The brain receives its arterial supply from two sources, the internal carotid and the vertebral arteries.
The common carotid arteries originate from the aortic arch on the left and the brachiocephalic trunk on the right. The common carotid arteries bifurcate at the level of the thyroid cartilage into the external and internal carotid arteries. The external carotid artery sends branches to the neck and face, whereas the internal carotid artery ascends to the base of the skull, entering the carotid canal. Upon exiting the carotid canal, the internal carotid artery courses horizontally over the foramen lacerum and enters the cavernous sinus and, after turning superiorly, divides into its terminal branches. The terminal branches of the internal carotid are as follows (Figure 16-3A–C):
- Ophthalmic artery. Courses through the optic canal to supply the retina, orbit and part of the scalp.
- Posterior communicating artery. Joins the posterior cerebral artery with the internal carotid artery.
- Anterior cerebral artery. Courses superior to the optic chiasma and enters the longitudinal cerebral fissures. The anterior cerebral artery courses superiorly and then posteriorly along the corpus callosum, providing blood supply to the medial sides of both cerebral hemispheres.
- Anterior communicating artery. Is a very short artery that connects the two anterior cerebral arteries.
- Middle cerebral artery. Courses into the lateral fissure between the parietal and temporal lobes. It sends many branches to the lateral sides of the cerebral hemispheres and central branches into the brain.
A. Cerebral arterial circle (of Willis). B. Anterior view of the brain showing the arteries. C. Lateral view of the brain showing the arteries (CN II, optic nerve; CN III, oculomotor nerve).
Each vertebral artery arises from the subclavian artery and ascends through the transverse foramina of C1–C6. The vertebral artery courses horizontally across C1 through the suboccipital triangle before entering the skull via the foramen magnum (Figure 16-3A–C). After penetrating the dura mater, the vertebral arteries then course along the inferior aspect of the medulla oblongata before converging into the basilar artery on the pons. Branches of the vertebral arteries travel to the spinal cord, the meninges, and the brainstem. The major branches are as follows:
- Posterior inferior cerebellar arteries. Course between the origins of cranial nerve (CN) X and CN XI en route to the inferior surface of the cerebellum. Often referred to by the acronym PICA.
- Posterior cerebral arteries. The terminal branches of the basilar artery provide vascular supply to that part of the brain base that is superior to the tentorium cerebelli. CN III and CN IV exit the brain between the superior cerebellar and the posterior cerebral arteries.
The basilar artery ascends along the ventral surface of the pons and gives rise to the following branches:
- Anterior inferior cerebellar artery. Courses along the inferior surface of the cerebellum.
- Superior cerebellar artery. Courses along the superior surface of the cerebellum.
The anterior communicating artery connects the two anterior cerebral arteries, and the posterior communicating arteries connect the internal carotid and posterior cerebral arteries. As a result of these connections, an arterial circle, known as the cerebral arterial circle (of Willis), is formed around the infundibulum (stalk connecting the pituitary gland to the hypothalamus).
The structure of the arterial supply of the brain into the circle of Willis provides a collateral circulatory pathway
in the cerebral circulation. As a result, if one part of the circle becomes narrowed or blocked or if one of the four arteries supplying the circle is narrowed or blocked, blood flow from the other vessels preserves cerebral perfusion. This collateral circulation is made possible by the lack of valves in arteries. However, some of the numerous arteries arising from this circle that penetrate the brain substance are small and are considered end arteries (without collateral circulation). Therefore, if an end artery becomes narrowed or blocked, ischemia may occur in the region of the brain that is uniquely supplied by that end artery.
A berry aneurysm
is a balloon-like outpouching of a cerebral arterial wall that is berry shaped (hence, the name). This outpouching most often reflects a gradual weakening of the arterial wall as a result of chronic hypertension or arteriosclerosis and places the artery at risk to rupture, causing a stroke
. Some cerebral vessels are inherently weak and susceptible to berry aneurysms, such as the arteries associated with the circle of Willis, where small communicating arteries connect larger cerebral arteries (internal carotid, vertebral, and basilar arteries). A ruptured berry aneurysm bleeds into the subarachoid space.