Difference between revisions of "Trigeminal Nervous System Segmentation"

[[File:Gray778 Trigeminal.png|thumb|The trigeminal nerve]]

Segmenting the part of the CNS involved in the masticating function results in dividing it into three macro areas: the ''cortical area'' (in which the response of the trigeminal motor action occurs - i.e. the Trigeminal Cortical Area); the ''peripheral area'', where the response of the trigeminal root is analysed; and the ''brainstem-pontine-related area'', which is involved in a broad genesis of trigeminal reflexes.

===Trigeminal Cortical Area===

When we try to assess the integrity and/or symmetry of the Trigeminal Motor Cortex (TMC), we certainly cannot completely trust the MR imaging techniques because they lack data about ‘functionality’: the image symmetry of the trigeminal motor cortical areas does not results in functional symmetry, even if we could try a kind of comparison with the functional MRI (fMR). This latest technology, however, is unlikely to be extemporaneous, because the speed of activation of neuronal excitability is too high. As a result, we are forced to employ electrophysiological stimulation techniques to evoke a response from the pyramidal bundle coming from neurons of the first order pf the TMC (Fig. 1).  

[[File:Trigeminal Cortical Area - C-MEPs.jpg|thumb|'''Figure 1:''' Transcranial Magnetic Stimulation |400x400px|left]]

Nowadays electrical transcranial stimulation is no longer used; it has been replaced by the Magnetic Transcranial Stimulation or Stimulation _mTCS.

===Magnetic Transcranial Stimulation (mTCS)===

In our laboratories, this procedure requires a unique experiment in which the electrodes are positioned on the patient and a series of tests is carried out, including the magnetic Transcranial Stimulation _mTCS, trigeminal reflexes, and the Electric Transcranial Stimulation of the trigeminal Root _eTCS, as described below.

In Figure 10, we can understand the genesis of the _mTCS and subsequent neuromotor responses named '''Cortex-Motor Evoked Potentials''' (C-MEPs) elicited by an induced current within the brain tissue; this current was determined by the magnetic field crossed from a coil placed on the subject’s skull vertex. As a routine, we use a Magstim BiStim² magnetic stimulator (Magstim Ltd, UK), attached to a circular coil (external diameter 10 cm), for magnetic fields up to 2 T (exit 100%). The intensity of stimulation is expressed as a percentage of the maximum power. The magnetic coil is positioned flat on the scalp; the optimum position is on the medial line, with the centre of the coil slightly forward to the top. This position is capable of evoking a motor action potential on both the masseter muscle, without evident depolarization of the trigeminal root. In order to evoke a Cortex-MEPs, subjects need to exercise a mild mandibular closure with EMG activity. The latency at the onset and the peak-to-peak width are measured at least on six tests<ref>{{cita libro  

===Trigeminal Nervous System excitability===

Transcranial magnetic stimulation activates the primary motor cortex by excitating the interneural fibres that project on pyramidal cells; it also evocates short series of potential of action called Indirect or I-waves. Magnetic high-intensity shocks can also depolarize the assonal portion of the pyramidal cells stimulating mixed events, including a direct D-wave action potential and some Indirect waves (I-wave)<ref name="roth">{{cita libro  

The stimulation of the facial motor cortex evokes a descendant burst that travels along the corticobulbar tract and reaches the trigeminal and facial motorneurons with a multi-synaptic connection, whereas there is a monosynaptic and almost completely controlateral connection at the masseteric motorneurons that is similar to the corticospinal projection on motor neurons of the hand muscles. Pre-innervation has also to be acknowledged when we have to deal with the trigeminal system: even with high intensity magnetic stimulation, no motor potential can be evoked without contracting the target muscles. During the contraction, potential masseterics show shorter latency, short duration, and synchronous responses, which reach an amplitude of approximately 30% of the motor response from direct stimulation of the masseter nerve named M-wave.