Arterial Supply Of The Brain PDF

ARTERIAL SUPPLY OF THE BRAIN  Among all causes of human neurological disorders, vascular disorders rank first in frequency and urgency At some point, every practicing physician can expect to examine patients with cerebrovascular complications and thus should be aware of the common typ...

ARTERIAL SUPPLY OF THE BRAIN  Among all causes of human neurological disorders, vascular disorders rank first in frequency and urgency At some point, every practicing physician can expect to examine patients with cerebrovascular complications and thus should be aware of the common types  Blood supply to the brain is derived from four arteries – Two internal carotid arteries – Two vertebral arteries Vertebral arteries join at the ventral part of the pons forming the basilar artery The carotid arteries supply about 80% of the total cerebral blood flow, while the vertebrals supply the remaining 20%.  The carotid arteries, and their associated branches, supply the anterior and middle portions of the cerebral hemispheres and the diencephalon – This is referred to as the anterior circulation (internal carotid system) The vertebral arteries, and their associated branches, supply the brain stem and cerebellum and the posterior portions of the cerebral hemispheres – This is therefore be referred to as the posterior circulation (vertebral-basilar system)  Anterior Circulation (internal carotid system) Although the carotid arteries fill from the aortic arch, the origins of the right and left carotids are asymmetrical On the right side, the aortic arch gives off the brachiocephalic trunk, which divides into the right common carotid artery and the right subclavian artery On the left side, the left common carotid artery arises directly from the aortic arch The common carotid arteries each bifurcate into an external and an internal carotid artery Anterior Circulation (internal carotid system) The internal carotid arteries course through the carotid canal in the base of the skull to enter the middle cranial fossa The arteries undergo a series of bends that represent the carotid siphon in a cerebral angiogram The internal carotids run forward in the cavernous sinus and then turn upward to enter the subarachnoid space The arteries then course backward below the optic nerve and immediately lateral to the optic chiasm divide into the middle and anterior cerebral arteries on each side  Important branches of the internal carotid artery – Superior and inferior hypophysial arteries – A. ophtalmica – A. choroidea anterior – A. communicans posterior – A. cerebri anterior – A. cerebri media Anterior Circulation (internal carotid system) Branches of the Internal Carotid Arteries Before the internal carotids terminate as the middle and anterior cerebral arteries, they give rise to several collateral branches Hypophysial artery arise from the internal carotid in the cavernous sinus – They supply the infundibulum, and in addition, enter the median eminence of the hypothalamus to form a network of capillary loops that are involved in the transport of hypothalamic releasing factors to the anterior pituitary Ophthalmic artery originates from the internal carotid immediately after it enters the subarachnoid space – This artery courses through the optic canal (lateral to the optic nerve) and enters the orbit to supply the eye and other structures of the orbit, the frontal area of the scalp, the frontal and ethmoid paranasal sinuses and parts of the nose – Also gives of the central artery of the retina which runs inside the optic nerve Anterior Circulation (internal carotid system) Branches of the Internal Carotid Arteries Posterior communicating artery arises from the internal carotid close to its terminal bifurcation – It runs caudally to join the proximal part of the posterior cerebral artery, thus forming part of the circle of Willis Anterior choroidal artery originates from the distal portion of the internal carotid or the proximal portion of the middle cerebral artery – It courses back along the optic tract and the choroid fissure at the medial edge of the temporal bone to supply the choroid plexus in the temporal horn of the lateral ventricle – In addition, the anterior choroidal artery gives off branches to the optic tract, uncus, amygdala, hippocampus, globus pallidus, lateral geniculate body, ventral portion of the thalamus, subthalamus, and the ventral portion of the internal capsule – The anterior choroidal artery is prone to thrombosis due to its small caliber and long course through the subarachnoid space – The globus pallidus and hippocampus are favored sites of neuronal degeneration as a result of circulatory deficiency related to the anterior choroidal artery Anterior Circulation (internal carotid system) Branches of the Internal Carotid Arteries Middle cerebral artery, which is the larger and more direct continuation of the internal carotid, courses deep in the lateral sulcus – This artery gives rise to frontal, parietal, and temporal branches, which emerge from the lateral sulcus and ramify over the lateral surface of the cerebral hemisphere Anterior Circulation (internal carotid system) Branches of the Internal Carotid Arteries Anterior