The Blood Supply And The Spinal Cord Biology Essay

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Cerebrovascular accidents (stroke) still remain the third leading cause of morbidity and death in the US. Consequently, it is important to know the areas of the cerebral cortex and spinal cord supplied by a particular artery and to understand the dysfunction that would result if the artery were blocked. The internal capsule that contains the major ascending the descending pathways to the cerebral cortex is commonly disrupted by arterial haemorrhage or thrombosis.

The ophthalmic artery arises as the internal carotid artery emerges from the cavernous sinus. It enters the orbit through the optic canal below and lateral to the optic nerve. It supplies the eye and other orbital structures, and its terminal branches supply the frontal area of the scalp, ethmoid and frontal sinuses, and the dorsum of the nose.

The posterior communicating artery is a small vessel that originates from the internal carotid artery close to its terminal bifurcation. The posterior communicating artery runs posteriorly above the occulomotor nerve to join the posterior cerebral artery, thus forming part of the circle of Willis.

The choroidal artery, a small branch, also originates from the internal carotid artery close to its terminal bifurcation. The choroidal artery passes posteriorly close to the optic tract, enters the inferior horn of the lateral ventricle, and ends in the choroid plexus. It gives off numerous small branches to surrounding structures, including the crus cerebri, the lateral geniculate body, the optic tract and the internal capsule.

The anterior cerebral artery is the smaller terminal branch of the internal carotid artery. It runs forward ad medially superior to the optic nerve and enters the longitudinal fissure of the cerebrum. Here, it is joined to the anterior cerebral artery of the opposite side by the anterior communicating artery. It curves backward over the corpus callosum and finally, anastomoses with the posterior cerebral artery. The cortical branches supply all the medial surface of the cerebral cortex as far back as the parietoocciputal sulcus. they also supply a strip of cortex about 2.5cm wide on the adjoining lateral surface. The anterior cerebral artery thus supplies the "leg area"of the precentral gyrus. A group of central branches pieces the anterior perforated substance and helps to supply parts of the lentiform and caudate nuclei and the internal capsule.

Middle cerebral artery, the largest branch of the internal carotid, runs laterally in teh lateral cerebral sulcus. Cortical branches supply the entire lateral surface of the hemisphere, except for the narrow strip supplied by the anterior cerebral artery, the occipital pole, and the inferolateral surface of the hemisphere, which are supplied by the posterior cerebral artery. This artery thus supplies all the motor area except the "leg area". Central branches enter the anterior perforated substance and supply the lentiform and caudate nucleus and the internal capsule.

Vertebral artery

The vertebral artery, a branch of the first part of the subclavian artery, ascends the neck by passing through the foramina in the transverse processes of the upper 6 cervical vertebrae. It enters the skull through the foramen magnum and pierces the dura mater and arachnoid to enter the subarachnoid space. It then passes upward, forward and medially on the medulla oblongata. At the lower border of the pons, it joins the vessel of the opposite side to form the basilar artery.

Branches of the cranial portion

The meningeal branches are small and supply the bone and dura in the posterior cranial fossa.

The posterior spinal artery - arise from vertebral artery or the posterior inferior cerebellar artery. It descends on the posterior surface of the spinal cord close to the posterior roots of the spinal nerves. The branches are reinforced by radicular arteries that enter the vertebral canal through the intervertebral foramina.

The anterior spinal artery - contributory branch from each vertebral artery near its termination. The single artery descends on the anterior surface of the medulla oblongata and spinal cord and is embedded in the pia mater along the anterior median fissure. The artery is reinforced by radicular arteries that enter the vertebral canal through the intervertebral foramina.

The posterior inferior cerebellar artery, the largest branch of the vertebral artery, passes on an irregular course between the medulla and the cerebellum. It supplies the inferior surface of the vermis, central nuclei of the cerebellum and undersurface of the cerebellar hemisphere. It also supples the medulla oblongata, choroid plexus of the 4th ventricle.

Medullary arteries - very small branches that are distributed to the medulla oblongata.

Basilar artery

The basilar artery is formed by the union of the two vertebral arteries, ascends in a groove on the anterior surface of the pons. At the upper border of the pons, it divides into the two posterior cerebral arteries.

Branches

Pontine arteries - numerous small vessels that enter the substance of the pons.

Labyrinthine artery is a long, narrow artery that accompanies the facial and the vestibulocochlear nerves into the internal acoustic meatus and supplies the internal ear. It often arises as a branch of the anterior inferior cerebellar artery.

Anterior inferior cerebellar artery passes posteriorly and laterally and supplies the anterior + inferior part of the cerebellum. A few branches pass to the pons and the upper part of the medulla oblongata.

Superior cerebellar artery - close to the termination of the basilar artery. It winds around the cerebral peduncle and supplies the superior surface of the cerebellum. It also supplies the pons, the pineal gland and the superior medullary velum.

