Figure 65b lower midbrain crosssection

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This cross-section includes the cerebral peduncles, still located anteriorly and the substantia nigra located immediately behind these fibers. The unique feature in the lower midbrain is the decussation (crossing) of the superior cer-ebellar peduncles, which occupies the central area of the section; this identifies the section as the inferior collicular level. Posteriorly the aqueduct is surrounded by the peri-aqueductal gray, and behind the aqueduct is the inferior colliculus. Often, the cross-section at this level includes some of the pontine nuclei. (as is seen in the histological section below). Therefore, one may see a somewhat confusing mixture of structures.

The arrangement of the fibers in the cerebral peduncle is the same as found in the upper midbrain. The tegmen-tum contains the ascending tracts, the medial lemniscus, the trigeminal pathway, and the anterolateral fibers (system), which are situated together at the outer edge of the lower midbrain (see Figure 40).

In sections through the lower levels of the midbrain, there is a brief appearance of a massive fiber system (as seen with a myelin-type stain) occupying the central region of the lower midbrain. These fibers are the continuation of the superior cerebellar peduncles, which are crossing (decussating) at this level (see Figure 10 and Figure 40). The fibers are coming from the deep cerebellar nuclei (the intracerebellar nuclei), mainly the dentate nucleus, and are headed for the ventral lateral nucleus of the thalamus, and then on to the motor cortex (discussed with Figure 57). Some of the fibers that come from the intermediate deep cerebellar nucleus will synapse in the red nucleus.

The nuclei of the reticular formation found in the central region (the tegmentum) at this level are function ally part of the ARAS and play a significant role in consciousness (see Figure 42A and Figure 42B). Between the cerebral peduncles is a small nucleus, the interpeduncular nucleus, which belongs with the limbic system. The peri-aqueductal gray surrounding the aqueduct of the midbrain is involved with pain and also with the descending pathway for the modulation of pain (see Figure 43).

The nucleus of CN IV, the trochlear nucleus, is located in front of the periaqueductal gray, next to the midline. Because it supplies only one extra-ocular muscle, it is a smaller nucleus than the oculomotor nucleus. CN IV heads dorsally and will exit from the brainstem below the inferior colliculus (see Figure 48), on the posterior aspect of the brainstem. The MLF lies just anterior to the trochlear nucleus. Some unusually large round cells are often seen at the edges of the periaqueductal gray; these cells are part of the mesencephalic nucleus of the trigeminal nerve, CN V (see Figure 8B).

The lateral lemniscus, the ascending auditory pathway, is still present at this level, and its fibers are terminating in the inferior colliculus, a relay nucleus in the auditory pathway (see Figure 37 and Figure 38). After synapsing here, the fibers are relayed to the medial gen-iculate nucleus via the brachium of the inferior colliculus, seen at the upper midbrain level (previous illustration).

Clinical Aspect

The presence of the pain and temperature fibers that are found at this level at the outer edge of the midbrain has prompted the possibility, in very select cases, to surgically sever the sensory ascending pathways at this level. This highly dangerous neurosurgical procedure would be done particularly for cancer patients who are suffering from intractable pain. Nowadays it would only be considered as a measure of last resort. Pain control is currently managed through the use of drugs, either as part of palliative care or in the setting of a pain "clinic," accompanied by other measures.

Inferior colliculus Lateral lemniscus

Mesencephalic nucleus of CN V

Anterolateral system Medial lemniscus

Trochlear nucleus

Interpeduncular nucleus

Inferior Colliculus

Inferior colliculus Lateral lemniscus

Mesencephalic nucleus of CN V

Anterolateral system Medial lemniscus

Trochlear nucleus

Interpeduncular nucleus

Aqueduct of midbrain

Substantia nigra

Parieto-, temporo- and occipito-pontine fibers

Cortico-spinal tract Fronto-pontine fibers

Aqueduct of midbrain

Periaqueductal gray Reticular formation

Decussation of the superior cerebellar peduncles

Substantia nigra

Parieto-, temporo- and occipito-pontine fibers

Cortico-spinal tract Fronto-pontine fibers

FIGURE 65B: Brainstem Histology — Lower Midbrain

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