Cranial Nerves and Their Nuclei

The Sulcus Limitans Intervenes Between Motor and Sensory Nuclei of Cranial Nerves

Axons from all four categories found in spinal nerves are also found in various cranial nerves, where they take care of the same functions for the head. However, some cranial nerves contain axons from additional categories, reflecting specialized structures and functions associated with the head. Thus there are special sensory fibers that, in the case of the cranial nerves attached to the brainstem, are related to the special senses of hearing and equilibrium. ^a

a The cranial nerve fibers conveying information from taste buds are often considered special afferents as well—special visceral afferents—but, as discussed in this and the next chapter, they have connections similar in many ways to those of other visceral afferents. Therefore in this account all visceral afferents are treated as one category.

In addition, motor axons in certain cranial nerves innervate striated muscles in and near the head and neck in humans and other mammals. Such muscles develop from the pharyngeal arches, known as branchial or gill arches in fish. Pharyngeal arch–derived musculature includes the muscles of the larynx, pharynx, jaw, and face. Functionally and histologically, pharyngeal arch–derived musculature is identical to ordinary skeletal muscle, but the motor neurons for such muscles have a distinctive location in the brainstem, different from that of ordinary somatic motor neurons. In recognition of their special development and location, they are classified as a separate category, here called pharyngeal motor neurons. ^b

b Because muscles derived from the pharyngeal arches tend to be concentrated around the mouth at a junction between visceral and somatic areas, the motor fibers that innervate them were historically referred to as special visceral efferent fibers. This classification is somewhat confusing (particularly because the sensory fibers from these muscles are called somatic afferent fibers), so it is not used here.

Therefore there are six different categories of nerve fibers in the cranial nerves attached to the brainstem ( Table 12.1 ).

As in the case of the spinal cord, the locations of the cell bodies where cranial nerve afferents terminate or cranial nerve efferents originate can be predicted, to some extent, from the embryology of the brainstem. The walls of the neural tube spread apart in the medulla and pons to form the floor of the fourth ventricle (see Fig. 2.9 ). The sulcus limitans runs longitudinally along the floor of the adult ventricle (see Fig. 11.3A ), still separating sensory alar plate derivatives (now lateral) from motor basal plate derivatives (now medial) (see Fig. 12.1B ). As in the case of the spinal cord, cells concerned with visceral functions are usually located nearer the sulcus limitans.

Ideally the cell columns subserving the special components of the cranial nerves would be located adjacent to those for the corresponding general components, as indicated in Fig. 12.1C . The actual arrangement in the adult brainstem is not quite as simple as in this idealized diagram, for two principal reasons. First, the cell columns of the brainstem are not continuous like those of the spinal cord; rather, they are interrupted and form a series of nuclei located at longitudinal levels roughly corresponding to the attachment points of the cranial nerves. As a result, all components are seldom present in a given transverse plane ( Fig. 12.2 ; see also Fig. 15.2 ). Second, in a few instances, portions of a cell column migrate away from their expected locations ( Fig. 12.3 ). For example, most pharyngeal motor neurons are located in the anterolateral part of the tegmentum rather than in the floor of the ventricle adjacent to other efferent neurons. The actual locations of cranial nerve nuclei in the rostral medulla are shown in Fig. 12.3 ; also indicated are the functional types of fibers in each of the cranial nerves of the brainstem. (This is only meant to be a convenient summary; not all cranial nerves project to or originate from the rostral medulla.)

It can be seen from Fig. 12.3 that no cranial nerve contains axons from all six categories. If the compositions of all the nerves are tabulated (as in Table 12.2 ), it becomes apparent that there are three types of cranial nerves. Some cranial nerves (III, IV, VI, XI and XII) contain motor axons for ordinary skeletal muscle and little or nothing else, so they may be referred to as somatic motor nerves. Others (I, II, and VIII) contain special sensory fibers and little or nothing else. The remaining nerves (V, VII, IX, and X) are somewhat more complex and typically contain several components. These innervate pharyngeal arch musculature and are called branchiomeric nerves because of the older nomenclature that refers to their development in the region of the gill (branchial) arches in fish.

TABLE 12.2

Contents of the Cranial Nerves

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The three types of cranial nerves are indicated by shading. Somatic motor nerves have no shading, special sensory nerves have light shading, and nerves serving muscles derived from pharyngeal arches have darker shading.

^a Most conveniently considered as one of the somatic motor nerves, despite the clinically important autonomic component.

^b Final destination after synapse in a parasympathetic ganglion.

A, Autonomic (visceral motor); P, pharyngeal motor; Sm, somatic motor; Sp, special sensory; Ss, somatic sensory; Vi, visceral sensory.

