Facial nerve

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22 Facial nerve

Chapter Summary

Facial nerve

Supranuclear connections
Nuclear connections
Corneal reflex
Nervus intermedius
CLINICAL PANEL

Lesions of the facial nerve
Syndromes of the cerebellopontine angle

Study Guidelines

1 VII is the most commonly paralyzed 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 supplies secretomotor fibers to glands in the eye, nose, mouth, and gustatory fibers 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.

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Figure 22.1 Transverse section of the pons, showing the facial nerve and the nervus intermedius (NI).

The nerve emerges at the lower border of the pons 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 posterior belly of the digastric. It then turns forward within the substance of the parotid gland while breaking up into the five named branches to the muscles of facial expression (Figure 22.2).

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Figure 22.2 Principal extracranial branches of the facial nerve.

Supranuclear connections

All of the cells 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 (occi-pitofrontalis and orbicularis oculi) receive an equal 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).

Clinical Panel 22.1 Lesions of the facial nerve

Supranuclear lesions

The commonest cause of a supranuclear lesion of the seventh nerve is a vascular stroke, in which corticonuclear and corticospinal fibers 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 one side combined with motor weakness of the limbs on the opposite side owing to concomitant involvement of the corticospinal tract.

Infranuclear lesions

Bell’s 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 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. The patient may experience hyperacousis: ordinary sounds may be unpleasantly loud owing to loss of the damping action of the stapedius muscle.

Tests may reveal blockage of nervus intermedius fibers, 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 (Ch. 9); recovery takes about 3 months and is often incomplete. During regeneration, some preganglionic fibers 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’).

Other causes of infranuclear palsy include a patch of demyelination within the pons in the course of multiple sclerosis (Figure CP 22.1.1), tumors in the cerebellopontine angle (Clinical Panel 22.2), middle ear disease, and tumors of the parotid gland. Herpes zoster oticus is a rare but well-recognized viral infection of the geniculate ganglion. Severe pain in one ear precedes a vesicular rash in and around the external acoustic meatus. Swelling of the geniculate ganglion may result in a complete facial palsy (Ramsay Hunt syndrome).

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Figure CP 22.1.1 Complete facial nerve paralysis, patient’s right side. The patient has been asked to smile and to look upward. To compare the two sides, cover the left and right halves of the photograph alternately with a card. On the normal side, (1) the frontalis muscle has raised the eyebrow, (2) the buccinator has retracted the lips, and (3) the platysma is in moderate contraction. On the right side, the lower eyelid is drooping due to paralysis of orbicularis oculi.

The muscles of facial expression are more responsive to emotional states than any other muscle group. A limbic contribution to the supranuclear supply is to be expected, and indeed two have been identified. One is the nucleus accumbens at the base of the forebrain, identified in Figure 33.1. The nucleus accumbens is a ventral part of the basal ganglia, which in turn influence the motor cortex. That circuit is compromised in Parkinson’s disease, which is often characterized by a mask-like facies (Ch. 33). The other occupies the affective area of the cingulate gyrus (illustrated in Figure 34.10), an emotionally responsive region in the territory of the anterior cerebral artery. It is active during production of a spontaneous smile, and this is of clinical interest, as explained in Clinical Panel 22.1.

Nuclear connections

Five reflex arcs engaging the facial nucleus are listed in Table 22.1. Most important clinically is the corneal reflex.

Table 22.1 Brainstem reflexes involving the facial nerve

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Corneal reflex

The usual test is to touch the cornea with a cotton wisp. This should elicit a bilateral blink response. The afferent limb of the reflex is the ophthalmic division of the trigeminal nerve (nasociliary branch). The efferent limb is the facial nerve (branch to palpebral element of orbicularis oculi). The reflex can still be elicited following a transection of the spinal tract of the trigeminal nerve (tactotomy, Ch. 21), because the ophthalmic afferents evidently synapse in the principal (pontine) nucleus of the trigeminal. Internuncials projecting from each principal nucleus to both facial nuclei complete the reflex arc.

The corneal reflex may be lost following a lesion of either the ophthalmic or facial nerve. A gradual compression of ophthalmic fibers in the sensory root of the trigeminal nerve may damage corneal neurons selectively. For this reason, the corneal reflex must be tested in patients under suspicion of an acoustic neuroma (Clinical Panel 22.2).

Clinical Panel 22.2 Syndromes of the cerebellopontine angle

The cerebellopontine angle is the recess between the hemisphere of the cerebellum and the lower border of the pons. The petrous temporal bone, laterally, completes a triangle having the V nerve at its upper corner and IX and X at its lower corner, and bisected by VII and VIII.

