Figure 65a upper midbrain crosssection

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The identifying features of this cross-section of the mid-brain include the cerebral peduncle ventrally, with the substantia nigra posterior to it. The aqueduct is surrounded by the periaqueductal gray. The remainder of the midbrain is the tegmentum, with nuclei and tracts. Dorsally, behind the aqueduct, is a colliculus.

The descending fiber systems are segregated within the cerebral peduncles (see Figure 45, Figure 46, and Figure 48). The substantia nigra consists, in fact, of two functionally distinct parts — the pars compacta and the pars reticulata. The pars reticulata lies adjacent to the cerebral peduncle and contains some widely dispersed neurons; these neurons connect the basal ganglia to the thalamus as one of the output nuclei of the basal ganglia (similar to the globus pallidus internal segment, see Figure 53). The pars compacta is a cell-rich region, located more dorsally, whose neurons contain the melanin-like pigment. These are the dopaminergic neurons that project to the neostriatum (discussed with Figure 52). Loss of these neurons results in the clinical entity Parkinson's disease (discussed with Figure 52).

The red nucleus is located within the tegmentum; large neurons are typical of the ventral part of the nucleus. With a section that has been stained for myelin, the nucleus is seen as a clear zone. The red nucleus gives origin to a descending pathway, the rubro-spinal tract, which is involved in motor control (see Figure 47 and Figure 48).

The oculomotor nucleus (CN III) is quite large and occupies the region in front of the periaqueductal gray, near the midline; this identifies the level as upper midbrain with the superior colliculus. These motor neurons are large in size and easily recognizable. The parasympathetic portion of this nucleus is incorporated within it and is known as the Edinger-Westphal (EW) nucleus (see Figure 8A). The fibers of CN III pass anteriorly through the medial portion of the red nucleus and exit between the cerebral peduncles, in the interpeduncular fossa (see Figure 6 and Figure 7).

The ascending (sensory) tracts present in the midbrain are a continuation of those present throughout the brain-stem. The medial lemniscus, the ascending trigeminal pathway, and the fibers of the anterolateral system incorporated with them (see Figure 36 and Figure 40) are located in the outer part of the tegmentum, on their way to the nuclei of the thalamus (see Figure 63).

The nuclei of the reticular formation are found in the central region of the brainstem (the tegmentum); they are functionally part of the ascending reticular activating system and play a significant role in consciousness (discussed with Figure 42A and Figure 42B). The periaqueductal gray surrounding the cerebral aqueduct is involved with the descending pathway for the modulation of pain (see Figure 43).

The superior colliculus is a subcortical center for certain visual movements (see Figure 41B). These nuclei give rise to a fiber tract, the tecto-spinal tract, a descending pathway that is involved in the control of eye and neck movements; it descends to the cervical spinal cord as part of the medial longitudinal fasciculus (MLF) (see Figure 51B).

The MLF stains heavily with a myelin-type stain and is found anterior to the cranial nerve motor nucleus, next to the midline, at this level as well as other levels of the brainstem. Also to be noted at this level is the brachium of the inferior colliculus, a part of the auditory pathway (see Figure 10, Figure 37, and Figure 38).

Clinical Aspect

A specific lesion involving a thrombosis of the basilar artery may destroy much of the brainstem yet leave the inner part of the midbrain intact. Few people actually survive this cerebrovascular damage, but those that do are left in a suspended (rather tragic) state of living, known by the name "locked-in" syndrome. The patient retains consciousness, with intellectual functions generally intact, meaning that they can think and feel as before. However, usually, all voluntary movements are gone, except perhaps for some eye movements, or occasionally some small movements in the hands and fingers. This means that they require a respirator to breathe and 24-hour total care. There may also be a loss of all sensations, or some sensation from the body may be retained.

Brachium of the inferior colliculus

Anterolateral system Medial lemniscus

Edinger-Westphal nucleus

Oculomotor nucleus

Oculomotor nerve (CN III)

Brachium of the inferior colliculus

Anterolateral system Medial lemniscus

Edinger-Westphal nucleus

Anterolateral Nucleus

Oculomotor nucleus

Oculomotor nerve (CN III)

Red nucleus

Substantia nigra

Parieto-, temporo- and occipito-pontine fibers

Cortico-spinal tract Fronto-pontine fibers

Aqueduct of midbrain Periaqueductal gray Reticular formation MLF

Red nucleus

Substantia nigra

Parieto-, temporo- and occipito-pontine fibers

Cortico-spinal tract Fronto-pontine fibers

Midbrain Cross Section

FIGURE 65A: Brainstem Histology — Upper Midbrain

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