Transverse Sections of the Brainstem





The brainstem contains the continuations of the long tracts seen in the spinal cord together with nuclei and tracts associated with cranial nerves and the cerebellum. These various tracts and nuclei surround, traverse, or are embedded in the reticular formation (named for its anatomical appearance—the Latin word reticulum means “network”), which forms a central core at all brainstem levels.


This chapter considers the level-to-level arrangements of structures as seen in transverse sections of the brainstem; many of the same structures are revisited in Chapter 8 as parts of functional systems. The sections were made by Pam Eller and stained, as were the spinal cord sections in the previous chapter, by the Klüver-Barrera method, using luxol fast blue for myelin and a neutral red counterstain (which, despite its name, is a basic stain with an affinity for nucleic acids). The result is blue-violet staining of white matter and red staining of large neurons with prominent Nissl substance (e.g., hypoglossal motor neurons in Fig. 3.10 ) and of areas tightly packed with small neurons (e.g., the granular layer of cerebellar cortex in Fig. 3.10 ). A parasagittal section of the brainstem ( Fig. 3.1 ) is used as a reference view throughout the chapter. It includes some of the features characteristic of each brainstem level ( Fig. 3.2 ), such as the superior and inferior colliculi of the midbrain , the basal pons , and a medullary pyramid .


The three major longitudinal pathways (lateral corticospinal tract, posterior columns, and spinothalamic tract) that were followed through the spinal cord in Chapter 2 extend into the brainstem in consistent ways, as indicated in Fig. 3.3 . Corticospinal fibers travel in the most ventral part of the brainstem, traversing the cerebral peduncle , basal pons , and medullary pyramid . At the spinomedullary junction, most of the fibers in each pyramid cross the midline (in the pyramidal decussation ) and form the lateral corticospinal tract. Each posterior column terminates in the posterior column nuclei ( nucleus gracilis and nucleus cuneatus ) of the medulla . Afferent fibers from these nuclei decussate in the medulla to form the medial lemniscus , which travels rostrally and ends in the thalamus. The medial lemniscus starts out near the midline and then moves progressively more laterally as it proceeds rostrally through the brainstem ( Fig. 3.4 ), rotating nearly 180 degrees in the process. The spinothalamic tract at all levels of the brainstem is at or near the lateral edge of the reticular formation. Cranial nerve nuclei are also arranged in reasonably consistent ways, as indicated schematically in Fig. 3.3 and in more detail in subsequent figures.


Figure 3.1


Parasagittal section of the brainstem and diencephalon.



Figure 3.2


Levels of the brainstem.



Figure 3.3


Arrangement of cranial nerve nuclei in the rostral medulla. The left side of the figure indicates how visceral sensory ( Vi) , somatic sensory (Ss) , and “special” sensory (Sp) (e.g., vestibular) nuclei are located lateral to the nuclei containing preganglionic autonomic neurons (A) , somatic motor neurons (Sm) , and motor neurons for muscles of branchial arch origin (B) (e.g., muscles of the larynx and pharynx). The cranial nerves containing each of these components are indicated on the right. (Not all of the nerves indicated actually emerge from the rostral medulla; they are included here for summary purposes.) CST , Corticospinal tract; ML , medial lemniscus; STT , spinothalamic tract.



Figure 3.4


Schematic views of six transverse sections of the brainstem, each enlarged about 3×, indicating major long tracts and cranial nerve nuclei. These are the same sections shown photographically in Figs. 3.9, 3.10, 3.12, 3.13, 3.15, and 3.16 , and they correspond to planes of section indicated in Fig. 3.6 . Abbreviations as in Fig. 3.6 . (A) Caudal medulla (see Fig. 3.9 ). (B) Rostral medulla (see Fig. 3.10 ). (C) Caudal pons (see Fig. 3.12 ). (D) Midpons (see Fig. 3.13 ). (E) Caudal midbrain (see Fig. 3.15 ). (F) Rostral midbrain (see Fig. 3.16 ).







Figure 3.5


Schematic views of six transverse sections of the brainstem, each enlarged about 3×, indicating areas of arterial supply. These are the same sections shown photographically in Figs. 3.9, 3.10, 3.12, and 3.14 through 3.16 , and they correspond to planes of section indicated in Fig. 3.6 . At each level, the brainstem supply is a series of wedge-shaped territories with anterolateral areas fed by midline arteries (e.g., vertebral, basilar) and posterolateral areas fed by circumferential branches (e.g., PICA, posterior cerebral). (A) Caudal medulla (see Fig. 3.9 ). (B) Rostral medulla (see Fig. 3.10 ). (C) Caudal pons (see Fig. 3.12 ). (D) Rostral pons (see Fig. 3.14 ). (E) Caudal midbrain (see Fig. 3.15 ). (F) Rostral midbrain (see Fig. 3.16 ).











Figure 3.6


The longitudinal arrangement of functional types of cranial nerve nuclei in the brainstem. The cranial nerves involved with each type of function are indicated on the left side of the diagram, and the actual nuclei are indicated on the right side. The dark blue line on each side represents the sulcus limitans, which (ideally, at least) separates motor nuclei medial to it from sensory nuclei lateral to it. Abbreviations for nuclei on the right side: Ab , Abducens nucleus; Ac , accessory nucleus; Am , nucleus ambiguus; C , cochlear nuclei; dV , dorsal motor nucleus of the vagus; EW , Edinger-Westphal nucleus (a subdivision of the oculomotor nucleus); Fa , facial nucleus; Hy , hypoglossal nucleus; Is , inferior salivary nucleus; mes , mesencephalic nucleus of the trigeminal; mT , motor nucleus of the trigeminal; Oc , oculomotor nucleus; Sol , nucleus of the solitary tract; spT , spinal nucleus of the trigeminal; Ss , superior salivary nucleus; sT , main sensory nucleus of the trigeminal; Tr , trochlear nucleus; Ves , vestibular nuclei. All of these nuclei (except the salivary nuclei) are indicated in one or more of the cross sections along the right side of the figure.

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Dec 29, 2019 | Posted by in NEUROLOGY | Comments Off on Transverse Sections of the Brainstem
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