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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.Breedlove, Watson, & Rosenzweig. Biological Psychology, 6th Edition, 2010, pp. 45-46
The name mesencephalon comes from the Greek mesos, "middle", and enkephalos, "brain".Mosby's Medical, Nursing & Allied Health Dictionary,≈ Fourth Edition, Mosby-Year Book 1994, p. 981
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.
It is involved in certain 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. The superior colliculi process some visual information, aid the decussate of several fibres of the optic nerve (some fibres remain ipsilateral), and are involved with Saccade. The tectospinal tract connects the superior colliculi to the 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 , is called the optic tectum; in those animals, the optic tectum integrates sensory information from the eyes and certain auditory reflexes.Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
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.
The nuclei of two pairs of 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,Duane E. Haines (2025). 9781605476537, Wolters Kluwer/ Lippincott Williams & Wilkins Health. . ISBN 9781605476537 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. The oculomotor nerve emerges from the nucleus by traversing the ventral width of the tegmentum, while the trochlear nerve emerges via the tectum, just below the inferior colliculus itself; the trochlear is the only cranial nerve to exit the brainstem dorsally. The Edinger-Westphal nucleus (which controls the shape of the lens and size of the pupil) is located between the oculomotor nucleus and the cerebral aqueduct.
The main bulk of the tegmentum contains a complex synaptic network of neurons, primarily involved in homeostasis and reflex actions. It includes portions of the reticular formation. A number of distinct between other parts of the brain pass through it. The medial lemniscus – a narrow ribbon of fibres – passes through in a relatively constant axial position; at the level of the inferior colliculus it is near the lateral edge, on the ventral side, and retains a similar position rostrally (due to widening of the tegmentum towards the rostral end, the position can appears more medial). The spinothalamic tract – another ribbon-like region of fibres – are located at the lateral edge of the tegmentum; at the level of the inferior colliculus it is immediately dorsal to the medial lemiscus, but due to the rostral widening of the tegmentum, is lateral of the medial lemiscus at the level of the superior colliculus.
A prominent pair of round, reddish, regions – the red nucleus (which have a role in motor co-ordination) – are located in the rostral portion of the midbrain, somewhat medially, at the level of the superior colliculus. The rubrospinal tract emerges from the red nucleus and descends caudally, primarily heading to the cervical portion of the spine, to implement the red nuclei's decisions. The area between the red nuclei, on the ventral side – known as the ventral tegmental area – is the largest dopamine-producing area in the brain, and is heavily involved in the reward system. The ventral tegmental area is in contact with parts of the forebrain – the mammillary body (from the Diencephalon) and hypothalamus (of the diencephalon).
The majority of each lobe constitutes the cerebral crus. The cerebral crus are the main tracts descending from the thalamus to caudal parts of the central nervous system; the central and medial ventral portions contain the corticobulbar and corticospinal tracts, while the remainder of each crus primarily contains tracts connecting the cortex to the pons. Older texts refer to the crus cerebri as the cerebral peduncle; however, the latter term actually covers all fibres communicating with the cerebrum (usually via the diencephalon), and therefore would include much of the tegmentum as well. The remainder of the crus pedunculi – small regions around the main cortical tracts – contain tracts from the internal capsule.
The portion of the lobes in connection with the tegmentum, except the most lateral portion, is dominated by a blackened band – the substantia nigra (literally black substance) – which is the only part of the basal ganglia system outside the forebrain. It is ventrally wider at the rostral end. By means of the basal ganglia, the substantia nigra is involved in motor-planning, learning, addiction, and other functions. There are two regions within the substantia nigra – one where neurons are densely packed (the pars compacta) and one where they are not (the pars reticulata), which serve a different role from one another within the basal ganglia system. The substantia nigra has extremely high production of melanin (hence the colour), dopamine, and noradrenalin; the loss of dopamine-producing neurons in this region contributes to the progression of Parkinson's disease.
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.
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.
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