The midbrain can be divided into three main parts: the tectum (quadrigeminal plate); the tegmentum, which is a continuation of the pons tegmentum; and the very large crus cerebri, which contains the corticofugal fibers. The midbrain contains two cranial nerve nuclei, the oculomotor and trochlear nuclei. The most prominent nuclear mass in the midbrain is the substantia nigra, a huge area darkly pigmented with melanin, a metabolic byproduct of dopamine breakdown. The substantia nigra, which sends dopaminergic projections to the basal ganglia, is very important clinically since its degeneration produces a loss of dopamine terminations in the basal ganglia, resulting in the extrapyramidal disorder Parkinson’s disease. The structure of the midbrain is most usually demonstrated using transverse sections at the level of the inferior and superior colliculi.
Transection at the level of the inferior colliculus reveals that the pontine tectum or covering, i.e., the superior medullary velum, is now replaced by the inferior and superior colliculi, swellings caused by the masses of nuclei serving as relay stations for transmission of auditory and other signals to the brain. At this level the cerebral aqueduct replaces the fourth ventricle and decussation of the fibers of the superior cerebellar peduncles is visible.
Several tegmental nuclear groups surround the cerebral aqueduct in the periaqueductal gray matter. These include the locus ceruleus, a pigmented cell mass which sends many norepinephrine-containing projections to the cerebellum and cerebral cortex. The locus ceruleus appears to be involved in modulation of cortical sensory and association areas, and in sleep activation. (Parts of several nuclei, including the nucleus ceruleus, are also seen in rostral sections of pontine areas; it is wrong to compartmentalize brain stemnuclei as strictly pontine or midbrain etc.) Also in this region is the mesencephalic nucleus of the trigeminal nerve, a collection of unipolar sensory neurons, and the dorsal nucleus of the raphe. The trochlear nucleus lies ventrally in the periaqueductal gray matter and sends efferents to the superior oblique muscle of the eye.
Several tracts can be seen in transverse section. The most prominent is the decussation of the cerebellar peduncles. The lateral lemniscus is seen where it enters the inferior colliculus and the medial lemniscus en route to the thalamus. Just medial is the ventral trigeminothalamic tract. Clustered medially are the dorsal trigeminothalamic tract, central tegmental tract, the medial longitudinal fasciculus, and the tectospinal tract. The ventrally placed crus cerebri contains the massive descending corticospinal and corticobulbar tracts, and temperopontine fibers.
Transection at the level of the superior colliculi shows the prominent bilateral red nucleus, so called because it appears pinkish red in freshly cut sections. The red nucleus runs continuous with the crossed superior cerebellar peduncle, and it is the origin of descending motor tracts, which decussate in the ventral tegmentum to become the rubrospinal tract.
The superior colliculi communicate through the posterior commissure and integrate auditory, cortical, spinal, and retinal afferents in the control of eye movements and reflex reflexes. The superior brachium carries the retinal inputs. The oculomotor nucleus lies ventrally in the periaqueductal gray matter, and its efferent projections cross the red nucleus, emerge in the interpeduncular fossa and run to optic and extra-optic muscle.