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The midbrain comprises the [[tectum]], [[tegmentum]], the [[cerebral aqueduct]], and the [[cerebral peduncle]]s, as well as several nuclei and fasciculi. [[wikt:caudal|Caudally]] the midbrain adjoins the [[metencephalon]] (afterbrain) ([[pons]] and [[cerebellum]]). [[wikt:rostral|Rostrally]] it adjoins the [[diencephalon]] ([[thalamus]], [[hypothalamus]], etc.).<ref>{{cite web |url=http://www.morris.umn.edu/~ratliffj/images/brain_slides/slide_5.htm |title=Archived copy |accessdate=2011-03-05 |deadurl=yes |archiveurl=https://web.archive.org/web/20110427225104/http://www.morris.umn.edu/~ratliffj/images/brain_slides/slide_5.htm |archivedate=2011-04-27 |df= }}</ref>
The principle regions of the midbrain are the [[tectum]], the [[cerebral aqueduct]], [[tegmentum]], and the [[basis pedunculi]]. [[wikt:rostral|Rostrally]] the midbrain adjoins the [[diencephalon]] ([[thalamus]], [[hypothalamus]], etc.), while [[wikt:caudal|Caudally]] it adjoins the [[metencephalon]] (hindbrain) ([[pons]] and [[cerebellum]]).<ref>{{cite web |url=http://www.morris.umn.edu/~ratliffj/images/brain_slides/slide_5.htm |title=Archived copy |accessdate=2011-03-05 |deadurl=yes |archiveurl=https://web.archive.org/web/20110427225104/http://www.morris.umn.edu/~ratliffj/images/brain_slides/slide_5.htm |archivedate=2011-04-27 |df= }}</ref>. In the rostral direction, the midbrain noticably 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 basis pedunculi 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.
Specifically, the midbrain consists of:


=== [[Tectum]] ===
=== Tectum ===
{{Main|Tectum}}
*[[Inferior colliculus|Inferior colliculi]]
The ''[[Tectum]]'' (Latin for ''roof'') is the dorsal side of the midbrain. It is involved in certain reflex actions in connection with visual or auditory stimuli. The [[reticulospinal tract]], which excerts 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|location=|pages=669}}</ref>, and travels both rostrally and caudally from it.
*[[Superior colliculus|Superior colliculi]]


The [[corpora quadrigemina]] ("quadruplet bodies") are four solid lobes on the surface of the tectum. The superior pair (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 is involved with [[Saccade|saccadic eye movements]]. The [[tectospinal tract]] connects the superior colliculi to the [[Cervical vertebrae|nerves of the neck]], and co-ordinates head and eye movements.
==== Medial and lateral pathways of tectum ====
* The medial descending pathway comprises the [[vestibulospinal tract]]s (lateral and medial tracts with control from the [[cerebellum]]), [[reticulospinal tract]], and the [[tectospinal tract]]. These tracts run through the [[Reticular formation|medial reticular formation]], the lateral and [[Medial vestibular nucleus|medial vestibular nuclei]].<ref>{{Cite book|title=Principles of Neural Science|last=Kandel|first=Eric|publisher=McGraw-Hill|year=2000|isbn=0-8385-7701-6|location=|pages=669}}</ref> These tracts extend rostrally and caudally of the tectum. The vestibulospinal tracts; the medial tract originate in the medial and inferior vestibular nuclei moving caudally to the [[anterior funiculus]] controlling muscle tone, while the lateral tract originates from lateral vestibular nuclei also referred to as [[Lateral vestibular nucleus|Deiters' nucleus]] named after [[Otto Deiters]] (1834–1863). The reticulospinal tract consists of dense cortical neurons which direct motor input as the tract terminates sequentially down the spine. The tectospinal tract provides stability and posture control.
* The lateral descending pathway consists of the [[rubrospinal tract]] originating from the [[red nucleus]] responsible for voluntary motor movement, and partially of the [[lateral corticospinal tract]] as this tract does not originate or terminate in the tectum but does play a critical role with afferent and efferent motor control along sidetone rubrospinal tract of the tectum.


