The midbrain is the upper and shortest part of the brainstem. It is about 2.5 cm long and 2.5 cm wide. It connects the hindbrain with the forebrain. Its cavity, the cerebral aqueduct (aqueduct of Sylvius) connects the third ventricle with the fourth ventricle. The midbrain passes through the tentorial notch. It is related on each side to the optic tract, parahippocampal gyrus, posterior cerebral artery, and basal vein; anteriorly to the interpeduncular structures, viz. mammillary bodies, tuber cinereum, etc.; posteriorly to the splenium of corpus callosum, great cerebral vein, pineal body and posterior ends of right and left thalami.
The midbrain like the other parts of the brainstem consists of grey and white matter. The ventral surface of midbrain presents two crura cerebri which emerges from the cerebral hemispheres and converge downwards to enter the pons forming the posterolateral boundaries of the interpeduncular fossa.
The superficial surface of the crus cerebri is finely corrugated by the underlying longitudinal fibres. It is crossed transversely from above downwards by optic tract, posterior cerebral artery, superior cerebellar artery and taenia pontis (white ridge).
The oculomotor nerve emerges from a groove on the medial side of the crus cerebri. The trochlear nerve emerges on the dorsal aspect of the midbrain and curls around the lateral aspect of the cerebral peduncle to appear on the ventral aspect of the midbrain lateral to the oculomotor nerve. These two nerves run forward between the posterior cerebral and the superior cerebellar arteries.
The dorsal surface of the midbrain presents four rounded elevations: two superior and two inferior colliculi (or corpora quadrigemina). The colliculi are separated from each other by a cruciform sulcus. The vertical limb of sulcus when traced above forms a surface depression which lodges the pineal body and when traced below, it becomes continuous with the frenulum veli (a median ridge on the dorsal surface of the superior medullary velum).
Thick ridges of white matter extending from lateral side of each colliculus constitute their brachia. The superior brachium connect the superior colliculus to the lateral geniculate body and the optic tract, and is made up of optic tract fibres. The inferior brachium connect the inferior colliculus to the medial geniculate body, and is made up of auditory fibres. The superior and inferior colliculi are concerned with reflex activities triggered by auditory and visual impulses respectively.
The internal structure of the midbrain is studied conveniently by examining its transverse sections. The transverse section of midbrain shows a tiny canal, called cerebral aqueduct. A coronal plane passing through the aqueduct divides the midbrain into two parts; a small posterior part and large anterior part. The small posterior part is called tectum and consists of four colliculi. The large anterior part is divided into two equal right and left halves by a vertical plane, the cerebral peduncle. Each cerebral peduncle is further subdivided into three parts, from dorsal to ventral these are: (a) tegmentum, (b) substantia nigra, and (c) crus cerebri.
The structures in the tectum and tegmentum vary at different levels of the midbrain, but those of the crus cerebri and substantia nigra are nearly the same throughout the length of the midbrain.
The crus cerebri is the part of cerebral peduncle situated anterolateral to the substantia nigra. It contains important descending tracts which connect the cerebral cortex to the anterior horn cells of the spinal cord, cranial nerve nuclei, and pontine nuclei. The corticospinal and corticonuclear fibres (pyramidal tract) occupy the middle two-thirds of the crus. The frontopontine fibres occupy the medial one-sixth of the crus. The temporopontine, parietopontine, and occipitopontine fibres occupy the lateral one-sixth of the crus.
The substantia nigra is a curved (crescent-shaped) pigmented band of grey matter (thicker medially than laterally) situated between tegmentum and crus cerebri. Its concavity is smooth and directed towards the tegmentum. From its convex margin spiky processes project into the substance of the crus cerebri. The substantia nigra is a large motor nucleus that extends throughout the length of midbrain. It is divided into two parts: (a) the dorsal part (pars compacta) containing medium sized cells and (b) a ventral part (pars reticularis) containing fewer cells. The pars reticularis is intermingled with the fibres of crus cerebri.
The substantia nigra is made up of deeply pigmented nerve cells which contain melanin (a polymerized form of dopamine) and iron. These cells synthesize dopamine which is carried through their axons (nigrostriatal fibres) to the corpus striatum.
The midbrain is supplied by following arteries: Basilar artery through its posterior cerebral and superior cerebellar arteries. Basilar artery also supplies midbrain through direct branches. Branches of posterior communicating and anterior choroidal arteries. The veins of midbrain drains into the great cerebral and the basal veins.