Annular Bump: What is it? Embryonic Function, Structure and Development

It is also called the Varolio Bridge, in honor of the Italian anatomist and surgeon Costanzo Varolio (1543-1575).

This region of the brainstem includes neural pathways and tracts that carry signals from the brain to the cerebellum and spinal cord, and tracts that carry sensory signals to the thalamus .

It is an anatomical region of the brain and is part of the nervous system within the skull.

The annulus is located in front of the cerebellum (transversely), above the brainstem and on the cerebral peduncles (lower region of the brain).

The bridge is characterized by a lump or increase in volume, like a whitish type of lump that follows from the spinal cord to the brain .

Together with the respiratory centers of the medulla oblongata, the bridge essentially contributes to the maintenance of normal respiration.

Function of the annular bulge

The pons contains nuclei that transmit signals from the forebrain to the cerebellum, along with nuclei that are primarily concerned with sleep, breathing, swallowing, bladder control, hearing, balance, taste, eye movement, facial expressions , facial sensation and posture.

Within the pons is the pneumotoxic center consisting of the subparabrachial and medial parabrachial nuclei. This center regulates the change from inhalation to exhalation.

The bulge is implicated in sleep paralysis and can also play a role in generating dreams.

Structure of the annular bulge

The bump in humans is about 2.5 centimeters long. Most appear as a broad anterior bulb rostral to the medulla.

Subsequently, it is mainly made up of two pairs of thick stalks called cerebellar peduncles. These connect the midbrain with the pons and cerebellum.

The bridge of Varolio or annular pons, is the middle part of the brainstem, which is located between the medulla oblongata (or the elongated medulla) and the midbrain.

The midbrain is the part of the adult brain that corresponds to the mid-region, including the quadrijumale tubercles and cerebral peduncles.

The medulla oblongata corresponds to a bulge in the upper part of the spinal cord where several important nerve centers are located, such as the respiratory centers (which allow the automatic functioning of the lungs).

Several cranial nerve nuclei are present in the pons:

  • The “main” or “pontine” nucleus of the sensory nucleus of the trigeminal nerve (V).
  • The motor nucleus of the trigeminal nerve (V).
  • The nucleus abducens (VI).
  • The nucleus of the facial nerve (VII).
  • The vestibular nuclei and cochlear nuclei (VIII).

Several pairs of cranial nerves exit the nuclei that are in the bridge.

These are essentially trigeminal nerves (V), abdominal nerves (VI) and facial nerves (VII).

Another important nucleus that belongs to the bridge: the pneumotaxic center that corresponds to the center of automatic respiration and that belongs to the reticular formation.

The reticular system integrates the sensations that reach the brain. It acts as a link between the responses of the nervous system of the will and those of the autonomic nervous system. The crosslinking also allows the coordination and synthesis of actions in general.

It controls the activities of the brain and spinal cord through the intervention of the ascending system and the descending inhibitory and facilitating system.

This anatomical zone also allows the creation and regulation of the tone of the posture, the organization of the muscular tension of an individual to allow him to stand upright. Finally, it intervenes in the waking state.

The pons is located in the brainstem, located between the midbrain and the medulla oblongata, and in front of the cerebellum.

The pons can be broadly divided into two parts: the basilar part of the pons (ventral pons) and the pontine tegmentum (dorsal pons).

The bridge is formed, in addition to pontic gray nuclei and ascending and descending pathways, mainly transverse fibers called cerebellar ponto fibers.

The bridge essentially consists of those projecting nerve fibers that are arranged longitudinally and transversely.

Longitudinal neurofibers, which are deep, provide communication between the higher brain centers located in the brain and the spinal cord.

Pontocerebellar fibers are the second-order neuronal fibers of the cortico-ponto-cerebellar tracts that cross to the other side of the pons and extend within the medial cerebellar peduncles, from the pons to the contralateral cerebellum.


During embryonic development, the metancephalon develops from the rhombencephalon and gives rise to two structures: the pons and the cerebellum.

The alar plate produces sensory neuroblasts, which will give rise to the solitary nucleus and its special visceral afferent column, the cochlear and vestibular nuclei, which form the special somatic afferent fibers of the vestibulocochlear nerve, the spinal nuclei and the main trigeminal nerve, which form the afferent column. General somatic of the trigeminal nerve and the pontine nuclei that relay to the cerebellum.

Basal plate neuroblasts give rise to the abducens nucleus, which forms the general somatic efferent fibers, the trigeminal facial and motor nuclei, which form the special visceral efferent column, and the superior salivary nucleus, which forms the general visceral efferent fibers of the nerve. facial.