MIDBRAIN: CHARACTERISTICS, FUNCTIONS AND PARTS - GEOGRAFÍA - 2024

The midbrain or midbrain is the upper structure of the brain stem. It is a brain region that is responsible for joining the brainstem bridge and the cerebellum with the diencephalon. It is located anteriorly and superiorly to the pons, confining itself correctly with this structure through an interpeduncular groove.

Thus, the midbrain is the most encephalic region of the brainstem. In the upper region of the midbrain the diencephalon is located, from which it is separated by the posterior commissure. Likewise, the midbrain is crossed by the midbrain aqueduct.

Illustration showing the midbrain and other brain areas

The main function of this region of the brain is to conduct motor impulses both from the top down and from the bottom up. That is, from the cerebral cortex to the brainstem bridge and from the spinal cord to the thalamus.

Characteristics of the midbrain

Main subdivisions of the brain of a vertebrate embryo. I, Nrets / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

The midbrain is a region of the brain that is also known as the midbrain, mainly because it is located in the middle of the brain.

Its structure is made up of the brain stem, which connects all the spaces of the brain (the brainstem bridge, the cerebellum and the different regions of the diencephalon).

This region is also characterized by having a conduit through which the cerebrospinal fluid travels. This brain substance, among other functions, is responsible for proportional mechanical stability to the body.

The specific function of the midbrain appears to be the conduction and control of motor impulses that are transmitted from the cerebral cortex (region superior to the midbrain) to the junction of the spine and the brainstem bridge (lower region of the midbrain).

Likewise, the midbrain is responsible for transmitting the sensory impulses that travel from the spinal cord to higher brain regions.

A specific region of the midbrain, the superior quadrigeminal tubercles, is responsible for carrying out the movements made by the ocular globules in response to perceived sensory stimuli. For this reason, the midbrain is characterized by containing the oculomotor nerve.

Location

The midbrain sits just above the brainstem and joins this structure with the cerebellum and diencephalon.

The longitudinal axis of the midbrain slopes posteriorly and the flagella move away from the midline of their ascent through the foramen of Pacchioni. Through this location, the midbrain penetrates into the cerebral hemispheres.

Anatomically, the midbrain is an easily identifiable brain structure. It borders the brainstem bridge through the pontomesonecephalic sulcus. Likewise, the upper limit of the midbrain is well defined through the optic bands.

On the other hand, one of the most important anatomical properties of the midbrain is that it is traversed by a narrow conduit known as the Silivian aqueduct.

The cerebrospinal fluid (a colorless brain fluid that bathes the brain and spinal cord) circulates through this aqueduct from the third ventricle to the fourth ventricle.

In general, the midbrain is made up of two main structures: the tectum (roof) and the tegmentum (shelter). The tectum is located in the dorsal portion of the midbrain, while the tegmentum is located just below the tectum of the midbrain.

The midbrain is also characterized by presenting different nuclei within it, which are mainly responsible for controlling eye movements. The most important appear to be the periaqueductal gray matter, the red nucleus, and the substantia nigra.

Faces of the midbrain

The three divisions of the brainstem, anterior-inferior view. 1. Brain, 2. Midbrain, 3. Varolian Bridge, 4. Medulla oblonga, 5. Cerebellum. Source: John A Beal, PhD

Dep't. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport.

Different faces can be distinguished in the midbrain. Each one of them has different anatomical characteristics.

In general, these faces are divided into external vision and internal vision. Within each of the visions different structures are located.

External vision

The external view of the midbrain is divided into two main structures: the anterior and the posterior.

Regarding the anterior aspect of the external vision of the midbrain, cerebral peduncles are located on both sides of the cerebral region. Between each of the peduncles the interpeduncular fossa is located.

The interpeduncular fossa is formed by a region of gray matter, which is perforated by small blood vessels.