Posterior cerebral artery - curves laterally and backward around the midbrain and is joined by the posterior communicating branch of the internal carotid artery. Cortical branches supply the inferolateral and medial surfaces of the temporal lobe and the lateral and medial surfaces of the occipital lobe. Thus, the posterior cerebral artery supplies the visual cortex. Central branches pierce the brain substance and supply parts of the thalamus and the lentiform nucleus as well as the midbrain, the pineal and the medial geniculate bodies. A choroidal branch enters the inferior horn of the lateral ventricle and supplies the choroid plexus of the 3rd ventricle.

Circle of Willis

The circle of Willis lies in the interpeduncular fossa at the base of the brain. It is formed by the anastomosis between the two internal carotid arteries and the two vertebral arteries. The anterior communicating, anterior cerebral, internal carotid, posterior communicating, posterior cerebral and basilar arteries all contribute to the circle. The circle of Willis allows blood that enters by either internal carotid or vertebral arteries to be distributed to any part of both cerebral hemispheres. Cortical and central branches arise from the circle and supply the brain substance.

Variations in the sizes of the arteries forming the circle are common, and the absence of one or both posterior communicating arteries has been reported.

Arteries to specific brain areas

The corpus striatum and the internal capsule are supplied mainly by the medial and lateral striate central branches of the middle cerebral artery; the central branches of the anterior cerebral artery supply the remainder of these structures.

The thalamus is supplied mainly by branches of the posterior communicating, basilar, and posterior cerebral arteries.

The midbrain is supplied by the posterior cerebral, superior cerebellar, and basilar arteries.

The pons is supplied by the basilar and the anterior, inferior and superior cerebellar arteries.

The medulla oblongata is supplied by the vertebral, anterior and posterior spinal, posterior inferior cerebellar, and basilar arteries.

The cerebellum is supplied by the superior cerebellar, anterior inferior cerebellar and posterior inferior cerebellar arteries.

Nerve supply of the cerebral arteries

The cerebral arteries receive a rich supply of sympathetic postganglionic nerve fibres. These fibres are derived from the superior cervical sympathetic ganglion. Stimulation of these nerves causes vasoconstriction of the cerebral arteries. However, under normal conditions, the local blood flow is mainly controlled by the concentrations of carbon dioxide, hydrogen ions, and oxygen present in the nervous tissue; a rise in the carbon dioxide and hydrogen ion concentrations and a lowering of the oxygen tension bring about a vasodilation.

Veins of the brain

The veins of the brain have no muscular tissue in their very thin walls, and they possess no valves. They emerge from the brain and lie in the subarachnoid space. They pierce the arachnoid mater and the meningeal layer of the dura and drain into the cranial venous sinuses.

External cerebral veins

The superior cerebral veins pass upward over the lateral surface of the cerebral hemisphere and empty into the superior saggital sinus.

The superficial middle cerebral vein drains the lateral surface of the cerebral hemisphere. It runs inferiorly in the lateral sulcus and empties into the cavernous sinus.

The deep middle cerebral vein drains the insula and is joined by the anterior cerebral and striate veins to form the basal vein. The basal vein ultimately joins the great cerebral vein, which in turn drains into the straight sinus.

Internal cerebral veins

There are two internal cerebral veins, and they are formed by the union of the thalamostriate vein and the choroid vein at the interventricular foramen. The two veins run posteriorly in the tela choroidea of the third ventricle and unite beneath the splenium of the corpus callosum to form the great cerebral vein, which empties into the straight sinus.

Veins of specific brain areas

The midbrain is drained by veins that open into the basal of great cerebral veins.

The pons is drained by veins that open into the basal vein, cerebellar veins or neighbouring venous sinuses.

The medulla oblongata is drained by veins that open into the spinal veins and neighbouring venous sinuses.

The cerebellum is drained by veins that empty into the great cerebral vein or adjacent venous sinuses.

Brain capillaries

The capillary blood supply to the brain is greater in the gray matter than in the white matter. This is to be expected, since the metabolic activity in the neuronal cell bodies in the gray matter is much greater than in the nerve processes in the white matter. The BBB isolates the brain tissue from the rest of the body and is formed by tight junctions that exist between the endothelial cells in the capillary beds.

Cerebral circulation

The blood flow to the brain must deliver oxygen, glucose, and other nutrients to the nervous tissue and remove carbon dioxide, lactic acid, and other metabolic by-products. The brain has been shown to be supplied with arterial blood from the two internal carotid arteries and the two vertebral arteries. The blood supply to half of the brain is provided by the internal carotid and vertebral arteries on that side, and their respective streams come together in the posterior communicating artery at a point where the pressure of the two is equal and they do not mix. If however, the internal carotid or vertebral artery is occluded, the blood passes forward or backward across that point to compensate for the reduction in blood flow. The arterial circle also permits the blood to flow across the midline, as shown when the internal carotid or vertebral artery on one side is occluded. It also has been shown that the two streams of blood from the vertebral arteries remain separate and on the same side of the lumen of the basilar artery and do not mix.

