NEUROANATOMY
Brain Stem
It is the oldest part of our central nervous system and composed of the mesencephalon,
metencephalon, and myelencephalon (these are embryologic terms; one may also state
that the brainstem is composed of the mesencephalon, pons, cerebellum, and medulla
oblongata)
Its dorsal aspect is mostly hidden from view in the whole brain, whereas its ventral and
lateral aspects are visible.
Mesencephalon
The mesencephalon (midbrain) is a relatively narrow band of the brainstem
surrounding the cerebral aqueduct, extending from the diencephalon to the pons.
The dorsal aspect of the midbrain is known as the tectum (L., “roof”) and
incorporates the paired superior and inferior colliculi (also known as the
corpora quadrigemina).
The trochlear nerve (CN IV) exits the dorsal aspect of the mesencephalon just
below the inferior colliculus.
All other cranial nerves exit the ventral aspect of the brainstem.
The region of the mesencephalon below the cerebral aqueduct is known as the
midbrain (mesencephalic) tegmentum (L., “cover”).
The cerebral hemispheres are connected to the brainstem by two large fiber tracts,
the cerebral peduncles, and the depression between the peduncles is known as the
interpeduncular fossa, the site of origin of the oculomotor nerve (CN III).
Metencephalon
The metencephalon is located below the cerebellum; its ventral bulge, the pons, is
clearly visible.
The cerebellum overlies and hides the dorsal aspect of the brainstem, but its ventral
aspect, the pons, is clearly evident.
Rostrally, the superior pontine sulcus acts as the boundary between the
metencephalon and the midbrain and the inferior pontine sulcus as the
boundary between the metencephalon and the myelencephalon.
Part of the floor of the fourth ventricle is formed by the dorsal aspect of the pons,
and is known as the pontine tegmentum, the structure that houses the nuclei of
the trigeminal, abducent, facial, and vestibulocochlear nerves.
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Cranial nerves VI, VII, and VIII leave the brainstem at the inferior pontine sulcus,
whereas the trigeminal nerve exits the brainstem through the middle cerebellar
peduncle.
Myelencephalon
The caudal-most portion of the brainstem is the myelencephalon, also known as
the medulla oblongata, extends from the inferior pontine sulcus to the spinal cord.
The boundary between them is the region where the lateral walls of the fourth
ventricle converge in a V shape at the midline obex (at the level of the foramen
magnum).
The ventral surface of the myelencephalon displays the anterior midline fissure,
bordered on each side by the pyramids and crossed by the pyramidal decussations,
connecting the right and left pyramids to each other.
The olives are olivepit-shaped swellings lateral to each pyramid.
The hypoglossal nerve is evident as a number of thin filaments on each side of the
brainstem, arising from the anterior lateral sulcus between the pyramids and
olives.
The glossopharyngeal, vagus, and accessory nerves arise from the groove dorsal to
the olives. The dorsal surface of the myelencephalon presents the posterior median
fissure, which is interposed between the right and left tuberculum gracilis,
swellings formed by the nucleus gracilis.
Just lateral to the tuberculum gracilis is another swelling, the tuberculum
cuneatus, a bulge formed by the underlying nucleus cuneatus.
The caudal continuation of the tuberculum gracilis is the fasciculus gracilis, and
the continuation of the tuberculum cuneatus is the fasciculus cuneatus. Just
lateral to the tuberculum cuneatus is another swelling, the tuberculum cinereum,
formed by the descending tract of the trigeminal nerve.
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References:
1. A Textbook of Neuroanatomy Maria A. Patestas & Leslie P. Gartner
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