It is the most superior of the three brainstem regions. It acts as a conduit between the anterior forebrain, the protuberance and the cerebellum.
The mesencephalon is an area of the brain that, as you may have guessed, is in the middle of two other regions: the forebrain and the hindbrain.
The forebrain is the ‘frontal’ brain (front) and is composed of the cerebral cortex, the area that most people consider the ‘brain’; It is the ‘supercomputer’ of the human body.
The posterior brain is composed of the cerebellum, the pons and the medulla oblongata (or medulla, for short) of the brainstem.
It is evolutionarily the oldest part of our brain, which controls the primary instincts and automatic actions of the body, such as our “fight or flight” response and heart rate.
External anatomy of the midbrain
The mesencephalon is the smallest of the three brain stem regions, measuring about 2 cm in length. As it ascends, the mesencephalon travels through the opening of the cerebellum.
It can be divided into two main parts:
- Tectum: located later to the cerebral aqueduct.
- Paired brain peduncles: located anteriorly and laterally.
Internally, the cerebral peduncles are separated by the substantia nigra in the crus cerebri and the tegmentum.
The tectum houses four rounded prominences called colliculi (collectively the quadrilateral bodies) that sit directly inferior to the pineal gland. The colliculi are separated by the cruciform groove; There are two superior and two inferior colliculi.
Extending laterally from each colliculus is the quadrigeminal brachium:
- The upper quadrigeminal brachium forms a pathway between the superior colliculus and the retina of the eye.
- The inferior quadrigeminal brachium transports fibers from the lateral lemniscus and the inferior colliculus to the internal geniculate body.
- Inferior to the colliculi, the trochlear nerve emerges before sweeping toward the anterior surface.
The pairs of cerebral peduncles extend from the cerebral hemispheres until converging when encountering the protuberance.
They are separated anteriorly in the midline by the interpeduncular fossa, whose floor is called posterior perforated substance (since many perforating blood vessels can be identified).
The ocular motor nerve is seen coming out from between the peduncles while the optic tract runs around the upper edge of the midbrain.
Internal anatomy of the midbrain
Two cross sections of the mesencephalon will be discussed: the level of the inferior colliculus and the level of the superior colliculus.
Lower Collicle Level:
The anterolateral surface of the midbrain harbors the matched crus cerebri. Four fiber tracts run inside the crus:
- Frontopontine fibers: located more medially.
- Corticospinal fibers : motor fibers of the primary motor cortex.
- Tracts corticobulbar: motor fibers of the primary motor cortex.
- Temporary fibers: located posterolaterally.
Then there is the substantia nigra, a pigmented nucleus that separates the two regions of the cerebral peduncles. It is subdivided into the pars reticulata (anterior) and the pars compact (posterior).
The tegmentum is located behind the black substance. It is continuous with that found in the protuberance of the same name. It is important to note that unlike the crus cerebri, the tegmentum is continuous in the middle line.
The cerebral aqueduct is a structure of the midline surrounded by the central gray matter, the periaqueductal gray matter.
Within this gray matter is the mesencephalic nucleus of the trigeminal nerve, as well as the trochlear nucleus with its fibers that continue around the gray substance to leave the mesencephalon. Previous to this, you can see the medial longitudinal fascicle.
The decussation of the superior cerebellar peduncles can be seen centrally at this level with some reticular formation (noted along the brainstem) that extends laterally.
Between the central gray matter and the black substance are four lemnisci. Moving from anterior to posterior are medial, spinal, trigeminal and lateral leminisci.
Colliculus Superior Level:
Much of the internal structure of the mesencephalon is not modified at this level and should be assumed to be present unless mentioned below.
The central portion that was previously occupied by the decussation of the superior cerebellar peduncles now contains the large red nuclei paired with some decussation of the anterior rubrospinal tract. The reticular formation now fan around the posterior edges of the red nuclei.
The trochlear nucleus is replaced with the oculomotor nucleus while the ocular motor nerve projects forward. The medial, spinal and trigeminal lemnisci are all present in the same location, however, the lateral lemnisci does not reach this level.
We will briefly examine the mesencephalon vasculature:
The supply to the area is derived from the basilar artery and its branches.
The main suppliers are:
- The posterior cerebral artery and its peduncular branch.
- The superior cerebellar artery, the posterior choroidal artery.
- The interpeduncular branches of the basilar artery.