Cranial Cavity, Venous Sinuses and Brain - LO 6

6. What are the two main arteries supplying the brain? What is the clinical importance of the cerebral arterial circle (circle of Willis)? What arteries make up the arterial circle?

• • Internal carotid a. - give rise to the:

    • Ophthalmic a.,

    • Posterior communicating a.,

    • Anterior cerebral a., and

    • Middle cerebral a.

  • Vertebral a.

    • Anterior spinal a. - paired branches that unite in the midline.

    • Posterior inferior cerebellar aa., from which arise the

      • Posterior spinal arteries

    • Basilar a. - formed by union of the vertebral arteries, it gives rise to:

      • Anterior inferior cerebellar aa.,

      • Superior cerebellar aa., and bifurcates into the

      • Posterior cerebral aa.

  • Cerebral arterial circle (Circle of Willis) - forms an important means of collateral circulation in case of obstruction. The circle itself is an example of collateral circulation, but branches of the circle are end arteries and there is little collateral circulation in the brain itself. Formed by the union of the anterior cerebral, anterior communicating, posterior communicating, and posterior cerebral arteries. A complete cerebral arterial circle is not necessarily typical. 

The oculomotor n. (CN III) courses between the Superior cerebellar a. and the Posterior cerebral a. An aneurysm in either of these arteries may impinge on the nerve, affecting extrinsic eye muscle function, causing symptoms of diplopia.