Cranial Nerves and Their Nuclei

Chapter 12 Cranial Nerves and Their Nuclei



The caudal medulla looks somewhat similar to the spinal cord, but this similarity seems to disappear at more rostral levels of the brainstem. One of the complicating factors is the arrangement of the tracts and nuclei associated with cranial nerves III to XII. These tracts and nuclei appear discouragingly intricate on first inspection, but there is a common way of systematizing the cranial nerves so that their central connections make sense. This involves categorizing the tracts and nuclei according to the kinds of afferent and efferent fibers contained within each nerve (often referred to as the functional components of each nerve).



Cranial Nerve Nuclei Have a Generally Predictable Arrangement


Spinal nerves contain sensory and motor fibers. Some of each kind are related to visceral structures and some to somatic structures. A given axon entering or leaving the spinal cord can therefore be placed in one of the following four categories:






By and large the cell bodies on which spinal afferents synapse and the cell bodies of spinal efferent fibers are located in portions of the spinal gray matter predictable from its embryological development (Fig. 12-1A). The sulcus limitans separates the alar plate (which develops into the posterior horn) from the basal plate (which develops into the anterior horn). Within both the alar and the basal plates, cells concerned with visceral function tend to be located nearer the sulcus limitans. This is shown most clearly in the adult central nervous system (CNS) by the location of the cell bodies of visceral motor neurons in the intermediolateral cell column. So for each of the four spinal axon categories there is a corresponding column of cells in the spinal gray matter. The somatic sensory and motor columns extend the length of the cord; the visceral sensory and motor columns are found at spinal levels T1 to L2-L3 and S2 to S4.




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. * In addition, motor axons in certain cranial nerves innervate striated muscles with a special embryological origin, referred to as branchiomeric muscles. Structures that develop into the gill arches (or branchial arches) in fish develop instead into various structures in and near the head and neck in humans and other mammals. Branchiomeric muscles (notably the muscles of the larynx, pharynx, jaw, and face) are associated with these branchial arch structures. Functionally and histologically, branchiomeric muscles are identical to ordinary skeletal muscle, but the motor neurons for branchiomeric 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 branchial motor neurons.* Hence there are six different categories of nerve fibers in the cranial nerves attached to the brainstem (Table 12-1).


Table 12-1 Categories of Nerve Fibers in Cranial Nerves of the Brainstem





















































  Structures Innervated Cranial Nerves*
Sensory
Somatic Skin, muscles, joints of the head V
Visceral Cranial, thoracic, abdominal viscera X
Taste buds VII, IX
Special Inner ear VIII
Motor
Somatic Extraocular muscles III, IV, VI
Tongue muscles XII
Visceral Parasympathetic ganglia for cranial, thoracic, and abdominal viscera X
Branchial Jaw muscles V
Facial muscles VII
Laryngeal and pharyngeal muscles X
Middle ear muscles V, VII
Sternocleidomastoid, trapezius XI

* Principal cranial nerves only. Smaller contributions that may nevertheless be clinically important (e.g., parasympathetics for the pupil in cranial nerve III) are not indicated here but are included in Table 12-2.


Does not include the efferents in cranial nerve VIII (described in Chapter 14) that innervate the receptor cells of the inner ear, which do not fit comfortably into any of these categories.


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. 2-10). 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) (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 Figure 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 branchial motor neurons are located in the ventrolateral 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 Figure 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 Figure 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 nerves (III, IV, VI, 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, X, and XI) are somewhat more complex and typically contain several components; all innervate branchial arch musculature, so they are called branchiomeric nerves.



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 more or less distinct 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 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.



Cranial Nerves III, IV, VI, and XII Contain Somatic Motor Fibers


The somatic motor nerves are the simplest of the cranial nerves because each contains fibers of only one category (except for cranial nerve III, which has a small but important complement of preganglionic parasympathetic fibers). * The nuclei of origin of all these nerves are located adjacent to the midline near the aqueduct or the floor of the fourth ventricle, as would be expected from their embryological origins.



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) and all the internal and external muscles of the ipsilateral eye except the lateral rectus, superior oblique, and pupillary dilator. The fibers originate in the wedge-shaped oculomotor nucleus, which is located at the ventral edge of the periaqueductal gray in the rostral midbrain (Fig. 12-4). They then proceed ventrally and arch through the midbrain tegmentum in several separate 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 on both sides. The column supplying the superior rectus projects to the contralateral eye. The columns supplying the medial rectus, inferior oblique, and inferior rectus all project to the ipsilateral eye. Finally, a column containing preganglionic parasympathetic neurons, known by the tenacious eponym 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 (although there are occasional cases of unilateral nuclear damage in which these deficits are found).


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;* the lens cannot be focused for near vision.



Along the course of the oculomotor nerve from brainstem to orbit, the preganglionic parasympathetic fibers from the Edinger-Westphal nucleus travel in a superficial location 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 might 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, of course, by defective eye movements and lateral strabismus.




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 nucleus (see Figs. 12-6 and 12-24), which is located in the ventrolateral part of the pontine tegmentum at about the same level as the abducens nucleus. The facial fibers travel dorsomedially, wrap around the abducens nucleus, and turn back ventrally 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. * 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 (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.


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Jul 31, 2016 | Posted by in NEUROLOGY | Comments Off on Cranial Nerves and Their Nuclei
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