General Arrangement of Cranial Nerve Nuclei

In the thoracic region of the developing spinal cord, four distinct cell columns can be identified in the gray matter on each side (Figure 17.1A, B). In the basal plate, the general somatic efferent column supplies the striated muscles of the trunk and limbs. The general visceral efferent column contains preganglionic neurons of the autonomic system. In the alar plate, the general visceral afferent column receives afferents from thoracic and abdominal organs. A general somatic afferent column receives afferents from the body wall.

Figure 17.1 Cell columns of the spinal cord and brainstem. (A) Embryonic spinal cord. (B) Adult spinal cord. (C) Embryonic hindbrain. (D) Adult hindbrain. Afferent cell columns: GSA, general somatic afferent; GVA, general visceral afferent; SSA, special somatic afferent; SVA, special visceral afferent. Efferent cell columns: GSE, general somatic efferent; GVE, general visceral efferent; SVE, special visceral efferent.

In the brainstem, these four cell columns can be identified, but they are fragmented, and not all contribute to each cranial nerve. Their connections are as follows.

Three additional cell columns (Figure 17.1C, D) serve branchial arch tissues and the inner ear, as follows.

Figure 17.2 shows the position of the various nuclei in a dorsal view of the brainstem.

Figure 17.2 Posterior view of adult brainstem, showing position of cranial nerve cell columns. L, S, I, M, lateral, superior, inferior, medial vestibular nuclei.

In this chapter, details of the internal anatomy of the brainstem accompany nine representative transverse sections and their captions. Connections (direct or indirect) with the right cerebral hemisphere have been highlighted in accordance with information to be provided.

Background Information

As stated earlier, exteroceptive and conscious proprioceptive information are transferred (by spinothalamic and dorsal column–medial lemniscal pathways, respectively) from left trunk and limbs to right cerebral hemisphere. It was also explained that corticospinal fibers of the pyramidal tract arising from motor areas of the cerebral cortex supply contralateral anterior horn cells and give a small ipsilateral supply of similar nature, and that those arising from the parietal lobe project to the contralateral posterior gray horn.

The same arrangement holds good for the brainstem. The pyramidal tract fibers terminating in the brainstem are corticonuclear. As shown in Figure 17.3, their distribution is predominantly contralateral to somatic and branchial motor nuclei, and entirely contralateral to the somatic sensory nuclei.

Figure 17.3 Posterior view of brainstem, showing distribution of corticonuclear fibers from the right cerebral cortex.

Absent from this figure are the three pairs of motor ocular nuclei. Why? Because these do not receive a direct corticonuclear supply. Instead, their predominantly contralateral supply synapses on adjacent cell groups known as gaze centers having the function of synchronizing conjugate (conjoint parallel) movements of the eyes.

For a basic understanding of neural relationships in the brainstem, it is also essential to appreciate hemisphere linkages to the inferior olivary nucleus and to the cerebellum (Figure 17.4).

Figure 17.4 Anterior view of the four principal motor decussations of the brainstem. Pathways are numbered in accordance with their sequence of activation in voluntary movements: (1) corticopontocerebellar; (2) dentatothalamocortical; (3) corticospinal; (4) olivocerebellar. Also shown is the rubroolivary connection.

The general layout of the reticular formation (Figure 17.5) is borrowed from a figure in Chapter 24 devoted to this topic. It may be consulted when reading under this heading in successive descriptions.

Figure 17.5 Layout of the reticular formation (RF).

Figure 17.6 depicts the main components of the medial longitudinal fasciculus (MLF). This fiber bundle extends the entire length of the brainstem, changing its fiber composition at different levels. This figure, too, may be consulted during study of the brainstem sections to be described, following inspection of C1 segment of the spinal cord.

Figure 17.6 Main fiber composition of the medial longitudinal fasciculus (MLF). PPRF, paramedian pontine reticular formation; RN, reticular nucleus; RST, reticulospinal tract; VN, vestibular nucleus.

Study guide

The presentation departs from the traditional method, which is to describe photographs or diagrams at successive levels in ascending order without highlights. In the present approach:

1 The various nuclei and pathways are highlighted and labeled on the side having primary affiliation with the right cerebral hemisphere.

2 The nuclei and pathways are color-coded by systems, for example red for motor, blue for sensory, green for connections of cerebellum and reticular formation.

3 Highlighting together with color-coding makes it possible to study individual systems in vertical, ‘multiple window’ mode. The descriptive text related to the brainstem sections enables a logical sequence of study whereby afferent pathways can be followed from below upward to thalamic level (commencing with Figure 17.10), and efferent pathways can be followed from above downward (commencing with Figure 17.19). It must be emphasized that, following study in the vertical mode, a horizontal approach must be undertaken, the location of the various systems to be identified at each level. This is because occlusion of a small artery of supply to the brainstem may affect function in a patch that may include several distinct nuclei or pathways.

At each level, miniature replicas of the diagrams in Figure 17.7 are inserted to assist left–right orientation.

Figure 17.7 (A) Anterior and (B) posterior view of brainstem, showing disposition of some major pathways.

Special note: Readers unfamiliar with the internal anatomy of the brainstem may be disconcerted by the amount of new information contained in the series of sections to be described. It may be reassuring to know that all the information will come up again in later chapters. Therefore a sensible approach could be to undertake an initial browse through the sections and to recheck the location of individual items during later reading.

Overview of three pathways in the brainstem

Figure 17.8 shows the posterior column–medial lemniscal and anterolateral pathways already described in Chapter 15. Recall that the latter comprises the lateral spinothalamic tract serving pain and temperature, and the anterior spinothalamic tract serving touch. Within the brainstem, the two are combined as the spinal lemniscus.

Figure 17.8 Posterior column–medial lemniscal and anterolateral pathways. VPN, ventral posterior nucleus of thalamus.

The corticospinal tract, treated in Chapter 16, is shown in Figure 17.9. Also included are corticonuclear projections to the facial and hypoglossal nuclei.

Figure 17.9 Corticospinal tract; two corticonuclear projections.

C1 Segment of Spinal Cord (Figure 17.10)

Blue

The gracile and cuneate fasciculi constitute the posterior column of the spinal cord on each side. Their axons are ipsilateral central processes of posterior root ganglion cells whose peripheral processes receive information from the large tactile nerve endings in the skin, including Meissner’s and Pacinian corpuscles, and from neuromuscular spindles and Golgi tendon organs. The fasciculi terminate in the gracile and cuneate nuclei (Figure 17.12).

Figure 17.10 C1 segment of spinal cord.

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