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The midbrain or mesencephalon (from
the Greek mesos, middle, and enkephalos, brain is a
portion of the central nervous system associated with
vision, hearing, motor control, sleep/wake, arousal
(alertness), and temperature regulation.
Structure
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 midbrain adjoins the metencephalon (afterbrain)
(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.
Specifically, the midbrain consists of:
tectum
inferior colliculi
superior colliculi
cerebral peduncle
midbrain tegmentum
crus cerebri
substantia nigra
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.y other
reports of large series of children with equal sex
distribution.
Cerebral peduncle
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.
Anatomical features of cross-sections through the midbrain
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
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.[5]
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.[6]
Both sections will show the cerebral aqueduct, which
connects the third and fourth ventricle and the
periaqueductal gray.[7]
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
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.[8]
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.
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
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.
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