Editing Midbrain (section)

==Structure ==
[[File:Human brainstem-thalamus posterior view description.JPG|thumb|upright=0.5|Brainstem (dorsal view) <br />A:[[Thalamus]] B:Midbrain C:[[Pons]]<br /> D:[[Medulla oblongata]]<br /> 7 and 8 are the [[corpora quadrigemina|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]].<ref name="Sinnatamby-2011" />
                  
The principal regions of the midbrain are the [[tectum]], the [[cerebral aqueduct]], [[tegmentum]], and the [[cerebral peduncle]]s. [[Rostral and caudal|Rostrally]] the midbrain adjoins the [[diencephalon]] ([[thalamus]], [[hypothalamus]], etc.), while [[Rostral and caudal|caudally]] it adjoins the [[hindbrain]] ([[pons]], [[Medulla oblongata|medulla]] and [[cerebellum]]).<ref>{{cite web |url=http://www.morris.umn.edu/~ratliffj/images/brain_slides/slide_5.htm |title=Slide 5 |access-date=2011-03-05 |url-status=dead |archive-url=https://web.archive.org/web/20110427225104/http://www.morris.umn.edu/~ratliffj/images/brain_slides/slide_5.htm |archive-date=2011-04-27 }}</ref> 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|Superior colliculus|Inferior colliculus}}
[[File:Gray719.png|thumb|left|Principal connections of the tectum]]
The tectum (Latin for ''roof'') is the part of the midbrain dorsal to the [[cerebral aqueduct]].<ref name="Sinnatamby-2011" />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 [[reflex]]es in response to visual or auditory stimuli. The [[reticulospinal tract]], which exerts some control over alertness, takes input from the tectum,<ref>{{Cite book|title=Principles of Neural Science|last=Kandel|first=Eric|publisher=McGraw-Hill|year=2000|isbn=0-8385-7701-6|pages=[https://archive.org/details/isbn_9780838577011/page/669 669]|url-access=registration|url=https://archive.org/details/isbn_9780838577011/page/669}}</ref> 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 colliculus|superior colliculi]] process some visual information, aid the [[decussate|decussation]] of several fibres of the [[optic nerve]] (some fibres remain ipsilateral), and are involved with [[Saccade|saccadic eye movements]]. The [[tectospinal tract]] connects the superior colliculi to the [[cervical nerve]]s 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 [[homology (biology)|homologous]] structure to the superior colliculus in non mammalian vertebrates including [[fish]] and [[amphibian]]s, is called the [[Superior colliculus#Optic tectum|optic tectum]]; in those animals, the optic tectum integrates sensory information from the eyes and certain auditory reflexes.<ref>Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005</ref><ref>{{cite journal|title= Functions of the optic lobes or corpora quadrigemina|pages=149–173|author=Ferrier, David|author-link=David Ferrier|year=1886|url=http://psycnet.apa.org/books/12789/005|doi=10.1037/12789-005|url-access=subscription}}</ref>

The [[inferior colliculus|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===
[[File:Brain-ventricle-anatomy-diagram.jpg|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]],<ref name="Martin">Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.</ref> 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 nerve]]s are similarly located at the ventral side of the periaqueductal grey – the pair of [[oculomotor nucleus|oculomotor nuclei]] (which control the eyelid, and most eye movements) is located at the level of the superior colliculus,<ref>{{cite book|last=Haines|first=Duane E.|title=Neuroanatomy : an atlas of structures, sections, and systems|url=https://archive.org/details/neuroanatomyatla00hain_570|url-access=limited|publisher=Wolters Kluwer/ Lippincott Williams & Wilkins Health|location=Philadelphia|isbn=978-1-60547-653-7|pages=[https://archive.org/details/neuroanatomyatla00hain_570/page/n55 42]|edition=8th|year=2012}}</ref> while the pair of [[trochlear nucleus|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.<ref name="Martin"/> 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.<ref name="Martin"/>

===Tegmentum===
[[File:Cn3nucleus-en.svg|thumb|right|Cross-section of the midbrain at the level of the superior colliculus]]
[[File:Midbrain - inferior colliculus.svg|thumb|right|Cross-section of the midbrain at the level of the inferior colliculus.]]

The [[midbrain tegmentum]] is the portion of the midbrain ventral to the cerebral aqueduct, and is much larger in size than the tectum. It communicates with the [[cerebellum]] by 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 [[decussation|decussate]], but they dissipate more rostrally.<ref name="Martin"/> 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 tract]]s 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|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.<ref name="Martin"/> 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|neural reward system]]. The ventral tegmental area is in contact with parts of the forebrain – the [[mammillary body|mammillary bodies]] (from the [[Diencephalon]]) and [[hypothalamus]] (of the [[diencephalon]]).

===Cerebral peduncles===
[[Image:Brain Anatomy - Mid-Fore-HindBrain.png|thumb|Brain anatomy – forebrain, midbrain, hindbrain.]]

The [[cerebral peduncle]]s 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|date=February 2017}}.

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 tract|corticobulbar]] and [[corticospinal tract]]s, 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'')<ref name="Martin"/> – which is the only part of the [[primate basal ganglia|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]].<ref>{{Cite journal|last1=Damier|first1=P.|last2=Hirsch|first2=E. C.|last3=Agid|first3=Y.|last4=Graybiel|first4=A. M.|date=1999-08-01|title=The substantia nigra of the human brainII. Patterns of loss of dopamine-containing neurons in Parkinson's disease|journal=Brain|volume=122|issue=8|pages=1437–1448|doi=10.1093/brain/122.8.1437|pmid=10430830|issn=0006-8950|doi-access=free}}</ref>