4160 · 4 Brainstem

Blink Reflex

If an object suddenly appears before the eyes, reflex eye closure occurs (blink reflex). The afferent impulses of this reflex travel from the retina directly to the midbrain tectum and then run, by way of the tectonuclear tract, to the facial nerve nuclei of both sides, whose efferent fibers then innervate the orbicularis oculi muscles. Further impulses may descend in tectospinal fibers to the anterior horn cells of the spinal cord, which innervate the cervical musculature to produce aversion of the head.

Trigeminal Nerve (CN V)

The trigeminal nerve is a mixed nerve. It possesses a larger component (portio major) consisting of sensory fibers for the face, and a smaller component (portio minor) consisting of motor fibers for the muscles of mastication.

Trigeminal ganglion and brainstem nuclei. The trigeminal (gasserian) ganglion is the counterpart of the spinal dorsal root ganglia for the sensory innervation of the face. Like the dorsal root ganglia, it contains pseudounipolar ganglion cells, whose peripheral processes terminate in receptors for touch, pressure, tactile discrimination, pain, and temperature, and whose central processes project to the principal sensory nucleus of the trigeminal nerve (for touch and discrimination) and to the spinal nucleus of the trigeminal nerve (for pain and temperature). The mesencephalic nucleus of the trigeminal nerve is a special case, in that its cells correspond to spinal dorsal root ganglion cells even though it is located within the brainstem; it is, in a sense, a peripheral nucleus that has been displaced into the central nervous system. The peripheral processes of neurons in this nucleus receive impulses from peripheral receptors in the muscle spindles in the muscles of mastication, and from other receptors that respond to pressure.

The three nuclei just mentioned extend from the cervical spinal cord all the way to the midbrain, as shown in Figure 4.30. The trigeminal ganglion is located at the base of the skull over the apex of the petrous bone, just lateral to the posterolateral portion of the cavernous sinus. It gives off the three branches of the trigeminal nerve to the different areas of the face, i.e., the ophthalmic nerve (V1), which exits from the skull through the superior orbital fissure; the maxillary nerve (V2), which exits through the foramen rotundum; and the mandibular nerve (V3), which exits through the foramen ovale.

Somatosensory trigeminal fibers. The peripheral trajectory of the trigeminal nerve is shown in Figure 4.29. Its somatosensory portion supplies the skin of the face up to the vertex of the head. Figure 4.30 shows the cutaneous territo-

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Superficial temporal branch

Ophthalmic n.

Maxillary n.

Mandibular n.

Trigeminal ganglion

Auriculotemporal n. Pterygopalatine ganglion

Buccal n.

Lingual n.

Inferior alveolar n.

A = orbital fissure

b = foramen rotundum c = foramen ovale

1 = lateral pterygoid m.

2 = medial pterygoid m.

3 = mylohyoid m. and anterior belly of digastric m.

Cranial Nerves · 161 4

	Nasociliary n.
Temporalis m.	Ciliary ganglion	Frontal n.

A

B

C

1

2

Masseter
m
.
Mylohyoid n.	3
	Mental m.

Fig. 4.29 Peripheral course of the somatosensory and motor fibers of the trigeminal nerve

ries supplied by each of the three trigeminal branches. The cutaneous distribution of the trigeminal nerve borders the dermatomes of the second and third cervical nerve roots. (The first cervical nerve root, C1, is purely motor and innervates the nuchal muscles that are attached to the skull and the upper cervical vertebrae.)

Furthermore, the mucous membranes of the mouth, nose, and paranasal sinuses derive their somatosensory innervation from the trigeminal nerve, as do the mandibular and maxillary teeth and most of the dura mater (in the

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4162 · 4 Brainstem

Spinal and trigeminal lemniscus

(masseter reflex)

(blink reflex, corneal reflex)

Medial lemniscus

Nucleus cuneatus and nucleus gracilis

C1

C2

Lateral spinothalamic tract (body)

Meningeal branch Nerve to tensor Lesser

tympani m. petrosal n.

Auriculotemporal n./medial and lateral pterygoid nn.

Nerve to tensor veli palatini m.

Thalamus

Mesencephalic nucleus and tract of the trigeminal

Motor nucleus of the trigeminal

Principal sensory nucleus of the trigeminal n.

Spinal nucleus

	Spinal tract	trige-
		minal n.
	Mandibular n.
	Maxillary n.
	Ophthalmic n.
	Substantia
	gelatinosa

Motor root of the trigeminal n.:

1to muscles of mastication

2mylohyoid m. and

3anterior belly of the digastric m.

