Neurovascular Anatomy II: Blood Supply To The Cerebral Hemispheres
Blood Supply To The Cerebral Hemispheres
The cerebral hemispheres receive their blood supply from both the internal carotid (75%) and the vertebral-basilar systems (25%) via the anterior, middle, and posterior cerebral arteries. These branches are often termed cortical branches to differentiate them from the deep branches supplying the diencephalon, basal ganglia and internal capsule. Knowledge of the approximate boundaries of the cortical regions supplied by the different cerebral arteries helps explain the functional disturbances that follow vascular obstruction, or other pathology, of the cerebral vessels.
The anterior cerebral artery (ACA) is one of two terminal branches of the internal carotid artery. It runs anteriorly in the longitudinal cerebral fissure between the two hemispheres. Its branches supply the medial surface and a small part of the lateral convexity of the frontal lobe. The artery sweeps upward around the genu of the corpus callosum to nourish the remainder of the medial surface and part of the lateral hemispheric surface as far caudally as the parieto-occipital sulcus. A number of small and medium-sized vessels (anterior medial striate arteries) are given off by the anterior cerebral and anterior communicating arteries. These striate arteries supply parts of the basal ganglia, the septal nuclei, and internal capsule.
The anterior cerebral artery is C-shaped, like many parts of the cerebral hemispheres. It originates where the internal carotid artery bifurcates and courses within the sagittal fissure and around the rostral end of the corpus callosum. Gross distribution of the Anterior Cerebral Artery (ACA) supplies the dorsal and medial portions of the frontal and parietal lobes.
The middle cerebral artery (MCA) is one of the two terminal branches of the internal carotid artery. It runs laterally along the base of the hemisphere and through the lateral sulcus before dividing into branches that supply the insula and most of the lateral surface of the frontal, parietal, temporal, and occipital lobes. It also covers the anterolateral-inferior surface of the frontal lobe. The middle cerebral artery provides almost all of the oxygen and nourishment for the lateral surface of the cerebral hemisphere. Occlusion of the branches of this vessel may deprive cortical connected with critical motor, somatosensory, auditory, and speech function.
The Middle Cerebral Artery (MCA) supplies blood to the lateral convexity of the cortex. The middle cerebral artery begins at the bifurcation of the internal carotid artery and takes an indirect course through the lateral sulcus along the surface of the insular cortex, and over the inner opercular surfaces of the frontal, temporal, and parietal lobes. It finally emerges on the lateral convexity. This complex configuration of the middle cerebral artery can be seen on angiograms.
The middle cerebral artery, also a terminal branch of the internal carotid artery, runs laterally in the lateral fissure, giving off delicate, hairlike arteries to the basal ganglia and internal capsule.
The posterior cerebral artery (PCA) arises as the terminal branch of the basilar artery. The posterior communicating artery connects the posterior cerebral and internal carotid arteries. The proximal part of the posterior cerebral artery curves over the lateral surface of the crus cerebri to reach the undersurface of the hemisphere. Its branches supply the entire occipital lobe and the inferior and medial surfaces of the temporal lobe. The calcarine branch of the posterior cerebral artery nourishes the primary visual cortex. Since deep branches of the posterior cerebral artery also supply the tectum, medial and lateral geniculate bodies, and pulvinar, note that virtually all these brain centers involved in the central processing of visual information receive their blood by a single major artery, the posterior cerebral.
The Posterior Cerebral Artery (PCA), originating where the basilar artery bifurcates courses around the lateral margin of the midbrain. This artery supplies the occipital lobe and portions of the medial and inferior temporal lobes.
The Anterior and Posterior circulations supply the diencephalon and cerebral hemispheres.
The internal carotid artery divides near the basal surface of the cerebral hemisphere to form the anterior cerebral and middle cerebral arteries.
The posterior cerebral artery is part of the vertebral basilar system and originates at the bifurcation of the basilar artery at the midbrain. The posterior cerebral artery develops from the anterior system, thereby receiving blood from the carotid arteries. However, later in development much more blood comes from the basilar artery, making the posterior cerebral artery functionally part of the posterior circulation in maturity.
Blood Supply to the Deep Forebrain
The vascular supply to structures of the deep forebrain is of great clinical significance. Even modest vascular accidents (hemorrhage or thrombosis) can produce severe, often lasting injury or even death.
The basal ganglia, including the caudate nucleus, the globus pallidus and putamen and the amygdala, the basal forebrain area, the internal capsule, and the diencephalon - represented by the thalamus and hypothalamus - are among the critical structures supplied by the deep branches of the cortical arteries.
The anterior cerebral artery, a terminal branch of the internal carotid artery sends medial striate arteries to the area of the anterior limb of the internal capsule and head of the caudate nucleus. These vessels reach their target area via the anterior perforated substance. Anterior communicating arteries from the anterior cerebral artery give off anterior medial arteries. These arteries enter the anterior perforated substance to supply the preoptic and supraoptic nuclei of the hypothalamus, the head of the caudate nucleus, and the anterior limb of the internal capsule. These are the lateral striate arteries, which include the lenticulostriate arteries.
