The vertebral arteries (VAs) normally arise as the first branches of the subclavian artery (see Fig..22-1 ). They course superiorly and medially to enter the transverse foramen of C6. The VAs course vertically through the transverse foramen of C3-C6 and then turn superolaterally to pass through the C2 foramen. After the arteries pass through the foramen of the atlas, they curve around the atlanto-occipital joint and lie in a horizontal groove along the posterior arch of C1. The VAs then curve cephalad to enter the skull through the foramen magnum.
The basilar artery is formed by the union of the two vertebral arteries, which takes place at the lower border of the ventral pons (see Fig 2215 ). It continues superiorly to terminate in the interpeduncular cistern by dividing into the posterior cerebral arteries. The branches of the basilar artery can be divided into paramedian arteries, short circumferential arteries, and long circumferential arteries. The paramedian vessels are four to six in number, and they penetrate into the pontine parenchyma to supply the medial basal pons. The short circumferential arteries enter the brachium pontis to supply the ventrolateral basis pontis. The long circumferential arteries include the superior cerebellar artery (SCA), anterior inferior cerebellar artery (AICA), and the internal auditory artery.
LONG CIRCUMFERENTIAL ARTERIES (PICA, AICA, SCA)
The arterial supply to the cerebellum includes the posterior inferior cerebellar artery (PICA), the anterior inferior cerebellar artery (AICA), and the superior cerebellar artery (SCA). PICA is the largest branch of the vertebral artery (.Figs..22-5 and 22-6. ). PICA encircles the medulla to supply the lateral medulla. The distal portion of PICA then bifurcates into a medial trunk that supplies the vermis and the adjacent cerebellar hemisphere and a lateral trunk that supplies the cortical surface of the tonsil and cerebellar hemisphere.
AICA most often arises directly from the basilar artery (see Figs 2.2.-5 and 22-6. ). Rarely, it may arise from the vertebral artery. In its normal course, it runs across the pons, enters the cerebellopontine angle cistern, and then forms a tight loop before running over the cerebellum. AICA supplies the lateral tegmentum of the lower two thirds of the pons and the ventrolateral cerebellum. The internal auditory artery arises from AICA to supply the auditory and facial nerves.
The SCA arises from the basilar artery near its terminal bifurcation (see Figs...22-5 and 2.2.-6 ). It courses posterolaterally in the perimesencephalic cistern to circle the brain stem in the region between the pons and the mesencephalon. The SCA supplies the superolateral cerebellar hemispheres, the superior cerebellar peduncle, the dentate nucleus, and part of the middle cerebellar peduncle.
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