BRAINSTEM: FUNCTIONS, PARTS, AND ANATOMY - NEUROPSYCHOLOGY - 2025

The brainstem or brainstem is a cylindrical region of the brain. It is the major route of communication between the brain and the spinal cord, and is made up of the midbrain, pons, and medulla oblongata.

This brain structure is responsible for controlling multiple basic processes such as breathing, heart rate, the cardiovascular system or the perception of sounds. Inside it is made up mainly of gray matter and white matter, and it is the part of the brain that is in a lower position and closer to the cervicals.

Brainstem (orange)

The brain stem, as its name suggests, takes the shape of a trunk and is located in the lower area of ​​the brain.

Above it are regions such as the diencephalon (formed by the epithalamus, the thalamus, the subthalamus, and the hypothalamus), the telencephalon, and the forebrain. Below it is the spinal cord, and laterally the cerebellum is located.

Brain stem. It can be seen next to the cerebellum, which has a certain cauliflower appearance

Thus, the brain stem has an anatomical location that indicates the connection between the brain and the spinal cord. All nerve impulses sent by the latter must pass through the brainstem regions to reach the structures of the brain.

Characteristics of the throne of the brain

Brainstem marked in red

The brainstem, also known as the brainstem, brainstem, or brainstem is one of the main structures of the brain. It is the most subcortical region of the brain, which is why it is the structure that directly connects with the spinal cord and peripheral nerves.

The different regions that make up the brainstem have similar characteristics: they are made up of white matter on the outside and have islets of gray matter scattered over their entire surface.

In the white matter there are thousands of nerve fibers that travel through different regions of the trunk. Likewise, these fibers have the special characteristic of being transported both to other brain regions and to the spinal cord.

With regard to gray matter, it is characterized by being distributed through masses of different sizes in different regions of the brain stem. Of all of them, the best known and most important is the red nucleus of the midbrain.

Likewise, despite the fact that the brain stem is characterized by well-defined areas of gray and white matter. It also contains a mixture of both substances, which is called a reticular formation.

Parts of the brainstem: anatomy

Encephalon Source: Xtabay / CC BY (https://creativecommons.org/licenses/by/3.0)

The brainstem is anatomically composed of three main regions: the midbrain, the pons, and the medulla oblongata.

- Midbrain

Midbrain marked red

The midbrain, also known as the midbrain, is the superior structure of the brain and is responsible for joining the bridge of Varolio and the cerebellum with the diencephalon.

The boundary of this region with the varolian bridge is well defined through a pontomencephalic sulcus. Likewise, the upper limit of the midbrain is delimited by the optic strips.

In the middle of the midbrain is the Silvio aqueduct, which crosses the brain stem region. The cerebrospinal fluid, a vital substance for brain function, circulates through this aqueduct.

Within the midbrain is the tectum (roof), a structure that is located in the dorsal portion of the midbrain and limits an upper part, and the tegmentum (integument), a portion located at its base that defines its lower part.

On the other hand, inside the midbrain there are several nuclei that control eye movements. These are: the periaqueductal gray substance, the red nucleus and the black substance.

The main functions of this structure is to conduct motor impulses from the cerebral cortex to the bridge of Varolio. As well as transporting sensory stimuli from the spinal cord to the thalamus.

Specifically, the quadrigeminal tubercles in the upper areas coordinate the movements of the eyeballs through perceived visual stimuli. The quadrigeminal tubercles of the lower areas coordinate the movements of the head and trunk through the perception of auditory stimuli.

In other words, the midbrain carries out information transmission functions in a bidirectional way: from the brain to the bridge of Varolio and from the medulla to the brain. Anatomically, the midbrain consists of four different faces: the anterior, the lateral and the posterior.

Previous face

In this region there is a deep depression known as the interpeduncular fossa. It borders on each side through the cerebral peduncle and constitutes a triangular space.

It is formed by the optic chiasm and the optic strips. The tuber cinerum is also found in the anterior part of the fossa, and in the posterior region it contains an area perforated by small blood vessels.

The origin of the common ocular motor nerve, which corresponds to the third cranial nerve, is also found on the anterior face of the midbrain.

Side faces

The midbrain contains two lateral faces, one on the left and one on the right. Through these areas the upper and lower conjunctival arms ascend.

The superior conjunctival arm is responsible for connecting the anterior quadrigeminal tubercle with the lateral geniculate body and the optic band. For its part, the lower conjunctival arm connects the lower quadrigeminal tubercles with the medial geniculate body.

Upper side

In the posterior region of the midbrain are the quadrigeminal tubercles, rounded eminences that are divided into anterior and posterior pairs. They divide through the cruciform groove and originate from the pathetic nerve.

- Annular protuberance

Varolio Bridge (red)

The annular pons, also known as the brainstem bridge or Varolius bridge, constitutes the portion of the brainstem that is located between the midbrain and the medulla oblongata.

In this way, it forms the medial part of the brainstem and its main function is to connect its other two regions: the midbrain with the medulla oblongata.

The pons is the most prominent segment of the brain stem. In its nucleus it contains the reticular formation (mass formed by white matter and gray matter) and includes important nuclei for sleep regulation and alert processes

It is separated inferiorly from the medulla oblongata by the bulboprotuberancial groove, and superiorly from the midbrain by the pontomesencephalic groove.

The anterior face of the pons is characterized by having multiple transverse fibers that converge to the sides and are responsible for forming the cerebellar peduncles.

