The pons (TA: pons; plural: pontes ³), also less commonly known as pons Varolii, is the middle portion of the three contiguous parts of the brainstem, sitting above the medulla and below the midbrain. It acts as a relay between the cerebellum and cerebral hemispheres. 

Gross anatomy

The pons has a bulbous shape and has two main components - the basis pontis (basal/ventral part) and the pontine tegmentum (dorsal part).

The basis pontis consists of white matter tracts (e.g. anterior and lateral corticospinal, corticobulbar and corticopontine tracts) with transverse fibres contributing to the bulk of the pons.

The pontine tegmentum is continuous with the tegmentum of the medulla and the midbrain. It contains multiple white matter tracts (e.g. medial longitudinal fasciculus, medial lemniscus, lateral lemniscus, etc.) and grey matter nuclei (e.g. cranial nerve nuclei). Within the dorsal tegmentum lie four cranial nerve nuclei:

• trigeminal nerve (CN V): many motor, sensory and mesencephalic nuclei extending from the pons to the upper cervical cord

  • mesencephalic nucleus 

  • main sensory nucleus

  • spinal nucleus

• abducens nerve (CN VI): motor nucleus

• facial nerve (CN VII): including superior salivary, motor and solitary tract nuclei

• vestibulocochlear nerve (CN VIII): including vestibular and cochlear nuclei

Posteriorly, the pons is connected to the cerebellum by the middle cerebellar peduncle. Inferiorly the posterior surface of the pons is slightly depressed, forming the diamond-shaped rhomboid fossa which is the floor of the fourth ventricle.

Relations

• anteriorly: prepontine cistern (contains CN V & VI)

• laterally: cerebellopontine angle cistern (contains CN VII & VIII)

• posteriorly: fourth ventricle (the pons makes up the anterior wall)

• superiorly: continuous with the midbrain

• inferiorly: continuous with the medulla

Arterial supply

Primarily by vertebrobasilar circulation:

• medial branches of the superior cerebellar artery

• pontine branches of basilar artery, thalamoperforator arteries

Venous drainage

The various pontine veins drain into the inferior petrosal sinuses and the basilar venous plexus ⁴.

History and etymology

Historically this structure was commonly called the pons Varolii, as the structure was first described by Costanzo Varolius (1543-1575), an Italian anatomist ².

Pons is the Latin word for bridge ³.

Related pathology

Related conditions include:

• pontine infarct

  • Brissaud-Sicard syndrome

  • facial colliculus syndrome

  • Gasperini syndrome: basilar artery or AICA

  • inferior medial pontine syndrome (Foville syndrome): basilar artery 

  • lateral pontine syndrome (Marie-Foix syndrome): basilar artery or AICA

  • locked-in syndrome: basilar artery

  • Millard-Gubler syndrome: basilar artery

  • Raymond syndrome: basilar artery

• pontine haemorrhage

• rhombencephalitis and causes thereof

Related radiological signs include:

• double panda sign

• hot cross bun sign

• split pons sign

• thumb sign (chordoma)

• trident sign (osmotic demyelination)