Blood supply of the brain

Blood Supply of the Brain

The continuous blood supply to the brain is of utmost importance because of its high metabolic demands for oxygen and glucose. It is highly sensitive to hypoxia (inadequate O₂) and hypoglycaemia (subnormal concentration of glucose in the blood). The consciousness is lost within 10 seconds of cessation of blood flow, and if the state continues, an irreversible brain damage starts to occur at about 4 minutes and is completed within 10 minutes.

The brain is one of the most metabolically active organs of the body as it depends on aerobic metabolism of glucose.

Although the brain constitutes only 2% (1/50) of the total body weight, it receives 20% (1/5) of the total cardiac output and consumes 20% of the total O₂ used by the body.

The cerebrovascular diseases (thrombosis, embolism and haemorrhage) are the third most common cause of death and the neurological signs depend on the site of lesion. Therefore an adequate knowledge of the blood supply of the brain is essential for proper diagnosis and treatment of these diseases.

N.B. The arterial occlusion by a thrombus which often leads to infarction of the portion of the brain supplied by the affected artery is the most common type of the cerebrovascular disease.

Arteries of The Brain

The brain is supplied by the paired internal carotid and vertebral arteries via an extensive system of branches (Fig. 15.1):

Fig. 15.1 Four arteries supplying the brain.

The two vertebral arteries unite at the lower border of the pons to form the basilar artery which ascends in the midline on the ventral surface of the pons and at its upper border terminates by dividing into right and left posterior cerebral arteries.

Each internal carotid artery ends in the region of anterior perforated substance by dividing into a larger middle cerebral artery, and a smaller anterior cerebral artery.

N.B. Thus the brain is supplied by two systems of arteries: (a) vertebral system, consisting of a pair of vertebral arteries, and (b) carotid system, consisting of a pair of internal carotid arteries.

Circle of Willis (circulus arteriosus)

The major arteries supplying the cerebrum (i.e. branches of basilar and internal carotid arteries) get interconnected to one another at the base of the brain to form a six-sided polygon of arteries called circulus arteriosus or circle of Willis (Figs. 15.2 and 15.3). The circle of Willis is formed around the interpeduncular fossa and lies in the interpeduncular subarachnoid cistern. It contributes most of the arterial blood supply to the brain.

Fig. 15.2 Polygonal anastomoses between the branches of internal carotid and basilar arteries.

Fig. 15.3 Circle of Willis and the branches of arteries supplying the brain. The central branches of cerebral arteries are shown by abbreviations: AM = anteromedial group, PM = posteromedial group, AL = anterolateral group, PL = posterolateral group.

Circle of Willis is formed:

Anteriorly, by the anterior communicating and the anterior cerebral arteries.

Posteriorly, by the basilar artery dividing into two posterior cerebral arteries.

Laterally on each side, by the posterior communicating artery connecting the internal carotid artery with the posterior cerebral artery.

Functional significance of circle of Willis

Normally there is little or no mixing of blood streams: (a) of two vertebral arteries in the basilar artery, (b) of two anterior cerebral arteries in the anterior communicating artery, and (c) of internal carotid and posterior cerebral arteries in the posterior communicating artery. Therefore, right half of the brain is supplied by right vertebral and right internal carotid arteries and left half of the brain is supplied by left vertebral and left internal carotid arteries.

However, if one of the major arteries forming the circle of Willis is blocked, the circle of Willis provides the various alternative roots for collateral circulation like an arterial traffic circle.

Clinical Correlation

• Congenital cerebral aneurysms

These occur mostly at the sites where two arteries join in the formation of the circle of Willis. The basic abnormality at these points is the congenital deficiency of the tunica media (elastic tissue) in the arterial wall. The aneurysms are berry-shaped, hence they are generally termed berry aneurysms.

• Subarachnoid haemorrhage

The subarachnoid haemorrhage commonly but not exclusively results from rupture of congenital berry aneurysms in the interpeduncular cistern.

The subarachnoid haemorrhage produces a sudden severe pain in head followed by mental confusion. The death may quickly occur, or the patient may survive the first bleeding only to die few days or weeks later.

Vertebral System

Vertebral artery (Fig. 15.4)

The vertebral artery, a branch of subclavian artery, ascends in the foramina transversaria of upper six cervical vertebrae. On reaching the base of skull, it winds backwards and medially around the lateral mass of the atlas and pierces posterior atlanto-occipital membrane, to enter the posterior cranial fossa through the foramen magnum where it runs on the anterolateral aspect of the medulla. Here the two vertebral arteries converge, and unite at the lower border of the pons to form the basilar artery (Fig. 15.4).

