A Synopsis of Cranial Nerves of the Brainstem

Overview

In general, the exit of a cranial nerve from the brainstem (“exit” is used here with reference to both efferent and afferent fibers of the nerve) is associated with the same brainstem area in which the nuclei of that nerve are found. The obvious exception is the trigeminal nerve, the sensory nuclei of which form a continuous cell column from rostral regions of the midbrain to the spinal cord–medulla interface.

This chapter reviews cranial nerves of the brainstem from caudal (hypoglossal) to rostral (oculomotor) and presents a number of clinical examples. The goal here is not simply to review the information covered in the last three chapters but to consider the cranial nerves in a somewhat broader perspective. Structure, function, and dysfunction are described in an integrated manner because this is how cranial nerves are evaluated in the clinical setting.

Motor Cell Columns and Nuclei

Early in development, the derivatives of the basal plate that form motor nuclei of the cranial nerves in the brainstem tend to form rostrocaudally oriented cell columns. As the brainstem enlarges, these cell columns become discontinuous. That is, they are in line with but are separated from each other in the adult brain (see Fig. 10.5, Fig. 10.6, Fig. 10.7 ). As we shall review here, those nuclei that are in line with each other and that have arisen from the same original cell column have developmental, structural, and functional characteristics in common.

The most medial cranial nerve motor nuclei in the brainstem are the hypoglossal (XII), abducens (VI), trochlear (IV), and oculomotor (III) nuclei ( Fig. 14.1 ). These nuclei share three characteristics. First, they are located adjacent to the midline and anterior to the ventricular space of their particular brain division. Second, the alpha motor neurons in these nuclei innervate skeletal muscle that originates from paraxial mesoderm that migrated into the occipital region (tongue muscles) and into the area of the orbit (extraocular muscles). Third, the functional component of the lower motor neurons in these nuclei is somatic efferent (SE), reflecting the fact that these motor neurons innervate skeletal muscles that originated from paraxial mesoderm and migrated through the pharyngeal arches.

Laterally adjacent to the SE cell column are the nuclei that collectively constitute the cranial part of the craniosacral division ( parasympathetic ) of the visceromotor nervous system. These nuclei, with the cranial nerves on which the preganglionic fibers travel, are (1) the dorsal motor vagal nucleus —vagus nerve; (2) the inferior salivatory nucleus —glossopharyngeal nerve; (3) the superior salivatory nucleus —the facial nerve, intermediate part; and (4) the Edinger-Westphal pg ( preganglionic cells ) nucleus —oculomotor nerve ( Fig. 14.1 ; see also Fig. 10.7 ). These nuclei share the following characteristics. First, they form a discontinuous column located slightly lateral to the SE nuclei. Second, the neurons in these nuclei give rise to preganglionic axons that terminate in a peripheral ganglion, the cells of which give rise to postganglionic fibers that innervate a visceral structure. Third, because these motor neurons are part of a pathway that innervates a visceral structure (tissue composed of smooth muscle, glandular epithelium, or cardiac muscle or a combination of these), they are classified as visceral efferent (VE). They can most accurately be identified as VE preganglionic parasympathetic as this term completely identifies their structural and functional relationships.

The most lateral motor cell column in the medulla and in the pontine tegmentum is formed by the nucleus ambiguus, the efferents of which travel on the vagus and glossopharyngeal nerves, and by the facial motor nucleus and the trigeminal motor nucleus, related to facial and trigeminal nerves, respectively ( Fig. 14.1 ; see also Fig. 10.7 ). These motor nuclei also share common characteristics. First, they form a discontinuous column in the more lateral part of the medulla and pontine tegmentum. Their position, as is the case for the SE and VE cell columns, reflects the differentiation of the basal plate in the brainstem (see Fig. 10.8 ). Second, the muscles innervated by these lower motor neurons originate from paraxial mesoderm that initially migrates into the pharyngeal arches. The muscles of mastication (trigeminal nerve innervation) originate through arch I; the muscles of facial expression (facial nerve innervation), through arch II; the stylopharyngeus muscle (glossopharyngeal nerve innervation), through arch III; and the constrictors of the pharynx, intrinsic laryngeal muscles, palatine muscles ( except the tensor veli palatini ), and the vocalis (vagal nerve innervation), through arch IV. Third, owing to the fact that these lower motor neurons innervate skeletal muscles that also arose from paraxial mesoderm (see Fig. 10.8 ), they are classified as SE.

