1-2): p. 31-9. </ref> For LSPs, the same parameters were used as described above, but the recording is performed on masseter muscles. The area was subtracted from the 100-ms curve prior to adjusted and mediated laser stimulus (pre-analysis). The duration of the registered EMG activity is 400 ms, of which 100 ms corresponds to pre-stimulus and 300 ms in the post-stimulus period. The EMG signals are amplified, filtered (20 Hz–1 kHz), and sampled at 4 kHz. Subjects are asked to tighten their teeth with the maximum muscular strength to determine the EMG activity corresponding to the Maximum Volunteer Contraction (MVC: Maximum Voluntary Contraction) of masseter muscles and at different levels (15–25%), (35–45%), (55-65%), and (75–85%) of MVC. Subjects receive visual feedback with markers on the computer screen, which clearly indicates when the default level is reached. As one can see in Figure 19, the results of the following work are schematized in accordance with the type of test performed. With a laser stimulation in the skin region corresponding to the emergence of the supraorbital nerve (V1), we can have a Blink Reflex as a reflected response (BR) and notice the perfect symmetry of the responses on sides R1 and R2. Stimulation in the perioral region will result in a reflex response from the masseterini muscles called Laser Silent Period (LSP: Laser Silent Period). We might note that, in this test, a slight asymmetry of the track—mainly caused by the different degree of motoneural recruitment in the maximum intercuspidation. The registration on the scalp determines the potential laser summers of the trigeminal somatosensory area (LEPs) and measure the negative and positive spikes (N and P). Here, too, we witness a high level of symmetry.