Although the cerebral arteries anastomose with one another at the circle of Willis and by means of branches o the surface of the cerebral hemispheres, once they enter the brain substance no further anastomoses occur.

The most important factor in forcing the blood through the brain is the arterial blood pressure. This is opposed by such factors as a raised intracranial pressure, increased blood viscosity, and narrowing of the vascular diameter. Cerebral blood flow remains remarkably constant despite changes in the general blood pressure. This autoregulation of the circulation is accomplished by a compensatory lowering of the cerebral vascular resistance when the arterial pressure is decreased and a raising of the vascular resistance when the arterial pressure is increased. Needless to say, this autoregulation does not maintain adequate blood blow when the arterial blood pressure falls to a very low level.

The diameter of the cerebral blood vessels is the main factor contributing to the cerebrovascular resistance. While it is known the cerebral blood vessels are innervated by sympathetic postganglionic nerve fibres and respond to norepinephrine, they apparently play little or no part in the control of cerebrovascular resistance in normal beings. The most powerful vasodilator influence on cerebral blood vessels is an increase in carbon dioxide or hydrogen ion concentration; a reduction in oxygen concentration also causes vasodilation. It has been shown, using PET, that an increase in neuronal activity un different parts of the brain causes a local increase in blood flow. For example, viewing an object will increase the oxygen and glucose consumption in the visual cortex of the occipital lobes. This results in an increase in the local concentrations of carbon dioxide and hydrogen ions and brings about a local increase in blood flow.

The cerebral blood flow in patients can be measured by the intracarotid injection or inhalation of radioactive krypton or xenon. A cerebral blood flow of 50-60 mL per 100g of brain per minute is considered normal.

Blood supply of the spinal cord

Arteries of the spinal cord

The spinal cord receives its arterial supply from three small arteries: the two posterior spinal arteries and the anterior spinal artery. These longitudinally running arteries are reinforced by small segmentally arranged arteries that arise from arteries outside the vertebral column and enter the vertebral canal through the intervertebral foramina. These vessels anastomose on the surface of the cord and send branches into the substance of the white and gray matter. Considerable variation exists as to the size and segmental levels at which the reinforcing arteries occur.

Posterior spinal arteries

The posterior spinal arteries arise either directly from the vertebral arteries inside the skull or indirectly from the posterior inferior cerebellar arteries. Each artery descends on the posterior surface of the spinal cord close to the posterior nerve roots and gives off branches that enter the substance of the cord. The posterior spinal arteries supply the posterior 1/3 of the spinal cord.

The posterior spinal arteries are small in the upper thoracic region, and the first 3 thoracic segments of the spinal cord are particularly vulnerable to ischaemia should the segmental or radicular arteries in this region be occluded.

Anterior spinal artery

The anterior spinal artery is formed by the union of the two arteries, each of which arises from the vertebral artery inside the skull. The anterior spinal artery then descends on the anterior surface of the spinal cord within the anterior median fissure. Branches from the anterior spinal artery enter the substance of the cord and supply the anterior 2/3 of the spinal cord.

In the upper and lower thoracic segments of the spinal cord, the anterior spinal artery may be extrememly small. Should the segmental or radicular arteries be occluded in these regions, the fouth thoracic and the first lumbar segments of the spinal cord would be particularly liable to ischaemic necrosis.

Segmental spinal arteries

At each intervertebral foramen, the longitudinally running posterior and anterior spinal arteries are reinforced by small segmental arteries on both sides. The arteries are branches of arteries outside the vertebral column (deep cervical, intercostals, and lumbar arteries). Having entered the vertebral canal, each segmental spinal artery gives rise to the anterior and posterior radicular arteries that accompany the anterior and posterior nerve roots to the spinal cord.

Addition feeder arteries enter the vertebral canal and anastomose with the anterior and posterior spinal arteries; however, the number and size of these arteries vary considerably from one individual to another. One large and important feeder artery, the great anterior medullary artery of Adamkiewicz, arises from the aorta in the lower thoracic or upper lumbar vertebral levels; it is unilateral and, in the majority of persons, enters the spinal cord from the left side. The importance of this surgery lies in the fact that it may be the major source of blood into the lower 2/3 of the spinal cord.

Veins of the spinal cord

The veins of the spinal cord drain into six tortuous longitudinal channels that communicate superiorly within the skull with the veins of the brain and the venous sinuses. They drain mainly into the internal vertebral venous plexus.

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