Midbrain

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Brain: Mesencephalon
Human brain inferior view description.JPG
Inferior view mesencephalon (2), above (3)
Human brainstem-thalamus posterior view description.JPG
Human brainstem mesencephalon (B)
Latin mesencephalon
Gray's p.800
NeuroNames hier-445
MeSH Mesencephalon
NeuroLex ID birnlex_1667
TA A14.1.03.005
FMA FMA:61993

The midbrain or mesencephalon (from the Greek mesos, middle, and enkephalos, brain[1]) is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation.[2]

Structure[edit]

Gray719.png

The midbrain comprises the tectum (or corpora quadrigemina), tegmentum, the cerebral aqueduct (or ventricular mesocoelia or "iter"), and the cerebral peduncles, as well as several nuclei and fasciculi. Caudally the mesencephalon adjoins the metencephalon (pons and cerebellum) while rostrally it adjoins the diencephalon (thalamus, hypothalamus, etc.). The midbrain is located below the cerebral cortex, and above the hindbrain placing it near the center of the brain.[3]

Specifically, the midbrain consists of:

Corpora quadrigemina[edit]

Main article: Corpora quadrigemina

The corpora quadrigemina ("quadruplet bodies") are four solid lobes on the dorsal side of the cerebral aqueduct, where the superior posterior pair are called the superior colliculi and the inferior posterior pair are called the inferior colliculi.

The four solid lobes help to decussate several fibres of the optic nerve. However, some fibers also show ipsilateral arrangement (i.e., they run parallel on the same side without decussating.)

The superior colliculus is involved with saccadic eye movements; while the inferior is a synapsing point for sound information. The trochlear nerve comes out of the posterior surface of the midbrain, below the inferior colliculus.

Cerebral peduncle[edit]

Main article: cerebral peduncles

The cerebral peduncles are paired structures, present on the ventral side of the cerebral aqueduct, and they further carry tegmentum on the dorsal side and cresta or pes on the ventral side, and both of them accommodate the corticospinal tract fibres, from the internal capsule (i.e., ascending + descending tracts = longitudinal tract.) the middle part of cerebral peduncles carry substantia nigra (literally "Black Matter"), which is a type of basal nucleus. It is the only part of the brain that carries melanin pigment.

Between the peduncles is the interpeduncular fossa, which is a cistern filled with cerebrospinal fluid. The oculomotor nerve comes out between the peduncles, and the trochlear nerve is visible wrapping around the outside of the peduncles. The oculomotor is responsible for pupil constriction (parasympathetic) and certain eye movements.[4]

Anatomical features of cross-sections through the midbrain[edit]

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

The midbrain is usually sectioned at the level of the superior and inferior colliculi.

Mesencephalon of human embryo

One mnemonic for remembering the structures of the midbrain involves visualizing the mesencephalic cross-section as an upside down bear face. The two red nuclei are the eyes of the bear and the cerebral crura are the ears. The tectum is the chin and the cerebral peduncles are the face and ears.

Development[edit]

During embryonic development, the midbrain arises from the second vesicle, also known as the mesencephalon, of the neural tube. Unlike the other two vesicles, the prosencephalon and rhombencephalon, the mesencephalon remains undivided for the remainder of neural development. It does not split into other brain areas. while the prosencephalon, for example, divides into the telencephalon and the diencephalon.[8]

Throughout embryonic development, the cells within the midbrain continually multiply and compress the still-forming Aqueduct of Sylvius or cerebral aqueduct. Partial or total obstruction of the cerebral aqueduct during development can lead to congenital hydrocephalus.[9]

Function[edit]

The mesencephalon is considered part of the brainstem. Its substantia nigra is closely associated with motor system pathways of the basal ganglia. The human mesencephalon 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 motivation and habituation of species from humans to the most elementary animals such as insects. Laboratory house mice from lines that have been selectively bred for high voluntary wheel running have enlarged midbrains.[10] The midbrain helps to relay information for vision and hearing.

Clinical significance[edit]

See also[edit]

References[edit]

  1. ^ Mosby’s Medical, Nursing and Allied Health Dictionary, Fourth Edition, Mosby-Year Book 1994, p. 981
  2. ^ Breedlove, Watson, & Rosenzweig. Biological Psychology, 6th Edition, 2010, pp. 45-46
  3. ^ http://www.morris.umn.edu/~ratliffj/images/brain_slides/slide_5.htm
  4. ^ Haines, Duane E. Neuroanatomy : an atlas of structures, sections, and systems (8th ed. ed.). Philadelphia: Wolters Kluwer/ Lippincott Williams & Wilkins Health. p. 42. ISBN 978-1-60547-653-7. 
  5. ^ Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.
  6. ^ Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.
  7. ^ Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.
  8. ^ Martin. Neuroanatomy Text and Atlas, Second Edition, 1996, pp. 35-36.
  9. ^ "Hydrocephalus Fact Sheet". National Institute of Neurological Disorders and Stroke. February 2008. Retrieved 2011-03-23. 
  10. ^ Kolb, E. M., E. L. Rezende, L. Holness, A. Radtke, S. K. Lee, A. Obenaus, and Garland T, Jr. 2013. Mice selectively bred for high voluntary wheel running have larger midbrains: support for the mosaic model of brain evolution. Journal of Experimental Biology 216:515-523.