Anatomy of Midbrain
The midbrain or mesencephalon is the shortest part of the
brain located in the middle portion. It is about 2cm in length. It is one of the
three regions of the brainstem. The other two regions are medulla oblongata and
pons.
The midbrain acts as a channel between the forebrain above and the pons
and the cerebellum below. It has four main parts: the tectum and the tegmentum, the cerebral aqueduct and the cerebral peduncle.
Tectum
The part of the midbrain
behind the cerebral aqueduct and at the posterior side of the midbrain is called
the tectum. It is smaller in size as compared to the tegmentum. The tectum
primarily consists of the two superior colliculi and two inferior colliculi, a
group of neurons that together form four bumps on the posterior surface of the
brainstem.
Tegmentum
The area in front of the cerebral aqueduct and at the
lateral side of the midbrain is called the tegmentum. It is larger as compared
to the tectum. The tegmentum contains a variety of ascending and descending
tracts like medial lemniscus and anterolateral tracts. The tegmentum usually
consists of the following parts;
Periaqueductal gray matter: It is gray matter
consisting of neurons surrounding the cerebral aqueduct within the tegmentum.
Spinothalamic tract: It is a ribbon-like region of fibers present at the lateral
border of the tegmentum.
Reticular formation: It is a network of interconnected
nuclei that spans from the lower end of the medulla and the upper end of the
midbrain.
Red nuclei: This is a pair of distinguished reddish regions located at
the anterior side of the midbrain.
Substantia nigra: Substantia nigra is a
crescent-shaped colored band of grey matter that is located within the
tegmentum. It is made up of deeply colored nerve cells that contain melatonin
and iron.
Cranial nerve nuclei: Two of the cranial nerve nuclei are found in the
tegmentum. These are trochlear nuclei and oculomotor nuclei.
Ventral tegmental
area: It is an area on the ventral side of the midbrain and between red nuclei
that is known for producing dopamine. It is also connected with the mammillary
body and hypothalamus.
Cerebral aqueduct
A cerebral aqueduct is a narrow tube
between the tectum and the tegmentum. It is a middle structure of the midbrain
surrounded by periaqueductal gray matter. It is a part of the ventricular system
that links the third ventricle with the fourth ventricle.
Cerebral peduncles
Cerebral peduncles are paired structures found at the anterior surface of the
midbrain. These consist of crus cerebri.
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