Sensory Cell Columns and Nuclei

The derivatives of the alar plate that give rise to cranial nerve sensory nuclei of the brainstem are located lateral to the sulcus limitans (see Fig. 10.5, Fig. 10.6 ). In contrast to the motor nuclei, which form rostrocaudally oriented but discontinuous cell columns, all three of the sensory nuclei in the brainstem form what can arguably be described as continuous cell columns in the adult. These sensory nuclei–cell columns are located in the lateral aspects of the brainstem.

The most medial of these cell columns is the solitary tract and nucleus, which is the visceral afferent (VA) center of the brainstem ( Fig. 14.1 ; see also Fig. 10.7 ). No matter what cranial nerve returns visceral afferent information to the brainstem, the central processes of these primary afferent fibers contribute to the solitary tract, the fibers of which terminate in the solitary nucleus ( Fig. 14.2 ). VA information is conveyed centrally on the facial, glossopharyngeal, and vagus nerves and consists of taste fibers and fibers conveying visceral sensations from salivary glands and viscera of the thorax and abdomen. The majority of taste input reaches rostral portions of the solitary nucleus (sometimes called the gustatory nucleus ), whereas most other visceral sensation enters the caudal portion of the solitary nucleus (sometimes referred to as the cardiorespiratory nucleus ) ( Fig. 14.2 ). Because the most rostral cranial nerve that contributes to the solitary tract and nucleus is the facial nerve (a nerve of the pons-medulla junction), the solitary tract and its nucleus are found throughout the medulla but do not extend rostrally beyond the pons-medulla junction.

Immediately and posteriorly adjacent to the solitary tract and nucleus are the medial and spinal vestibular nuclei. These continue rostrally, are joined by the anterior and posterior cochlear nuclei at the pons-medulla junction, and interface in the caudal pons with the superior and lateral vestibular nuclei (see Fig. 14.9 ). This cell column receives sensory input from the vestibulocochlear nerve (cranial nerve VIII) that subserves balance and equilibrium ( somatic afferent [SA] proprioception) and the sense of hearing (SA, exteroception).

The nuclei of the trigeminal sensory system form a continuous cell column extending from the spinal cord–medulla junction to the rostral midbrain ( Fig. 14.1 ; see also Fig. 10.7 ). The trigeminal sensory nuclei are divided into (1) the spinal trigeminal nucleus (consisting of a pars caudalis, pars interpolaris, and pars oralis ), located in the lateral medulla and extending into the caudal pons; (2) the principal sensory nucleus, located at the midpontine level; and (3) the mesencephalic nucleus, extending rostrally into the midbrain at the lateral aspect of the periaqueductal gray ( Fig. 14.1 ; see also Fig. 13.8 ). As is the case for the solitary tract and nucleus (the visceral receiving center of the brainstem), the principal sensory nucleus and especially the spinal trigeminal nucleus constitute the somatic sensory receiving center of the brainstem. Although SA pain and thermal sensations enter the brainstem on four different cranial nerves (trigeminal, facial, glossopharyngeal, and vagus), the central processes of these primary afferent fibers enter the spinal trigeminal tract and terminate in the medially adjacent spinal trigeminal nucleus.

Cranial Nerves of the Medulla Oblongata

The cranial nerves that are commonly identified as exiting the medulla are the hypoglossal nerve (cranial nerve XII) through the abducens nerve (cranial nerve VI) ( Figs. 14.3 and 14.4 ). However, in the subsequent discussion, the abducens (VI), facial (VII), and vestibulocochlear (VIII) nerves are considered the nerves of the pons-medulla junction. Consequently, the cranial nerves and nuclei that are generally associated with only the medulla are the hypoglossa l (XII), vagus (X), and glossopharyngeal (IX) nerves ( Figs. 14.3 and 14.4 ): the unique situation of the accessory nerve is addressed later in this chapter.

