After passing through the foramen magnum the vertebral artery gives rise to numerous branches before uniting with the opposite vertebral artery at the vertebral-basilar junction. Multiple small perforating arteries arise from the distal vertebral artery which supplies the lateral regions of the medulla. The posterior inferior cerebellar artery (PICA) usually arises from the distal vertebral artery 1-2 cm below the vertebral-basilar junction. These arteries are often asymmetric and their size can vary. There is a reciprocal relationship between the size of the PICA and that of the ipsilateral anterior inferior cerebellar artery (AICA) which arises from the basilar artery. When the AICA is large, the ipsilateral PICA is often small or sometimes can be absent. In this situation a large medial branch of the AICA supplies the PICA vascular territory. The PICA divides into branches supplying the ipsilateral cerebellar tonsil, inferior vermis, and the inferior cerebellar hemisphere. Small perforators from the proximal PICA can supply the lateral and posterior medulla. The anterior spinal artery usually arises from the distal 1 cm of the vertebral artery. This vessel often unites with the anterior spinal artery branch from the contralateral vertebral artery to form the main anterior spinal artery. The posterior meningeal artery which supplies the falx cerebelli arises from the distal vertebral artery near the foramen magnum. This vessel courses both posteriorally and superiorally.
The vertebral arteries unite to form the basilar artery ventral to the superior medulla/inferior pons.