Overview of the Blood Supply to the Brain
Most of the blood supply to the brain comes from paired vessels, the internal carotid and the vertebral arteries ( ). The internal carotid supplies the majority of the brain while the vertebral arteries provide blood to the posterior aspect of the cerebrum, the brain stem, and the cerebellum. A large anastomosis on the inferior surface of the brain, the circle of Willis, connects the anterior and posterior circulation. The blood–brain barrier is a physiological and anatomical system present in the circulatory system of the brain that controls the bidirectional movement of molecules. Superficial and deep cerebral veins drain the brain tissue and empty into dural venous sinuses, which in turn drain into the internal jugular vein and venous plexi.
Anterior Circulation of the Brain
The anterior circulation of the brain is derived from the internal carotid artery (ICA) (a). The internal carotid originates from the common carotid, which is a branch of the subclavian artery. After ascending through the neck, the ICA enters the carotid canal and begins its intracranial course (b). The pathway of the ICA is commonly divided into four segments:
Fig. 6.2 (a) Internal carotid artery. Left lateral view. (b) The four anatomical divisions of the internal carotid artery. Anterior view of the left internal carotid artery. The internal carotid artery consists of four topographically distinct parts between the carotid bifurcation and the point where it divides into the anterior and middle cerebral arteries. The parts are as follows: (1) Cervical part located in the lateral pharyngeal space; (2) petrous part located in the carotid canal of the petrous bone; (3) cavernous part which follows an S-shaped curve in the cavernous sinus; (4) cerebral part located in the chiasmatic cistern of the subarachnoid space. (c) The petrous part of the internal carotid artery (traversing the carotid canal) and the cavernous part (traversing the cavernous sinus) have a role in supplying extracerebral structures of the head. They give off additional small branches that supply local structures and are usually named for the areas they supply. Of the branches not supplying the brain, of special importance is the ophthalmic artery, which arises from the cerebral part of the internal carotid artery. Note: The ophthalmic artery forms an anastomosis with the artery of the pterygoid canal derived from the maxillary artery. (d) The ICA supplies most of the cerebrum. After giving off the ophthalmic artery, it bifurcates into the anterior and middle cerebral arteries.
(e) Cerebral arteries. Medial view of the right hemisphere. (f) The MCA runs laterally into the lateral sulcus of the brain. (g) After entering the lateral sulcus, the MCA sends branches to the insula. ICA, internal carotid artery; MCA, middle cerebral artery. (a, e, and f: Reproduced with permission from Gilroy AM, MacPherson BR. Atlas of Anatomy. Third Edition. © Thieme 2016. Illustrations by Markus Voll and Karl Wesker. b and 6.2g: Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker. c: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker. e: Modified with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)
The cervical portion begins at the bifurcation of the common carotid (at the level of C4) in the neck and continues until it reaches the carotid canal.
The petrous portion begins in the carotid canal, which is located in the petrous part of the temporal bone. After entering the canal, it curves forward and medially. As it exits, it travels superior to foramen lacerum, which in life is filled with cartilage, and then enters the cavernous sinus.
The cavernous portion lies between layers of dura forming the cavernous sinus. It initially ascends toward the posterior clinoid process, then curves forward along the side of the body of the sphenoid bone, and finally turns upward and ascends alongside the anterior clinoid process. The resulting S-shaped configuration of the ICA, which is made up of the cavernous and, to a smaller extent, the cerebral portion, is referred to as the carotid siphon and is often used as a radiographic landmark.
The ophthalmic artery is given off just as the ICA emerges from the cavernous sinus (c).
It enters the orbit through the optic foramen, inferior and lateral to the optic nerve.
Once in the orbit, it passes superior to the optic nerve along with the nasociliary nerve.
The cerebral portion begins after the vessel perforates the dura medial to the anterior clinoid process. Here, the ICA passes between the optic and occulomotor nerves and travels toward the lateral cerebral fissure where it gives off its terminal branches, the anterior and middle cerebral arteries.
The Anterior Cerebral Artery (ACA)
The ICA supplies the majority of the cerebrum. After giving off the ophthalmic artery, the ICA continues in a superior direction alongside the optic chiasm. Eventually, it will bifurcate into the anterior and middle cerebral arteries but before it does so, the anterior choroidal and posterior communicating arteries are given off (d).
The anterior choroidal artery supplies the optic tract, the choroid plexus, portions of the internal capsule, the thalamus, and the hippocampus.
The posterior communicating artery passes posteriorly and joins the posterior cerebral artery.
The anterior cerebral artery enters the longitudinal fissure of the brain that separates the cerebral hemispheres where it branches and arches posteriorly.
The ACA closely follows the corpus collosum and supplies the medial aspect of the frontal and parietal lobes of the brain (e).
Some of the branches supply the dorsolateral surface of each hemisphere.
The anterior communicating artery joins the anterior cerebral arteries from both sides.
Distal to the anterior communicating artery, the anterior cerebral artery continues as the pericallosal artery, which runs alongside the corpus callosum.
The Middle Cerebral Artery (MCA)
The middle cerebral artery (MCA) is the larger of the two terminal branches of the ICA. It runs laterally into the lateral sulcus of the brain (f).
Posterior Circulation of the Brain
The posterior circulation of the brain is provided by paired vertebral arteries that join to form a single basilar artery. For that reason, it is often referred to as the vertebrobasilar circulation.
The vertebral artery is a branch of the subclavian artery (a).
The cervical part of the vertebral arteries enters the transverse foramina of C6 vertebrae and continues to ascend through the foramina of C1 vertebrae.
After exiting the C1 foramen, the vertebral artery perforates the dura and the arachnoid meninges and ascends through the foramen magnum.
The vertebral arteries join to form the basilar artery at the caudal border of the pons (b).
The basilar artery traverses the clivus (occipital bone), and then divides into two posterior cerebral arteries (PCAs) at the level of the midbrain (c).
Before bifurcating, the PCA gives off numerous branches including the anterior inferior cerebellar artery (AICA) the superior cerebellar artery and pontine arteries.
The AICA supplies the inferior surface of the cerebellum and parts of the pons.
The posterior cerebral arteries supply the medial and inferior surfaces of the occipital and temporal lobes of the brain (e).