In the medial area of ​​the pons there is a shallow groove that contains the basilar artery. Finally, on the anterolateral aspect, the trigeminal nerve is observed.

The upper surface of the pons is part of the floor of the fourth ventricle. It is bordered outwards by the cerebellar peduncles and has an elongated eminence.

- Medulla oblongata

The brainstem or brainstem is the lowest region of the brain stem. Above it is the annular bulge and below it is the spinal cord.

In this way, the spinal canister constitutes the last region (below) of the brain. This structure is characterized by being in direct contact with the spinal cord, which is why it is a key structure for communicating the brain with the body.

It has a truncated cone shape and its functions include the transmission of multiple nerve impulses. Specifically, the brainstem controls functions such as the regulation of digestive juices, coughing, vomiting, sneezing, swallowing, blood pressure, or breathing.

In fact, injury to the medulla oblongata causes immediate death in the person from cardiac and / or respiratory arrest.

The medulla oblongata borders the spinal cord through the decussation of the pyramids and the annular pons through the bulbo-protuberant groove. While the delimitation with the latter is clear and observable, the boundary with the spinal cord is confusing and not very visible.

Regarding its structure, it consists of three main faces: the anterior face, the lateral face and the posterior face.

Previous face

In this area, the medulla oblongata contains a longitudinal groove that connects directly with the medial anterior groove of the spinal cord and ends in an area known as the caecum or foramen cecum.

On the sides of this groove there are two reliefs: the pyramids. These regions constitute the pyramidal pathway that allows the exchange of nerve fibers between the brain and the spinal cord.

Side face

This face of the medulla oblongata begins from the hypoglossal nerve. It contains the bulbar olive and the glossopharyngeal nerve belonging to the ninth cranial nerve, the vagus nerve referring to the tenth cranial nerve and the accessory nerve of the eleventh cranial pair.

Upper side

The posterior face has a groove known as the posterior medial groove, which is divided by another small Goll's cord groove.

Brainstem Nuclei

Brainstem marked in red

Apart from the three main regions that make up the brain stem, it can be subdivided into three different nuclei: somatomotor nuclei, somatosensory nuclei, and visceromoteric nuclei.

Somatomotor nuclei

The somatomotor nuclei are responsible for collecting somatic information from the spinal cord and initiating processes related to movement. Specifically, these nuclei have a dorsal column and a ventral column.

1. The dorsal spine is located in the midline and is responsible for the motor intervention of the extrinsic motor muscles of the eye. It includes the oculomotor nerve, the pathetic nerve, the external ocular nerve, and the greater hypoglossal nerve.
2. The ventral column is located on the outside and towards the front. It contains the trigeminal motor that performs the functions of chewing; the facial nerve that controls the muscles of the face; the ambiguous nerve that constitutes the origin of the somatomotor fibers and the spinal nerve.

Somatosensory nuclei

These nuclei perform the important function of collecting sensitive information from the body in order to regulate the corresponding mental processes.

The dorsal column of the somatosensory nuclei has the statoacoustic nuclei that are related to body balance and the integration of the perception of auditory stimuli.

Likewise, it contains the nucleus of the solitary fascicle that performs perceptual activities of the sense of taste, and in the ventral column it presents the trigeminal nucleus, which extends throughout the entire brainstem.

Visceromotor nuclei

These latter nuclei form a last column in the brain stem and are characterized by carrying out different activities related to the functioning of the eye. Visceromotor nuclei include:

1. The nuclei of intrinsic motility of the eye, which are made up of the median nucleus of Perlia, which are responsible for converging the gaze, and the nuclei of Edinger, which allow accommodation and pupillary contraction.
2. The lacrimal-muconasal nucleus is made up of fibers that join those of the seventh cranial nerve and regulate tear secretion and the mucosa of the nasal passages.
3. The salivary nuclei that intervene in the submaxillary and sublingual gland (superior nucleus) and innervate the parotid gland (inferior nucleus).
4. The cardioneumonteric nucleus that has the origin of the cardiac, respiratory and digestive fibers.
5. The vescerosensisitic nuclei that correspond to the gray wing of the fourth ventricle.

Pathways of association of the brainstem

Brainstem marked in red

Finally, the connection between the fibers of the nuclei of the brain stem, allow to define the existence of different association pathways. Mainly, there are two types of pathways that determine the functioning of this brain area. These are:

Rear longitudinal girdle

This first route includes the fibers that go from the end of the aqueduct of Silvio to the cervical medulla. It is especially related to the control of the reflexes of the movements of the head and the eye.

Central fascicle of the shell

This second pathway is part of the extrapyramidal pathways. It is located just anterior to the medial longitudinal band and contains descending fibers that connect the nuclei of the midbrain with the brainstem and spinal cord regions.

References

1. Bear, MF, Connors, B. i Paradiso, M. (2008) Neuroscience: exploring the brain (3rd edition) Barcelona: Wolters Kluwer.
2. Carlson, NR (2014) Physiology of behavior (11 edition) Madrid: Pearson Education.
3. Morgado Bernal, I. (Coordinator) (2005) Psychobiology: from genes to cognition and behavior. Barcelona: Ariel.
4. Kalat, JW (2004) Biological Psychology. Madrid: Thomson Paraninfo.
5. Rosenzweig, MR; Breedlove, SM; Watson, NV (2005) Psychobiology. An introduction to Behavioral, Cognitive and Clinical Neuroscience. Barcelona: Ariel (Ch. 2).