Midbrain

Source: Wikipedia, the free encyclopedia.
This article is about the midbrain in vertebrates. For the midbrain in insects, see
olfactory deutocerebrum
.
Midbrain
sagittal view of one cerebellar hemisphere. B: Pons. C: Medulla. D: Spinal cord. E: Fourth ventricle. F: Arbor vitae. G: Nodule. H: Tonsil. I: Posterior lobe. J: Anterior lobe. K: Inferior colliculus. L: Superior colliculus.
Inferior view in which the midbrain is encircled blue.
Details
PronunciationUK: /ˌmɛsɛnˈsɛfəlɒn, -kɛf-/, US: /ˌmɛzənˈsɛfələn/;[1]
Part ofBrainstem
Identifiers
Latinmesencephalon
MeSHD008636
NeuroNames462
NeuroLex IDbirnlex_1667
TA98A14.1.03.005
TA25874
FMA61993
Anatomical terms of neuroanatomy]

The midbrain or mesencephalon is the

rostral-most portion of the brainstem[2] connecting the diencephalon and cerebrum with the pons.[3] 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.[2]

The name comes from the Greek mesos, "middle", and enkephalos, "brain".[4]

Anatomy

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.[3]

Structure

Brainstem (dorsal view).
A:Thalamus B:Midbrain C:Pons
D:Medulla oblongata
7 and 8 are the colliculi.

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).[5]
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

Further information: Superior colliculus and Inferior colliculus
Principal connections of the tectum

The tectum (Latin for roof) is the part of the midbrain dorsal to the cerebral aqueduct.[3] 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

reticulospinal tract, which exerts some control over alertness, takes input from the tectum,[6]
and travels both rostrally and caudally from it.

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.[7][8]

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

Ventricular system anatomy showing the cerebral aqueduct, labelled centre right.

The

periaqueductal grey,[9] 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

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.[9]

Tegmentum

Cross-section of the midbrain at the level of the superior colliculus
Cross-section of the midbrain at the level of the inferior colliculus.

The

superior cerebellar peduncles, which enter at the caudal end, medially, on the ventral side; the cerebellar peduncles are distinctive at the level of the inferior colliculus, where they decussate, but they dissipate more rostrally.[9] Between these peduncles, on the ventral side, is the median raphe nucleus
, which is involved in memory consolidation.

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 nerve tracts 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 nuclei (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.[9] 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 neural reward system. The ventral tegmental area is in contact with parts of the forebrain – the mammillary bodies (from the Diencephalon) and hypothalamus (of the diencephalon).

Cerebral peduncles

Brain anatomy – forebrain, midbrain, hindbrain.

The cerebral peduncles each form a lobe ventrally of the tegmentum, on either side of the midline. Beyond the midbrain, between the lobes, is the interpeduncular fossa, which is a cistern filled with cerebrospinal fluid [citation needed].

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

noradrenalin; the loss of dopamine-producing neurons in this region contributes to the progression of Parkinson's disease.[11]

Vasculature

Arterial supply

The midbrain is supplied by the following arteries:

Venous drainage

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.[12]

Development

Further information: Isthmic organizer
Mesencephalon of human embryo

During

telencephalon and the diencephalon.[13]

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.[14] The tectum is derived in embryonic development from the alar plate of the neural tube.

Function

The mesencephalon is considered part of the

wheel running have enlarged midbrains.[15]
The midbrain helps to relay information for vision and hearing.

Related terms

The term "tectal plate" or "quadrigeminal plate" is used to describe the junction of the gray and white matter in the embryo. (ancil-453 at NeuroNames)

See also

Wikimedia Commons has media related to Mesencephalon.
Look up midbrain in Wiktionary, the free dictionary.

References

  1. ^ "mesencephalon". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  2. ^ a b Breedlove, Watson, & Rosenzweig. Biological Psychology, 6th Edition, 2010, pp. 45-46
  3. ^
    ISBN 978-0-7295-3752-0
    .
  4. Mosby's Medical, Nursing & Allied Health Dictionary
    ,≈ Fourth Edition, Mosby-Year Book 1994, p. 981
  5. ^ "Slide 5". Archived from the original on 2011-04-27. Retrieved 2011-03-05.
  6. ISBN 0-8385-7701-6
    .
  7. ^ Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
  8. doi:10.1037/12789-005. {{cite journal}}: Cite journal requires |journal= (help
    )
  9. ^ a b c d e f Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.
  10. ISBN 978-1-60547-653-7
    .
  11. PMID 10430830
    .
  12. ISBN 978-0-7295-3752-0
    .
  13. ^ Martin. Neuroanatomy Text and Atlas, Second Edition, 1996, pp. 35-36.
  14. ^ "Hydrocephalus Fact Sheet". National Institute of Neurological Disorders and Stroke. February 2008. Retrieved 2011-03-23.
  15. PMID 23325861
    .
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