A presentation of the cranial nerves involves too much material for one comfortable sitting, so it is spread out over several chapters. The remainder of this chapter is divided into two sections, discussing first the somatic motor nerves and then most components of the branchiomeric nerves. Chapter 13 discusses the chemical senses of taste and smell subserved by some brainstem cranial nerves and the olfactory nerve. Chapter 14 deals with the eighth cranial nerve, the special sensory nerve subserving hearing and equilibrium. (The remaining special sensory nerve, the optic nerve, is an outgrowth of the diencephalon and is really a tract of the CNS. It is considered separately in Chapter 17 .) Finally, as mentioned previously, Chapter 15 contains a series of brainstem sections with labels and summary descriptions indicating the locations and contents of cranial nerve nuclei and other important brainstem structures.

The Oculomotor Nerve (III) Innervates Four of the Six Extraocular Muscles

Cranial nerve III supplies the levator palpebrae superioris (the principal elevator of the eyelid); medial, superior and inferior recti; and the inferior oblique. The fibers originate in the wedge-shaped oculomotor nucleus, which is located at the anterior edge of the periaqueductal gray in the rostral midbrain ( Fig. 12.4 ). They then proceed anteriorly and arch through the midbrain tegmentum in bundles that join to form the nerve just as they emerge into the interpeduncular fossa.

The oculomotor nucleus actually consists of a series of longitudinal cell columns, or subnuclei, much like the columns of spinal cord motor neurons that supply individual muscles. The column supplying the levator palpebrae superioris is located in the midline and innervates this muscle bilaterally. The column supplying the superior rectus projects to the contralateral eye. The columns supplying the medial rectus, inferior oblique, and inferior rectus project to the ipsilateral eye. Finally, a column containing preganglionic parasympathetic neurons, known as the accessory oculomotor (Edinger-Westphal) nucleus, straddles the midline and projects to the ipsilateral ciliary ganglion. The ciliary ganglion in turn innervates the pupillary sphincter and the ciliary muscle.

The partly crossed–partly uncrossed nature of the oculomotor nerve is a curious fact but one of limited clinical significance. This is because the oculomotor nuclei of the two sides are so close to each other that a central lesion in this vicinity is likely to damage both nuclei. However, once a given oculomotor nerve emerges from the brainstem, it supplies only ipsilateral muscles, so a lesion of the third nerve, or of fibers curving through the midbrain tegmentum on their way to the third nerve, affects only one eye. Therefore the dissociated findings of paralysis of the superior rectus on one side and of other extraocular muscles on the opposite side are rarely encountered.

Damage to one oculomotor nerve causes a series of deficits (see Fig. 12.31B to E ). The eye ipsilateral to the lesion deviates laterally because the medial rectus is now paralyzed and the lateral rectus is unopposed. This is called lateral strabismus, indicating that the eyes are misaligned because one of them deviates laterally from midposition. As a result, the patient complains of diplopia (double vision) and is unable to move the affected eye medially; vertical movements are also impaired because of paralysis of the superior and inferior recti and the inferior oblique. The ipsilateral levator palpebrae superioris is paralyzed, so ptosis occurs. In addition, the pupillary sphincter and ciliary muscle are nonfunctional. The pupil on the affected side is dilated (mydriasis) as a result of the now-unopposed pupillary dilator, and it does not constrict in response to light. ^d

d Both pupils normally constrict when light is shone into either eye. This is the pupillary light reflex, which is discussed further in Chapter 17 .

The lens cannot be focused for near vision; allowing the lens to “round up” for near vision is known as accommodation.

Along the course of the oculomotor nerve from brainstem to orbit, the preganglionic parasympathetic fibers from the accessory oculomotor nucleus travel in a superficial location within the nerve and are therefore especially susceptible to external pressures. A dilated pupil, unresponsive to light, may be the first clinically detectable sign of something pressing on the third nerve.

Because ptosis and pupils of unequal size accompany Horner's syndrome, one may think that this syndrome could be confused with third nerve damage. However, in Horner's syndrome the ptosis is on the same side as a nonfunctional pupillary dilator, hence on the same side as the smaller pupil. In contrast, the ptosis caused by third nerve damage is on the same side as a nonfunctional pupillary sphincter, hence on the same side as the larger pupil. Also, the ptosis caused by third nerve damage is more pronounced and is usually accompanied by defective eye movements and lateral strabismus.

The Trochlear Nerve (IV) Innervates the Superior Oblique

Cranial nerve IV, the trochlear nerve, supplies the superior oblique and is named for the sling of connective tissue (the trochlea —Latin for “pulley”) through which the tendon of the superior oblique passes (see Fig. 21.2 ). Its cell bodies of origin are located in the contralateral trochlear nucleus. This is a small nucleus (because it has only one small muscle to supply) located at the level of the inferior colliculus, where it indents the medial longitudinal fasciculus (MLF) ( Fig. 12.5 ). Fibers leaving the nucleus turn caudally in the periaqueductal gray then arch posteriorly to decussate and leave the brainstem at the pons-midbrain junction. The trochlear nerve is thus unique in two respects: it is the only cranial nerve attached to the posterior surface of the brainstem and the only one to originate entirely from a contralateral nucleus. ^e

e As discussed by Zee (Ann Neurol 4:384, 1978), this may reflect an adaptation to maintain certain relationships between head movements and eye movements. Eye movements are discussed in more detail in Chapter 21 , but consider the following example: tilting your head toward your left shoulder evokes a reflex counterrotation of your eyes. The principal muscles that must contract in this counterrotation are the left superior oblique and superior rectus, and the right inferior oblique and inferior rectus. Because fibers to the superior oblique and superior rectus cross before leaving the brainstem, all the lower motor neurons needed for this counterrotation are located on the right side of the brainstem.

