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Physiology III
Vascular System

(Haines, Ch. 13)


Internal Carotid
the first of only two blood supplies to the brain, the second being the vertebral arteries. The internal carotids enter the skull and then divide into the anterior and middle cerebral arteries. The internal carotid artery consists of three segments, petrous, cavernous, and cerebral. The petrous part is located in the carotid canal and has no significant branches. The cavernous part passes through the cavernous sinus and gives rise to hypophysial and meningeal branches. The cerebral part begins where this vessel penetrates the dura just ventral to the optic nerve. Its branches are the ophthalmic, posterior communicating, and anterior choroidal arteries. The internal carotid ends by dividing into the anterior and middle cerebral arteries.
Vertebral Artery
the second of only two blood supplies to the brain, the vertebral artery leaves the transverse foramen of C1, loops caudally and medially around the lateral mass of the atlas, and then pierces the atlanto-occipital membrane, to which it is anchored. This circuitous portion of the vertebral artery is vulnerable to injury. Once inside the subarachnoid space, the vertebral artery is located in the lateral cerebellomedullary cistern. Branches of the vertebral artery supply the medulla, parts of the cerebellum, and the dura of the posterior fossa. The first major branch is the posterior inferior cerebellar artery (PICA).
Anterior Cerebral Artery
one of the first branches of the internal carotid once it enters the skull. The anterior cerebral artery passes superiorly over the optic chiasm and is joined to its counterpart by the anterior communicating artery. The part of the ACA between its origin from the internal carotid and the anterior communicating artery is the A1 segment. The anterior communicating artery and the distal parts of the A1 segments are located in the cistern of the lamina terminalis and give rise to small branches that serve structures in the immediate area. Distal to the anterior communicator, the anterior cerebral artery branches over the medial surface of the hemisphere to about the level of the parieto-occipital sulcus; collectively these branches form the A2 segment. The main branches of A2 lie within the callosal cistern.
Middle Cerebral Artery
usually the larger of the terminal branches of the internal carotid artery. The part of this vessel located between its origin from the internal carotid and the point where it branches in the Sylvian fissure is the M1 segment. Branches from M1 serve adjacent medial and rostral aspects of the temporal lobe and via lenticulostriate arteries, structures located inside the hemisphere. The M1 segment is located in medial portions of the Sylvian cistern. On the ventromedial aspect of the insular cortex, the M1 segment usually bifurcates into superior and inferior trucks. These trunks and their distal branches collectively form the M2 segment. As these vessels pass through the Sylvian cistern, they give rise to branches that serve the insular cortex. Distal branches of the superior and inferior trunks exit the lateral fissure and serve cortical areas located above and below this fissure.
Posterior Cerebral Artery
at the pons-midbrain junction, the basilar artery bifurcates in the interpeduncular cistern and gives rise to the posterior cerebral arteries. Each posterior cerebral artery passes laterally just rostral to the root of the oculomotor nerve, wraps around the midbrain in the ambient cistern, then joins the ventral and medial surface of the temporal lobes. The posterior cerebral artery sends branches to the midbrain and thalamus and to the ventral and medial surfaces of the temporal and occipital lobes as far as the level of the parieto-occipital sulcus. The posterior cerebral artery is divided into segments called P1 to P4. The P1 segment is located between the basilar bifurcation and the posterior communicating artery. It gives rise to small perforating vessels and to quadrigeminal and thalamoperforating arteries. The portion of the posterior cerebral artery between the posterior communicator and the inferior temporal branches if the P2 segment. Medial and lateral posterior choroidal and thalamogeniculate arteries, as well as small perforating branches to the midbrain, originate from P2. The P3 segment is the portion of the artery that gives rise to its temporal branches, and the parieto-occipital can calcarine arteries form the P4 segment.
Basilar Artery
lies in a shallow groove on the ventral surface of the pons in the prepontine cistern. Its first large branch, the anterior inferior cerbellar artery (AICA), arises from the lower one-third of the basilar artery and passses through the cerebellopontine cistern as it wraps around the caudal aspect of the middle cerebellar peduncle. The AICA serves ventral and lateral surfaces if the cerebellum, parts of the pons, and the portion of choroid plexus that extends out of the foramen of Luschka into the cerebellopontine angle. The labyrinthine artery is usually a branch of the AICA. The basilar artery gives rise to numerous pontine arteries. The last major branches of the basilar artery are the superior cerebellar arteries. These vessels pass laterally just caudal to the root of the oculomotor nerve and wrap around the brainstem in the ambient cistern to ultimately serve caudal parts of the midbrain and the entire superior surface of the cerebellum.
Circle Of Willis
a roughly shaped heptagon of arteries located on the ventral surface of the brain. This loop of vessels passes around the optic chiasm and the optic tract, crosses the crus cerebri of the midbrain, and joins at the pons-midbrain junction. Important structures located inside this circle include the optic chiasm and tracts, infundibulum and tuber cinereum, the mammillary bodies, the hypothalamus, and structures of the interpeduncular fossa. Arteries forming the Circle of Willis give rise to numerous perforating (central or ganglionic) branches, which serve structures located deep to their origin and to the large cortical branches (anterior, middle, and posterior cerebral arteries).
Cerebellar Arteries
Posterior Inferior Cerebellar Atery (PICA) first major branch of the vertebral artery , arches around the dorsolateral medulla and sends branches to this part of the brainstem. Dorsally, the PICA is located in the cisterna magna. It serves the choriod plexus of the fourth ventricle and then branches over medial parts of the inferior cerebellar surface. In about 75% of brains, the posterior spinal artery is a branch of the PICA; in the other 25%, it arises from the vertebral artery. The posterior spinal artery serves dorsolateral regions of the medulla caudal to the area served by the PICA. The vertebral artery supplies the ventrolateral medulla and, just before joining its counterpart on the opposite side, gives rise to the anterior spinal artery. The anterior spinal artery is found in the premedullary cistern.
Venous Sinuses
only one source of venous drainage from the brain through the dural sinuses to the internal jugular vein. The superior and inferior sagittal sinuses are located in the attached and free edges of the falx cerebri, and drain the cerebral veins on the lateral surface of the hemisphere. The straight sinus is found where the falx cerebri attaches to the tentorium cerebelli, and drains the basal vein, the corpus callosum and the interior of the hemisphere. The confluence of sinuses is formed by the junction of the straight sinus, the superior sagittal sinus, and both transverse sinuses. The transverse and sigmoid sinuses form a shallow groove on the internal surface of the occipital and temporal bones, and receive several tributaries. Venous blood travels from the transverse and petrosal sinuses into the sigmoid sinus and then into the internal jugular vein at the jugular foramen.
Meningeal Veins

Superior Sagittal Sinus

Transverse Sinus

Internal Jugular Vein
the one major vessel that drains all blood from superficial and deep veins of the brain.
Blood Brain Barrier
physiologic barrier to the movement of many substances into or out of the brain. The endothelial cells of brain capillaries form a continuous lining membrane; they are joined by numerous tight (occluding) junctions and have no intercellular pores or fenestrations. Endothelial cells of brain capillaries rest on a continuous basement membrane (basal lamina) which is surrounded by the end feet of astrocytes. Under normal, healthy conditions, the blood-brain barrier prohibits the movement of high molecular weight substances (such as proteins) into the brain. In some disease states, however, the barrier breaks down.


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