The lenticulostriate arteries, known clinically as the "stroke arteries," are frequently the site of occlusion (thrombosis) or rupture (hemorrhage). They are prone to catastrophic vascular accidents for two reasons: They are much smaller than their artery of origin (the middle cerebral artery), thus experiencing relatively high flow forces directly from their parent vessel, and their relatively thin walls make them susceptible to rupture. In the event of the rupture of these vessels, the motor pathways compressed within the internal capsule are largely deprived of blood, resulting in paralysis on the opposite side of the body (hemiplegia).
The posterior cerebral artery originates as a terminal branch of the basilar artery and communicates with the internal carotid artery via the posterior communicating artery. The posterior cerebral artery sends thalamic branches to the posterior thalamus including the geniculate bodies and pulvinar, and the posterior limb of the internal capsule. If these vessels are occluded or ruptured, contralateral anesthesia and hemiplegia occur. An unusual complication of such an occlusion or hemorrhage is the thalamic pain syndrome, characterized by severe and intractable pain following the slightest touch of the affected part of the body. The cause is not clear but may be due to the loss of inhibitory neurons in the nucleus reticularis thalami. These neurons normally inhibit sensory relays from the thalamus to the cerebral cortex.
Deep Branches of the Anterior and Posterior Circulations supply subcortical structures
The arterial supply of the diencephalon, basal ganglia, and internal capsule derives from both the anterior and posterior circulations. This supply is complex, and there are many individual variations. The branches supplying these structures emerge from the proximal portions of the cerebral arteries or directly from the internal carotid artery. The internal capsule, the structure through which axons pass to and from the cerebral cortex, contains three separate parts; the anterior limb, the genu, and the posterior limb.Each part contains axons with different functions. Each part of the internal capsule also has a somewhat different arterial supply. The superior halves of the anterior and posterior limbs and the genu are supplied primarily by branches of the middle cerebral artery. The inferior half of the internal capsule is supplied primarily by the anterior cerebral (anterior limb and part of the genu) and anterior choroidal arteries. The basal ganglia receive their arterial blood supply from the anterior and middle cerebral arteries and the anterior choroidal artery. The thalamus is nourished by branches of the posterior cerebral and posterior communicating arteries. The hypothalamus is fed by branches of the anterior and posterior cerebral arteries and the two communicating arteries.
The vertebral and basilar arteries supply blood to the brain stem
Each of the three divisions of the brain stem and the cerebellum receives its arterial supply from the posterior circulation. In contrast to the spinal arteries, which are located both ventrally and dorsally, arteries supplying most of the brain stem arise from the ventral surface only. Branches emerge from these ventral arteries and either penetrate directly or run around the circumference of the brain stem to supply dorsal brain stem structures and the cerebellum. Three groups of branches arise from the vertebra and basilar arteries: paramedian, short circumferential, and long circumferential. The paramedian branches supply regions close to the midline. The short circumferential branches supply lateral, often wedge-shaped regions and the long circumferential branches supply the dorsolateral portions of the brain stem and cerebellum.
MEDULLA SUPPLY
Even though the spinal arteries primarily supply the spinal cord, they also supply a small portion of the caudal medulla. The spinal arteries lie close to the dorsal and ventral midline and nourish the most medial areas. The more lateral area is served by the vertebral arteries, which are equivalent to the more rostral short circumferential branches.
Most of the medulla is supplied by the vertebral arteries on the ventral surface. Small branches that exit from the main artery supply the medial medulla. Because these arteries supply axons of the corticospinal tract and the medial lemnisucus, when the arteries become occluded patients develop impairments in voluntary limb movement and mechanosensation. The major laterally emerging branch from the vertebral artery, the posterior inferior cerebellar artery (PICA), nourishes the most dorsolateral region. This region of the medulla does not receive blood from any other artery. The absence of a collateral arterial supply makes the posterior inferior cerebellar artery particularly important because occlusion almost always results in tissue damage. When this occurs, patients commonly develop characteristic sensory and motor impairments due to destruction of nuclei and tracts in the dorsolateral medulla. Common neurological signs include loss of facial pain sensation and uncoordinated limb movements, both on the side of the occlusion.
PONS SUPPLY
The vertebral arteries join to for the basilar artery at the pontomedullary junction, from which paramedian and short circumferential arteries supply the base of the pons, where corticospinal fibers are located. The dorsolateral portion of the caudal pons is supplied by a long circumferential branch of the basilar artery, termed the anterior inferior cerebellar artery (AICA). the region in the pons rostral to that supplied by the anterior inferior cerebellar artery is nourished by the superior cerebellar artery, another long circumferential branch of the basilar artery.
MIDBRAIN SUPPLY
The posterior cerebral artery nourishes most of the midbrain. Paramedian and short circumferential branches supply the base and tegmentum, whereas long circumferential branches supply the tectum. The colliculi, the most dorsal portion of the tectum, also receive a small supply by the superior cerebellar artery.
CEREBELLAR SUPPLY
Long circumferential branches of the vertebral and basilar arteries supply the cerebellum. The posterior inferior cerebellar artery supplies the caudal portion of the cerebellum. More rostral portions are supplied by the anterior inferior cerebellar artery and the superior cerebellar artery.
Cerebral Veins drain into the dural sinuses
Venous drainage of the cerebral hemispheres tis provided by superficial and deep cerebral veins. Superficial veins, arising from the cerebral cortex and underlying white matter, are variable in distribution. Among the more prominent and consistent are the superior anastomotic vein lying across the parietal lobe, and the inferior anastomotic vein on the surface of the temporal lobe. The deep cerebral veins, such as the internal cerebral vein, drain the more interior portions of the white matter, including the basal ganglia and parts of the diencephalon. Many deep cerebral veins drain into the great cerebral vein.
