The midbrain or mesencephalon is the uppermost 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 mesencephalon comes from the Greek mesos, "middle", and enkephalos, "brain".

Structure

thumb|upright=0.5|Brainstem (dorsal view) <br />A:[[Thalamus B:Midbrain C:Pons<br /> D:Medulla oblongata<br /> 7 and 8 are the four colliculi.]]

The midbrain is the shortest segment of the brainstem, measuring less than 2cm in length. It is situated mostly in the posterior cranial fossa, with its superior part extending above the tentorial notch. 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.

Tectum

thumb|left|Principal connections of the tectum

The tectum (Latin for roof) is the part of the midbrain dorsal to the cerebral aqueduct. 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. The superior colliculi process some visual information, aid the decussation of several fibres of the optic nerve (some fibres remain ipsilateral), and are involved with saccadic eye movements. The tectospinal tract connects the superior colliculi to the cervical nerves of the neck, and co-ordinates head and eye movements. Each superior colliculus also sends information to the corresponding lateral geniculate nucleus, with which it is directly connected. The homologous structure to the superior colliculus in non mammalian vertebrates including fish and amphibians, is called the optic tectum; in those animals, the optic tectum integrates sensory information from the eyes and certain auditory reflexes.

The inferior colliculi – located just above the trochlear nerve – process certain auditory information. Each inferior colliculus sends information to the corresponding medial geniculate nucleus, with which it is directly connected.

Cerebral aqueduct

thumb|upright=1.5|Ventricular system anatomy showing the cerebral aqueduct, labelled centre right.

The cerebral aqueduct is the part of the ventricular system which links the third ventricle (rostrally) with the fourth ventricle (caudally); as such it is responsible for continuing the circulation of cerebrospinal fluid. The cerebral aqueduct is a narrow channel located between the tectum and the tegmentum, and is surrounded by the periaqueductal grey, which has a role in analgesia, quiescence, and bonding. The dorsal raphe nucleus (which releases serotonin in response to certain neural activity) is located at the ventral side of the periaqueductal grey, at the level of the inferior colliculus.

The nuclei of two pairs of cranial nerves are similarly located at the ventral side of the periaqueductal grey – the pair of oculomotor nuclei (which control the eyelid, and most eye movements) is located at the level of the superior colliculus, while the pair of trochlear nuclei (which helps focus vision on more proximal objects) is located caudally to that, at the level of the inferior colliculus, immediately lateral to the dorsal raphe nucleus.

Blood supply

The midbrain is supplied by the following arteries:

  • The tectum is supplied by the superior cerebellar artery.
  • The central part of the tegmentum is supplied by the paramedian branches of the basilar artery.
  • The lateral part of the midbrain is supplied by the posterior cerebral artery.

Venous blood from the midbrain is mostly drained into the basal vein as it passes around the peduncle. Some venous blood from the colliculi drains to the great cerebral vein.

Development

thumb|Mesencephalon of human embryo

During embryonic development, the midbrain (also known as the mesencephalon) arises from the second vesicle of the neural tube, while the interior of this portion of the tube becomes the cerebral aqueduct. Unlike the other two vesicles – the forebrain and hindbrain – the midbrain does not develop further subdivision for the remainder of neural development. It does not split into other brain areas. While the forebrain, for example, divides into the telencephalon and the diencephalon.

Throughout embryonic development, the cells within the midbrain continually multiply; this happens to a much greater extent ventrally than it does dorsally. The outward expansion compresses the still-forming cerebral aqueduct, which can result in partial or total obstruction, leading to congenital hydrocephalus. The tectum is derived in embryonic development from the alar plate of the neural tube.

Function

The midbrain is the uppermost part of the brainstem. Its substantia nigra is closely associated with motor system pathways of the basal ganglia. The human midbrain is archipallian in origin, meaning that its general architecture is shared with the most ancient of vertebrates. Dopamine produced in the substantia nigra and ventral tegmental area plays a role in movement, movement planning, excitation, motivation and habituation of species from humans to the most elementary animals such as insects. Laboratory mice from lines that have been selectively bred for high voluntary wheel running have enlarged midbrains. The midbrain helps to relay information for vision and hearing.

The term "tectal plate" or "quadrigeminal plate" is used to describe the junction of the gray and white matter in the embryo. ()

See also

  • List of regions in the human brain
  • Pretectal area (Pretectum)

References