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Caudal Medulla: Level of the Motor Decussation

At the level of the motor decussation (pyramidal decussation), about 90% of corticospinal fibers cross the anterior midline to form the contralateral lateral corticospinal tract of the cord (Figs. 11-5 to 11-7). Posteriorly, at this level the gracile and cuneate nuclei first appear in their respective fasciculi (Figs. 11-6 and 11-7). Because the gracile and cuneate fasciculi are collectively called the posterior (or dorsal) columns, their respective nuclei are frequently referred to as the posterior column nuclei. Laterally, the spinal trigeminal tract (visible on the surface of the medulla as the trigeminal tubercle or tuberculum cinereum) is located on the medullary surface. Internal to the spinal trigeminal tract is the spinal trigeminal nucleus, pars caudalis (Fig. 11-6).

The spinal trigeminal tract is composed of central processes of primary sensory fibers that enter the brain mainly in the trigeminal nerve. This tract also receives fibers that originate from cranial nerves VII, IX, and X. These primary sensory fibers terminate on cells of the spinal trigeminal nucleus, which, in turn, projects to the contralateral thalamus as the anterior (ventral) trigeminothalamic tract.

In the lateral medulla, the anterolateral system and rubrospinal tract are found medial to the superficially located posterior and anterior spinocerebellar tracts (Figs. 11-6 and 11-7). It is important to emphasize that anterolateral system fibers (conveying pain and temperature input from the contralateral side of the body) and spinal trigeminal tract fibers (conveying pain and temperature from the ipsilateral face) are located adjacent to each other throughout the lateral area of the medulla.

The anterior medulla contains the most rostral part of the accessory nucleus (cranial nerve XI), remnants of the medial motor cell column of C1, and the medial longitudinal fasciculus and tectobulbospinal system. The most rostral remnants of the accessory nucleus (cranial nerve XI) and the medial motor cell column of C1 are seen at the spinal cord/medulla junction but do not extend into the medulla. Immediately adjacent to these cell groups are the small fiber bundles of the medial longitudinal fasciculus and the tectobulbospinal system (Figs. 11-6 and 11-7). At this level, the tectospinal fibers in the tectobulbospinal system are incorporated into the medial longitudinal fasciculus. These small bundles are displaced laterally by the motor decussation compared with their medial positions at more rostral levels. The central gray surrounds the central canal of the medulla and contains the caudal extremes of the hypoglossal (XII) and dorsal motor vagal nuclei (X) (Fig. 11-6). When the ventricle flares open at the level of the obex, these nuclei occupy the medial floor of the ventricular space.

Figure 11-6 Cross section of the medulla at the level of the motor decussation. Correlate with Figure 11-5.

Caudal Medulla: Level of the Sensory Decussation

Cells of the posterior column nuclei (gracile and cuneate nuclei) give rise to axons that swing anteromedially, as internal arcuate fibers, to cross the midline immediately rostral to the motor decussation (Fig. 11-5). This crossing of fibers at the midline constitutes the sensory decussation, so named because it is the point at which a major ascending sensory pathway (posterior column-medial lemniscus) crosses the midline.

At this level the posterior columns (gracile and cuneate fasciculi) are largely replaced by the gracile and cuneate nuclei (Figs. 11-8 and 11-9). Fibers conveying tactile and vibratory sensations from lower and upper levels of the body terminate, respectively, in the gracile and cuneate nuclei. The axons of these cells, in turn, form the internal arcuate fibers, which cross the midline as the sensory decussation and collect to form the medial lemniscus on the contralateral side (Figs. 11-5, 11-8, and 11-9). Information from lower extremities (gracile cell axons) is conveyed in the anterior part of the medial lemniscus, and information from the upper extremities (cuneate cell axons) is conveyed in the posterior part of the medial lemniscus (see Fig. 12-13). The accessory cuneate nucleus is located lateral to the cuneate nucleus (Fig. 11-8). Its cells receive primary sensory input via cervical spinal nerves and project to the cerebellum as cuneocerebellar fibers. In doing so, they represent the upper extremity equivalent of the posterior spinocerebellar tract.

The spinal trigeminal tract and nucleus (pars caudalis) maintain their position in the

lateral medulla. The pars caudalis is that portion of the spinal trigeminal nucleus located caudal to the level of the obex. At this level, however, posterior spinocerebellar fibers have migrated posteriorly to cover the spinal tract, heralding the beginnings of the restiform body (Figs. 11-8 and 11-9). Just medial to the spinal trigeminal nucleus, a small column of motor neurons, the nucleus ambiguus, appears (Fig. 11-8). The axons of these SVE cells travel in the glossopharyngeal (IX) and vagus (X) nerves. Fibers of the anterolateral system and rubrospinal tract are located in the anterolateral medulla (Fig. 11-8). The lateral reticular nucleus, a distinct cell group adjacent to the anterolateral system, receives spinal input and projects to the cerebellum.

