The trigeminal nerve supplies the muscles of mastication and sensation to the face, including the corneal reflex (figure 18). The lingual branch carries taste to the anterior two-thirds of the tongue and parasympathetic fibres to the submandibular ganglion, both derived from the facial nerve. Herpes zoster (shingles) infection, may cause severe damage to the cornea and may also involve the geniculate and otic ganglia, resulting in deafness, vertigo and facial palsy (Ramsay Hunt syndrome). Trigeminal neuralgia is a condition of unknown origin, it is characterised by severe intense pain over one or more divisions of the nerve.
Touch, pain and temperature are tested over the temple, cheek and jaw, corresponding to the phthalmic, maxillary and mandibular divisions of the trigeminal nerve (figure 19a–c).
7. Greater occipital
10. External nasal
12. Lesser occipital
14. Great auricular
15. Transverse cervical
C – cervical roots Trigeminal:
III mandibular divisions
To test the corneal reflex, twist the corner of a piece of cotton wool into a point (figure 19d). Ask the subject to look towards the other side, and then stroke the cotton wool gently over the exposed cornea. Be sure not to touch the eyelashes and remain out of the line of vision. Note any contact lenses; ask the subject to remove them when testing this reflex. The muscles controlling a blink reflex are innervated by the facial nerve; the trigeminal provides the sensory component.
The motor fibers of the trigeminal nerve supply the muscles of mastication (figure 20; table 1). Ask the subject to bite hard and palpate the contracting masseter and temporalis muscles over the angle of the jaw and the temple respectively (figure 21a,b).
Protrusion of the jaw and movements to each side, are by the pterygoid muscles and can be assessed against resistance (figure 22a-c).
Figure 20 Muscles of mastication
1. Parietal bone
2. Superior temporal line
3. Frontal bone
4. Occipital bone
5. Temporal bone
7. Zygomatic bone
8. Mastoid process
10. Styloid process
11. Posterior belly of digastric
15. Middle constrictor
17. Anterior belly of digastric 18. Hyoglossus
20. Inferior constrictor
Table 1 Muscles of mastication
Opening the mouth is partly through the action of the lateral pterygoid muscle drawing the mandibular condyle onto the articular eminence; the muscle pulls the fibrocartilaginous disc forward, flattening it, in this movement (figure 23a,b). The digastric, geniohyoid and mylohyoid muscles support the movement.
The temporomandibular joint is subcutaneous and palpable laterally (figure 24). The condyle can be felt to glide over the articular eminence. The joint may click, and become painful from trauma and arthritic changes. It is very stable in the closed position, but may be dislocated by an injury to the open mouth. Jaw fractures usually involve the body of the mandible in the region of the first molar: fractures of the angle of the mandible are usually anterior to the attachment of the masseter.