Study guidelines
- 1.
Cranial Nerve VII is the most commonly paralysed of all peripheral nerves, owing to the great length of its canal in the temporal bone, where it is at risk of compression when swollen. Because VII supplies the muscles of facial expression, the effects of peripheral facial nerve paralysis are obvious to all.
- 2.
Learn the distinctions between upper and lower motor neuron lesions of VII.
- 3.
Note that VII participates in several important reflex arcs.
Facial nerve
The facial nerve supplies the muscles derived from the second branchial arch. These include the muscles of facial expression and four others mentioned below. It is accompanied during part of its course by the nervus intermedius , which is the sensory and parasympathetic part of the facial nerve. The nervus intermedius supplies secretomotor fibres to glands in the eyes, nose, and mouth, and gustatory fibres to the tongue and palate.
The facial nerve arises from the branchial (special visceral) efferent cell column caudal to the motor nucleus of the trigeminal nerve ( Figure 17.2 ). The facial nucleus occupies the lateral region of the tegmentum in the caudal part of the pons ( Figures 17.15 , 22.1 ). Before emerging from the brainstem, it loops, as the internal genu , around the abducens nucleus, creating the facial colliculus in the floor of the fourth ventricle.
The nerve emerges at the lower border of the pons at the pontomedullary junction together with the nervus intermedius. Both nerves cross the subarachnoid space in company with the vestibulocochlear nerve, to the internal acoustic meatus. Above the vestibule of the labyrinth, it enters a 7-shaped bony canal having a backward bend at the external genu of the facial nerve. Prior to escaping the canal at the stylomastoid foramen, it supplies the stapedius muscle. Upon escape, it supplies the posterior belly of the occipitofrontalis, the stylohyoid, and the occipital belly of the digastric. It then turns forward within the substance of the parotid gland while dividing into the five named branches to the muscles of facial expression ( Figure 22.2 ).
Supranuclear connections
All of the cell bodies of the motor nucleus receive a corticonuclear supply from the ‘face’ area of the contralateral motor cortex. In addition, those to the muscles of the upper face (occipitofrontalis and orbicularis oculi) receive a bilateral supply from the ipsilateral motor cortex. The bilateral supply for the upper facial muscles is reflected in their habitual paired activities in wrinkling the forehead, blinking, and squeezing the eyes closed. The muscles around the mouth, on the other hand, are often activated unilaterally for some expressive purpose. The partial bilateral supply to the facial muscles helps to distinguish a supranuclear from a nuclear or infranuclear lesion of the nerve ( Clinical Panel 22.1 ).
Supranuclear lesions
The commonest cause of a supranuclear lesion of the seventh nerve is a vascular stroke, in which corticobulbar and corticospinal fibres are interrupted at or above the level of the internal capsule. The usual effect of a stroke is to produce a contralateral motor weakness of the lower part of the face and of the limbs. (The lower part of the face may appear to recover momentarily when participating in a spontaneous smile, as mentioned earlier.) The upper face escapes because of the bilateral supranuclear supply to the upper part of the facial nucleus.
Nuclear lesions
The main motor nucleus may be involved in thrombosis of one of the pontine branches of the basilar artery. As might be anticipated from the relationships depicted in Figure 22.1 , the usual result of such a lesion is an alternating (crossed) hemiplegia : complete paralysis of the facial and/or abducens nerve on the side of the lesion combined with motor weakness of the limbs on the opposite side owing to concomitant involvement of the corticospinal tract.
Infranuclear lesions
Bell palsy is a common disorder caused by a neuritis (possibly viral in origin) of the facial nerve. The inflammation causes the nerve to swell and conduction is compromised by the close fit of the nerve in its bony canal in the interval between the geniculate ganglion and stylomastoid foramen. There may be some initial pain in the ear, but the condition is otherwise painless.
Facial paralysis is usually complete. On the affected side, the patient is unable to raise the eyebrow, close the eye, or retract the lip ( Figure 22.3 ). The patient may experience hyperacusis : ordinary sounds may be unpleasantly loud because of loss of the damping action of the stapedius muscle.
Tests may reveal dysfunction of nervus intermedius fibres, with ipsilateral reduced lacrimal and salivary secretions and loss of taste from the anterior part of the tongue.
Four out of five patients recover completely within a few weeks because the nerve has only suffered a conduction block ( neuropraxia ). In the remainder, the nerve undergoes Wallerian degeneration ( Chapter 9 ); recovery takes about 3 months and is often incomplete. During regeneration, some preganglionic fibres of the nervus intermedius may enter the greater petrosal nerve instead of the chorda tympani, with the result that the lacrimal gland becomes active at mealtimes (so-called ‘crocodile tears’).
