Mesencephalon: Structure, Position, and Function

The mesencephalon of the midbrain is part of the brain located in the most rostral part of the Truncus encephali or the brain stem, between the hindbrain and the forebrain (1). Mesencephalon connects these two parts of the brain. At the same time, this is the most superior brain region located in the brainstem.

Its key peculiarity is numerous nerve tracts that play the key role in connecting the cerebellum and the cerebrum, as well as other brain segments. Its main function is visual, auditory and movement processing and control.

This includes motoric activities, as well as movements of the eye and muscles. As a result, trauma or lesions in this part of the brain can lead to the development of Parkinson's disease (1).

In this article, which is focused on
the mesencephalon, we will elaborate on its structure, position, and function.
Moreover, we will describe its anatomy, the paths of its blood supply, its
relation to the cranial nerves, and explain some basic consequences of the
mesencephalon damage.

Position and Structure of the Mesencephalon

 The mesencephalon is associated with the III and the IV cranial nerve. Those are the oculomotor and trochlear nerves. The definitive brain cavity is the Mesencephalic aqueduct. There are many important brain structures in the midbrain. Two of them are of the utmost importance. Those are the Tectum and the Tegmentum. Except for these two structures, there are four more crucial mesencephalon segments. Those are:

• substantia nigra,
• cranial nerves,
• cerebral peduncle,
• crus cerebri (1).

 The Tectum

This is one of the two key mesencephalon structures. This part of the mesencephalon that got its name based on the Latin word meaning “roof”. The reason is simple – this portion of the mesencephalon builds up its rear part.

Moreover, it contains the structures called colliculi. These are rounded bulges that are important for the success of the hearing and seeing processes.

The Tegmentum

The Tegmentum is the other key midbrain segment. It is one of the structures forming the midbrain base. It is anatomically positioned in front of the structure called tectum. Tegmentum contains fibre tracts. Three regions differ in color. Namely, there are the red nucleus, the substantia nigra, or the black matter, and the periaqueductal gray region.

First
of all, the red nucleus is located in the center of the tegmentum. This
structure is relatively large and is responsible for the coordination of the
sensorimotor information. It is the red nucleus where the crossed fibres of the
major output system in the cerebellum end. These also have access to the motor
cortex. 

Secondly, the substantia nigra contains pars compacta (compact segment) and pars reticulata. This is also a large pigmented region that contains neuron clusters and is named substantia nigra thanks to the pigment melanin, which is dark.

These structures and cells are of the utmost importance as they synthesize dopamine. Most noteworthy, they are related to the process of movement control and coordination.

Third, the periaqueductal gray area is the third "colored" region in the tegmentum. As its name itself implies, this zone contains gray matter. This matter is a neural tissue that is covered in myelin.

As mentioned at the beginning of this article, it is the periaqueductal gray region that surrounds the cerebral aqueduct. This canal is a short brain structure that connects the 3^rd and the 4^th brain ventricles.

When it comes to the function of this area, its key role in pain suppression. Scientists found that this results from a high endorphin concentration in this brain region. 

Substantia Nigra

The substantia nigra is a mesencephalon segment that is connected to the frontal lobe. Moreover, it is related to brain segments that are important for motoric functions. The most noteworthy, the substantia nigra is a part of the midbrain where dopamine is produced.

It is a chemical messenger. It is responsible for the coordination of muscle movements.

Cranial nerves

III and IV cranial nerves are related to the mesencephalon. Namely, the III cranial nerve exits from the peduncles. The optic tract encircles the upper midbrain border. This is one of the oculomotor nerves and its key functions include autonomic and parasympathetic innervation of the lens and the pupil, as well as the upper eyelid.

Moreover, it somatically innervates the eye muscles important for gaze fixation, as well as visual tracking.

The IV cranial nerve is also called the trochlear nerve. It is the ocular motor nerve. As a result, this nerve is responsible for control over the eye movements. At the same time, this is the smallest cranial nerve.

Most importantly, it has a long intracranial course. Thanks to the fact that it has a dorsal exit from the brainstem, namely the cerebral peduncle, its path are rather long. This nerve extends to the superior oblique muscle (2).

