The sensory fibers include the modalities discriminative touch as well as pain and temperature. The sensory input comes from the face, particularly from the lips, all the mucous membranes inside the mouth, the conjunctiva of the eye, and the teeth. The fiber sizes and degree of myeli-nation are similar to the sensory inputs below the neck. The cell bodies of these fibers are found in the trigeminal ganglion inside the skull.
The fibers enter the brainstem along the middle cere-bellar peduncle (see Figure 6 and Figure 7). Within the CNS there is a differential handling of the modalities, comparable to the previously described pathways in the spinal cord.
Those fibers carrying the sensations of discriminative touch will synapse in the principal (main) nucleus of CN V, in the mid-pons, at the level of entry of the nerve (see Figure 8B and Figure 66B). The fibers then cross the midline and join the medial lemniscus, terminating in the ventral posteromedial (VPM) nucleus of the thalamus (see Figure 12 and Figure 63). They are then relayed via the posterior limb of the internal capsule to the postcentral gyrus, where the face area is represented on the dorsolat-eral surface (see Figure 14A); the lips and tongue are very well represented on the sensory homunculus.
Those fibers carrying the modalities of pain and temperature descend within the brainstem. They form a tract that starts at the mid-pontine level, descends through the medulla, and reaches the upper level of the spinal cord (see Figure 8B) called the descending or spinal tract of V, also called the spinal trigeminal tract. Immediately medial to this tract is a nucleus with the same name. The fibers terminate in this nucleus and, after synapsing, cross to the other side and ascend (see Figure 40). Therefore, these fibers decussate over a wide region and do not form a compact bundle of crossing fibers; they also send collaterals to the reticular formation. These trigeminal fibers join with those carrying touch, forming the trigeminal pathway in the mid-pons. They terminate in the VPM and other thalamic nuclei, similar to those of the anterolateral system (see Figure 34; also Figure 12 and Figure 63). The trigeminal pathway joins the medial lemniscus in the upper pons, as does the anterolateral pathway (see Figure 36 and Figure 40).
The cross-sectional levels for this pathway include the three medullary levels of the brainstem, the mid-pons, and the lower midbrain.
The principal nucleus of CN V is seen at the mid-pontine level (see also Figure 66B). The descending trigeminal tract is found in the lateral aspect of the medulla, with the nucleus situated immediately medially (see Figure 67A and Figure 67B). The crossing pain and temperature fibers join the medial lemniscus over a wide area and are thought to have completely crossed by the lower pontine region (see Figure 66A). The collaterals of these fibers to the reticular formation are shown.
Trigeminal neuralgia is an affliction of the trigeminal nerve of uncertain origin which causes severe "lightning" pain in one of the branches of CN V; often there is a trigger such as moving the jaw, or an area of skin. The shooting pains may occur in paroxysms lasting several minutes. An older name for this affliction is tic douloureux. Treatment of these cases, which cause enormous pain and suffering, is difficult, and used to involve the possibility of surgery involving the trigeminal ganglion inside the skull, an extremely difficult if not risky treatment; nowadays most cases can be managed with medical therapy.
A vascular lesion in the lateral medulla will disrupt the descending pain and temperature fibers and result in a loss of these sensations on the same side of the face, while leaving the fibers for discriminative touch sensation from the face intact. This lesion, known as the lateral medullary syndrome (of Wallenberg), includes other deficits (see Figure 40 and discussed with Figure 67B). A lesion of the medial lemniscus above the mid-pontine level will involve all trigeminal sensations on the opposite side. Internal capsule and cortical lesions cause a loss of trigem-inal sensations from the opposite side, as well as involving other pathways.
FIGURE 35: Trigeminal Pathways — Discriminative Touch, Pain, and Temperature
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