Mesencephalon

Located between these two structures of the cerebral cortex, the mesencephalon (or the midbrain), is one of the three major divisions of the human brain. This brain structure serves as a vital connection between the forebrain and the hindbrain.

What Are the Three Major Divisions of the Human Brain?

The human brain is primarily divided into three large processing centers, comprising of a large number of structures:

 

• Forebrain (The Prosencephalon) – The forebrain comprises the cerebrum, thalamus, hypothalamus, pituitary gland, limbic system, and the olfactory bulb, as well as the 4 lobes of the brain: the frontal, temporal, parietal, and the occipital lobe.
• Midbrain (The Mesencephalon) – A part of the brainstem, the mesencephalon consists of various cranial nerve nuclei, tectum, tegmentum, and the cerebral aqueduct.
• Hindbrain (The Rhombencephalon) – The lower part of the brain, comprising the cerebellum, pons, and medulla oblongata.

 

What Is the Function of the Mesencephalon?

The translation of the sensory impulses into motor movements and the audio-visual processing of sensory stimuli is the main function of the mesencephalon, i.e. the midbrain. It also regulates and controls sleep and wakefulness, the movement of both eyes and eyelids, arousal (alertness), and temperature regulation.

 

The mesencephalon is one of the most important elements of the central nervous system because it serves as a passage for all major neuronal transmissions of the body. Thus, the mesencephalon is associated with many more functions, assisted by the structures of the dedicated areas in the cerebral cortex.

 

More specifically, the mesencephalon connects the pons and cerebellum with the thalamus and the cerebral hemispheres of the brain. In this way, it is involved in the control of body movement, the modulation of movements, of visual and auditory sensory information, pain suppression, the regulation of the Circadian rhythm, as well as the reward-based learning patterns.

Anatomy and Structure of the Mesencephalon

There are several major functional areas of the mesencephalon. It comprises of the following structures:

 

• • Tectum (lat. roof – forming the upper part of the midbrain)
  • Tegmentum (lat. covering – forming the lower part of the midbrain)
  • Cerebral aqueduct (a channel in the midbrain which is filled with cerebrospinal fluid)
  • Cerebral peduncles (parts of the midbrain which link the brainstem to the cerebrum, thus helping the transport of nerve signals to the other parts of our nervous system)

• Cranial Nerve Nuclei (a network of cells in the brain which aid the transfer of signals from the parts of the brain to the parts of the body)

• Colliculi (bundles of nerve fibers)

Cranial Nerve Nuclei

The cranial nerve nuclei are a part of all structures comprising the brain stem, including the mesencephalon. They include the following neural pathways:

 

• The Olfactory nerve (CN I),
• The Optic nerve (CN II)
• The Oculomotor nerve (CN III)
• The Trochlear nerve (CN IV)
• The Trigeminal nerve (CN V)
• The Abducens nerve (CN VI)
• The facial nerve (CN VII)
• The Vestibulocochlear nerve (CN VIII)
• The Glossopharyngeal nerve (CN IX)
• The Vagus nerve (CN X)
• The Accessory nerve (CN XI)
• The Hypoglossal nerve (CN XII)

 

The cranial nerves are responsible for innervation of the special senses, the transmission of sensory signals to all parts of the brain and body, for receiving signals from and relaying them to the skeletal muscles and the skin, as well as for carrying information about the internal organs.

Tegmentum

The tegmentum is one of the most superior areas of the mesencephalon, located in front of the tectum. It’s comprised of two elements:

 

• The Red Nucleus – This nucleus of the mesencephalon controls and coordinates the movements of the body. It also takes part in the coordination of the eyes with the movement of the head, as well as the processing of the sensory information received through the retina of the eyes.

 

• The Periaqueductal Gray Nucleus – The gray nucleus of the mesencephalon is in charge of the inhibition of pain, the regulation of heart rate, blood pressure,  smooth muscle contraction, and the fight-or-flight response.

 

This gray matter structure encircles the cerebral aqueduct. Based upon its connections with the other structures of the brain, this nucleus can be divided into dorsomedial, dorsolateral, lateral, and ventrolateral columns.

Cerebral Aqueduct

This is one of the most notable parts of the mesencephalon since it provides a pathway for the cerebrospinal fluid to flow between the third and the fourth ventricle in the brain.

Tectum

With the help of the superior and inferior colliculi, the tectum controls the major systems of coordination in the brain. This is supported by its role in the perception and recognition of physical forms, the conditioning of the visual stimuli, as well as in visual memory.

Colliculi

There are 4 structures that resemble swellings on the back part of the mesencephalon. These relay stations for visual and auditory stimuli are labeled as colliculi. They are separated from each other by a crucial sulcus.

 

There are two types of colliculi:

 

• Inferior colliculi – Hemispheral in shape, the inferior colliculi play a part in the processing of the stimuli obtained from the organs of hearing.
• Superior colliculi – Larger and darker in color than the inferior colliculi, the superior colliculi have an oval shape and are involved in the processing of stimuli obtains through the sense of sight.

What Is the Structure and Function of the Cerebral Peduncles?

