21 Trigeminal nerve
Trigeminal Nerve
The trigeminal nerve has a very large sensory territory which includes the skin of the face, the oronasal mucous membranes and the teeth, the dura mater, and major intracranial blood vessels. The nerve is also both motor and sensory to the muscles of mastication. The motor root lies medial to the large sensory root at the site of attachment to the pons (Figure 17.16). The trigeminal (Gasserian) ganglion, near the apex of the petrous temporal bone, gives rise to the sensory root and consists of unipolar neurons.
Details of the distribution of the ophthalmic, maxillary, and mandibular divisions are available in gross anatomy textbooks. Accurate appreciation of their respective territories on the face is essential if trigeminal neuralgia is to be distinguished from other sources of facial pain (Clinical Panel 21.1).
Clinical Panel 21.1 Trigeminal neuralgia
Trigeminal neuralgia is an important condition occurring in middle age or later, characterized by attacks of excruciating pain in the territory of one or more divisions of the trigeminal nerve (usually II and/or III). The patient (who is usually more than 60 years old) is able to map out the affected division(s) accurately. Because it must be distinguished from many other causes of facial pain, the clinician should be able to mark out a trigeminal sensory map (Figure CP 21.1.1). Attacks are triggered by everyday sensory stimuli, e.g. brushing teeth, shaving, and chewing, and the tendency of patients to wince at the onset of attacks accounts for the French term tic doloureux.
Episodes of paroxysmal facial pain occurring in young adults should raise a suspicion of multiple sclerosis as the cause. Postmortem histology in such cases has revealed demyelination of the sensory root of the trigeminal nerve where it enters the pons. Demyelination of large sensory fibers receiving tactile signals from skin or mucous membranes in trigeminal territory may cause their exposed axons to come into direct contact with unmyelinated axons serving pain receptors. Animal experiments have shown that this type of contact can initiate ephaptic transmission of action potentials between them. It is now widely accepted that the most frequent etiology in later years is vascular compression, usually by a ‘sagging’ posterior cerebral artery in transit around the brainstem. The trigeminal CNS/PNS transition zone (Ch. 6) is several millimeters lateral to the entry zone into the pons, and postmortem histology has provided evidence of the demyelinating effect of chronic pulsatile compression.
Motor nucleus (Figures 17.16 and 21.1)
The motor nucleus is the special visceral efferent nucleus supplying the muscles derived from the embryonic mandibular arch. These comprise the masticatory muscles attached to each half of the mandible (Figure 21.2), along with tensor tympani, tensor palati, mylohyoid, and anterior belly of digastric muscle. The nucleus occupies the lateral pontine tegmentum. Embedded in its upper pole is a node of the reticular formation, the supratrigeminal nucleus, which acts as a pattern generator for masticatory rhythm.
Voluntary control is provided by corticonuclear projections from each motor cortex to both motor nuclei, but mainly the contralateral one (Figure 17.3).