Damage to the trochlear nerve results in much less drastic and noticeable deficits than does damage to either the oculomotor or the abducens nerve. The superior oblique helps move the adducted eye downward, as when descending stairs or reading. Lesions may result in diplopia that is particularly evident when engaged in such activities. Furthermore, the superior oblique is an aid in eye intorsion; lesions lead to an extorted eye, resulting in an affected individual tilting his or her head away from the lesioned side to compensate.

The Abducens Nerve (VI) Innervates the Lateral Rectus

Cranial nerve VI, the abducens nerve, supplies the lateral rectus, which abducts the eye (hence the name of the nerve). The fibers originate from the ipsilateral abducens nucleus, which is located in the caudal pons beneath the floor of the fourth ventricle ( Fig. 12.6 ). Medial to this nucleus are two bundles of fibers. The more medial of the two is the MLF. Between the MLF and the abducens nucleus are motor fibers of the facial nerve, which take an unusual course in leaving the brainstem. They originate in the facial motor nucleus (see Figs. 12.6 and 12.24 ), which is located in the anterolateral part of the pontine tegmentum at about the same level as the abducens nucleus. The facial fibers travel posteromedially, wrap around the abducens nucleus, and turn anteriorly to exit from the brainstem ( Fig. 12.7 ). The place where these fibers wrap around the abducens nucleus is called the internal genu of the facial nerve. ^f

f “Internal” to distinguish it from the bend in the peripheral course of the facial nerve at the level of the geniculate ganglion.

The abducens nucleus, together with the internal genu, is responsible for the facial colliculus in the floor of the fourth ventricle (see Fig. 11.3A ).

The Abducens Nucleus Also Contains Interneurons That Project to the Contralateral Oculomotor Nucleus

Damage to the abducens nerve causes a medial strabismus (i.e., the affected eye deviates medially) as a result of the action of the now-unopposed medial rectus. The individual may be able to move the affected eye from the adducted position to midposition (but not past it) by relaxing its medial rectus ( Fig. 12.8A ). Damage to the abducens nucleus causes the same deficit, but with a significant addition: the ipsilateral eye will not abduct past midposition, and the contralateral eye will not adduct past midposition (see Fig. 12.8B ; see also Fig. 12.10C ). This is called lateral gaze paralysis, and it occurs because the abducens nucleus contains not only lateral rectus motor neurons but also an approximately equal number of internuclear neurons with axons that ascend through the MLF.

The function of the MLF in lateral gaze becomes apparent if you consider that both eyes normally work together : when we look to one side, for example, one lateral rectus and the contralateral medial rectus contract simultaneously . The pathway that interconnects the abducens, trochlear, and oculomotor nuclei to make these sorts of movements possible is the MLF. Vertical movements and the higher centers that direct coordinated eye movements are discussed in Chapter 21 , but for purely horizontal movements, the crucial interconnecting fibers are those that arise from the internuclear neurons in the abducens nucleus ( Fig. 12.9 ). These cells send their axons across the midline as they emerge from the abducens nucleus; they then join the contralateral MLF and ascend to the oculomotor nucleus. There they make excitatory synapses on medial rectus motor neurons. Simultaneous firing of abducens motor neurons and internuclear neurons thus results in coordinated lateral gaze.

Damage to one MLF removes this excitatory influence from medial rectus motor neurons, so the eye ipsilateral to the lesion fails to move medially past midposition during attempted horizontal gaze (see Fig. 12.8C ). Because both abducens nuclei are intact, full lateral movements of both eyes are still possible. In addition, although the affected medial rectus fails to contract during attempted horizontal gaze, it still functions normally when used without the opposite lateral rectus (i.e., during convergence). This condition has the ponderous but logical name internuclear ophthalmoplegia ^g

g Literally, “paralysis of the eye caused by damage between the nuclei.”

(often abbreviated as INO).

Another eye movement disorder, clinically called a one-and-a-half, is rarely seen but is nevertheless instructive. It is caused by damage in the vicinity of the abducens nucleus and is characterized by the patient's inability to move either eye toward the side of the lesion in lateral gaze, or to move the eye on the side of the lesion in gaze toward the opposite side (see Fig. 12.8D ). Thus, of the two directions of horizontal gaze (right and left), only half of one is intact. This is caused by destruction of one abducens nucleus plus destruction of fibers from the contralateral internuclear neurons as they join the MLF on the side of the lesion. The nearby internal genu of the facial nerve may also be affected ( Fig. 12.10 ).