VAROLIUM BRIDGE (ANNULAR PONS): ANATOMY, FUNCTIONS - GEOGRAFÍA - 2025

The pons, brainstem bump ring or bridge, is a region of the brain that is located on the trunk of the brain, between the medulla oblongata and midbrain. Its main function is to connect these two structures of the brain.

The bridge of Varolio is characterized as the most prominent segment of the brain stem. Inside it contains a portion of the reticular formation, so it also performs important activities related to sleep and the alert process.

The corticobulbar tract. Source: Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist.

Anatomically, the Varolio bridge contains a series of central nuclei, which are divided into proper nuclei and nuclei associated with cranial nerves.

Characteristics of the Varolio bridge

Illustration of the Varolio bridge

The bridge of Varolio is a portion of the brain stem that is located between the medulla oblongata and the midbrain.

The brainstem is a brain region that is made up of the midbrain, the bridge of Varolio itself, and the medulla oblongata. These three structures constitute the major communication pathway between the forebrain and the spinal cord.

The main function of the Varolio bridge is to communicate the brain with the spinal cord, as well as the two other structures of the brainstem (the midbrain and the medulla oblongata).

In general, the Varolio bridge has three main characteristics. These are:

Point of origin of the constitutive fibers.

The Varolio bridge is the point of origin of most of the constituent fibers of the middle cerebellar peduncles. These fibers are characterized by containing white matter from the brain, which passes to the cortex of the lamellae.

Contains Purkinje fibers

On the other hand, the Varolio bridge stands out for containing Purkinje fibers of the cerebellum, which, after crossing the nucleus of the raphe, descend vertically through the brainstem bridge and become longitudinal in the reticular substance on the opposite side.

Action on the cerebellum

The two previous characteristics of the Varolio bridge show that this structure has a mechanism of action on the cerebellum.

Cerebellum (light blue)

The brain sends information to the body through the pyramidal pathway, which includes the Varolio bridge. When the nerve impulses reach this structure, some of them are diverted towards the cerebellum through the fibers of pontine origin.

Purkinje cells and their subordinate corpuscles seem to be involved in this process, which initiate a coordinating action of the motor drive, descending to the motor nuclei of the bulb and the spinal cord.

Anatomy

Varolio Bridge marked in red

Varolio's bridge is a structure that is located in the brain stem. Specifically, it is in the middle of the two other regions that are part of this region of the brain, the medulla oblongata and the midbrain.

Thus, the bridge of Varolio is the most prominent segment of the brainstem. Inside it contains a portion of the reticular formation, including some important nuclei in sleep regulation and the alert process.

Brain stem

The bridge of Varolio is separated from the medulla oblongata through the pons and the midbrain through the pontomesencephalic sulcus.

The anterior region of the bridge of Varolio is convex and contains a large number of transverse fibers that converge laterally to give rise to the middle cerebellar peduncles.

In the middle region, the bridge of Varolio presents the basilar sulcus, a shallow and wide cleft that houses the basilar artery, which originates at the junction of the right and left vertebral arteries and provides oxygenated blood to the brain.

Source: Sanador2.0 / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

In the outer region and on the contralateral side, the Varolio bridge gives rise to the trigeminal nerve of the central nervous system.

Trigeminal (V)

For its part, the posterior surface of the bridge of Varolio forms the upper part of the floor of the fourth ventricle and presents a triangular shape with an upper vertex that is hidden by the cerebellum.

The posterior region of the bridge of Varolio stands out for being limited outward by the superior cerebellar peduncles and divided by the middle sulcus or stalk of the calamus scriptorius.

Outside this stem, there is an elongated eminence that subdivides the floor into an internal white wing and, above, ends in a rounded elevation, the eminence teres.

Finally, further outward, the bridge presents a gray triangular depression that corresponds to the nuclei of origin of the glossopharyngeal nerve. Continuing outward is a second white area that forms the outer white wing.

Structure

The Varolio bridge is shaped like a thick roller similar to a transversely arranged bridge. In this bridge two main faces are distinguished, the anterior face and the posterior face.

The posterior aspect of the bridge of Varolio is hidden by the cerebellum and constitutes the upper part of the floor of the fourth cerebral ventricle. The anterior face for its part is visible and presents a vertical median depression called the «basilar groove».

On the other hand, the Varolio bridge is characterized by presenting the emergence of the trigeminal nerves in its lateral limits, which indicate the places where the bridge continues with the middle cerebellar peduncles, which extend towards the cerebellum.

Finally, three pairs of cranial nerves emerge from the inferior pontine groove: above the pyramids the abductor nerves and above the lateral phonicules the facial nerves and the cochlear vestibule.

Nuclei of the Varolio bridge

Varolio Bridge (red)

The Varolio Bridge has a series of central nuclei that perform different functions related to movement and the transmission of the senses.

These nuclei can be divided into two main categories: the proper nuclei (which are not associated with any cranial nerve) and the nuclei associated with cranial nerves.

Own nuclei

The nuclei of the bridge of Varolio are located in the ventral part of the brain structure.

They are important regions of the bridge of Varolio, since they are the structures to which the afferences of the cerebral cortex reach. Likewise, through these nuclei the efferences that are transmitted to the cerebellum start.

That is, these nuclei are responsible for receiving the information that the cerebral cortex emits towards the brain stem and transmitting these nerve impulses to the cerebellum.

Nuclei associated with cranial nerves

The nuclei associated with cranial nerves, as their name suggests, are characterized by being related to some cranial nerve of the brain.

In this sense, the Varolio bridge has much more nuclei than its own. The nuclei associated with the most important cranial nerves are:

• Trigeminal pontine nucleus: it is a nucleus that is associated with the fifth cranial nerve and that performs merely sensitive functions.
• Special trigeminal motor nucleus: this nucleus is associated with the fifth cranial nerve, it performs special visceral motor functions.
• Somatic motor nucleus of the abductor: it is a nucleus associated with the sixth cranial nerve of the brain.
• Special motor nucleus of the facial: it constitutes a nucleus associated with the seventh cranial nerve, resulting in a special visceral motor structure.
• Superior salivary nucleus: this nucleus is also associated with the seventh cranial nerve and performs vegetative or motor functions.
• Nucleus of the solitary tract: it is a sensitive nucleus that is associated with the seventh, the ninth and the tenth cranial nerves.
• Vestibular and cochlear nuclei: they constitute a group of six sensory nuclei (two cochlear and four vestibular) that are associated with the eighth cranial nerve.

Varolio bridge functions

The main functions of the Varolio Bridge consist of conducting information. In fact, this brain region acts as a bridge for the transmission of brain impulses.

The Varolio bridge can be defined as a conducting organ and a functional center:

Driving body

Sensory pathways that go from the spinal cord to the brain and vice versa circulate through the Varolio bridge.

It is a basic communication structure between the brain and the spinal cord. All fibers that are transmitted from one region to another must cross the Varolio Bridge to reach their destination.

Functional center

The Varolio Bridge also acts as a station center for the transmission of information.

Without the action of this brain structure, people would not be able to remain standing and would fall immediately, so the Varolio bridge plays important functions in the muscular activity of the body.

Likewise, the Varolio bridge is an association center that develops activities related to emotions and certain physiological phenomena such as respiratory or cardiovascular functioning.

References

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