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Blood Supply To The Brain Notes

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Core: common carotid, external carotid, internal carotid, vertebral (and its subclavian origin)
 The arch of the aorta gives off the braciocephalic artery, the left common carotid artery and the left subclavian artery.
 The braciocephalic artery branches into the right subclavian and right common carotid.
 The common carotids divide into external and internal carotid arteries.
 The vertebral arteries are branches of the subclavian arteries.
 The internal carotid arteries and vertebral arteries are the four main arteries supplying the brain with blood.
 These form an internal carotid system and a vertebrobasilar system, respectively, with the former system supplying the anterior and middle parts of the brain, and the latter supplying posterior regions
 The internal carotids enter the skull through the carotid canal and form the internal carotid system, which supplies anterior and middle parts of the brain. 80% of blood supply.
 The vertebral arteries enter the skull through the foramen magnum and join to form the basilar artery and vertebro-basilar system supplying posterior parts of the brain. 20% of blood supply. Core: circle of Willis Extension: arterial anastamoses
 These two systems are joined by communicating arteries, forming a large anastomosis at the base of the brain called the circle of Willis.
 The circle of Willis is an important protective feature, as blood supply to a particular region can be maintained, at least to a certain extent, by flow through other vessels if one becomes occluded.
 Blood flow through these anastomoses is limited, however, such that occlusion of an artery can lead to serious ischaemia.

The vertebro-basilar system The vertebral artery and its branches
 The vertebral arteries run in the transverse foramina of the upper six cervical vertebrae then pass through the foramen magnum and run over the ventral surface of the medulla.
 They give off a number of branches: Core: posterior inferior cerebellar
 The posterior inferior cerebellar arteries are the largest branches of the vertebral artery.
 They branch off at the level of the caudal end of the medulla, running dorsally and laterally around the brainstem, supplying the lateral parts of the medulla.
 They have a tortuous course amongst the glossopharyngeal, hypoglossal and vagus nerve rootlets.
 They then run towards the ventral, caudal region of the cerebellum, which they supply.
 They also have a branch supplying the choroid plexus of the fourth ventricle. Core: posterior spinal
 The posterior spinal arteries may branch directly from the vertebral artery but more commonly branch from the posterior inferior cerebellar arteries.
 They run caudally, through the foramen magnum and over the posterior surface of the spinal cord. Core: anterior spinal
 The anterior spinal arteries branch off just before the formation of the basilar artery and run caudally and medially, uniting in the midline to form a single artery.
 This runs in the anterior medial fissure of the medulla, giving off fine branches that supply some regions of the medulla.
 It then exits through the foramen magnum and runs caudally over the anterior surface of the spinal cord, which it supplies.

Core: reinforcement of spinal supply - artery of Adamkiewicz
 It typically arises from a left posterior intercostal artery which branches from the aorta and supplies the lower two thirs of the spinal cord via the anterior spinal artery Spinal Blood Supply
 As shown in the diagram of the arterial supply to the brain, the vertebral artery gives off branches supplying the spinal cord as well as branches supplying the brain: the anterior spinal artery and the paired posterior spinal arteries.
 Although these vessels carry blood cranio-caudally along the entire length of the spinal cord, it is only the cervical region that receives a sufficient supply.
 Anastomoses with radicular arteries that branch from segmental arteries such as the ascending cervical, intercostal and lumbar arteries, fulfil the requirements of more caudal regions of the cord.
 The venous drainage runs closely with the arterial supply: there is a single posterior spinal vein, a pair of posterolateral spinal veins and a single anterior spinal vein.
 These drain via radicular veins to the internal vertebral venous plexus, from where blood passes into the external vertebral venous plexous, returning to the main circulation in the lumbar, azygous and hemi-azygous veins.

The basilar artery and its branches

Core: basilar
 At the inferior end of the pons the vertebral arteries join to form the basilar artery, which runs rostrally in the basilar groove of the pons. Extension: anterior inferior cerebellar artery
 Extension: the anterior inferior cerebellar artery branches from the caudal part of the basilar artery.
 It runs laterally and posteriorly to the antero-lateral parts of the inferior aspect of the cerebellum and anastomoses with the posterior inferior cerebellar artery. Core: pontine
 The pontine arteries are a number of small branches that enter and supply the pons.
 Extension: the labyrynthine artery is a small branch that enters the internal auditory meatus with the facial and vestibulocochlear nerves, providing the supply to the inner ear.
 If occluded, vertigo and ipsilateral deafness can occur.
 The superior cerebellar artery branches close to the termination of the basilar artery.
 It winds laterally over the cerebral peduncle (roughly corresponds to the mesencephalon) and runs over the superior surface of the cerebellum. Core: posterior cerebral
 The occulomotor nerve and free edge of the tentorium cerebelli separate it from the posterior cerebral artery.
 The terminal branches of the basilar artery are the posterior cerebral arteries, which can be seen at the rostral border of the pons.
 They run laterally around the cerebral peduncles to the posterior surface of the cerebrum, supplying parts of the occipital and temporal cortex, including the visual cortex.
 There are therefore often losses of vision if this artery is occluded.

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