The Cerebral Peduncle

This structure is built of a bundle of nerves. These serve to connect the hindbrain and the forebrain. These are paired structures that connect the cerebral hemispheres and the pons. Interpeduncular fossa separates these structures anteriorly, by the midline. It is the cerebral peduncle that houses the oculomotor nerve (the III cranial nerve).

Crus cerebri

The crus cerebri serves to connect the cerebellum and the cerebrum. These are, basically, tracts that consist of neurons. They also contain reticular formation which is a kind of a neural network. The key role of this brain part is controlling and coordination alertness and arousal.

The front-side mesencephalon surface
is where the paired crus cerebri are located. There are four fibre tracts
within this structure. Those include:

• Frontopontine fibres
• Corticospinal fibres
• Corticobulbar tracts
• Temporopontine fibres.

Blood supply of the mesencephalon

In this section, we will talk about the midbrain vasculature. Simply put, the blood supply of the mesencephalon depends on the basilar artery. It is supported by its branches. The key blood suppliers in this area are:

• posterior cerebral artery
• peduncular branch of the posterior
  cerebral artery,
• basilar artery branches,
• superior cerebellar artery,
• posterior choroidal artery.

Functions of the Mesencephalon

The mesencephalon is responsible for
many different bodily functions, including:

• Controlling the process of vision,
  i.e. regulating responses to sight,
• Pupil dilation
• Eye movement
• Hearing
• Regulates movements of muscles.

Damage of the Mesencephalon

It is particularly interesting to observe what happens with bodily functions in the case of damage to the substantia nigra cells. Namely, neurodegeneration of these cells leads to low dopamine production levels.

As a result, when dopamine levels decrease by 60 to 80%, Parkinson's disease may develop. This is a nervous system disorder that causes loss of coordination and motor control in patients. Symptoms of this disease include balance problems, tremors, muscle stiffness, and slow movements. 

Another condition related to the dysfunction of a structure found in the mesencephalon is the impairment of the oculomotor nerves. Certain traumas and diseases can lead to damage to these nerve fibers.

As a consequence, this prevents normal vision. Besides, certain conditions can serve as signs during the diagnostics of a neoplasm or an aneurysm.

Finally, it is useful to take a look at the results of a well-known neuroscience study on midbrain lesions and relation to head injuries (3). Namely, a study analyzed 35 head-injured patients whose injuries led to a lethal consequence.

The results showed that 23 of 35 patients had a midbrain lesion as a result of a head injury. The specialists performing the autopsy proved that the analyzed lesions were results of the head injury impact.

Those were characterized as primary lesions. As a result, the study showed that mesencephalon damage is almost always a result of a hemispheric injury (3).

Conclusion

Mesencephalon or midbrain is part of the brain stem which is located between the hindbrain and the forebrain. It has two main parts. Those are the tectum and tegmentum. Besides, it has other important structures that are responsible for different functions. Namely, those include the substantia nigra, cerebral nerves, the cerebral peduncle, and the crus cerebri.

Mesencephalon or the midbrain has many important functions, including controlling eye movements, motor movements, and visual and auditory processing. Moreover, this part of the brain is responsible for vision control, i.e. regulating responses to sight, pupil dilation, eye movement control, the process of hearing, as well as regulating movements of muscles.

The mesencephalon is a relatively small brain segment. However, its dysfunctions can lead to serious disorders, including Parkinson's disease. Moreover, important studies in neuroscience and neurosurgery found that the head injuries do lead to mesencephalon damage.

These injuries can be lethal. Lesions that are formed in different parts of the midbrain damage various patient's bodily functions, depending on the affected region. Detailed mesencephalon studies on its dysfunctions and damage continue.

References

1. Barkovich AJ. Developmental disorders of the midbrain and hindbrain. Front Neuroanat. 2012 Mar 6;6:7. doi: 10.3389/fnana.2012.00007. PMID: 22408608; PMCID: PMC3294267.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3294267/
2. Laine FJ. Cranial nerves III, IV, and VI. Top Magn Reson Imaging. 1996 Apr;8(2):111-30. Found online at: https://www.ncbi.nlm.nih.gov/pubmed/8784967
3. Rosenblum WI, Greenberg, RP, Seelig JM, Becker DP. Midbrain lesions: frequent and significant prognostic feature in closed head injury. Neurosurgery. 1981 Dec;9(6):613-20. Found online at: https://www.ncbi.nlm.nih.gov/pubmed/7322325