Blood supply of the Brain
Internal carotid artery
The Internal Carotid Artery is one of the two terminal branches of the common carotid artery. It begins at the level of the C4 vertebra and then passes deeply into the neck, where it gives off no branches before entering the carotid canal in the temporal bone. After traversing the carotid canal it opens into the cranial cavity at the sides of foramen lacerum. It then forms an S-shaped configuration as it passes through the cavernous sinus, where it gives off branches to the hypophysis (pituitary gland) and orbit (ophthalmic artery) before piercing the dura mater. It then gives off the slender posterior communicating artery before bifurcating into its two terminal branches, the anterior and middle cerebral arteries, just lateral to the optic chiasm.
The internal carotid artery supplies the hypophysis, eye and orbit, the cortex of the frontal lobe, the lateral surface of the parietal, temporal and occipital lobes, lateral ventricles, internal capsule, basal ganglia and diencephalon.
Vertebral artery
The Vertebral Artery arises from the subclavian artery. It passes deeply to enter the transverse foramen of the C6 vertebra and then ascends through the transverse foramina of all higher cervical vertebrae. After emerging from the atlas (CI), it winds around the lateral mass of the atlas to enter the foramen magnum on the ventral surface of the medulla. It then ascends on the medulla, passing gradually towards the midline where it unites with the vertebral artery of the opposite side at the pontomedullary junction.
It supplies blood to the spinal cord (cervical levels), medulla (see diagram Medulla - Blood Supply) and cerebellum via the following branches:
(1) Posterior Spinal Artery - which passes down the dorsal surface of the spinal cord.
(2) Anterior Spinal Artery - which passes toward the midline where it unites with its opposite artery to form a single anterior spinal artery which occupies the ventral median fissure of the spinal cord.
The anterior and posterior spinal arteries continue down the entire length of the spinal cord but below the cervical levels they do not derive their blood from the vertebral arteries, but receive it instead from radicular arteries which arise from the segmental branches of the aorta.
(3) Posterior Inferior Cerebellar Artery (PICA) - which is the largest and most clinically significant branch off the vertebral artery. It supplies the dorsolateral part of the medulla as well as the cerebellum (see separate card for more details).
(4) Medullary Branches - which are small perforating arteries which arise from the vertebral and anterior spinal arteries to supply the ventral surface of the medulla.
Lesions involving the Vertebral Artery.
(1) The cervical part of the vertebral artery may be injured as it ascends through the transverse foramina when one vertebra is suddenly displaced from its neighbour, as may occasionally happen during chiropractic manipulations of the neck.
(2) The vertebral artery is vulnerable to compression as it winds around the lateral mass of the atlas. If the two vertebral arteries are normal this does not have much effect because there is compensation by the opposite artery, but if one artery is narrowed or blocked the person will complain of feeling faint and dizzy when the head is turned hard to one side. This is due to diminished blood supply to the medulla.
Basilar artery
The basilar artery arises by union of the two vertebral arteries at the caudal border of the pons and runs along the basilar groove on the ventral surface of the pons. It terminates at the rostral border of the pons where it divides into the two posterior cerebral arteries.
Branches:
(1) Pontine aa, which supply most of the pons. These are divided into paramedian and circumferential groups.
(2) Anterior inferior cerebellar a., which supplies the dorsolateral region of the pons and the inferior surface and deep nuclei of the cerebellum
(3) Labyrinthine (Internal Acoustic) a., which enters the internal acoustic meatus to supply the inner ear and the facial n. (CN VII) and vestibulocochlear n. (CN VIII)
(4) Superior Cerebellar a., which supplies the superior surface and deep nuclei of the cerebellum.
Lesions of the basilar artery:
Complete or partial thrombosis (occlusion) of the basilar artery may occur suddenly and is usually accompanied by severe headache, vomiting, generalised loss of muscle tone and loss of consciousness (deep comatose state). Complete thrombosis is usually fatal.