Drainage of blood from the central nervous system into the major vessels emptying into the heart - the systemic circulation - is achieved through either a direct or an indirect path. Spinal cord and caudal medullary veins drain directly, through a network of veins and plexuses, into the systemic circulation. By contrast, the rest of the central nervous system drains by an indirect path: The veins first empty into the dural sinuses before returning blood to the systemic circulation. The dural sinuses functions low-pressure channels for venous blood flow back to the systemic circulation. They are located between the periosteal and meningeal layers of the dura.
The superficial cerebral veins drain into the superior and inferior sagittal sinuses. The superior sagittal sinus runs along the midline at the superior margin of the falx cerebri. The inferior sagittal sinus courses along the inferior margin of the falx cerebri just above the corpus callosum. The inferior sagittal sinus, together with the great cerebral vein return venous blood to the straight sinus. At the occipital pole the superior sagittal sinus and the straight sinus join to form the two transverse sinuses. Finally, these sinuses drain into the sigmoid sinuses, which return blood to the internal jugular veins. The cavernous sinus, into which drain the ophthalmic and facial veins, is also illustrated in figure 4-16.
Veins of the midbrain drain into the great cerebral vein, which empties into the straight sinus, whereas the pons and rostral medulla drain into the superior petrosal sinus. Cerebellar veins drain into the great cerebral vein and the superior petrosal sinus.
The Blood Brain Barrier
The blood brain barrier isolates the chemical environment of the central nervous system from that of the rest of the body
The intravascular compartment is isolated from he extracellular compartment of the central nervous system. This feature, the blood brain barrier was discovered when intravenous dye injection stained most tissues and organs of the body but not the brain. This permeability barrier protects the brain from neuroactive compounds in the blood as well as rapid changes in the ionic constituents of the blood that can affect neuronal excitability.
The blood brain barrier is thought to result from two characteristics of endothelial cells in the capillaries of the brain and spinal cord. first, in peripheral capillaries, endothelial cells have fenestrations (pores) that allow large molecules to flow into the extracellular space. Moreover, the intercellular spaces between adjacent endothelial cells are leaky. In contrast, in central nervous system capillaries, adjacent endothelial cells are tightly joined, preventing movement of compounds into the extracellular compartment the central nervous system. Second, there is little transcellular movement of compounds from intravascular to extracellular compartments in the central nervous system because the endothelial cells lack the required transport mechanisms. Moreover, relatively nonselective transport may occur by pinocytosis in peripheral but not central nervous system capillaries.
Although most of the central nervous system is protected by the blood-brain barrier, eight brain structures lack a blood-brain barrier. These structures are close to the midline and because they are closely associated with the ventricular system, are collectively termed circumventricular organs At each of these structures, either neurosecretory products are secreted into the blood or local neurons detect blood-borne compounds as part of a mechanism for regulating the body's internal environment. Regions that lack a blood-brain barrier are not isolated from the rest of the brain. Rather, it has been proposed that rapid venous drainage from these areas protects surrounding regions.
The cerebral hemispheres receive their blood supply from both the internal carotid (75%) and the vertebral-basilar systems (25%) via the anterior, middle, and posterior cerebral arteries. These branches are often termed cortical branches to differentiate them from the deep branches supplying the diencephalon, basal ganglia and internal capsule. Knowledge of the approximate boundaries of the cortical regions supplied by the different cerebral arteries helps explain the functional disturbances that follow vascular obstruction, or other pathology, of the cerebral vessels.
The anterior cerebral artery (ACA) is one of two terminal branches of the internal carotid artery. It runs anteriorly in the longitudinal cerebral fissure between the two hemispheres. Its branches supply the medial surface and a small part of the lateral convexity of the frontal lobe. The artery sweeps upward around the genu of the corpus callosum to nourish the remainder of the medial surface and part of the lateral hemispheric surface as far caudally as the parieto-occipital sulcus. A number of small and medium-sized vessels (anterior medial striate arteries) are given off by the anterior cerebral and anterior communicating arteries. These striate arteries supply parts of the basal ganglia, the septal nuclei, and internal capsule.
The anterior cerebral artery is C-shaped, like many parts of the cerebral hemispheres. It originates where the internal carotid artery bifurcates and courses within the sagittal fissure and around the rostral end of the corpus callosum. Gross distribution of the Anterior Cerebral Artery (ACA) supplies the dorsal and medial portions of the frontal and parietal lobes.
The middle cerebral artery (MCA) is one of the two terminal branches of the internal carotid artery. It runs laterally along the base of the hemisphere and through the lateral sulcus before dividing into branches that supply the insula and most of the lateral surface of the frontal, parietal, temporal, and occipital lobes. It also covers the anterolateral-inferior surface of the frontal lobe. The middle cerebral artery provides almost all of the oxygen and nourishment for the lateral surface of the cerebral hemisphere. Occlusion of the branches of this vessel may deprive cortical connected with critical motor, somatosensory, auditory, and speech function.