The cerebral peduncles (also: pedunculus cerebri) is a bundle of nerve fibers located on each side of the midbrain. They connect the forebrain to the hindbrain. Located in the far back part of the midbrain, they connect the leftover part of the brainstem to the lobes of the thalamus.

 

These midbrain structures have a cylindrical form and are located behind the temporal lobes of the cerebrum. The cerebral peduncles support the transport of the stimuli received from the sensory organs in the cerebral cortex and the brain stem, or in the other areas of the central nervous system.

 

By translating and processing the motor stimuli, the cerebral peduncles help refine our movements and learn new motor skills, and they support body posture and balance.

 

There are several vitally important fiber tracts that pass through the cerebral peduncles:

 

• The corticospinal nerve tract

• The corticopontine nerve tract

• The corticobulbar tracts

 

In addition, 2 out of the 12 cranial nerves are housed in this structure of the mesencephalon. Namely, the cranial nerves 3 (the oculomotor nerve) and 4 (the trochlear nerve) encircle the cerebral peduncles. These nerves support the movement of both the eyes and the eyelids.

Blood Supply of the Mesencephalon

The mesencephalon receives its oxygen, glucose, and nutrition via the basilar and vertebral arteries and their branches:

 

• The posterior cerebral artery
• The peduncular branch of the posterior cerebral artery
• The superior cerebellar artery
• The posterior choroidal artery

What Are the Other Brain Structures That Connect to the Mesencephalon?

As a subdivision of the brain stem, the mesencephalon links some of the most active information-processing structures of the brain that support processes that are of vital importance for survival. The list includes the substantia nigra, hypothalamus, the pons of the brain, and the cerebellum.

Substantia Nigra

Substantia nigra is a layer of pigmented nucleus composed of gray matter. Located between the reticular formation of the mesencephalon and the cerebral crura, this brain structure is responsible for the production of dopamine in the brain, as well as for the control of the reward-seeking processes.

Hypothalamus

The hypothalamus is an area of the brain responsible for the control of the body’s autonomic nervous system, as well as the regulation of various vitally important biological processes, such as body temperature, blood pressure, thirst, hunger, the sleep-wake cycle, important aspects of parenting and attachment behaviors, fatigue, etc.

 

Located at the upper end of the brainstem, the hypothalamus monitors and regulates some of the most vital internal conditions such as:

 

• nutrient levels
• water-salt balance
• blood flow
• the sleep-wake cycle

• body temperature
• daily physiological cycles
• appetite
• sexual behavior
• emotional responses
• secretion of gonadotropin-releasing hormones
• production of antidiuretic hormone
• inhibition of prolactin release

The Pons of the Brain

Located on the upper part of the brain stem, right above the medulla oblongata and the midbrain, the pons function as a relay center of all information coming from the brain or descending from it.

 

This brain structure is involved in the regulation and control of a long list of functions, including facial expression, chewing, biting, swallowing, as well as the transmission of sound from the ears to the brain.

 

What’s more important, the pons is responsible for the regulation of deep sleep and breathing by controlling breathing rate and the pattern of inhaling and exhaling air into the lungs.

Cerebellum

The cerebellum is the part of the cerebral cortex which integrates sensory and other inputs from many regions of both the brain and spinal cord. It’s located at the back of the brain and sits directly above the pons and spinal cord in the brain, i.e. right below the occipital lobe and the temporal lobes of the cerebral cortex.

 

This so-called little brain is in charge of the timing, planning, execution, and accuracy of our movements, as well as of the control of balance and posture of the human body.

 

The list of functions supported by the cerebellum also includes the coordination, precision, and timing of voluntary movements, muscle tone, motor learning, participation in cognitive functions (control of speech), as well as control of emotions and emotional responses.

Fun Facts

Did you know?

 

1. The mesencephalon, or the midbrain, is the first part of the brainstem that can be seen when an intact adult brain is examined.
2. Cranial nerves are the nerves that carry signals to our eyes, nose, throat, ears, and the skin on the scalp.
3. The brainstem is the main ‘communicator’ of the brain – all of the brain parts send their information through this part of the brain in order to reach, control, and regulate the parts of our body. In this manner, the brain stem serves as a kind of ‘bridge’ between the head and the body.
4. Logically, we cannot survive without the brain stem, since it regulates the most vital and life-sustaining functions of the body. When a person’s brain stem becomes damaged, that person falls into a condition of prolonged sleepiness or unconsciousness called coma.
5. The word tegmentum originates from the Latin expression for ‘hill’.
6. The basis pedunculi or crusta is a collective name that refers to both the crus cerebri and substantia nigra.
7. Crus cerebri (plural: crura cerebri) is the anterior part of the cerebral peduncle which contains the nerve tracts and fibers.
8. The interpeduncular ganglion is a median collection of nerve cells located in the ventral part of the tegmentum.
9. The cerebral aqueduct is also known as the Sylvian Aqueduct, or The Aqueduct of Sylvius – named after the anatomist Franciscus Sylvius who first discovered it.
10. The cerebral aqueduct is 15 mm long. It’s also T-shaped – triangular above, and oval in the middle of the structure.