Somatic afferent	proprioception
Somatic afferent	touch
Somatic afferent	pain, temperature
Branchial efferent	motor

Fig. 4.30 a Central connections of the various trigeminal fibers and their corresponding nuclei

(schematic drawing). b Motor root of the trigeminal nerve.

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Cranial Nerves · 163 4

anterior and middle cranial fossae). Around the external ear, however, only the anterior portion of the pinna and the external auditory canal and a part of the tympanic membrane are supplied by the trigeminal nerve. The rest of the external auditory canal derives its somatosensory innervation from the nervus intermedius and the glossopharyngeal and vagus nerves.

Proprioceptive impulses from the muscles of mastication and the hard palate are transmitted by the mandibular nerve. These impulses are part of a feedback mechanism for the control of bite strength.

All trigeminal somatosensory fibers terminate in the principal sensory nucleus of the trigeminal nerve, which is located in the dorsolateral portion of the pons (in a position analogous to that of the posterior column nuclei in the medulla). The axons of the second neurons cross the midline and ascend in the contralateral medial lemniscus to the ventral posteromedial nucleus of the thalamus (VPL, Fig. 4.30).

The somatosensory fibers of the trigeminal nerve are a component of several important reflex arcs.

Corneal reflex. Somatosensory impulses from the mucous membranes of the eye travel in the ophthalmic nerve to the principal sensory nucleus of the trigeminal nerve (afferent arm). After a synapse at this site, impulses travel onward to the facial nerve nuclei and then through the facial nerves to the orbicularis oculi muscles on either side (efferent arm). Interruption of this reflex arc in either its afferent component (trigeminal nerve) or its efferent component (facial nerve) abolishes the corneal reflex, in which touching the cornea induces reflex closure of both eyes.

Sneeze and suck reflexes. Other somatosensory fibers travel from the nasal mucosa to the trigeminal nuclear area to form the afferent arm of the sneeze reflex. A number of different nerves make up its efferent arm: cranial nerves V, VII, IX, and X, as well as several nerves that are involved in expiration. The suck reflex of infants, in which touching of the lips induces sucking, is another reflex with a trigeminal afferent arm and an efferent arm that involves several different nerves.

Pain and temperature fibers of the trigeminal nerve. Fibers subserving pain and temperature sensation travel caudally in the spinal tract of the trigeminal nerve and terminate in the spinal nucleus of the trigeminal nerve, whose lowest portion extends into the cervical spinal cord. This nucleus is the upper extension of the Lissauer zone and the substantia gelatinosa of the posterior horn, which receive the pain and temperature fibers of the upper cervical segments.

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4164 · 4 Brainstem

The caudal portion (pars caudalis) of the spinal nucleus of the trigeminal nerve contains an upside-down somatotopic representation of the face and head: the nociceptive fibers of the ophthalmic nerve terminate most caudally, followed from caudal to rostral by those of the maxillary and mandibular nerves The spinal tract of the trigeminal nerve also contains nociceptive fibers from cranial nerves VII (nervus intermedius), IX, and X, which subserve pain and temperature sensation on the external ear, the posterior third of the tongue, and the larynx and pharynx (see Figs. 4.48 and 4.49).

The midportion (pars interpolaris) and rostral portion (pars rostralis) of the spinal nucleus of the trigeminal nerve probably receive afferent fibers subserving touch and pressure sensation (the functional anatomy in this area is incompletely understood at present). The pars interpolaris has also been reported to receive nociceptive fibers from the pulp of the teeth.

The second neurons that emerge from the spinal nucleus of the trigeminal nerve project their axons across the midline in a broad, fanlike tract. These fibers traverse the pons and midbrain, ascending in close association with the lateral spinothalamic tract toward the thalamus, where they terminate in the ventral posteromedial nucleus (Fig. 4.30). The axons of the thalamic (third) neurons in the trigeminal pathway then ascend in the posterior limb of the internal capsule to the caudal portion of the postcentral gyrus (Fig. 2.19, p. 46).

Motor trigeminal fibers. The motor nucleus from which the motor fibers (portio minor) of the trigeminal nerve arise is located in the lateral portion of the pontine tegmentum, just medial to the principal sensory nucleus of the trigeminal nerve. The portio minor exits the skull through the foramen ovale together with the mandibular nerve and innervates the masseter, temporalis, and medial and lateral pterygoid muscles, as well as the tensor veli palatini, the tensor tympani, the mylohyoid muscle, and the anterior belly of the digastric muscle (Figs. 4.29 and 4.30).

