Pons

At a glance

English

Pons

Latin (TA2)

Pons

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the pons

The pons, whose name is Latin for bridge, is the part of the brainstem between the midbrain above and the medulla oblongata below. It lies in front of the cerebellum, separated from it by the fourth ventricle, and rests on the clivus of the skull base. It is also known as the pons Varolii after the sixteenth-century Italian anatomist Costanzo Varolio. In adults it is about two and a half centimetres long. The superior pontine sulcus marks its junction with the midbrain and the inferior pontine sulcus its junction with the medulla, where the abducens, facial and vestibulocochlear nerves emerge. The trigeminal nerve leaves its lateral side.

In section it has two main parts. The front, or basilar part, forms the large ventral bulge. Its shallow midline basilar sulcus holds the basilar artery. This part contains descending corticospinal and corticobulbar fibres and the scattered pontine nuclei, which receive input from the cerebral cortex and send their axons across the midline into the cerebellum through the thick middle cerebellar peduncles. That crossing traffic gives the pons its bridge-like transverse ridges. The back part, the pontine tegmentum, continues the reticular formation and the long ascending sensory pathways, and forms the upper floor of the fourth ventricle. Behind, the superior cerebellar peduncles connect the cerebellum to the midbrain.

The tegmentum houses several cranial nerve nuclei: the principal sensory and motor nuclei of the trigeminal nerve in the mid-pons, and lower down the abducens nucleus, the facial motor nucleus, the superior salivatory nucleus and parts of the vestibular and cochlear nuclei. These serve facial sensation, chewing, eye movement, facial expression, the secretion of tears and saliva, hearing and balance. Pontine respiratory groups, including the parabrachial region once called the pneumotaxic centre, help switch breathing from inspiration to expiration, and other networks take part in sleep, the paralysis of dreaming sleep, bladder control and posture. In development the pons and the cerebellum both arise from the metencephalon.

Clinically, a large pontine stroke or haemorrhage can leave a patient conscious but able to move only the eyes up and down, the locked-in syndrome. Central pontine myelinolysis, linked with too rapid correction of a low blood sodium, damages the basilar part and disturbs balance, walking, swallowing and speech.

Function and supply

Function

Relays signals from the cerebral cortex to the cerebellum, carries the long motor and sensory tracts, contains the nuclei of the fifth to eighth cranial nerves, and helps regulate breathing, sleep and bladder control.

Blood supply

Pontine branches of the basilar artery, with contributions from the anterior inferior and superior cerebellar arteries.