17 Brainstem
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.
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.
Background Information
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.
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Figure 17.3 Posterior view of brainstem, showing distribution of corticonuclear fibers from the right cerebral cortex.
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).
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 17.6 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.
Study guide
At each level, miniature replicas of the diagrams in Figure 17.7 are inserted to assist left–right orientation.
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.
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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.
C1 Segment of Spinal Cord (Figure 17.10)
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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).