The motor nuclei (and, through them, the muscles of mastication) are under the influence of cortical centers that project to them by way of the corticonuclear tract. This supranuclear pathway is mostly crossed, but there is also a substantial ipsilateral projection. This accounts for the fact that a unilateral interruption of the supranuclear trigeminal pathway does not produce any noticeable weakness of the muscles of mastication.

The supranuclear pathway originates in neurons of the caudal portion of the precentral gyrus (Fig. 3.2, p. 58; Fig. 4.30).

Lesions of the motor trigeminal fibers. A nuclear or peripheral lesion of the motor trigeminal pathway produces flaccid weakness of the muscles of mastication.

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Cranial Nerves · 165 4

This type of weakness, if unilateral, can be detected by palpation of the masseter and temporalis muscles while the patient clamps his or her jaw: the normally palpable muscle contraction is absent on the side of the lesion. When the patient then opens his or her mouth and protrudes the lower jaw, the jaw deviates to the side of the lesion, because the force of the contralateral pterygoid muscle predominates. In such cases, the masseteric or jaw-jerk reflex is absent (it is normally elicitable by tapping the chin with a reflex hammer to stretch the fibers of the masseter muscle).

Disorders Affecting the Trigeminal Nerve

Trigeminal neuralgia. The classic variety of trigeminal neuralgia is characterized by paroxysms of intense, lightninglike (shooting or “lancinating”) pain in the distribution of one or more branches of the trigeminal nerve. The pain can be evoked by touching the face in one or more particularly sensitive areas (“trigger zones”). Typical types of stimuli that trigger pain include washing, shaving, and tooth-brushing. This condition is also known by the traditional French designation, tic douloureux (which is somewhat misleading, because any twitching movements of the face that may be present are a reflex response to the pain, rather than a true tic). The neurological examination is unremarkable; in particular, there is no sensory deficit on the face.

The pathophysiology of this condition remains imperfectly understood; both central and peripheral mechanisms have been proposed. (The older term “idiopathic trigeminal neuralgia” for the classic condition is no longer widely used, because this issue is still unsettled.) Gardner (1959) and, later, Jannetta (1982) attributed trigeminal neuralgia to compression of the trigeminal root by a blood vessel, usually the superior cerebellar artery, looping around the proximal, unmyelinated portion of the root immediately after its exit from the pons (Fig. 4.31). This hypothesis is supported by the observation that a pain-free state can be achieved in up to 80% of patients with a neurosurgical procedure known as microvascular decompression, in which the vascular loop is exposed and dissected free of the nerve, and a small sponge made of synthetic material is inserted between these two structures to keep them apart.

The pain can be significantly diminished, or even eliminated, in 80­90% of cases by medical treatment alone, either with carbamazepine or with gabapentin, which has recently come into use for this purpose. Neurosurgical intervention is indicated only if the pain becomes refractory to medication. The options for neurosurgical treatment include, among others, microvascular decompression (mentioned above) and selective percutaneous thermocoagulation of the nociceptive fibers of the trigeminal nerve.

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4 166 · 4 Brainstem

III	Posterior cerebral a.
IV
VI		Superior cerebellar a.
V sensory			Trigeminal n.
(2.0–6.0 mm)
VII			Site of origin of
			trigeminal neuralgia
(0.5–4.0 mm)
VIII			Anterior inferior
			cerebellar a.
IX
(0.1–1.1 mm)
			Posterior inferior
X			cerebellar a.
XI			Vertebral a.
XII

Fig. 4.31 Unmyelinated portions of the cranial nerve roots (orange, left) and nearby vascular loops (dark red, right) that may irritate the nerve roots at these sites. In particular, the diagram shows a loop of the superior cerebellar artery that may cause trigeminal neuralgia.

The most common cause of symptomatic trigeminal neuralgia is multiple sclerosis: 2.4% of all MS patients develop trigeminal neuralgia; among these patients, 14% have it bilaterally.

Other, rarer causes of symptomatic pain in the distribution of the trigeminal nerve include dental lesions, sinusitis, bony fractures, and tumors of the cerebellopontine angle, the nose, or the mouth. Pain in the eye or forehead should also arouse suspicion of glaucoma or iritis. The pain of acute glaucoma can mimic that of classic trigeminal neuralgia.

Gradenigo syndrome consists of pain in the distribution of the ophthalmic nerve accompanied by ipsilateral abducens palsy. It is caused by infection in the air cells of the petrous apex.

Differential Diagnosis: Disorders with Facial Pain in the Absence of a Trigeminal Lesion

Charlin neuralgia consists of pain at the inner canthus of the eye and root of the nose accompanied by increased lacrimation. It is thought to be due to irritation of the ciliary ganglion.

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