Posterior Commissure

The 'posterior commissure' is a small but important bundle of white matter fibers located in the dorsal part of the upper midbrain, just above the cerebral aqueduct and posterior to the third ventricle. It serves primarily to connect the left and right pretectal nuclei, enabling bilateral coordination of the pupillary light reflex.

• Located at the junction of the midbrain and diencephalon
• Lies below the pineal gland and anterior to the superior colliculi
• Runs transversely across the dorsal aspect of the upper brainstem
• One of the key fiber tracts crossing the midline in this region, alongside the habenular commissure

• Mediates the consensual pupillary light reflex, allowing light in one eye to cause constriction in both pupils
• Transmits signals from the pretectal nuclei to both Edinger-Westphal nuclei
• Involved in reflexive eye movements and vertical gaze control

• Lesions affecting the posterior commissure can result in:
  • Parinaud syndrome (dorsal midbrain syndrome)
  • Vertical gaze palsy
  • Loss of pupillary light reflex with preserved accommodation (light-near dissociation)
• Commonly affected in pineal region tumors or tectal plate gliomas

• Pretectal area
• Edinger-Westphal nucleus
• Pineal gland
• Vertical gaze center

Nieuwenhuys R, Voogd J, van Huijzen C. The Human Central Nervous System. Springer, 2007.