Structures characteristic of the anterior surface of the medulla at this level include the pyramid, fibers of the hypoglossal nerve, and the caudal end of the inferior olivary complex (Figs. 11-8 and 11-9). The inferior olivary nuclei (internal to the olivary eminence), which become larger at more rostral levels, receive input from a variety of areas and project primarily to the cerebellum. The inferior olivary nuclei are sometimes described as the inferior olivary complex since they collectively consist of principal, medial accessory, and posterior accessory nuclei. Internal to the pyramid, and along the midline from anterior to posterior, are the medial lemniscus, tectobulbospinal fibers, and medial longitudinal fasciculus (Fig. 11-8). At this level, medial longitudinal fasciculus fibers are characteristically found adjacent to the midline and anterior to structures of the central gray.

The central gray is larger than at the level of the motor decussation, and caudal parts of the hypoglossal and dorsal motor vagal nuclei and the solitary nucleus and tract can be clearly identified along its perimeter (Figs. 11-8 and 11-9). Hypoglossal (GSE) motor neurons innervate the ipsilateral half of the tongue. These fibers course anterolaterally along the lateral edge of the medial lemniscus and pyramid and share a common blood supply with these structures. The GVE cells of the dorsal motor vagal nucleus provide preganglionic parasympathetic fibers to visceromotor ganglia (autonomic ganglia), the postganglionic fibers of which innervate viscera in the thorax and abdomen. The solitary tract and nucleus receive GVA and SVA (taste) input from cranial nerves VII, IX, and X. At this caudal level of the medulla, input to the solitary nucleus is primarily GVA in nature and originates mainly from thoracic and abdominal viscera (via cranial nerve X) and the carotid sinus (via cranial nerve IX).

The fourth ventricle flares open at the level of the obex (Fig. 11-10). The area postrema is an emetic (vomiting) center located in the wall of the ventricle at this level. Especially noticeable changes at this point compared with more caudal levels include enlargement of the inferior olivary complex and restiform body.

Figure 11-8 Cross section of the medulla at the level of the sensory decussation. Correlate with Figure 11-5.

Figure 11-10 Cross section of the medulla at the level of the obex.

Midmedullary Level

Rostral to the obex, the structures in the medial floor of the fourth ventricle are the hypoglossal and dorsal motor vagal nuclei and, lateral to the sulcus limitans, the vestibular nuclei (Figs. 11-11 and 11-12). The latter cell groups consist, at this level, of medial and inferior (or spinal) vestibular nuclei. They receive input from cranial nerve VIII and interconnect with areas of the brain concerned with balance and eye movement. The solitary tract and nucleus occupy their characteristic position immediately inferior to the vestibular nuclei.

Laterally, the restiform body forms a prominent elevation on the posterolateral aspect of the medulla (Figs. 11-11 and 11-12). This structure contains posterior spinocerebellar, cuneocerebellar, olivocerebellar, reticulocerebellar, and other cerebellar afferents. In the base of the cerebellum, these fibers join with the juxtarestiform body to form the inferior cerebellar peduncle.

The spinal trigeminal tract and nucleus (pars interpolaris) are internal to the restiform body (Figs. 11-11 and 11-12). The pars interpolaris is the part of the spinal trigeminal nucleus located between the levels of the obex and the rostral end of the hypoglossal nucleus. Other structures in the lateral medulla are comparable to those seen more caudally. These include the nucleus ambiguus and the lateral reticular nucleus as well as the anterolateral system, anterior spinocerebellar tract, and rubrospinal tract (Fig. 11-11). At all medullary levels, neurons of the nucleus ambiguus contribute axons to cranial nerves IX and X, which innervate pharyngeal and laryngeal muscles, including those of the vocal folds.

Anterolaterally, the inferior olivary complex is prominent at midmedullary levels and is composed of a large, saccular principal olivary nucleus and diminutive medial and posterior accessory olivary nuclei (Figs. 11-11 and 11-12). These cell groups receive input from a variety of central nervous system nuclei and project primarily to the contralateral cerebellum (as olivocerebellar fibers) through the restiform body. Anteriorly and medially, the orientation of the pyramid, medial lemniscus, medial longitudinal fasciculus, and tectobulbospinal system remains essentially the same as at more caudal levels (Figs. 11-11 and 11-12).

Figure 11-11 Cross section of the medulla at midolivary levels. Correlate with Figure 11-5. The anatomic orientation is flipped to illustrate internal structures in a clinical orientation; the clinically important tracts and nuclei are shown on a T2-weighted MR image at a comparable level of the mid medulla.

Rostral Medulla and Pons-Medulla Junction

Comparison of Figures 11-11 and 11-13 shows that many of the structures seen in the mid medulla are present in essentially the same locations in the rostral medulla. Therefore, we shall emphasize the features that are different in the rostral medulla.

In the floor of the fourth ventricle, the positions occupied by the hypoglossal and dorsal motor vagal nuclei at more caudal levels are taken by the prepositus (hypoglossal) nucleus and the inferior salivatory nucleus (Fig. 11-13). The prepositus nucleus is a small, somewhat flattened cell group that is easily distinguished from the hypoglossal nucleus. The GVE cells of the inferior salivatory nucleus are located immediately inferior to the medial vestibular nucleus and medial to the solitary tract and nucleus. Axons of these salivatory nucleus cells distribute to the otic ganglion via peripheral branches of the glossopharyngeal nerve.

