Anatomical Blood Supply to the Brain PDF

Lecture 2 Anatomical Blood Supply to the Brain Dr. Martin  CEREBRAL ARTERIES Two pairs of arteries enter the skull, delivering blood to the brain. Internal Carotid and Vertebral Arteries The arteries travel independently until entering the cranium, where th...

Lecture 2 Anatomical Blood Supply to the Brain Dr. Martin  CEREBRAL ARTERIES Two pairs of arteries enter the skull, delivering blood to the brain. Internal Carotid and Vertebral Arteries The arteries travel independently until entering the cranium, where they interface with the anastomotic arterial circle of Willis.  INTERNAL CAROTID The right common carotid artery arises from the brachiocephalic trunk, and the left arises from the aortic arch. At the level of the thyroid cartilage, the common carotids bifurcate, yielding the external and internal carotid arteries. The internal carotid arteries ascend through the deep neck, the carotid canals of the petrous bones, and the cavernous sinus.  The ICA emit the ophthalmic and posterior communicating arteries. They divide, terminating as the anterior and middle cerebral arteries.  VERTEBRAL ARTERIES The vertebral arteries are the first branches of the subclavian arteries. They ascend through the transverse foramina of the upper 6 cervical vertebrae and enter the skull through the foramen magnum. They emit the posterior inferior cerebellar arteries (PICA), which usually emit the posterior spinal arteries.  The vertebral arteries then emit the anterior spinal artery before unifying to form the basilar artery.  BASILAR ARTERY The basilar artery gives rise to the anterior inferior cerebellar arteries (AICA). The superior cerebellar arteries arise from the basilar artery, just prior to its bifurcation into the two posterior cerebral arteries near the caudal midbrain.  THE ARTERIAL CIRCLE OF WILLIS The arterial circle of Willis is formed by a group of arteries whose perforating branches supply deep structures.  ANTERIOR AND MIDDLE CEREBRAL ARTERIES Anterior and middle cerebral arteries are the terminal divisions of the internal carotid artery, also emitting the posterior communicating artery to anastamose with the vertebral arterial circulation. The anterior cerebral artery runs medially anterior to the optic chiasm to the longitudinal cerebral fissure to supply the frontal lobes.  The anterior communicating artery is located in the fissure, joining the anterior cerebral arteries at the midline. Major branches of the anterior cerebral arteries pass to the medial aspect of the frontal and parietal lobes and corpus callosum (often feeding all but the most posterior zones of this commissure).  The middle cerebral artery extends laterally in lateral fissure over the insula, giving off branches to the lateral aspect of frontal, temporal, and parietal lobes.  POSTERIOR CEREBRAL ARTERIES The posterior cerebral arteries are the terminal branches of the basilar artery. They receive the posterior communicating arteries from the internal carotid. Cerebral branches supply medial temporal, occipital, and parietal structures. VENTRICULAR SYSTEM  VENTRICULAR SYSTEM At an early stage, the embryonic brain comprises three parts, the prosencephalon, mesencephalon and rhombencephalon.  The prosencephalon splits into the telencephalon, related to the (left and right) lateral ventricles, and the diencephalon, which surrounds the third ventricle. The mesencephalon is the cerebral aqueduct. The rhombencephalon splits into the metencephalon and myelencephalon, both of which relate to the fourth ventricle.  The caudal myelencephalon and the myelon (spinal cord) contain the central canal. The four interconnected ventricles are lined by ependyma and filled with CSF, which is secreted by the choroid plexus. The left and right lateral ventricles communicate with the third ventricle via the interventricular foramina (of Monro).  The third ventricle communicates with the fourth ventricle via the cerebral aqueduct. The fourth ventricle communicates directly with the central canal of the medulla (myelencephalon), which is continuous with the central canal of the spinal cord. The spinal central canal terminates as the tiny terminal ventricle, which is located in the conus medullaris  LATERAL VENTRICLES Each lateral ventricle has four distinct parts: Anterior (Frontal) Horn Body (corpus) Posterior (Occipital) Horn Inferior (Temporal) Horn  THIRD VENTRICLE The third ventricle is a thin vertical cavity, occupying the midline between the diencephalic hemispheres. Roof is formed by choroid plexus and the body of the fornix.  FOURTH VENTRICLE The fourth ventricle is a rhomboid cavity overlying the pons and rostral medulla. Is connected to the third ventricle by the mesencephalic cerebral aqueduct, which is frequently involved in obstructive hydrocephalus. This ventricle communicates with the subarachnoid space via the two foramina of Luschka (lateral near the cerebellopontine angle) and the foramen of Magendie (at the caudal midline).  CHOROID PLEXUS Choroid Plexus occupies all ventricles: The Choriod Plexus is a rich network of blood vessels of the pia mater that projects into each ventricle to form a semi permeable filter between arterial blood and the CSF. An epithelial layer of ependyma covers the plexus and lines the ventricles.  