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.[2]
The midbrain is typically sectioned axially at either the superior or inferior colliculi levels. Visualizing these cross-sections as an upside-down bear face helps remember its structures, with the peduncles forming ears, aqueducts mouth, and tectum chin.
The tectum (Latin for roof) is the part of the midbrain dorsal to the cerebral aqueduct.[2] 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 reflexes in response to visual or auditory stimuli. The reticulospinal tract, which exerts some control over alertness, takes input from the tectum,[6] 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 colliculi process some visual information, aid the decussation of several fibres of the optic nerve (some fibres remain ipsilateral), and are involved with saccadic eye movements. The tectospinal tract connects the superior colliculi to 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]
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,[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 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,[10] 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, 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. 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.[9]
Tegmentum
Cross-section of the midbrain at the level of the superior colliculusCross-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 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).
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 substantia nigra (literally black substance)[9] – 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 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.[11]
Blood supply
The midbrain is supplied by the following arteries:
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.[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 midbrain is the uppermost part of the brainstem. Its substantia nigra is closely associated with motor system pathways of the basal ganglia. The human midbrain 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 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 been selectively bred for high voluntary 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.
The sense of balance or equilibrioception is the perception of balance and spatial orientation. It helps prevent humans and nonhuman animals from falling over when standing or moving. Equilibrioception is the result of a number of sensory systems working together; the eyes, the inner ears, and the body's sense of where it is in space (proprioception) ideally need to be intact.
Articles related to anatomy include:
The brainstem is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is continuous with the thalamus of the diencephalon through the tentorial notch, and sometimes the diencephalon is included in the brainstem.
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of the eye and that raise the eyelid. The nerve also contains fibers that innervate the intrinsic eye muscles that enable pupillary constriction and accommodation. The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in control of eye movement.
The cerebral aqueduct is a small, narrow tube connecting the third and fourth ventricles of the brain. The cerebral aqueduct is a midline structure that passes through the midbrain. It extends rostrocaudally through the entirety of the more posterior part of the midbrain. It is surrounded by the periaqueductal gray, a layer of gray matter.
The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle. From this nucleus the fibers pass forward through the tegmentum, the red nucleus, and the medial part of the substantia nigra, forming a series of curves with a lateral convexity, and emerge from the oculomotor sulcus on the medial side of the cerebral peduncle.
The medial longitudinal fasciculus (MLF) is a prominent bundle of nerve fibres which pass within the ventral/anterior portion of periaqueductal gray of the mesencephalon (midbrain). It contains the interstitial nucleus of Cajal, responsible for oculomotor control, head posture, and vertical eye movement.
The midbrain is anatomically delineated into the tectum (roof) and the tegmentum (floor). The midbrain tegmentum extends from the substantia nigra to the cerebral aqueduct in a horizontal section of the midbrain. It forms the floor of the midbrain that surrounds below the cerebral aqueduct as well as the floor of the fourth ventricle while the midbrain tectum forms the roof of the fourth ventricle. The tegmentum contains a collection of tracts and nuclei with movement-related, species-specific, and pain-perception functions. The general structures of midbrain tegmentum include red nucleus and the periaqueductal grey matter.
The inferior colliculus (IC) is the principal midbrain nucleus of the auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The inferior colliculus has three subdivisions: the central nucleus, a dorsal cortex by which it is surrounded, and an external cortex which is located laterally. Its bimodal neurons are implicated in auditory-somatosensory interaction, receiving projections from somatosensory nuclei. This multisensory integration may underlie a filtering of self-effected sounds from vocalization, chewing, or respiration activities.
The periaqueductal gray (PAG), also known as the central gray, is a brain region that plays a critical role in autonomic function, motivated behavior and behavioural responses to threatening stimuli. PAG is also the primary control center for descending pain modulation. It has enkephalin-producing cells that suppress pain.
The anterior choroidal artery is a bilaterally paired artery of the brain. It is typically a branch of the internal carotid artery which supplies the choroid plexus of lateral ventricle and third ventricle as well as numerous structures of the brain.
The tegmentum is a general area within the brainstem. The tegmentum is the ventral part of the midbrain and the tectum is the dorsal part of the midbrain. It is located between the ventricular system and distinctive basal or ventral structures at each level. It forms the floor of the midbrain (mesencephalon) whereas the tectum forms the ceiling. It is a multisynaptic network of neurons that is involved in many subconscious homeostatic and reflexive pathways. It is a motor center that relays inhibitory signals to the thalamus and basal nuclei preventing unwanted body movement.
The trochlear nucleus is the motor nucleus of the trochlear nerve. It is located in the medial midbrain.
The central tegmental tract is a tract that carries ascending and descending fibers, situated in the midbrain tegmentum, and the pontine tegmentum. The tract is situated in the central portion of the reticular formation.
The rostromedial tegmental nucleus (RMTg), also known as the tail of the ventral tegmental area (tVTA), is a GABAergic nucleus which functions as a "master brake" for the midbrain dopamine system. This region was discovered by the researchers, M. Barrot, J.Kaufling and T. Jhou. It is poorly differentiated from the rest of the ventral tegmental area (VTA) and possesses robust functional and structural links to the dopamine pathways. Notably, both acute and chronic exposure to psychostimulants have been shown to induce FosB and ΔFosB expression in the RMTg; no other drug type has been shown to induce these proteins in the RMTg.
Central arteries of the brain are numerous small arteries branching from the Circle of Willis and adjacent arteries that often enter the substance of the brain through the anterior and posterior perforated substances, and supply structures of the base of the brain and internal structures of the cerebral hemisphere. They are separated into four principal groups: anteromedial central arteries; anterolateral central arteries ; posteromedial central arteries ; and posterolateral central arteries.
In neuroanatomy, corticomesencephalic tract is a descending nerve tract that originates in the frontal eye field and terminates in the midbrain. Its fibers mediate conjugate eye movement.
The interstitial nucleus of Cajal is a collection of neurons in the mesencephalon (midbrain) which are involved in integrating eye position-velocity information in order to coordinate head-eye movements - especially those related to vertical and torsional conjugate eye movements (gaze). It also mediates vertical gaze holding.
The nucleus of Darkschewitsch is an accessory oculomotor nucleus situated in the ventrolateral portion of the periaqueductal gray of the mesencephalon (midbrain) near its junction with the diencephalon. It is involved in mediating vertical eye movements. It projects to the trochlear nucleus, receives afferents from the visual cortex, and forms a reciprocal (looping) connection with the cerebellum by way of the inferior olive.
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