Several kinds of space-occupying lesions may compromise one or more of the nerves. The most frequent is an acoustic neuroma (Figure CP 22.2.1), a slow-growing, benign tumor of Schwann cells (neurolemmoma). The tumor originates on the vestibular nerve within the internal acoustic meatus, but the initial symptoms are more often cochlear than vestibular. An acoustic neuroma must be suspected in every middle-aged or elderly patient presenting with auditory or vestibular symptoms. Early diagnosis is important because of the difficulty of removing a large neuroma extending into the posterior cranial fossa; also because the cumulative motor and sensory disturbances may not show significant improvement after surgery.

The following is a fairly typical sequence of symptoms and signs in a case escaping early detection:

• Tinnitus is experienced on the affected side, in the form of a high-pitched ringing or fizzing sound.
• Deafness on the affected side is slowly progressive over a period of months or years.
• Vertigo occurs episodically. Severe vertigo with nystagmus signifies compression of the brainstem.
• Loss of the corneal reflex is an early sign of distortion of the V nerve by a tumor emerging from the internal acoustic meatus into the posterior cranial fossa.
• Weakness of the masticatory muscles is a later sign of V nerve involvement. The jaw deviates toward the affected side when the mouth is opened, because the normal lateral pterygoid is unopposed. Wasting of the masseter may be detected by palpation.
• Weakness of the facial musculature develops as the VII nerve becomes stretched.
• Anesthesia of the oropharynx signifies involvement of the IX nerve.
• Ipsilateral ‘cerebellar signs’ in the arm and leg appear when the cerebellum is compressed.
• ‘Upper motor neuron signs’ in the limbs signify compression of the brainstem.
• Signs of raised intracranial pressure (headache, drowsiness, papilledema) signify obstruction of cerebrospinal fluid circulation either inside or around the brainstem.

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Figure CP 22.2.1 An acoustic neuroma invading the right posterior cranial fossa.

Nervus Intermedius

Nervus intermedius aligns with the facial nerve distal to the internal genu. It comprises two sets of parasympathetic and two sets of special sense fibers (Figure 22.3).

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Figure 22.3 The nervus intermedius and its branches. Arrows indicate direction of impulse traffic.

The parasympathetic root of the nerve arises from the superior salivatory nucleus in the pons. This is the motor component of the greater petrosal and chorda tympani nerves. The greater petrosal synapses in the pterygopalatine ganglion (‘the ganglion of hay fever’), whose postganglionic fibers stimulate the lacrimal and nasal glands as well as palatine and nasopharyngeal glands. The motor component of chorda tympani synapses in the submandibular ganglion, whose postganglionic fibers stimulate the submandibular and sublingual glands.

The special sense root of this nerve has unipolar cell bodies in the geniculate ganglion of the facial nerve. The peripheral processes of these ganglion cells supply taste buds in the palate via the great petrosal nerve, and taste buds in the anterior two-thirds of the tongue via the chorda tympani. The central processes enter the gustatory part of the solitary nucleus, which also receives fibers from the glossopharyngeal nerve (Ch. 18), as well as the vagus nerve (which carries taste fibers from the epiglottis). From here, second-order neurons project to the thalamus on the same side, for relay to the anterior parts of insula and cingulate cortex.

A few cells of the geniculate ganglion supply skin in and around the external acoustic meatus (Clinical Panel 22.1).

Core Information

Upon leaving its nucleus, the facial nerve whirls around the abducent nerve nucleus, producing the facial colliculus, emerges at the lower border of pons, and at the internal auditory meatus enters a long bony canal opening at the stylomastoid foramen. It supplies the muscles of facial expression, the occipital portion of occipitofrontalis, the stapedius, and the posterior digastric. The upper half of the facial nucleus receives a bilateral supranuclear supply from the motor cortex; the lower half receives only a contralateral supply.

The nervus intermedius travels in part with VII. The superior salivatory nucleus provides the motor components of greater petrosal (for lacrimal and nasal glands by the pterygopalatine ganglion) and chorda tympani (for submandibular and sublingual glands via the submandibular ganglion). The geniculate ganglion of VII has unipolar neurons receiving taste from the palate via greater petrosal nerve and tongue via chorda tympani. A few unipolar neurons supply skin in and around the external acoustic meatus.

References

Fischer U, Hess CW, Rosler KM. Uncrossed corticonuclear projections in humans are abundant to facial muscles of the upper and lower face, but may differ between sexes. J Neurol. 2005;252:21-26.

Gilchrist JM. Seventh cranial neuropathy. Semin Neurol. 2009;29:5-13.

Lang J. Clinical anatomy of the cerebellopontine angle and internal acoustic meatus. Adv Oto-Rhino-Laryng. 1984;34:8-24.

Lemon CH, Katz DB. The neural basis of taste. BMC Neuroscience. 2007;8(S3):1-8.

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