⚫
The inferior pair ([[inferior colliculus|inferior colliculi]]) - located just above the trochlear nerve - process certain auditory information. The [[homology (biology)|homologous]] structures are called ''optic lobes'' in some [[lower vertebrate]]s ([[fish]] and [[amphibian]]s) where they integrate 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|author=Ferrier, David|authorlink=David Ferrier|year=1886|url=http://psycnet.apa.org/books/12789/005|doi=10.1037/12789-005}}</ref>
=== Fourth ventricle ===
The [[ventricular system]] comprises the [[choroid plexus]], which produces [[Cerebrospinal fluid|cerebral spinal fluid]] (CSF), the lateral, third, and fourth ventricles responsible for circulating the CSF. The [[fourth ventricle]] is the most caudal aspect of the cerebral ventricular system, and is formed by the pons and medulla. The fourth ventricle is connected to the third ventricle via the [[cerebral aqueduct]], and is the smallest ventricle in the ventricular system. Cerebral spinal fluid originates via the choroid plexus, circulating through the ventricular system and recycled into the [[subarachnoid space]]. In the fourth ventricle, CSF is recycled through spinal nerve sheaths through the [[Epidural venous plexus|epidural vein.]]<ref>{{Cite web|url=https://www.ncbi.nlm.nih.gov/books/NBK27998/|title=Blood—Cerebrospinal Fluid Barrier|last=Laterra|first=John|date=|website=NCBI|access-date=}}</ref>


=== Cerebral peduncle ===
=== Cerebral aqueduct ===
*[[Midbrain tegmentum]]
*[[Cerebral crus|Crus cerebri]]


The [[cerebral aqueduct]] is the part of the [[ventricular system]] which links the [[third ventricle]] (rostally) with the [[fourth ventricle]] (caudally); as such it is responsible for continuing the circulation of [[cerebrospinal fluid]]. The cerebral aqueduct is the smallest ventricle in the ventricular system. It is located between the tectum and the tegmentum, and is surrounded by the [[periaqueductal grey]]<ref name="auto">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 [[seretonin]] in response to certain neural activity) is located at the ventral side of the periaqueductal grey, at the level of the inferior colliculus.
==== Substantia nigra ====
The [[substantia nigra]] is located in the midbrain, with a left and right region, with two primary regions; the [[Substantia nigra pars compacta|substantia nigra pars compact]] and the [[Substantia nigra pars reticulata|substantia nigra pars reticular]]. This region contains three of the four primary dopaminergic tracts and is responsible for coordination of eye movement and voluntary motor movement. The region undergoes extremely high metabolic synthesis of dopamine and norepinephrine through the metabolic conversion of tyrosine to [[L-DOPA]] via enzymatic reaction [[Tyrosine hydroxylase|Tyrosine Hydroxlyase]] (cat-2), to dopamine through [[Aromatic L-amino acid decarboxylase|Aromatic L-Amino Acid Decarboxylase]], and if not used processed further through [[Dopamine beta-hydroxylase|Dopamine Beta-Hydroxylase]] into [[Norepinephrine]] or [[Epinephrine]] via an additional reaction. This region of the brain has been extensively studied in biomedical research since the loss of dopaminergic neurons in this region contributes to the progression of [[Parkinson's disease]].<ref name=":0">{{Cite journal|last=Damier|first=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|url=https://academic.oup.com/brain/article/122/8/1437/504337|journal=Brain|volume=122|issue=8|pages=1437–1448|doi=10.1093/brain/122.8.1437|issn=0006-8950}}</ref> Currently, there is not a cure for Parkinson's disease but there are short-term effective treatments. Exogenous L-DOPA has been shown to be an effective treatment, however long-term use can result in increased dyskinesia and have deleterious effects on the survival of dopaminergic neurons.<ref name=":0" /><ref>{{Cite journal|last=Smith|first=L.M.|last2=Parr-Brownlie|first2=L.C.|last3=Duncan|first3=E.J.|last4=Black|first4=M.A.|last5=Gemmell|first5=N.J.|last6=Dearden|first6=P.K.|last7=Reynolds|first7=J.N.J.|title=Striatal mRNA expression patterns underlying peak dose l-DOPA-induced dyskinesia in the 6-OHDA hemiparkinsonian rat|url=http://linkinghub.elsevier.com/retrieve/pii/S0306452216002359|journal=Neuroscience|language=en|volume=324|pages=238–251|doi=10.1016/j.neuroscience.2016.03.012}}</ref> Extensive biomedical research is underway after the initiation of the [[BRAIN Initiative|BRAIN initiative]] formed by the Obama administration in 2013 with the audacious goal of creating cures and preventive measures by 2023.