CEREBRO-SPINAL FLUID (CSF) CSF is a clear, colorless, almost protein-free filtrate of blood that forms in the ventricles and circulates though the subarachnoid space. Function of CSF CSF serves to support and cushion the CNS against injury. It may serve nutritive functions and remove metabolic waste products. Its normal pressure (intracranial pressure, ICP) is 65 - 200 mm H2O (5 – 15mm Hg)  CONTENTS OF CSF Produced by choroid plexus, ependyma, glia, pial and arachnoid vessels. Ions are actively transported with passive movement of water (~0.5 ml/min). CSF is clear and low in glucose, protein, K+, Ca++ and cells Na+, Cl- , and Mg++ higher than in serum.  It is constantly formed (approximately 600- 700 ml/day). The total volume in the ventricles and subarachnoid space is about 140 ml.  CIRCULATION OF CSF The brain and spinal cord effectively float within a shock-absorbing cushion of CSF. CSF flows through the ventriclular system: 1. From the lateral ventricles into the third ventricle via the interventricular foramina (of Monro). 2. From the third ventricle into the cerebral aqueduct. 3. From the cerebral aqueduct into the fourth ventricle. 4. Very small amounts of CSF trickle between the fourth ventricle and thecentral canal.  CSF exits the fourth ventricle into the subarachnoid space via the foramina of Luschka (lateral) and Magendie (medial). Extraventricular CSF follows the subarachnoid space. It returns to the venous system through small membranous villi, the arachnoid granulations, located along the superior sagittal sinus. The arachnoid villi react to pressure gradients between the subarachnoid space and venous system to form one way valves, ensuring the unidirectional movement of fluid into the venous system.  BRAIN SINUSES AND VEINS The venous dural sinuses are epithelium-lined spaces or channels between the two layers of dura of the brain, and drainage of blood from the brain is chiefly into these sinuses. The dural sinuses do not have valves and are usually triangular in shape. The sinuses receive blood from three major sources: The cerebral veins are the major source of blood drainage The short midline great cerebral vein (of Galen) forms at the junction of the two internal cerebral veins and drains into the straight sinus. The diploic veins lie between the layers of cranial bone.  They anastomose freely with each other and also communicate with the meningeal veins internally and the superficial veins externally. The emissary veins connect extracranial and intracranial veins. The venous sinuses have little tendency to collapse as do most other veins because of the fibrous consistency of the dura mater. Unpaired dural venous sinuses: Superior longitudinal (sagittal) sinus begins as a continuation of the nasal vein and receives blood from cerebral, diploic, and emissary veins. Inferior sagittal sinus receives blood from veins on the medial surface of the brain. Straight sinus occupies the area between the falx cerebri and Tentorium cerebelli.  The Straight sinus is the posterior continuation of the great cerebral vein of Galen and joins the superior sagittal sinus to form the confluence of sinuses. The small occipital sinus begins near the foramen magnum and ascends in the attached margin of the falx cerebelli to end in the confluence of sinuses. It also receives the inferior cerebellar veins. Paired dural venous sinuses: Transverse sinuses receive blood from the confluence of sinus and drain into the internal jugular veins via the sigmoid sinuses. Cavernous sinuses are located lateral to the body of the sphenoid bone and the sella turcica and receive blood from the superficial middle cerebral veins and superior ophthalmic veins. Cavernous sinuses cont’d The internal carotid artery and the abducens nerve pass through these sinuses. In addition, the oculomotor, trochlear, ophthalmic, and maxillary nerves pass forward in the lateral wall of these sinuses. Communicate with the pterygoid venous plexus by emissary veins and receive the superior ophthalmic vein.  Cavernous Sinus Pituitary Gland CN III CN IV CN VI Internal Carotid CN V1 CN V2 Dura  They communicate with each other via intercavernous sinuses. Also have communication with pterygoid plexus of veins. These sinuses are the largest and most clinically important of the deep interconnecting sinuses and drain into the superior and inferior petrosal sinuses. Sigmoid sinuses are a continuation of the transverse sinuses. In their S-shaped course toward the jugular foramen, they receive blood from the inferior cerebrum, cerebellum, and emissary veins.  Superior and inferior petrosal sinuses drain from the cavernous sinuses into the transverse sinuses and internal jugular veins, respectively. Sphenoparietal sinuses, having received blood from the superficial middle cerebral veins, extend along the crest of the lesser wing of the sphenoid bone and drain into the cavernous sinuses.  References Moore KL, Dalley AF, Clinically Oriented Anatomy, Seventh Edition. 2014. Chapter 7, Head (pages 492 - 498) Frank H. Netter. Atlas of Human Anatomy 6th, 2014 Chapter 1, Head and Neck (plates 4 – 18)