The Middle Cerebral Artery (MCA) supplies blood to the lateral convexity of the cortex. The middle cerebral artery begins at the bifurcation of the internal carotid artery and takes an indirect course through the lateral sulcus along the surface of the insular cortex, and over the inner opercular surfaces of the frontal, temporal, and parietal lobes. It finally emerges on the lateral convexity. This complex configuration of the middle cerebral artery can be seen on angiograms.
The middle cerebral artery, also a terminal branch of the internal carotid artery, runs laterally in the lateral fissure, giving off delicate, hairlike arteries to the basal ganglia and internal capsule.
The posterior cerebral artery (PCA) arises as the terminal branch of the basilar artery. The posterior communicating artery connects the posterior cerebral and internal carotid arteries. The proximal part of the posterior cerebral artery curves over the lateral surface of the crus cerebri to reach the undersurface of the hemisphere. Its branches supply the entire occipital lobe and the inferior and medial surfaces of the temporal lobe. The calcarine branch of the posterior cerebral artery nourishes the primary visual cortex. Since deep branches of the posterior cerebral artery also supply the tectum, medial and lateral geniculate bodies, and pulvinar, note that virtually all these brain centers involved in the central processing of visual information receive their blood by a single major artery, the posterior cerebral.
The Posterior Cerebral Artery (PCA), originating where the basilar artery bifurcates courses around the lateral margin of the midbrain. This artery supplies the occipital lobe and portions of the medial and inferior temporal lobes.
The Anterior and Posterior circulations supply the diencephalon and cerebral hemispheres.
The internal carotid artery divides near the basal surface of the cerebral hemisphere to form the anterior cerebral and middle cerebral arteries.
The posterior cerebral artery is part of the vertebral basilar system and originates at the bifurcation of the basilar artery at the midbrain. The posterior cerebral artery develops from the anterior system, thereby receiving blood from the carotid arteries. However, later in development much more blood comes from the basilar artery, making the posterior cerebral artery functionally part of the posterior circulation in maturity.
Blood Supply to the Deep Forebrain
The vascular supply to structures of the deep forebrain is of great clinical significance. Even modest vascular accidents (hemorrhage or thrombosis) can produce severe, often lasting injury or even death.
The basal ganglia, including the caudate nucleus, the globus pallidus and putamen and the amygdala, the basal forebrain area, the internal capsule, and the diencephalon - represented by the thalamus and hypothalamus - are among the critical structures supplied by the deep branches of the cortical arteries.
The anterior cerebral artery, a terminal branch of the internal carotid artery sends medial striate arteries to the area of the anterior limb of the internal capsule and head of the caudate nucleus. These vessels reach their target area via the anterior perforated substance. Anterior communicating arteries from the anterior cerebral artery give off anterior medial arteries. These arteries enter the anterior perforated substance to supply the preoptic and supraoptic nuclei of the hypothalamus, the head of the caudate nucleus, and the anterior limb of the internal capsule. These are the lateral striate arteries, which include the lenticulostriate arteries.
The lenticulostriate arteries, known clinically as the "stroke arteries," are frequently the site of occlusion (thrombosis) or rupture (hemorrhage). They are prone to catastrophic vascular accidents for two reasons: They are much smaller than their artery of origin (the middle cerebral artery), thus experiencing relatively high flow forces directly from their parent vessel, and their relatively thin walls make them susceptible to rupture. In the event of the rupture of these vessels, the motor pathways compressed within the internal capsule are largely deprived of blood, resulting in paralysis on the opposite side of the body (hemiplegia).
The posterior cerebral artery originates as a terminal branch of the basilar artery and communicates with the internal carotid artery via the posterior communicating artery. The posterior cerebral artery sends thalamic branches to the posterior thalamus including the geniculate bodies and pulvinar, and the posterior limb of the internal capsule. If these vessels are occluded or ruptured, contralateral anesthesia and hemiplegia occur. An unusual complication of such an occlusion or hemorrhage is the thalamic pain syndrome, characterized by severe and intractable pain following the slightest touch of the affected part of the body. The cause is not clear but may be due to the loss of inhibitory neurons in the nucleus reticularis thalami. These neurons normally inhibit sensory relays from the thalamus to the cerebral cortex.
Deep Branches of the Anterior and Posterior Circulations supply subcortical structures
The arterial supply of the diencephalon, basal ganglia, and internal capsule derives from both the anterior and posterior circulations. This supply is complex, and there are many individual variations. The branches supplying these structures emerge from the proximal portions of the cerebral arteries or directly from the internal carotid artery. The internal capsule, the structure through which axons pass to and from the cerebral cortex, contains three separate parts; the anterior limb, the genu, and the posterior limb.Each part contains axons with different functions. Each part of the internal capsule also has a somewhat different arterial supply. The superior halves of the anterior and posterior limbs and the genu are supplied primarily by branches of the middle cerebral artery. The inferior half of the internal capsule is supplied primarily by the anterior cerebral (anterior limb and part of the genu) and anterior choroidal arteries. The basal ganglia receive their arterial blood supply from the anterior and middle cerebral arteries and the anterior choroidal artery. The thalamus is nourished by branches of the posterior cerebral and posterior communicating arteries. The hypothalamus is fed by branches of the anterior and posterior cerebral arteries and the two communicating arteries.