⚫
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|publisher=Wolters Kluwer/ Lippincott Williams & Wilkins Health|location=Philadelphia|isbn=978-1-60547-653-7|pages=42|edition=8th}}</ref>
<gallery>
, 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, immediatetly lateral to the dorsal raphe nucleus<ref name="auto"/>. 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="auto"/>.
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File:Brain Anatomy - Mid-Fore-HindBrain.png|Brain Anatomy - Mid-fore-hindbrain.
</gallery>


===Corpora quadrigemina===
=== Tegmentum ===
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[[File:Midbrainsuperiorcolliculus.png|thumb|right|Cross-section of the midbrain at the level of the superior colliculus]]
{{Main|Corpora quadrigemina}}
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[[File:Midbrain - inferior colliculus.svg|thumb|right|Cross-section of the midbrain at the level of the inferior colliculus.]]
⚫
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 colliculus|superior colliculi]] and the inferior posterior pair are called the [[inferior colliculus|inferior colliculi]]. The [[homology (biology)|homologous]] structures are called ''optic lobes'' in some [[lower vertebrate]]s ([[fish]]es and [[amphibian]]s) where they integrate 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|author=Ferrier, David|authorlink=David Ferrier|year=1886|url=http://psycnet.apa.org/books/12789/005|doi=10.1037/12789-005}}</ref>


The [[Midbrain tegmentum|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="auto"/>. Between these peduncles, on the ventral side, is the [[median raphe nucleus]], which is involved in memory consolidation.
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 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 lemiscus]] - a narrow ribbon of fibres - passes through in a relatively constant coronal 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 [[anterolateral tracts]] - 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.
The superior colliculus is involved with [[Saccade|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.


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="auto"/>. 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 [[dopamin]] 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 [[telencephalon]]) and [[hypothalamus]] (of the [[diencephalon]]).
===Cerebral peduncle===
{{Main|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]]{{citation needed|date=February 2017}} (literally "Black Matter"), which is a type of basal nucleus. It is the only part of the brain that carries melanin pigment.


=== Basis pedunculi ===
⚫
Between the peduncles is the [[interpeduncular fossa]], which is a cistern filled with [[cerebrospinal fluid]]. {{citation needed|date=February 2017}} 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.<ref>{{cite book|last=Haines|first=Duane E.|title=Neuroanatomy : an atlas of structures, sections, and systems|publisher=Wolters Kluwer/ Lippincott Williams & Wilkins Health|location=Philadelphia|isbn=978-1-60547-653-7|pages=42|edition=8th}}</ref>
⚫
[[Image:Brain Anatomy - Mid-Fore-HindBrain.png|thumb|Brain Anatomy - Mid-fore-hindbrain.]]