Finally, between each cerebral peduncle and the interpeduncular fossa, the midbrain contains the groove of the common ocular motor nerve, the region from which the fibers of the third cranial nerve depart.

Regarding the posterior aspect of external vision, the midbrain presents the four colliculi. The upper colliculi define the rostral limit of the midbrain while the lower ones establish the caudal limit.

The fourth cranial nerve is also found in this region of the midbrain, which (unlike the rest of the cranial nerves of the midbrain) arises through the posterior region of the brainstem and runs anteriorly around the brainstem.

Human brain stem and thalamus - posterior view: 1. Taenia choroidea (and lateral: Lamina affixa, Stria terminalis), 2. Thalamus, Pulvinar thalami, 3. (Ventriculus tertius), 4. Stem of Glandula pinealis, 5. Habenula, 6 Stria medullaris, 7. Superior colliculus, 8. Brachium colliculi superioris, 9. Inferior colliculus, 10. Brachium, colliculi inferioris, 11. Corpus geniculatum mediale, 12. Sulcus medianus, 13. Superior pedunculus cerebellaris, 14. Inferior pedunculus cerebellaris, 15. Pedunculus cerebellaris medius, 16. Tuberculum anterius thalami, 17. Obex, area postema. A: thalamus, B: midbrain, C: pons, D: medulla oblongata. Source: John A Beal, PhD Dep't. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport

Internal vision

The internal division of the midbrain is somewhat more complex than the external one. However, it can generally be divided into three major regions: the tectum (roof), the foot and the tegmentum.

More specifically, a large number of different structures can be distinguished on the inner side of the midbrain. The most important ones are:

- Quadrigeminal plate: the quadrigeminal plate or tectum is a structure that is located in the dorsal region of the midbrain. This brain area contains the cudrigeminal tubercles, four rounded structures (two lower and two upper) that are called collicles.

-Cerebral peduncles: they are located in the ventral region of the midbrain. They mainly contain corticopontine, cotrospinal and temporopontine fibers.

-Black substance: this substance is responsible for separating the peduncular region into two parts: an antero-inferior region (foot) and a posterior-superior region (tegmentum).

-Central mesencephalic tegmentum: this region is formed by the mesencephalic aqueduct and the periaqueductal gray matter. The central part of the mesencephalic aqueduct is located in the midbrain, while the ends are located in other brain regions.

Features

Midbrain

The midbrain is a brain structure that acts primarily as a communicator. That is, it is responsible for relating and communicating different structures and regions of the brain.

The main function of the midbrain is to transmit motor impulses from the cerebral cortex to the brainstem bridge. Likewise, it is responsible for conducting sensory impulses from the spinal cord to the thalamus.

The tubers of the midbrain are responsible for coordinating movements of different regions of the body. Specifically, the superior quadrigeminal tubercles coordinate the movements of the eyeballs in response to visual and other stimuli.

On the other hand, the lower quadrigeminal tubercles are responsible for coordinating the movements of the head and the rest of the body in response to auditory stimuli.

Finally, the midbrain is also related to some aspects of vision, hearing, sleep and wakefulness.

The midbrain perceives and transmits stimuli from different sensory modalities and is responsible for regulating important bodily functions such as body temperature, the sleep cycle, and homeostatic processes.

Anatomical parts

This brain region is characterized by housing a large number of different structures / parts.

Each of the mesoencephalic structures is located in a different area of ​​the midbrain. Likewise, each of these regions has different anatomical properties and functions.

The midbrain has 17 different structures inside. These are:

Superior colliculus

The superior colliculus, also called the pretectum, constitutes an upper conjunctival arm. It is in communication with the lateral geniculate nucleus and develops visual functions.

The superior colliculi receive afferents from the optic tract, acting as a relay station between it and the lateral geniculate nucleus.

Lower colliculus

The inferior colliculi encompass the central, pericentral and external nuclei of the midbrain. They constitute the inferior conjunctival arm and communicate the inferior colliculus with the medial geniculate nucleus.

