Posterior commissure
Commissura posterior
A rounded band of crossing white fibres at the back of the third ventricle, just above the entrance to the cerebral aqueduct.
At a glance
- English
- Posterior commissure
- Latin (TA2)
- Commissura posterior
- System
- Brain and spinal cord
- Region
- Head
- Side
- Single, on the midline
About the posterior commissure
The posterior commissure, also called the epithalamic commissure, is a short bundle of fibres that crosses the midline at the junction of the diencephalon and the midbrain. It lies in the back wall of the third ventricle, below the stalk of the pineal gland and directly above the opening of the cerebral aqueduct. Above it, in the upper lamina of the pineal stalk, runs the habenular commissure; between the two the small pineal recess of the third ventricle pushes back into the stalk.
Its fibres are among the first in the brain to gain their myelin, but their connections are still not fully known. Many arise in a small cell group named after the commissure, which lies in the grey matter around the top of the aqueduct, and in the neighbouring interstitial nucleus and nucleus of Darkschewitsch. Others come from the pretectal nuclei, the posterior thalamus and the superior colliculus, and some pass down to join the medial longitudinal fasciculus. Beneath the commissure the ependyma forms a specialised patch, the subcommissural organ.
The best known traffic through it belongs to the pupillary light reflex. Light on one retina reaches the olivary pretectal nucleus, which sends fibres to the Edinger-Westphal nuclei on both sides; those going to the opposite side cross in the posterior commissure. As a result both pupils constrict when only one eye is lit, the consensual response. Other crossing fibres help coordinate vertical eye movements and the position of the eyelids.
Lesions here, typically from tumours of the pineal region pressing forward, produce Parinaud (dorsal midbrain) syndrome: loss of upward gaze, pupils that react poorly to light but better to near focus, retraction of the eyelids and convergence-retraction jerks. The line joining the anterior and posterior commissures is the standard reference plane for stereotactic brain mapping.
Function and supply
- Function
- Carries crossing fibres for the consensual pupillary light reflex and for vertical gaze, linking the pretectal and periaqueductal nuclei of the two sides.
- Blood supply
- Small branches of the posterior cerebral artery, including posterior medial choroidal and paramedian midbrain branches.
Anthromica
See the posterior commissure where it really sits, with every layer of the body around it.
- Every layer, one body. Peel from muscle to bone, nerve, vessel and organ, and turn it any way.
- English and Latin. Every structure named the way your exam names it.
- Quiz until it sticks. Short drills on the structures you keep missing.
Free to start. No card.
