Formed by the junction of the 2 vertebral arteries.
V4 vertebral artery segment pierces the dura (location somewhat variable) and immediately enters the subarachnoid space. Joins the contralateral VA at the vertebral confluens located at the lower pontine border to form the basilar artery (BA).
The two vertebral arteries and the basilar artery are sometimes together called the vertebrobasilar system, which supplies blood to the posterior part of circle of Willis and anastomoses with blood supplied to the anterior part of the circle of Willis from the internal carotid arteries.
It ascends in the central gutter (sulcus basilaris) inferior to the pons and divides into the posterior cerebral arteries and the superior cerebellar artery just inferior to the pituitary stalk.
The basilar arterial bifurcation is the most common site for aneurysms arising from the posterior circulation.
From the basilar artery arises the anterior inferior cerebellar artery (supplying the superior and inferior aspects of the cerebellum), as well as smaller branches for the supply of the pons (the pontine branches).
In under 15% of people the basilar artery gives rise to the labyrinthine artery.
Proximally, the basilar artery joins the two internal carotid arteries and other communicating arteries to form a complete anastomotic ring at the base of the brain known as the circle of Willis.