cerebral artery, which is the smaller terminal branch of the internal carotid, is first directed medially above the optic nerve The two anterior cerebral arteries are joined at the midline by the anterior communicating artery The anterior cerebral artery then ascends in the longitudinal fissure and courses caudally around the genu of the corpus callosum The anterior cerebral arteries provide the blood supply for the medial aspect of the parietal and frontal cerebral cortex and middle cerebral arteries supplies the lateral part of the cerebral cortex; apart from the area which is supplied by the posterior cerebral artery Due to the occlusion of the branches of the middle cerebral artery, one or more of the following signs can be observed  Signs and symptoms Structures involved Paresis of face, arm, and Somatic motor area for face leg; contralaterally and arm Paresis of leg, Fibers descending from the contralaterally leg area to enter the corona radiata Sensory impairment over face and arm; contralaterally Somatic sensory area for face and arm Sensory impairment over trunk and leg; contralaterally Thalamocortical projections to somatic sensory area for trunk and leg  Signs and symptoms Structures involved Motor speech disorder (motor Broca's area and related aphasia) frontal lobe areas of the dominant hemisphere (more commonly, the left hemisphere) Wernicke or arcuate Sensory or conductive aphasia fasciculus Contralateral neglect, agnosia Usually nondominant parietal for the contralateral external lobe space Contralateral homonymous Optic radiation hemianopia  Signs and symptoms Structures invoved Contralateral superior Optic radiation deep to quadrantanopia second temporal convolution (Meyer's Loop) Paralysis of conjugate Frontal eye field and its efferent fibers gaze to the opposite side Capsular (pure motor) Upper portion of the posterior limb of the internal hemiplegia capsule and the adjacent corona radiata Leg Foot Due to the occlusion of the branches of the anterior cerebral artery, one or more of the following signs can be observed  Signs and symptoms Structures invoved Paresis of foot and leg Somatic motor area for leg Lesion is contralateral to paralysis Fibers coursing from arm area of Paresis of arm cortex through corona radiata Lesion is contralateral to paresis Sensory impairment over toes, Somatic sensory area for foot and foot, and leg leg Lesion is contralateral to paresthesia or anesthesia Urinary incontinance Sensory area for pelvic structures, specifically urinary bladder (mostly in bilateral lesion)  Posterior Circulation (vertebral-basilar system) The vertebral arteries, which also fill from the aortic arch, arise from the the right and left subclavian arteries They ascend in the foramen of the transverse processes of the upper six cervical vertebrae At the base of the skull, the arteries twist around the lateral mass of the atlas, pierce the posterior atlanto-occipital membrane, and enter the subarachnoid space at the level of the foramen magnum The vertebral arteries course forward (beneath the medulla oblongata) and join at the caudal border of the pons to form the basilar artery The basilar artery courses forward in the midline of the pons and divides into the posterior cerebral arteries Posterior Circulation (vertebral-basilar system) Branches of the Vertebral Arteries The upper portion of the cervical spinal cord receives small branches directly from the vertebral arteries A single anterior spinal artery is formed by a contribution from each vertebral artery and is continuous throughout the length of the spinal cord A posterior spinal artery arises from each vertebral or from each PICA (see below) – These are also continuous throughout the length of the spinal cord Posterior inferior cerebellar arteries (PICA) are the largest branches of the vertebral arteries – They send branches to the posterior part of the cerebellar hemisphere, inferior vermis, central nuclei of the cerebellum, choroid plexus of the fourth ventricle, and the lateral medulla – Occlusion of the posterior inferior cerebellar artery results in lateral medullary syndrome (Wallenberg's syndrome) Posterior Circulation (vertebral-basilar system) Branches of the Vertebral Arteries Basilar Artery The basilar artery gives rise to a number of branches before it divides into the posterior cerebral arteries Anterior inferior cerebellar artery (AICA) arises from the caudal end of the basilar artery – It supplies the cortex of the inferior surface of the cerebellum anteriorly and the underlying white matter – It also provides collateral supply to the deep nuclei of the cerebellum, the upper medulla, and lower pons Labyrinthine artery is a branch of either the anterior inferior cerebellar artery or the basilar artery – It courses through the internal acoustic meatus and ramifies throughout the labyrinth of the internal ear – Occlusion of the labyrinthine artery can result in deafness of the corresponding ear and signs of vestibular dysfunction Posterior Circulation (vertebral-basilar system) Branches of the Vertebral Arteries Basilar Artery Pontine arteries are slender branches arising from the basilar artery – They supply the ventral portion of the