| Midbrain | |
|---|---|
 Figure shows the midbrain (A) and surrounding regions;sagittal view of onecerebellar 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 | |
| Pronunciation | UK:/ˌmɛsɛnˈsɛfəlɒn,-kɛf-/,US:/ˌmɛzənˈsɛfələn/;[1] |
| Part of | Brainstem |
| Identifiers | |
| Latin | mesencephalon |
| MeSH | D008636 |
| NeuroNames | 462 |
| NeuroLex ID | birnlex_1667 |
| TA98 | A14.1.03.005 |
| TA2 | 5874 |
| FMA | 61993 |
| Anatomical terms of neuroanatomy | |
Themidbrain ormesencephalon is the uppermost portion of thebrainstem connecting thediencephalon andcerebrum with thepons.[2] It consists of thecerebral peduncles,tegmentum, andtectum.
It is functionally associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness), and temperature regulation.[3]
The namemesencephalon comes from the Greekmesos, "middle", andenkephalos, "brain".[4]
The midbrain is the shortest segment of the brainstem, measuring less than 2cm in length. It is situated mostly in theposterior cranial fossa, with its superior part extending above thetentorial notch.[2]
The principal regions of the midbrain are thetectum, thecerebral aqueduct,tegmentum, and thecerebral peduncles.Rostrally the midbrain adjoins thediencephalon (thalamus,hypothalamus, etc.), whilecaudally it adjoins thehindbrain (pons,medulla andcerebellum).[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.
The tectum (Latin forroof) is the part of the midbrain dorsal to thecerebral aqueduct.[2]The position of the tectum is contrasted with thetegmentum, which refers to the region in front of theventricular system, or floor of the midbrain.
It is involved in certainreflexes in response to visual or auditory stimuli. Thereticulospinal tract, which exerts some control over alertness, takes input from the tectum,[6] and travels both rostrally and caudally from it.
Thecorpora quadrigemina are four mounds, called colliculi, in two pairs – a superior and an inferior pair, on the surface of the tectum. Thesuperior colliculi process some visual information, aid thedecussation of several fibres of theoptic nerve (some fibres remain ipsilateral), and are involved withsaccadic eye movements. Thetectospinal tract connects the superior colliculi to thecervical nerves of theneck, and co-ordinates head and eye movements. Each superior colliculus also sends information to the correspondinglateral geniculate nucleus, with which it is directly connected. Thehomologous structure to the superior colliculus in non mammalian vertebrates includingfish andamphibians, is called theoptic tectum; in those animals, the optic tectum integrates sensory information from the eyes and certain auditory reflexes.[7][8]
Theinferior colliculi – located just above thetrochlear nerve – process certain auditory information. Each inferior colliculus sends information to the correspondingmedial geniculate nucleus, with which it is directly connected.
Thecerebral aqueduct is the part of theventricular system which links thethird ventricle (rostrally) with thefourth ventricle (caudally); as such it is responsible for continuing the circulation ofcerebrospinal fluid. The cerebral aqueduct is a narrow channel located between the tectum and the tegmentum, and is surrounded by theperiaqueductal grey,[9] which has a role in analgesia, quiescence, and bonding. Thedorsal raphe nucleus (which releasesserotonin 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 ofcranial nerves are similarly located at the ventral side of the periaqueductal grey – the pair ofoculomotor nuclei (which control the eyelid, and most eye movements) is located at the level of the superior colliculus,[10] while the pair oftrochlear 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.[9] 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. TheEdinger-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]
Themidbrain tegmentum is the portion of the midbrain ventral to the cerebral aqueduct, and is much larger in size than the tectum. It communicates with thecerebellum by thesuperior 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 theydecussate, but they dissipate more rostrally.[9] Between these peduncles, on the ventral side, is themedian 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 thereticular formation. A number of distinctnerve tracts between other parts of the brain pass through it. Themedial 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). Thespinothalamic 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 – thered 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] Therubrospinal 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 theventral tegmental area – is the largestdopamine-producing area in the brain, and is heavily involved in theneural reward system. The ventral tegmental area is in contact with parts of the forebrain – themammillary bodies (from theDiencephalon) andhypothalamus (of thediencephalon).
Thecerebral peduncles each form a lobe ventrally of the tegmentum, on either side of the midline. Beyond the midbrain, between the lobes, is theinterpeduncular fossa, which is a cistern filled withcerebrospinal fluid[citation needed].
The majority of each lobe constitutes thecerebral crus. The cerebral crus are the main tracts descending from thethalamus to caudal parts of the central nervous system; the central and medial ventral portions contain thecorticobulbar andcorticospinal tracts, while the remainder of each crus primarily contains tracts connecting the cortex to thepons. Older texts refer to the crus cerebri as thecerebral 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 theinternal capsule.
The portion of the lobes in connection with the tegmentum, except the most lateral portion, is dominated by a blackened band – thesubstantia nigra (literallyblack substance)[9] – which is the only part of thebasal 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 (thepars compacta) and one where they are not (thepars 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, andnoradrenalin; the loss of dopamine-producing neurons in this region contributes to the progression ofParkinson's disease.[11]
The midbrain is supplied by the followingarteries:
Venous blood from the midbrain is mostly drained into thebasal vein as it passes around the peduncle. Some venous blood from the colliculi drains to thegreat cerebral vein.[12]
Duringembryonic development, the midbrain (also known as the mesencephalon) arises from the secondvesicle of theneural tube, while the interior of this portion of the tube becomes the cerebral aqueduct. Unlike the other two vesicles – theforebrain andhindbrain – 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 thetelencephalon and thediencephalon.[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 congenitalhydrocephalus.[14] The tectum is derived in embryonic development from thealar plate of the neural tube.
The midbrain is the uppermost part of thebrainstem. Itssubstantia nigra is closely associated withmotor system pathways of thebasal ganglia. The human midbrain isarchipallian in origin, meaning that its general architecture is shared with the most ancient ofvertebrates.Dopamine produced in the substantia nigra andventral tegmental area plays a role in movement, movement planning, excitation, motivation and habituation of species from humans to the most elementary animals such as insects.Laboratory mice from lines that have beenselectively bred for high voluntarywheel running have enlarged midbrains.[15] The midbrain helps to relay information for vision and hearing.
The term "tectal plate" or "quadrigeminal plate" is used to describe the junction of the gray and white matter in the embryo. (ancil-453 atNeuroNames)
{{cite journal}}:Cite journal requires|journal= (help)