The vertebral and basilar arteries supply blood to the brain stem
Each of the three divisions of the brain stem and the cerebellum receives its arterial supply from the posterior circulation. In contrast to the spinal arteries, which are located both ventrally and dorsally, arteries supplying most of the brain stem arise from the ventral surface only. Branches emerge from these ventral arteries and either penetrate directly or run around the circumference of the brain stem to supply dorsal brain stem structures and the cerebellum. Three groups of branches arise from the vertebra and basilar arteries: paramedian, short circumferential, and long circumferential. The paramedian branches supply regions close to the midline. The short circumferential branches supply lateral, often wedge-shaped regions and the long circumferential branches supply the dorsolateral portions of the brain stem and cerebellum.
MEDULLA SUPPLY
Even though the spinal arteries primarily supply the spinal cord, they also supply a small portion of the caudal medulla. The spinal arteries lie close to the dorsal and ventral midline and nourish the most medial areas. The more lateral area is served by the vertebral arteries, which are equivalent to the more rostral short circumferential branches.
Most of the medulla is supplied by the vertebral arteries on the ventral surface. Small branches that exit from the main artery supply the medial medulla. Because these arteries supply axons of the corticospinal tract and the medial lemnisucus, when the arteries become occluded patients develop impairments in voluntary limb movement and mechanosensation. The major laterally emerging branch from the vertebral artery, the posterior inferior cerebellar artery (PICA), nourishes the most dorsolateral region. This region of the medulla does not receive blood from any other artery. The absence of a collateral arterial supply makes the posterior inferior cerebellar artery particularly important because occlusion almost always results in tissue damage. When this occurs, patients commonly develop characteristic sensory and motor impairments due to destruction of nuclei and tracts in the dorsolateral medulla. Common neurological signs include loss of facial pain sensation and uncoordinated limb movements, both on the side of the occlusion.
PONS SUPPLY
The vertebral arteries join to for the basilar artery at the pontomedullary junction, from which paramedian and short circumferential arteries supply the base of the pons, where corticospinal fibers are located. The dorsolateral portion of the caudal pons is supplied by a long circumferential branch of the basilar artery, termed the anterior inferior cerebellar artery (AICA). the region in the pons rostral to that supplied by the anterior inferior cerebellar artery is nourished by the superior cerebellar artery, another long circumferential branch of the basilar artery.
MIDBRAIN SUPPLY
The posterior cerebral artery nourishes most of the midbrain. Paramedian and short circumferential branches supply the base and tegmentum, whereas long circumferential branches supply the tectum. The colliculi, the most dorsal portion of the tectum, also receive a small supply by the superior cerebellar artery.
CEREBELLAR SUPPLY
Long circumferential branches of the vertebral and basilar arteries supply the cerebellum. The posterior inferior cerebellar artery supplies the caudal portion of the cerebellum. More rostral portions are supplied by the anterior inferior cerebellar artery and the superior cerebellar artery.
Cerebral Veins drain into the dural sinuses
Venous drainage of the cerebral hemispheres tis provided by superficial and deep cerebral veins. Superficial veins, arising from the cerebral cortex and underlying white matter, are variable in distribution. Among the more prominent and consistent are the superior anastomotic vein lying across the parietal lobe, and the inferior anastomotic vein on the surface of the temporal lobe. The deep cerebral veins, such as the internal cerebral vein, drain the more interior portions of the white matter, including the basal ganglia and parts of the diencephalon. Many deep cerebral veins drain into the great cerebral vein.
Drainage of blood from the central nervous system into the major vessels emptying into the heart - the systemic circulation - is achieved through either a direct or an indirect path. Spinal cord and caudal medullary veins drain directly, through a network of veins and plexuses, into the systemic circulation. By contrast, the rest of the central nervous system drains by an indirect path: The veins first empty into the dural sinuses before returning blood to the systemic circulation. The dural sinuses functions low-pressure channels for venous blood flow back to the systemic circulation. They are located between the periosteal and meningeal layers of the dura.
The superficial cerebral veins drain into the superior and inferior sagittal sinuses. The superior sagittal sinus runs along the midline at the superior margin of the falx cerebri. The inferior sagittal sinus courses along the inferior margin of the falx cerebri just above the corpus callosum. The inferior sagittal sinus, together with the great cerebral vein return venous blood to the straight sinus. At the occipital pole the superior sagittal sinus and the straight sinus join to form the two transverse sinuses. Finally, these sinuses drain into the sigmoid sinuses, which return blood to the internal jugular veins. The cavernous sinus, into which drain the ophthalmic and facial veins, is also illustrated in figure 4-16.
Veins of the midbrain drain into the great cerebral vein, which empties into the straight sinus, whereas the pons and rostral medulla drain into the superior petrosal sinus. Cerebellar veins drain into the great cerebral vein and the superior petrosal sinus.
The Blood Brain Barrier
The blood brain barrier isolates the chemical environment of the central nervous system from that of the rest of the body
The intravascular compartment is isolated from he extracellular compartment of the central nervous system. This feature, the blood brain barrier was discovered when intravenous dye injection stained most tissues and organs of the body but not the brain. This permeability barrier protects the brain from neuroactive compounds in the blood as well as rapid changes in the ionic constituents of the blood that can affect neuronal excitability.