Hypoglossal Nerve

The hypoglossal nucleus is located internal to the hypoglossal trigone. Axons of hypoglossal motor neurons pass anteriorly in the medulla along the lateral aspect of the medial lemniscus and the pyramid (see Fig. 11.11 ) to exit as a series of rootlets from the preolivary fissure as the hypoglossal nerve ( Figs. 14.3 and 14.5 ). They continue through the hypoglossal canal and distribute to the intrinsic muscles of the tongue plus the hyoglossus, palatoglossus, and genioglossus muscles ( Fig. 14.6 ). In addition to the hypoglossal nerve, the hypoglossal canal may also contain an emissary vein and a small meningeal branch to the dura of the posterior fossa from the ascending pharyngeal artery.

The blood supply to the hypoglossal nucleus and its exiting fibers is via penetrating branches of the anterior spinal artery. Occlusion of these branches (as in the medial medullary syndrome ) may result in paralysis of the genioglossus muscle with deviation of the tongue toward the side of the lesion ( the weak side ) on protrusion. In addition, the patient experiences a contralateral hemiparesis (corticospinal tract involvement) and a contralateral loss of position sense, vibratory sense, and two-point discrimination (medial lemniscus involvement) because the anterior spinal artery also serves these structures.

Other lesions that may affect hypoglossal function include a lesion of the root of the nerve only (causing tongue deviation to the side of the lesion with no other deficits) and injury to the internal capsule. In the latter case, corticonuclear fibers to hypoglossal motor neurons innervating the genioglossus muscle are predominantly crossed. Consequently, internal capsule lesions may result in a deviation of the tongue to the contralateral side (side opposite the lesion) on protrusion, in concert with other deficits such as a contralateral hemiplegia and a drooping of the facial muscles in the lower quadrant of the contralateral side of the face. See Chapter 25 for examples of lesions that result in hypoglossal nerve dysfunction.

Accessory Nerve

This cranial nerve was historically described as having a cranial part (from the medulla) and a spinal part (from the cervical spinal cord). However, studies have shown that the SE motor neurons that innervate the sternocleidomastoid and trapezius muscles are located in the cervical cord only; these muscles are not innervated by motor neurons located in the medulla. There is not a cranial part of the accessory nerve. For consistency and in recognition of wide usage, the accessory nerve is considered here as a cranial nerve associated with the medulla.

The accessory nerve originates from motor neurons in the cervical spinal cord extending from C1 to C6 ( Fig. 14.7 ). The axons of these neurons exit the lateral aspect of the cord, coalesce to form the nerve ( Fig. 14.3 ), and ascend to enter the cranial cavity via the foramen magnum. As these accessory fibers course through the posterior fossa, they are briefly joined by vagal fibers that originate from the caudal portion of the nucleus ambiguus. These vagal fibers diverge from this temporary association and exit the skull on the tenth cranial nerve. The accessory fibers form the eleventh cranial nerve, receive no contributions from the medulla, and along with cranial nerves IX and X exit the cranial cavity via the jugular foramen ( Fig. 14.8 ).

The relationship of the accessory nerve to the vagus nerve is similar to the relationship of the seventh nerve to the trigeminal nerve via the chorda tympani. In the latter case, the taste fibers from the anterior two thirds of the tongue travel on the trigeminal nerve and then join the seventh nerve via the chorda tympani. However, throughout their extent, these taste fibers are considered part of the seventh nerve, not the fifth. In like manner, fibers of the accessory nerve temporarily join the vagus and then leave it to exit the skull ( Fig. 14.7 ). These accessory nerve fibers do not originate from the medulla, do not distribute peripherally with the vagus, and have their cells of origin in the cervical spinal cord. What has classically been called the cranial part of the accessory nerve is actually a misnomer; these fibers represent the caudal portions of the vagus nerve to which the accessory nerve temporarily relates as it traverses the posterior fossa. Reflecting the fact that the sternocleidomastoid and trapezius muscles in the human originate from paraxial mesoderm caudal to the fourth arch (not in the fourth arch), the functional component associated with these motor neurons is SE.

Lesions of the root of the accessory nerve result in drooping of the shoulder (trapezius paralysis) on the ipsilateral side and difficulty in turning the head to the contralateral side (sternocleidomastoid paralysis) against resistance. Weakness of these muscles is not especially obvious in cervical cord lesions because a hemiplegia (indicating damage to corticospinal fibers) is the overwhelmingly obvious deficit. However, a lesion of the internal capsule may also result in deficits similar to those described previously owing to interruption of the corticonuclear fibers to the accessory nucleus; these corticonuclear fibers are primarily uncrossed.