It is the most significant part of the brainstem, located above the spinal cord and below the midbrain.
The bridge of Varolio develops from the embryonic metencephalon (part of the hindbrain, created from the rhombencephalon), together with the cerebellum.
Varolio’s bridge is a horseshoe-shaped collection of nerve fibers located in the anterior part of the posterior cranial fossa. Their anatomical relationships are as follows:
- Posteriorly: the cerebellum, separated by the fourth ventricle.
- Inferiorly: the medulla oblongata.
- Superior: the midbrain is located just above the Varolius bridge.
The anterior or ventral surface of the bridge of Varolio is marked by a bulge formed by the transverse cerebellar fibers. These fibers wrap around the vertically oriented brainstem. It measures about 2.5 cm in adults.
The basilar groove demarcates the midline of the ventral surface and is where the basilar artery is located.
The pontomedullary junction is an important anatomical landmark defined by the angle between the lower edge of the bridge of Varolio and the upper edge of the medulla. Several cranial nerves originate on the ventral surface of the bridge of Varolio:
- Cranial nerve V, trigeminal: derives from the lateral aspect of the middle of the bridge of Varolio.
- Cranial nerve VI, abducens: originates at the pontomedullary junction, near the midline.
- Cranial nerve VII, facial: derives from the cerebellopontine angle, the most lateral part of the pontomedullary junction.
- Cranial nerve VIII, vestibulocochlear: creates laterally to the facial nerve.
The bridge of Varolio is closely related to the cerebellum and is connected to it by the cerebellar peduncles in the middle. Removal of the cerebellum will reveal the underlying fourth ventricle.
The floor of the fourth ventricle is composed of the dorsal surface of the bridge of Varolio and the medulla. Here are some essential anatomical landmarks:
- The medial eminence marks the midline of the floor.
- The facial colliculus is a bulge formed by the facial nerve fibers that surround the nucleus abducens.
- The medullary stria of the fourth ventricle is a bundle of nerve fibers that cross transversely from the lateral aspect to the midline. They mark the posterior border between the bridge of Varolio and the medulla.
The angle formed at the junction of the Carolina bridge, the medulla, and the cerebellum is another anatomical landmark called the cerebellopontine angle.
The cerebellar floc, ventricular choroid plexus, and emerging CN VII and VIII surround the lateral openings of the fourth ventricle (the foramen of Luschka).
The Varolio bridge comprises two main components: the ventral Varolio bridge and the tegmentum. The ventral bridge of Varolio contains the pontine nuclei, which are responsible for coordinating movement.
The fibers of the pontine nuclei cross the midline and from the peduncles of the middle cerebellum on their way to the cerebellum.
The tegmentum is the evolutionarily oldest part of the Carolina bridge that is part of the reticular formation, a set of nuclei found throughout the brainstem and responsible for arousal and attention.
Damage to this part of the Varolio bridge can cause hemiplegia anosognosia, where patients are unaware of their paralysis.
The rest of the Varolio bridge is made up of sections that pass through the command square, including:
- Descending corticospinal tracts: responsible for voluntary motor control of the body.
- Descending corticobulbar tracts: responsible for voluntary motor control of the face, head, and neck.
- Ascending lots of medial lemnisci: responsible for a delicate touch, vibration, and proprioception.
- Ascending spinothalamic tracts: accountable for pain and temperature sensation
Cranial Nerve Nuclei
The Varolio bridge contains important cranial nerve nuclei. The primary sensory nucleus and the trigeminal motor nucleus are located in the middle, at the level where the fibers originate on the lateral face of the bridge of Varolio.
The primary sensory nucleus receives somatosensory information from the face. Two other cores receive sensory information from the trigeminal nerve:
- Trigeminal spinal nucleus: extends caudally into the spinal cord.
- Midbrain nucleus: extends rostrally to the midbrain.
The abducens nucleus controls the nerve, which supplies the ipsilateral lateral rectus muscle. It is located on the Caudal Bridge of Varolio, on the medial aspect of its dorsal surface.
The facial nucleus lies more anteriorly and laterally at the same level as the abducens nucleus. Control the muscles of facial expression. Its fibers take an unusual course and revolve around the nucleus abducens before exiting the brainstem through its ventrolateral surface.
The cochlear and vestibular nuclei sit dorsolaterally from the inferior Varolio’s bridge to the superior medulla.
The blood supply of the Varolio bridge is made up of branches of the vertebrobasilar system:
- Most of the bridge of Varolio is supplied by the pontine arteries, components of the basilar artery
- A minor part of your blood supply comes from the anterior inferior cerebellar artery and the superior cerebral artery (AICA and SCA).