The basis pedunculi 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}}.
===Anatomical features of cross-sections through the midbrain===
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[[File:Midbrain - superior colliculus.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.]]
[[File:Midbrainsuperiorcolliculus.png|thumb|right]]
The midbrain is usually sectioned at the level of the superior and inferior colliculi.
⚫
[[File:Human embryo 8 weeks 4.JPG|thumb|Mesencephalon of human embryo]]


The majority of each lobe constitutes the [[Cerebral crus|Crus cerebri]], which 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 the 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]].
*A horizontal (transverse) cross-section at the level of the superior colliculus shows the [[red nucleus]], the nuclei of the [[oculomotor nerve]] (and associated [[Edinger-Westphal nucleus]]), the cerebral peduncles or [[crus cerebri]], and the [[substantia nigra]].<ref name="auto">Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.</ref>
*A horizontal (transverse) cross-section at the level of the inferior colliculus still shows the substantia nigra. Also apparent are the [[trochlear nerve]] nucleus, and the [[decussation]] of the superior cerebellar peduncles.<ref name="auto"/>
*Both sections will show the [[cerebral aqueduct]], which connects the [[third ventricle|third]] and [[fourth ventricle]] and the [[periaqueductal gray]].<ref name="auto"/>


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="auto"/> - 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 aren't (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 name=":0">{{Cite journal|last=Damier|first=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|url=https://academic.oup.com/brain/article/122/8/1437/504337|journal=Brain|volume=122|issue=8|pages=1437–1448|doi=10.1093/brain/122.8.1437|issn=0006-8950}}</ref>
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==
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[[File:Human embryo 8 weeks 4.JPG|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]].<ref>Martin. Neuroanatomy Text and Atlas, Second Edition, 1996, pp. 35-36.</ref>
⚫
===Development===
⚫
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 [[forebrain]] and [[hindbrain]], the midbrain remains undivided 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]].<ref>Martin. Neuroanatomy Text and Atlas, Second Edition, 1996, pp. 35-36.</ref>


Throughout embryonic development, the cells within the midbrain continually multiply and compress the still-forming [[cerebral aqueduct]]. Partial or total obstruction of the cerebral aqueduct during development can lead to congenital [[hydrocephalus]].<ref>{{cite web |url=http://www.ninds.nih.gov/disorders/hydrocephalus/detail_hydrocephalus.htm |title=Hydrocephalus Fact Sheet |publisher=National Institute of Neurological Disorders and Stroke |date=February 2008 |accessdate=2011-03-23}}</ref>
Throughout embryonic development, the cells within the midbrain continually multiply; this happens to a much greater extend 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]]<ref>{{cite web |url=http://www.ninds.nih.gov/disorders/hydrocephalus/detail_hydrocephalus.htm |title=Hydrocephalus Fact Sheet |publisher=National Institute of Neurological Disorders and Stroke |date=February 2008 |accessdate=2011-03-23}}</ref>.


==Function==
==Function==

Revision as of 17:22, 11 October 2018

Midbrain
Inferior view midbrain (2), above (3)
Human brainstem midbrain (B)
thalamus (A) pons (C)
medulla oblongata (D)
Details
Identifiers
Latinmesencephalon
MeSHD008636
NeuroNames462
NeuroLex IDbirnlex_1667
TA98A14.1.03.005
TA25874
FMA61993
Anatomical terms of neuroanatomy

The midbrain or mesencephalon (UK: /ˌmɛsɛnˈsɛfəlɒn, -kɛf-/, US: /ˌmɛzənˈsɛfələn/;[1] from Greek mesos 'middle', and enkephalos 'brain'[2]) is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation.[3]

Structure

The principle regions of the midbrain are the tectum, the cerebral aqueduct, tegmentum, and the basis pedunculi. Rostrally the midbrain adjoins the diencephalon (thalamus, hypothalamus, etc.), while Caudally it adjoins the metencephalon (hindbrain) (pons and cerebellum).[4]. In the rostral direction, the midbrain noticably 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 basis pedunculi 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

Main article: Tectum

The Tectum (Latin for roof) is the dorsal side of the midbrain. It is involved in certain reflex actions in connection with visual or auditory stimuli. The reticulospinal tract, which excerts some control over alertness, takes input from the tectum[5], and travels both rostrally and caudally from it.