pons and the pontine tegmentum Superior cerebellar artery arises from the upper end of the basilar artery – It ramifies over the dorsal surface of the cerebellum and supplies the cortex, medullary core, and central nuclei of the cerebellum – Branches from the proximal portion of the superior cerebellar artery supply the pons, superior cerebellar peduncle, and inferior colliculus of the midbrain Posterior cerebral arteries are the terminal branches of the basilar artery – They curve around the midbrain and reach the medial surface of the cerebral hemisphere – Supplies the medial and inferior surfaces of the occipital and temporal lobes of the brain, rostral part of the midbrain and the caudal part of the diencephalon Posterior Circulation (vertebral-basilar system) Branches of the Vertebral Arteries Medial Medullary Syndrome Results from infarct of paramedian penetrating branch, generally arise from vertebral, occasionally from anterior spinal or basilar Lateral Medullary Syndrome (Wallenberg's Syndrome) Generally due to occlusion of the direct branches of the vertebral artery supplying the lateral part of the medulla,vertebral artery itself or PICA  Arterial Circle of Willis The major arteries supplying the cerebral hemispheres are joined at the base of the brain to form the arterial circle of Willis These arteries include – Anterior communicating artery – Anterior cerebral artery – Internal carotid artery (or middle cerebral artery) – Posterior communicating artery – Posterior cerebral artery Arterial Circle of Willis In the normal situation, there is little exchange of blood through the anterior and posterior communicating arteries However, when one of the major arteries is compromised by occlusion, the arterial circle provides an alternate route for the continued flow of blood These anastomoses are frequently inadequate (especially in the elderly) in that the communicating arteries may be narrowed because of vascular disease  BLOOD BRAIN BARRIER The blood-brain barrier is the physical barrier between the central nervous system and the blood vessels in the central nervous system The barrier is formed by the wall of blood capillaries in the brain This barrier controls both the kinds of substances that enter the extracellular fluid of the brain and the rates at which they enter The blood-brain barrier protects the brain from common infection, thus an infection of the brain is very rare – Tight junctions between the endothelial cells – Continuous basal membrane outer to the endothelial cells – Astrocyte processes BLOOD BRAIN BARRIER Unlike peripheral capillaries, the blood-brain barrier strictly limits transport into the brain through both physical (tight junctions) and metabolic (enzymes) barriers Endothelial cells are packed much tighter together in the brain, allowing almost nothing to pass in and out BLOOD BRAIN BARRIER Substances in the blood that gain rapid entry into the brain include glucose, the important source of energy, certain ions that maintain a proper medium for electrical activity, and oxygen for cellular respiration Small fat soluble molecules, like ethanol pass through the blood- brain barrier However, some water soluble molecules pass into the brain with the aid of protein carriers in the plasma membrane of the endothelial cells The barrier is impermeable to high size molecules, such as plasma proteins and large organic molecules BLOOD BRAIN BARRIER In addition to its blood supply, the central nervous system is perfused by the cerebrospinal fluid (CSF) A barrier is present here too, between the blood in the capillaries of the choroid plexuses and the CSF The choroid plexuses also play a role in protecting the brain by trapping toxic heavy metals such as lead Clinical note Cerebrovascular accident or attack (stroke) May be due to occlusion (ischemic attack) or rupture of a vessel (hemorrhagic attack) Clinical note Ischemic attack Most cerebrovascular accidents are in this form Might be due to atherosclerosis or embolus Hemorrhagic attack Duvarı zayıf olan bir damarın rüptüre olup, kanamasına bağlı olarak gelişir Doku içinde biriken kan, biriktiği bölge civarına baskı yaparak o bölgenin kanlanmasının bozulmasına yol açar Damar duvarındaki zayıflık, genellikle anevrizma veya arteriovenöz malfromasyonlara bağlıdır Transient ischemic attack A short term occlusion of a vessel which resolves usually within minutes without causing damage Is a risk factor for eventually having a stroke Clinical note Cerebral Aneurysm A cerebral aneurysm is an abnormal dilation or ballooning of a brain artery Aneurysms can occur anywhere in the brain Factors that can cause a cerebral aneurysm are brain trauma, infection, high blood pressure, atherosclerosis, but most seem to arise from a congenital or developmental defect Unruptured aneurysms may be discovered when they cause neurological symptoms which depend on the location and size of the aneurysm – i.e. headaches, double vision As the wall of the blood vessel is abnormally thin, aneurysms tend to rupture Immediately before an aneurysm ruptures, an individual may experience a sudden, extremely severe headache, accompanied by nausea, vomiting and loss of consciousness. Middle cerebral artery