Functionally, they are characterized by transmitting auditory information. They receive afferents from the coclar nuclei, acting as a relay station between these structures and the medial geniculate nucleus.

Periaqueductal gray matter

The periaqueductal gray matter is located around the mesencephalic aqueduct. It receives input from the hypothalamic nuclei and its main function is to modulate pain sensations.

Midbrain aqueduct

The mesencephalic aqueduct is a structure that is located in the center of the midbrain. The function of this region is to communicate the third ventricle with the fourth cerebral ventricle.

Nucleus of the third cranial nerve

The nucleus of the third cranial nerve is located ventrally to the periaqueductal gray matter. The fibers of the third cranial nerve bundle cross the medial longitudinal bundle and the decusive fibers of the superior cerebellar peduncle.

This nucleus is responsible for innervating the rectus muscles and innervating the pupillary parasympathetic activity.

Nucleus of the fourth cranial nerve

The nucleus of the fourth cranial nerve is located in the ventral region of the periaqueductal gray matter. The fourth cranial nerve runs posterior and inferior to the aqueduct and decuses in the posterior part of the midbrain.

This nucleus of the midbrain is characterized by innervating the superior oblique muscle.

Midbrain nucleus

The mesencephalic nucleus of the fifth cranial nerve is located lateral to the mesencephalic aqueduct. It is a sensitive region that receives proprioceptive information from the chewing muscles.

Pretectal nucleus

The pretectal nucleus is located just above the superior colliculus, at the level of the posterior commissure. It is a structure that is involved in the pupillary reflex.

Medial longitudinal fasciculus

The longitudinal bundle is located anterior and lateral to the nucleus of the third cranial nerve. It is characterized by transmitting the fibers to the oculomotor nuclei and the medullary and medullary motor nuclei.

Likewise, this booklet plays an important role in body movement. It is involved in the reflex control of head and eye movements.

Black substance

The midbrain also contains a black substance inside. This is located in the dorsal region of the cerebral peduncles and contains two main parts: the pars compacta and the reticulated pars.

The substantia nigra of the midbrain is part of the basal ganglia and plays an important role in the development of motor activities.

Red core

The red nucleus of the midbrain is located dorsal to the substantia nigra. Inside it contains a rubrospinal tract that crosses the nucleus through its axons.

The red nucleus acts as a relay center for cerebellar and striated reflex pathways. It is an essential part of the cortical extrapyramidal pathways and is responsible for controlling muscle tone.

Reticular formation

The reticular formation is located between the periaqueductal gray matter, the lateral lemniscus, and the spinothalamic tract. It constitutes the locomotor center, the cardiorespiratory center, the vomiting center and the regulation of circadian rhythms.

Medial longitudinal fasciculus

The medial longitudinal fasciculus is a structure of the midbrain that is situated anterior and lateral to the nucleus of the third cranial nerve.

This midbrain region is responsible for transmitting fibers to the oculomotor nuclei and medullary and medullary motor nuclei. Likewise, it is a structure involved in the reflex control of the movements of the head and eyes.

Lateral spinothalamic tract.

The lateral spinothalamic tract is located lateral to the reticular formation. It is an especially important route in the transmission of pain and the regulation of body temperature.

Medial lemniscus

The medial lemniscus is a small structure located lateral to the reticular formation. It is a path of deep conscious sensitivity and epicritic sensitivity.

Cerebellar peduncles

The cerebellar peduncles form a decussation of the midbrain that is located in the central portion of the tegmentum, anterior to the mesencephalic aqueduct.

The function of this brain structure is to connect the midbrain with the cerebellum.

Brain peduncles

The cerebral peduncles are located ventrally in the midbrain. They contain three bundles (the corticobulbar, the corticospinal, and the corticopontine) and make up a motor pathway.

The corticospinal tract presents somatotopic organization for the musculature of the head and extremities.

References

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