The corpora quadrigemina ("quadruplet bodies") are four solid lobes on the surface of the tectum. The superior pair (the superior colliculi) process some visual information, aid the decussation of several fibres of the optic nerve (some fibres remain ipsilateral), and is involved with saccadic eye movements. The tectospinal tract connects the superior colliculi to the nerves of the neck, and co-ordinates head and eye movements.

The inferior pair (inferior colliculi) - located just above the trochlear nerve - process certain auditory information. The homologous structures are called optic lobes in some lower vertebrates (fish and amphibians) where they integrate sensory information from the eyes and certain auditory reflexes[6][7]

Cerebral aqueduct

The cerebral aqueduct is the part of the ventricular system which links the third ventricle (rostally) with the fourth ventricle (caudally); as such it is responsible for continuing the circulation of cerebrospinal fluid. The cerebral aqueduct is the smallest ventricle in the ventricular system. It is located between the tectum and the tegmentum, and is surrounded by the periaqueductal grey[8], which has a role in analgesia, quiescence, and bonding. The Dorsal raphe nucleus (which releases seretonin 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[9] , 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, immediatetly lateral to the dorsal raphe nucleus[8]. 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[8].

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 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 decussate, but they dissipate more rostrally[8]. 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 lemiscus - a narrow ribbon of fibres - passes through in a relatively constant coronal 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 anterolateral tracts - 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[8]. 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 dopamin 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 telencephalon) and hypothalamus (of the diencephalon).

Basis pedunculi

Brain Anatomy - Mid-fore-hindbrain.

The basis pedunculi 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 Crus cerebri, which 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 the 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)[8] - 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 aren't (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.[10]

Development

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.[11]

Throughout embryonic development, the cells within the midbrain continually multiply; this happens to a much greater extend 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[12].

Function

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 excitation, 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.[13] The midbrain helps to relay information for vision and hearing.

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. ^ Mosby's Medical, Nursing & Allied Health Dictionary,≈ Fourth Edition, Mosby-Year Book 1994, p. 981
  3. ^ Breedlove, Watson, & Rosenzweig. Biological Psychology, 6th Edition, 2010, pp. 45-46
  4. ^ "Archived copy". Archived from the original on 2011-04-27. Retrieved 2011-03-05. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)CS1 maint: archived copy as title (link)
  5. ^ Kandel, Eric (2000). Principles of Neural Science. McGraw-Hill. p. 669. ISBN 0-8385-7701-6.
  6. ^ Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
  7. ^ Ferrier, David (1886). "Functions of the optic lobes or corpora quadrigemina". doi:10.1037/12789-005. {{cite journal}}: Cite journal requires |journal= (help)
  8. ^ a b c d e f Martin. Neuroanatomy Text and Atlas, Second edition. 1996, pp. 522-525.
  9. ^ Haines, Duane E. Neuroanatomy : an atlas of structures, sections, and systems (8th ed.). Philadelphia: Wolters Kluwer/ Lippincott Williams & Wilkins Health. p. 42. ISBN 978-1-60547-653-7.
  10. ^ Damier, P.; Hirsch, E. C.; Agid, Y.; Graybiel, A. M. (1999-08-01). "The substantia nigra of the human brainII. Patterns of loss of dopamine-containing neurons in Parkinson's disease". Brain. 122 (8): 1437–1448. doi:10.1093/brain/122.8.1437. ISSN 0006-8950.
  11. ^ Martin. Neuroanatomy Text and Atlas, Second Edition, 1996, pp. 35-36.
  12. ^ "Hydrocephalus Fact Sheet". National Institute of Neurological Disorders and Stroke. February 2008. Retrieved 2011-03-23.
  13. ^ 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.
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