Where it is
- It crosses the midline on the dorsal (back) side of the front (rostral) end of the cerebral aqueduct, and is counted as part of the epithalamus. [1]
- It sits on the back border of the third ventricle, just below (caudal to) the pineal gland, near the corner where the third ventricle narrows into the cerebral aqueduct. [2]
What it does
- It is important in the pupillary light reflex on both sides: the olivary pretectal nucleus sends fibres to the Edinger–Westphal nuclei of both sides, and the fibres reaching the other side cross in the posterior commissure. [1]
- That is why light in one eye also narrows the other pupil (the consensual light reflex); the Edinger–Westphal nucleus sends its signal out along the oculomotor nerve. [3]
- Honest limit: its fibres get their myelin sheaths early, but their connections have not been definitively worked out. [1]
Connections
- Some of its fibres are thought to come from the back of the thalamus and from the superior colliculus; others are thought to continue downward into the medial longitudinal fasciculus. [1]
Inside it
- Its own nucleus, the nucleus of the posterior commissure (nucleus of Darkschewitsch), lies in the periaqueductal grey at the front end of the cerebral aqueduct, in front of the oculomotor nucleus. [1]
Other names
- Also called the epithalamic commissure; Latin commissura posterior. [1]
Size
- On this model the posterior commissure measures about 5 mm front to back, 3 mm top to bottom and 15 mm side to side (the box it fits in, not its volume). [4]
- Centre of the posterior commissure compared with the centre of the pineal gland: 10 mm further forward; 10 mm apart in a straight line. [4]
- Centre of the posterior commissure compared with the centre of the anterior commissure: 29 mm further back, 8 mm lower; 30 mm apart in a straight line. [4]