Mesencephalon
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The midbrain /mesencephalon/ is the most rostral part of the brainstem , connected to the pons .
It connects the rhombencephalon with the diencephalon. It measures about 2 cm in length. It is practically entirely covered by the hemispheres of the cerebrum , only its ventral part is visible as the so-called crura cerebri (partes anteriores pedunculi cerebri) – massive stems containing white matter. The aquaeductus mesencephali (Sylvii) – a narrow canal leading cerebrospinal fluid , after leaving the IV. ventricle – runs through the midbrain .
The aquaeductus m. is lined with a layer of gray matter, the substantia grisea centralis.
The midbrain can be divided into several parts – the tectum mesencephali and the pedunculus cerebri (which consists of the tegmentum mesencephali and the crura cerebri ).
The pedunculus cerebri is ventral to the aqueductus m.
Crura cerebri ( partes anteriores pedunculi cerebri ) – also part of the pedunculus cerebri . They are lateral and insert under the optic tract into the base of the brain.
Between the two peduncles is the fossa interpeduncularis . Its surface is perforated by the course of a number of blood vessels, therefore it is called the substantia perforanta posterior (interpeduncularis). Be careful not to confuse it with the substantia perforanta anterior , which is part of telencephalon !
In the interpeduncular fossa, medial to the peduncle, the oculomotor nerve III emerges.
Ventrally from the fossa interpeduncularis are the corpora mamillaria , which already belong to the diencephalon . The following cranial nerves – III , IV – depart from the midbrain (II here already runs as a path, not the optic nerve). Furthermore, the cranial ends of the reticular formation extend here .
The tectum lies dorsal to the mesencephalic aqueduct. It contains two pairs of tubercles – the superior and inferior colliculi .
They are involved in the visual and auditory pathways and continue as the brachium colliculi superioris et inferiori into the corpus geniculatum laterale and mediale diencephala .
Caudally from the colliculus, the superior cerebellar peduncles extend, another in the series of cerebellar tracts. As with the inferior cerebellar peduncles, the velum, more precisely the velum medullare superius, forming the cranial part of the roof of the fourth ventricle, is also stretched here. Cranially from the tectum is the area pretectalis, which already belongs to the diencephalon .
Nuclei of the tectum mesencephali:
- predominantly sensory pathways (vision and hearing)
a) Nc. colliculi superioris - fibers from the retina of the eye, both motor and somatosensitive
b) No. inferior colliculus - auditory cortex center
c) No. posterior suture
d) Ncc. pretectals - nc. optical tract
Tegmentum ( posterior parts of the cerebral peduncle ) – ventrally from the roof, part of the cerebral peduncle .
The border between the crura and the tegmentum is formed by the substantia nigra.
Substantia nigra - pigment in the perikarya, the outer part faces the crura and has a net-like appearance (pars reticularis) and the inner part faces the tegmentum (pars compacta) produces dopamine .
It contains a number of important pathways and nuclei. On the lateral side, a fine elevation is visible – trigonum lemnisci, where the lemniscus medialis runs .
Nuclei of the tegmentum mesencephali:
- mainly motor pathways
a) Nc. ruber - oval, large, reddish, between the substantia nigra and the aquaeductus, regulation of limb movements
b) Nc. nervi oculomotorii - near the aquaeductus, a set of several nuclei, sends somatomotor fibers to n. oculomotorius, which innervate 4 of the 6 oculomotor muscles, controls eyeball movements
c) Nc. accessorius n. oculomotorii - dorsal from nc. nervi oculomotorii, visceromotor parasympathetic fibers, m. ciliaris and sphincter pupillae, eyeball movements
d) Nc. interstitialis Cajal - small, dorsal to nc. nervi oculomotorii, connected to the gray matter substantia grisea centralis and to the rostral part of the formatio reticularis, part of fasciculus longitudinalis medialis
e) Nc. Darkševiči - small, fibers from the cerebellum, telencephalic cortex, vestibular nuclei of the rhombencephalon
f) Nc. nervi trochlearis - small, near the substantia grisea centralis, caudal to n. III, somatomotor fibers innervate m. obliquus superior bulbi
g) Nc. mesencephalicus nervi trigemini - long and slender, comes here from the rhombencephalon, lateral to the aquaeductus, sensitive muscle and joint receptors of the masticatory muscles, artic. temporomandibular and oculomotor muscles
Pathways of the mesencephalon:
- afferent to ncc. collicules superiores from the retina of the eye, from the spinal cord, rhombencephalon, from the visual cortex
- to colliculi inferioris from ncc. cochleares
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Source[edit | edit source]
- ČIHÁK, Radomír a Miloš GRIM. Anatomie 3. 2., upr. a dopl vydání. Praha : Grada, 2004. 673 s. sv. 3.