The blood brain barrier is thought to result from two characteristics of endothelial cells in the capillaries of the brain and spinal cord. first, in peripheral capillaries, endothelial cells have fenestrations (pores) that allow large molecules to flow into the extracellular space. Moreover, the intercellular spaces between adjacent endothelial cells are leaky. In contrast, in central nervous system capillaries, adjacent endothelial cells are tightly joined, preventing movement of compounds into the extracellular compartment the central nervous system. Second, there is little transcellular movement of compounds from intravascular to extracellular compartments in the central nervous system because the endothelial cells lack the required transport mechanisms. Moreover, relatively nonselective transport may occur by pinocytosis in peripheral but not central nervous system capillaries.
Although most of the central nervous system is protected by the blood-brain barrier, eight brain structures lack a blood-brain barrier. These structures are close to the midline and because they are closely associated with the ventricular system, are collectively termed circumventricular organs At each of these structures, either neurosecretory products are secreted into the blood or local neurons detect blood-borne compounds as part of a mechanism for regulating the body's internal environment. Regions that lack a blood-brain barrier are not isolated from the rest of the brain. Rather, it has been proposed that rapid venous drainage from these areas protects surrounding regions.
Circulus Arteriosus aka The Circle of Willis
The circulus arteriosus or Circle of Willis is a site of multiple vascular interconnections between the anterior and posterior arterial systems. It is a vascular loop or network of interconnected arteries (anastamosis) formed from the vertebral-basilar and internal carotid arterial systems (proximal portions of the cerebral and the communicating arteries) located on the anterior-inferior (ventral) surface of the upper brain stem. The rostral portion of the Circle of Willis consists of the two anterior cerebral arteries, which arise from the terminal part of the internal carotid arteries. The paired anterior cerebral vessels are joined just anterior to the optic chiasm by the anterior communicating artery, completing the rostral part of the loop. The anterior cerebral arteries proceed anteriorly between the hemispheres. Under normal conditions, there is no or very little mixing between left and right vessels of the loop.
The Circle of Willis gives rise to several arteries that supply both central and cortical brain areas. Central arteries, coming off the circle at several sites, as well as the lenticulostriate branches of the middle cerebral arteries, penetrate and supply the diencephalon, the corpus striatum, and the internal capsule. Obstruction of these small vessels brings on serious neurological deficits due to inadequate collateral circulation within their areas of distribution. Cortical branches, such as the anterior, middle, and posterior cerebral arteries, are larger and branch more freely. Occlusion of one of these vessels can accordingly be partially compensated for by anastomotic vessels.
The posterior communicating artery allows blood to flow between the middle and posterior cerebral arteries, and the anterior communicating artery allows blood to flow between the anterior cerebral arteries on both sides of the cerebral hemispheres. When either the posterior or the anterior arterial circulation becomes occluded, collateral circulation may occur through the circle of Willis to rescue the region deprived of blood. Many individuals, however, lack one of the components of the circle of Willis. In these individuals, a functional "circle" may not be achieved, resulting in incomplete cerebral perfusion by the surviving system.
The posterior communicating arteries arise at the junction of the internal carotid and middle cerebral arteries. These posterior communicating vessels are directly inferiorly and posteriorly on either side of the hypothalamic surface and the stalk of the pituitary gland, and they anastamose (connect) with the initial segment of the posterior cerebral arteries. The posterior cerebral vessels originate from the bifurcation of the basilar artery. The irregular loop (or circle) thus formed surrounds the inferior surface of the diencephalon, including the optic chiasm, pituitary stalk, and mammillary bodies.
The basilar artery is formed by the union of the left and right vertebral arteries. Located inferior to the circulus arteriosus on the anterior-inferior surface of the pons, the basilar artery is the connecting vessel between the vertebral and carotid systems. It terminates at the pontine-midbrain junction by dividing into the posterior cerebral arteries. Branches of the basilar artery include (from caudal to rostral) the anterior inferior cerebellar arteries, which supply the inferior aspect of the cerebellum anterolaterally and communicate with branches of the posterior inferior cerebellar artery (J; from the vertebral artery); the labyrinthine arteries to the internal ear; the pontine arteries to the pons; and the superior cerebellar arteries to the pons, pineal gland, and superior part of the cerebellum.
When the shit hits the fan, background and events
What happens when the shit hits the fan?
The terminal ends of the cerebral arteries anastomose on the lateral convexity of the cerebral hemisphere. These interconnections or networks occur between branches only when they are located on the cortical surface, not when the artery has penetrated the brain. When a major artery becomes occluded, these anastomoses limit the extent of damage. For example, if a branch of the posterior cerebral artery becomes occluded, tissue with compromised blood supply in the occipital lobe may be rescued by collateral circulation from the middle cerebral artery that connects anastomotically with the blocked vessel.
This collateral circulation can rescue the gray matter of the cerebral cortex. In contrast, little collateral circulation exists between the regions perfused by the cerebral arteries in the white matter. Although collateral circulation provides the cerebral cortex with a margin of safety during arterial occlusion, the anastomotic network that provides such insurance also creates a vulnerability. When systemic blood pressure is reduced, the region served by this network is particularly susceptible to ischemia because such anastomoses occur at the terminal ends of the arteries, regions where perfusion is lowest. The peripheral borders of the territory supplied by major vessels are termed border zones, and an infarction occurring in these regions is termed a border zone infarct.
