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Concept
Midbrain
Summary
The midbrain or mesencephalon is the rostral-most portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum.
It is functionally associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation.
The name comes from the Greek mesos, "middle", and enkephalos, "brain".
The midbrain is the shortest segment of the brainstem, measuring at less than 2cm in length. It is situated mostly in the posterior cranial fossa, with its superior part extending above the tentorial notch.
The principal regions of the midbrain are the tectum, the cerebral aqueduct, tegmentum, and the cerebral peduncles. Rostrally the midbrain adjoins the diencephalon (thalamus, hypothalamus, etc.), while caudally it adjoins the hindbrain (pons, medulla and cerebellum). In the rostral direction, the midbrain noticeably splays laterally.
Sectioning of the midbrain is usually performed axially, at one of two levels – that of the superior colliculi, or that of the inferior colliculi. One common technique for remembering the structures of the midbrain involves visualizing these cross-sections (especially at the level of the superior colliculi) as the upside-down face of a bear, with the cerebral peduncles forming the ears, the cerebral aqueduct the mouth, and the tectum the chin; prominent features of the tegmentum form the eyes and certain sculptural shadows of the face.
The tectum (Latin for roof) is the part of the midbrain dorsal to the cerebral aqueduct. The position of the tectum is contrasted with the tegmentum, which refers to the region in front of the ventricular system, or floor of the midbrain.
It is involved in certain reflexes in response to visual or auditory stimuli. The reticulospinal tract, which exerts some control over alertness, takes input from the tectum, and travels both rostrally and caudally from it.
The corpora quadrigemina are four mounds, called colliculi, in two pairs – a superior and an inferior pair, on the surface of the tectum.
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