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This cross-sectional level is often presented as "typical" of the medulla. The pyramids and inferior olive are easily recognized anteriorly.

The medial lemniscus occupies the area between the olives, on either side of the midline (see Figure 40). The MLF lies behind (dorsal) the medial lemniscus, also situated adjacent to the midline. The fibers of the anterolat-eral system are situated dorsal to the olive. The descending nucleus and tract of the trigeminal system have the same location as seen previously in the lateral aspect of the tegmentum.

The hypoglossal nucleus (CN XII) is found near the midline and in front of the ventricle; its fibers exit anteriorly, between the pyramid and the olive (see Figure 6 and Figure 7). CN IX and CN X are attached at the lateral aspect of the medulla (see Figure 6 and Figure 7). Their efferent fibers are derived from two nuclei (indicated by the dashed lines): the dorsal motor nucleus, which is para-sympathetic, and the nucleus ambiguus, which is motor to the muscles of the pharynx and larynx (see Figure 8A). The dorsal motor nucleus lies adjacent to the fourth ventricle just lateral to the nucleus of XII. The nucleus ambig-uus lies dorsal to the olivary nucleus; in a single cross-section only a few cells of this nucleus are usually seen, making its identification difficult (i.e., "ambiguous") in actual sections. The taste and visceral afferents that are carried in these nerves synapse in the solitary nucleus, which is located in the posterior aspect of the tegmentum, surrounding the tract of the same name.

The reticular formation occupies the central core of the tegmentum; the nucleus gigantocellularis is located in this part of the reticular formation (see Figure 42B). These cells give rise to a descending tract, the lateral reticulo-spinal tract as part of the indirect voluntary motor system (see Figure 49B); there is also a strong influence on the excitability of the lower motor neuron, influencing the stretch reflex and muscle tone.

The inferior cerebellar peduncle is found at the lateral edge of this section, posteriorly, carrying fibers to the cerebellum (see Figure 55). The fourth ventricle is still a rather large space, behind the tegmentum, with the choroid plexus attached to its roof in this area; often the ventricle appears "open," likely because this thin tissue has been torn. There is no cerebellar tissue posteriorly since the section is below the level of the cerebellum (see the sagittal schematic accompanying this figure).

Clinical Aspect

Vascular lesions in this area of the brainstem are not uncommon. The midline area is supplied by the parame-dian branches from the vertebral artery (see Figure 58). The structures included in this territory are the cortico-spinal fibers, the medial lemniscus, and the hypoglossal nucleus.

The lateral portion is supplied by the posterior inferior cerebellar artery, a branch of the vertebral artery (see Figure 58, Figure 59A, and Figure 61), called PICA by neuroradiologists. This artery is prone to infarction for some unknown reason. Included in its territory are the cranial nerve nuclei and fibers of CN IX and X, the descending trigeminal nucleus and tract, fibers of the ante-rolateral system, and the solitary nucleus and tract, as well as descending autonomic fibers. The inferior cerebellar peduncle or vestibular nuclei may also be involved. The whole clinical picture is called the lateral medullary syndrome (of Wallenberg).

Interruption of the descending autonomic fibers gives rise to a clinical condition called Horner's syndrome. In this syndrome, there is loss of the autonomic sympathetic supply to one side of the face, ipsilaterally. This leads to ptosis (drooping of the upper eyelid), a dry skin, and constriction of the pupil. The pupillary change is due to the competing influences of the parasympathetic fibers, which are still intact. Other lesions elsewhere that interrupt the sympathetic fibers in their long course can also give rise to Horner's syndrome.

Note to the Learner: It is instructive to work out the clinical symptomatology of both of these vascular lesions, using a drawing, indicating which function is lost with each of the tracts or nucleus involved in the lesion, and which side of the body would be affected.

Accessory cuneate n.

Dorsal motor n. Hypoglossal n.

Vagus nerve

Anterolateral system Medial lemniscus

Accessory cuneate n.

Dorsal motor n. Hypoglossal n.

Vagus nerve

Nucleus Ambiguus Wallenberg Sendromu

Anterolateral system Medial lemniscus

Cortico-spinal fibers

Choroid plexus 4th ventricle

Solitary n. Solitary t.

Inferior cerebellar peduncle

N. ambiguus MLF

Reticular formation Inferior olivary n.

Hypoglossal nerve (CN XII)

Cortico-spinal fibers

Choroid plexus 4th ventricle

Solitary n. Solitary t.

Inferior cerebellar peduncle

N. ambiguus MLF

Reticular formation Inferior olivary n.

Hypoglossal nerve (CN XII)

FIGURE 67B: Brainstem Histology — Mid-Medulla

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