Arteriograms of the cerebellar and spinal branches of the vertebral arteries are used to search for space-occupying lesions associated the posterior cranial fossae and cervical spinal cord, respectively. As these avascular masses form, they may distort the normal pattern of these vessels. Introduction of contrast medium into the vertebral system is usually accomplished by injection into the subclavian artery or percutaneous (through the skin) catheterization through the femoral artery of the thigh.
The principal source of nourishment for the central nervous system is glucose, and because neither glucose nor oxygen is stored in appreciable amounts, when the blood supply of the central nervous system is interrupted, even briefly, brain functions become severely disrupted.
Following massive constriction of one of the vessels of the loop, some collateral flow occurs across the mid-line, as evidenced by widening of the communicating vessels.
Saclike outpocketings or localized dilations of arteries are known as aneurysms (widenings). Such abnormalities, usually congenital (present at birth) in origin, range from microscopic dimensions to the size of a golf ball. They are frequently found to be associated with vessels of the Circle of Willis. About 90 percent of such aneurysms occur in the anterior part of the circle. Visualization of cerebral aneurysms has been enhanced by the technique of digital subtraction angiography (in which the vessels in front and in back of the vessel to be studied are eliminated from the angiogram by a digital computer). Rupture of aneurysms may occur spontaneously or may be related to short periods of increased blood pressure; subsequent hemorrhage can result in serious neurological deficits or in death. Aneurysms can often be treated surgically by deploying strands of metal that structurally stabilize the aneurysm or placing stents.
Under normal conditions, cessation of blood flow (ischemia) to the brain for 5 to 10 seconds is sufficient to cause temporary changes in neuronal activity and interruption of flow for 5 to 10 minutes can produce irreversible neuronal damage.
Brain vasculature disorders constitute a major class of nervous system disease.
At the point of carotid bifurcation, the disease atherosclerosis (gruel-hardening) frequently occurs. The formation of granular, lipid-containing, plate-like atheromatous plaques in the inner layer of certain arteries. These plaques can build up in the internal carotid arteries to such an extent that they obstruct the blood supply to the brain (cerebrovascular disease). Cardiovascular surgeons can often remove these plaques from the carotid bifurcation by a technique known as a carotid endarterectomy, restoring the blood supply to the brain.
Decreased blood supply occurs when an artery becomes occluded or when systemic blood pressure drops substantially. An occlusion commonly occurs because of an acute blockade, such as from an embolus, or the gradual narrowing of the arterial lumen (stenoses) as in atherosclerosis. A brief reduction in blood flow produces transient neurological signs, attributable to lost functions of the blood-deprived area. This event is termed a transient ischemic attack. If ischemia is persistent and is uncorrected for several minutes, it can produce tissue death (infarction) which can result in more enduring or even permanent impairments. Under special circumstances the local reduction in arterial blood flow may not produce an ischemia or infarction because the tissue receives a redundant supply from another artery (collateral circulation.)
Hemorrhagic stroke can occur when an artery ruptures, thereby releasing blood into the surrounding tissue. A hemorrhagic stroke not only produces a loss of downstream flow but also can damage brain tissue at the rupture site because of the volume now occupied by the blood outside of the vessel. A common cause of a hemorrhagic stroke is when an aneurysm, or ballooning of an artery due to weakening of the muscular wall, ruptures.
The Circle of Willis gives rise to several arteries that supply both central and cortical brain areas. Central arteries, coming off the circle at several sites, as well as the lenticulostriate branches of the middle cerebral arteries, penetrate and supply the diencephalon, the corpus striatum, and the internal capsule. Obstruction of these small vessels brings on serious neurological deficits due to inadequate collateral circulation within their areas of distribution. Cortical branches, such as the anterior, middle, and posterior cerebral arteries, are larger and branch more freely. Occlusion of one of these vessels can accordingly be partially compensated for by anastomotic vessels.
The posterior communicating artery allows blood to flow between the middle and posterior cerebral arteries, and the anterior communicating artery allows blood to flow between the anterior cerebral arteries on both sides of the cerebral hemispheres. When either the posterior or the anterior arterial circulation becomes occluded, collateral circulation may occur through the circle of Willis to rescue the region deprived of blood. Many individuals, however, lack one of the components of the circle of Willis. In these individuals, a functional "circle" may not be achieved, resulting in incomplete cerebral perfusion by the surviving system.
The posterior communicating arteries arise at the junction of the internal carotid and middle cerebral arteries. These posterior communicating vessels are directly inferiorly and posteriorly on either side of the hypothalamic surface and the stalk of the pituitary gland, and they anastamose (connect) with the initial segment of the posterior cerebral arteries. The posterior cerebral vessels originate from the bifurcation of the basilar artery. The irregular loop (or circle) thus formed surrounds the inferior surface of the diencephalon, including the optic chiasm, pituitary stalk, and mammillary bodies.
The basilar artery is formed by the union of the left and right vertebral arteries. Located inferior to the circulus arteriosus on the anterior-inferior surface of the pons, the basilar artery is the connecting vessel between the vertebral and carotid systems. It terminates at the pontine-midbrain junction by dividing into the posterior cerebral arteries. Branches of the basilar artery include (from caudal to rostral) the anterior inferior cerebellar arteries, which supply the inferior aspect of the cerebellum anterolaterally and communicate with branches of the posterior inferior cerebellar artery (J; from the vertebral artery); the labyrinthine arteries to the internal ear; the pontine arteries to the pons; and the superior cerebellar arteries to the pons, pineal gland, and superior part of the cerebellum.
