Transverse sections throughout the midbrain, Section through midbrain at the level of lower colliculus, Section through midbrain at the level of upper colliculus, Connections of the red nucleus, Inferior Colliculus, Connections of the Superior Colliculus, Connections of the substance nigra, Some fiber bundles seen in the brainstem. The Midbrain – Some features of the internal structure of the midbrain have been considered. The subdivision of the midbrain into the tectum, the tegmentum, the substantia nigra, and the crus cerebri has been noted. The superior and inferior colliculi, the red nucleus, and the reticular formation have been identified. Transverse sections throughout the midbrain – A transverse section throughout the midbrain at the level of the inferior colliculus is shown in Fig.
The crus cerebri
11.8 and a section at the level of the superior colliculus in Fig. 11.9. We’ll first consider those features which are common to both these levels. The crus cerebri consists of fibers descending from the cerebral cortex. The intermediate two-thirds of the crus cerebri are occupied by corticospinal and corticonuclear fibers. The fibers for the leg is more lateral and those for the head are most medial. The substantia nigra lies instantly behind and medial to the basis pedunculi. The pars compacta contain dopaminergic and cholinergic neurons. The majority of the neurons in the pars reticularis are GABAergic. Superiorly, the pars reticularis becomes continuous with the globus pallidus.
The substantia nigra
The substantia nigra is closely connected, functionally, with the corpus striatum. The midbrain is crossed by the cerebral aqueduct which is encompassed by central grey matter. Ventrally, the central gray matter is related to cranial nerve nuclei. The region between the substantia nigra and the central gray matter is occupied by the reticular formation. Section through midbrain at the level of lower colliculus – A section throughout the midbrain at the level of the inferior colliculus shows the following additional features. The inferior colliculus is a large mass of grey matter lying in the rectum. Fibers arising in this nucleus follow an unusual course. They run dorsally and decussate before emerging on the dorsal facet of the brainstem. The mesencephalic nucleus of the trigeminal nerve lies in the lateral part of the central grey matter. A compact bundle of fibres lies in the tegmentum dorsomedial to the substantia nigra. The medial lemniscus includes fibres of the ventral spinothalamic tract whilst the spinal lemniscus includes fibres of the spinotectal tract. More dorsally, the lateral lemniscus forms a bundle ventrolateral to the inferior colliculus. The medial longitudinal fasciculus. Lies ventral to the trochlear nucleus, and ventral to the fasciculus there’s the tectospinal tract.