Study guidelines
- 1.
The motor nucleus supplies the muscles of mastication.
- 2.
The mesencephalic unipolar neurons are proprioceptive.
- 3.
The neurons of the principal sensory nucleus receive sensory inputs from the face and underlying mucous membranes.
- 4.
The spinal nucleus is of special clinical importance because of its huge nociceptive territory.
Trigeminal nerve
The trigeminal nerve has a very large sensory territory that 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 ).
Trigeminal neuralgia is an important condition occurring in middle age or later, characterised 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 21.1 ). Attacks are triggered by everyday sensory stimuli such as 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 fibres receiving tactile signals from skin
Antiepileptic drugs that exert a blocking effect on sodium and/or calcium channels (e.g. carbamazepine) may suffice to keep ephapses at bay. Surgery is indicated for those who fail to respond.
A procedure that can be performed under local anaesthesia is electrocoagulation of the affected division, through a needle electrode inserted through the foramen rotundum or ovale from below. The intention is to heat the nerve sufficiently to destroy only the finest fibres, in which case analgesia is produced but touch (including the corneal reflex) is preserved.
The final option is to decompress the afflicted nerve root through an intracranial approach whereby neighbouring vessels are lifted away from it.
A surgical procedure of historic interest is medullary tractotomy , whereby the spinal root was sectioned through the dorsolateral surface of the medulla. In successful cases, pain and temperature sensitivity was lost from the face but touch (mediated by the pontine nucleus) was preserved. This procedure was abandoned owing to a high mortality rate associated with compromise of underlying respiratory and cardiovascular centres.
Suggested reference
Motor nucleus ( Figures 17.16 and 21.2 )
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 21.3 ), along with the 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 ).
Sensory nuclei
Three sensory nuclei are associated with the trigeminal nerve: mesencephalic , pontine ( principal ) , and spinal .
Mesencephalic nucleus
The mesencephalic nucleus is unique in being the only nucleus in the central nervous system (CNS) that contains the cell bodies of primary unipolar sensory neurons. Their peripheral processes enter the sensory root via the mesencephalic tract of the trigeminal nerve. Some travel in the mandibular division to supply stretch receptors (neuromuscular spindles) in the masticatory muscles. Others travel in the maxillary and mandibular divisions to supply stretch receptors (Ruffini endings) in the suspensory, periodontal ligaments of the teeth.
The central processes of the mesencephalic afferent neurons descend through the pontine tegmentum in the small tract of Probst . Most fibres of this tract terminate in the supratrigeminal nucleus; others end in the motor nucleus or in the pontine sensory nucleus; a few travel as far as the dorsal nucleus of the vagus.
Pontine nucleus
The pontine (principal sensory) nucleus ( Figure 17.2 ) is homologous with the dorsal column nuclei (gracile and cuneate). It processes discriminative tactile information from the face and oronasal cavity.
Spinal nucleus
The spinal nucleus extends from the lower part of the pons to the third cervical segment of the spinal cord (hence the term ‘spinal’). Two minor nuclei in its upper part (called pars oralis and pars interpolaris ) receive afferents from the mouth. The main spinal nucleus ( pars caudalis ) receives nociceptive and thermal information from the entire trigeminal area, and even beyond.
In section, the main spinal nucleus is seen to be an expanded continuation of the outer laminae (I-III) of the dorsal horn of the cord ( Figure 21.4 ). The inner three laminae (IV-VI) are relatively compressed. Laminae III and IV are referred to as the magnocellular part of the nucleus. In animals, nociceptive-specific interneurons are found in lamina I. ‘Polymodal’ neurons are in the magnocellular nucleus and correspond to lamina V neurons lower down. They respond to tactile stimuli applied to the trigeminal skin area and also to noxious mechanical stimuli (e.g. pinching the skin with a forceps), whereas the nociceptive-specific neurons have small receptive fields confined to one territory (a patch of skin or mucous membrane). Many of the polymodal neurons show the phenomenon of convergence to a marked degree. In anaesthetised animals a single neuron may be responsive to noxious stimuli applied to a tooth, to facial skin, or to the temporomandibular joint. This finding provides a plausible basis of explanation for erroneous localisation of pain by patients. Examples are given in Clinical Panel 21.2 .