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The terminal ends of the cerebral arteries anastomose on the lateral convexity of the cerebral hemisphere. These interconnections or networks occur between branches only when they are located on the cortical surface, not when the artery has penetrated the brain. When a major artery becomes occluded, these anastomoses limit the extent of damage. For example, if a branch of the posterior cerebral artery becomes occluded, tissue with compromised blood supply in the occipital lobe may be rescued by collateral circulation from the middle cerebral artery that connects anastomotically with the blocked vessel.
This collateral circulation can rescue the gray matter of the cerebral cortex. In contrast, little collateral circulation exists between the regions perfused by the cerebral arteries in the white matter. Although collateral circulation provides the cerebral cortex with a margin of safety during arterial occlusion, the anastomotic network that provides such insurance also creates a vulnerability. When systemic blood pressure is reduced, the region served by this network is particularly susceptible to ischemia because such anastomoses occur at the terminal ends of the arteries, regions where perfusion is lowest. The peripheral borders of the territory supplied by major vessels are termed border zones, and an infarction occurring in these regions is termed a border zone infarct.
Arteriograms of the cerebellar and spinal branches of the vertebral arteries are used to search for space-occupying lesions associated the posterior cranial fossae and cervical spinal cord, respectively. As these avascular masses form, they may distort the normal pattern of these vessels. Introduction of contrast medium into the vertebral system is usually accomplished by injection into the subclavian artery or percutaneous (through the skin) catheterization through the femoral artery of the thigh.
The principal source of nourishment for the central nervous system is glucose, and because neither glucose nor oxygen is stored in appreciable amounts, when the blood supply of the central nervous system is interrupted, even briefly, brain functions become severely disrupted.
Following massive constriction of one of the vessels of the loop, some collateral flow occurs across the mid-line, as evidenced by widening of the communicating vessels.
Saclike outpocketings or localized dilations of arteries are known as aneurysms (widenings). Such abnormalities, usually congenital (present at birth) in origin, range from microscopic dimensions to the size of a golf ball. They are frequently found to be associated with vessels of the Circle of Willis. About 90 percent of such aneurysms occur in the anterior part of the circle. Visualization of cerebral aneurysms has been enhanced by the technique of digital subtraction angiography (in which the vessels in front and in back of the vessel to be studied are eliminated from the angiogram by a digital computer). Rupture of aneurysms may occur spontaneously or may be related to short periods of increased blood pressure; subsequent hemorrhage can result in serious neurological deficits or in death. Aneurysms can often be treated surgically by deploying strands of metal that structurally stabilize the aneurysm or placing stents.
Under normal conditions, cessation of blood flow (ischemia) to the brain for 5 to 10 seconds is sufficient to cause temporary changes in neuronal activity and interruption of flow for 5 to 10 minutes can produce irreversible neuronal damage.
Brain vasculature disorders constitute a major class of nervous system disease.
At the point of carotid bifurcation, the disease atherosclerosis (gruel-hardening) frequently occurs. The formation of granular, lipid-containing, plate-like atheromatous plaques in the inner layer of certain arteries. These plaques can build up in the internal carotid arteries to such an extent that they obstruct the blood supply to the brain (cerebrovascular disease). Cardiovascular surgeons can often remove these plaques from the carotid bifurcation by a technique known as a carotid endarterectomy, restoring the blood supply to the brain.
Decreased blood supply occurs when an artery becomes occluded or when systemic blood pressure drops substantially. An occlusion commonly occurs because of an acute blockade, such as from an embolus, or the gradual narrowing of the arterial lumen (stenoses) as in atherosclerosis. A brief reduction in blood flow produces transient neurological signs, attributable to lost functions of the blood-deprived area. This event is termed a transient ischemic attack. If ischemia is persistent and is uncorrected for several minutes, it can produce tissue death (infarction) which can result in more enduring or even permanent impairments. Under special circumstances the local reduction in arterial blood flow may not produce an ischemia or infarction because the tissue receives a redundant supply from another artery (collateral circulation.)
Hemorrhagic stroke can occur when an artery ruptures, thereby releasing blood into the surrounding tissue. A hemorrhagic stroke not only produces a loss of downstream flow but also can damage brain tissue at the rupture site because of the volume now occupied by the blood outside of the vessel. A common cause of a hemorrhagic stroke is when an aneurysm, or ballooning of an artery due to weakening of the muscular wall, ruptures.
Runoff
The cerebral cortex is supplied by the distal branches of the anterior, middle, and posterior cerebral arteries. These branches are often termed cortical branches to differentiate them from the deep branches supplying the diencephalon, basal ganglia and internal capsule. Knowledge of the approximate boundaries of the cortical regions supplied by the different cerebral arteries helps explain the functional disturbances that follow vascular obstruction, or other pathology, of the cerebral vessels.
The cerebral cortex is supplied by the distal branches of the anterior, middle, and posterior cerebral arteries. These branches are often termed cortical branches to differentiate them from the deep branches supplying the diencephalon, basal ganglia and internal capsule. Knowledge of the approximate boundaries of the cortical regions supplied by the different cerebral arteries helps explain the functional disturbances that follow vascular obstruction, or other pathology, of the cerebral vessels.