Circle of Willis
The Circle of Willis is a ring of arteries on the base of the brain. It surrounds the optic chiasm and inferior surface of the diencephalon. It is formed by anastomoses between branches of the vertebral and internal carotid arteries and provides for an alternate route for blood to the brain if either of these arteries is occluded.
Components of the Circle of Willis include the internal carotid, posterior communicating, posterior cerebral and anterior communicating arteries.
A number of very small arteries, termed the central or ganglionic arteries, arise directly from the circle of Willis. These arteries are important because they penetrate and supply the structures which overlie the circle, in particular the diencephalon, corpus striatum and internal capsule.
Posterior cerebral artery
The Posterior Cerebral Arteries are the terminal branches of the basilar artery. Each arises at the rostral end of the pons and passes laterally across the crus cerebri to reach the inferior surface of the temporal lobe. It turns upwards, just below the splenium of the corpus callosum, onto the medial surface of the parietal and occipital lobes.
The posterior cerebral artery distributes perforating branches to the midbrain (including paramedian, circumferential and collicular branches), posterior thalamus and subthalamus, and cortical branches to the inferior surface of the temporal lobe and medial surface of the parietal and occipital lobes (including the primary visual area of the cortex).
The major effect of occlusion of the posterior cerebral a. results from damage to the visual cortex. Vision is lost in the contralateral half of the visual field although central region (viewed by the fovea) is often unaffected. This condition is known clinically as 'contralateral hemianopsia with macular sparing'. Bilateral occlusion of the posterior cerebral arteries may cause disturbances of memory as a result of damage to the inferior part of the temporal lobes.
If perforating branches to the midbrain and thalamus are affected, deficits may also include partial or complete loss of contralateral sensation and hemiplegia. The sensory loss is often accompanied by pain due to damage to the posterior thalamus (thalamic syndrome). Contralateral hemiballismus (sudden, involuntary, flailing movements) may occur if the subthalamic branches are affected.
Middle cerebral artery
The Middle Cerebral Artery is the larger of the two terminal branches of the internal carotid artery and arises just lateral to the optic chiasm. It passes laterally above the temporal lobe into the lateral fissure where it gives cortical branches to the insula and to the frontal, temporal, parietal and occipital lobes. Via these branches the middle cerebral artery supplies the cortex of most of the lateral surface of the cerebral hemisphere, including primary somatosensory and motor areas for the trunk, arm and head and the primary auditory area, as well as the association areas of the frontal, parietal, temporal and occipital lobes. In the dominant (usually the left) hemisphere the middle cerebral artery also supplies the speech areas.
Before entering the lateral fissure the middle cerebral artery gives off several small but clinically important branches, the anterior choroidal and lateral striate arteries, which together provide the major supply to the corpus striatum and internal capsule in addition to several other structures (see individual information cards for more details).
Major effects of occlusion of the middle cerebral artery include paralysis of the muscles of the trunk, upper limb and head on the opposite (contralateral) half of the body and decreased sensation in the same regions. If the dominant hemisphere is affected aphasia (difficulty in the execution and/or comprehension of speech) will also occur.
Anterior cerebral artery
The Anterior Cerebral Artery is the smaller of the two terminal branches of the internal carotid artery and arises just lateral to the optic chiasm. It passes medially towards the longitudinal fissure before turning upwards onto the medial surface of the frontal lobe. Just below the genu of the corpus callosum it divides into pericallosal and callosomarginal branches. The pericallosal a. follows the callosal sulcus and supplies the adjacent corpus callosum. The callosomarginal a. follows the cingulate sulcus and then enters the marginal sulcus. The callosomarginal a. provides the blood supply to the paracentral lobule, the primary motor and sensory region for the leg and foot.
The anterior cerebral artery supplies the cortex of the inferior and medial surfaces of the frontal lobe and the medial surface of the parietal lobe, and an adjoining strip about 1 cm wide on the medial edge of the lateral surface of these lobes. Its area of supply includes the lower limb area of the primary motor and somatosensory areas of cortex.
Occlusion of the anterior cerebral artery causes paralysis of the muscles and diminished sensation in the leg and foot of the opposite side.