The medial and inferior (or spinal) vestibular nuclei are prominent at this level and are joined, in this plane of section, by the posterior and anterior cochlear nuclei (Figs. 11-12 and 11-13). The latter nuclei are located on the posterior and lateral aspects of the restiform body at the pons-medullary junction. The medial vestibular nucleus appears homogeneous in fiber-stained sections, and the inferior vestibular nucleus has a salt-and-pepper appearance (Fig. 11-12). This appearance results because small descending bundles of myelinated fibers (pepper) are intermingled with cells (salt) in the inferior nucleus. Medial to the restiform body is the spinal trigeminal tract and the pars oralis of the spinal trigeminal nucleus (Fig. 11-13). The pars oralis is that part of the nucleus located rostral to the level of the hypoglossal nucleus.

At this rostral level of the medulla, the solitary tract and nucleus retain their positions immediately inferior to the medial and inferior vestibular nuclei. However, in contrast to the more caudal parts of this nucleus (which receives mainly GVA input originating mainly from cranial nerves IX and X), this rostral portion of the nucleus receives primarily SVA (taste) input from nerves VII and IX. The solitary tract and nucleus do not extend craniad to the root of the facial nerve as this is the most rostral of the cranial nerves to contribute to these structures.

Although structures in anterior and medial areas of the medulla are unchanged from those at midmedullary levels, some changes take place at the pons-medullary junction that merit comment (Fig. 11-14). Fibers of the restiform body arch posteriorly to enter the cerebellum, where they are joined by fibers of the juxtarestiform body to form (collectively) the inferior cerebellar peduncle. The facial motor nucleus (SVE cells) appears anterolaterally, and the trapezoid body and superior olivary nucleus (both conveying auditory information) appear adjacent to the facial nucleus and the spinal trigeminal tract and nucleus (Fig. 11-14). The inferior olivary complex disappears, and the central tegmental tract, one source of input to the inferior olive, appears about where the latter cell group was located (Fig. 11-14). Finally, the medial lemniscus begins

to shift anterolaterally and to rotate from a posteroanterior orientation, which it exhibits in the medulla, toward a horizontal orientation more characteristic of the pons (Fig. 11-14). At the pons-medulla junction, the cross section of the medial lemniscus is oriented obliquely (posteromedial to anterolateral); by the level of the mid pons, it is horizontal.

Figure 11-13 Cross section of the medulla at rostral olivary (and medullary) levels. Correlate with Figure 11-5. The anatomic orientation is flipped to illustrate internal structures in a clinical orientation; the clinically important tracts and nuclei are shown on a T2-weighted MR image at a comparable level of the rostral medulla.

Figure 11-14 Cross section of the medulla at the pontomedullary junction. Fibers of the restiform body sweep up (arrows) into the cerebellum at this level.

Reticular and Raphe Nuclei

The word reticulum is Latin for "little net" (diminutive of rete, "net") and denotes mesh-like structures. The reticular nuclei of the brainstem are diffuse and ill defined and have little apparent internal organization. Collectively, they make up the reticular formation, which may be thought of simply as including all of the cells that are interspersed among the more compact and named structures of the brainstem.

Raphe is a Greek word for "suture" or "seam." Thus, the raphe nuclei are bilaterally symmetrical cell groups in the brainstem that are located directly adjacent to the midline.

The medial medullary reticular area consists of the central nucleus of the medulla at caudal medullary levels and the gigantocellular reticular nucleus rostrally; the latter cell group extends into the pons (Fig. 11-15). The lateral medullary reticular area contains a

compact column of cells, the lateral reticular nucleus, and a diffuse population of cells that forms the parvocellular nucleus and the ventrolateral reticular area (area reticularis superficialis ventrolateralis) (Fig. 11-15). The latter cells function in the control of heart rate and respiration. Consequently, a sudden onset of central apnea, indicating damage to these respiratory areas, is often a prime early sign of medullary compression. The raphe nuclei of the medulla are the nucleus raphes pallidus and nucleus raphes obscurus and, at rostral levels, the nucleus raphes magnus (Fig. 11-15). The nucleus pallidus and nucleus obscurus are located at mid- to rostral medullary levels along the posterior and anterior midline, respectively. The nucleus raphes magnus begins in the rostral medulla and extends into the caudal pons (Fig. 11-15). In general, the raphe nuclei of the medulla and medulla-pons junction (magnus, pallidus, obscurus) project caudally to spinal cord targets. Cells of these raphe nuclei receive input from several areas, including the central gray of the mesencephalon, and project to the spinal cord. Raphespinal fibers from the raphe magnus are especially important for the inhibition of pain transmission in the posterior horn of the spinal cord. The principal neurotransmitter associated with these nuclei and their fibers is serotonin, although cholecystokinin-containing cells are also found in all three of these raphe nuclei and enkephalin is found in magnus neurons.

Figure 11-15 Posterior (dorsal) (A) view of the brainstem and caudal (B) and rostral (C) cross sections showing the raphe and reticular nuclei of the medulla.

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