Fig. 15.4 Origin and course of internal carotid and vertebral arteries.

Clinical Correlation

The loop of vertebral artery around the lateral mass of atlas may damp down the arterial pulsations within the cranial cavity.

Branches of the cranial part of the vertebral artery (Fig. 15.3)

• Anterior spinal artery is a small branch arising near the termination of the vertebral artery. It descends in front of the medulla and unites with its fellow of the opposite side at the level of the lower end of the olive to form a single median trunk that descends along the anterior longitudinal fissure of the spinal cord.

• Posterior spinal artery arises from vertebral artery and sometimes from posterior inferior cerebellar artery. It passes downwards on the posterior surface of the spinal cord, after dividing into two branches; one along the medial side, and the other along the lateral side of the dorsal roots of the spinal nerves.

• Posterior inferior cerebellar artery is the largest branch of the cranial (4th) part of the vertebral artery. It arises near the lower end of the olive, winds backwards around the medulla oblongata, and then ascends to the pon-tomedullary junction.

• Meningeal branches are small and supply the dura mater of the posterior cranial fossa.

• Medullary arteries are several minute vessels which supply the medulla oblongata.

Basilar artery (Figs 15.1–15.3)

Basilar artery is formed by the union of two vertebral arteries at the lower border of the pons. It ascends in the basilar sulcus on the ventral aspect of the pons in the cis-terna pontis and terminates at the upper border of the pons by dividing into right and left posterior cerebral arteries.

Branches of basilar artery

• Pontine branches are numerous short slender parame-dian vessels which pierce the pons to supply it.

• Anterior inferior cerebellar artery arises close to the lower border of the pons and runs backwards and laterally usually ventral to the VIIth and VIIIth cranial nerves. Then it forms a loop over the flocculus of the cerebellum and peeps into the internal acoustic meatus for a variable distance lying below the VIIth and VIIIth cranial nerves. After exit from the meatus it supplies the anterolateral portion of the inferior surface of the cerebellum.

• Labyrinthine artery is a long slender branch which arises either from basilar artery or from anterior inferior cere-bellar artery. It accompanies the vestibulocochlear nerve and enters the internal auditory meatus to supply the internal ear. It is an end artery.

• Superior cerebellar artery arises close to the superior border of the pons, runs laterally below the oculomotor nerve (which is interposed between this artery and the posterior cerebral artery), and winds round the cerebral peduncle below the trochlear nerve to reach the superior surface of the cerebellum which it supplies.

• Posterior cerebral artery passes laterally parallel to the superior cerebellar artery, curves around the midbrain to reach the medial surface of the cerebral hemisphere, beneath the splenium of corpus callosum (Figs 15.5 and 15.6). The artery gives off temporal branches which ramify over the inferior surface of the temporal lobe, and calcarine and parieto-occipital branches which run along the corresponding sulci.

Fig. 15.5 Arteries on the inferior surface of the cerebral hemisphere.

Fig. 15.6 Arteries on the inferomedial aspect of the cerebral hemisphere. (C = central branches, PCA = posterior choroidal artery.)

1. Anterior temporal branch passes forwards.

2. Posterior temporal branch passes posteriorly towards the occipital pole.

3. Parieto-occipital branch runs towards the parieto-occipital sulcus.

4. Calcarine branch (often called calcarine artery) passes posteriorly in the calcarine sulcus.

Carotid System

Internal carotid artery (Fig. 15.4)

The internal carotid artery, a terminal branch of the common carotid artery, traverses the carotid canal in the base of the skull and enters the middle cranial fossa beside the dorsum sellae of the sphenoid bone. Here it first runs forwards along the floor and medial wall of the cavernous sinus and then turns upwards on the medial side of the anterior clinoid process. At this point the artery pierces the dural roof of the cavernous sinus and also the arachnoid mater to enter the subarachnoid space. Now it first runs backwards and then upwards to come to lie lateral to the optic chiasma just underneath the anterior perforated substance of the brain, where it terminates by dividing into two branches, a larger middle cerebral artery and a smaller anterior cerebral artery (Figs 15.2 and 15.3).

Clinical Correlation

The intracavernous and supracavernous course of internal carotid artery forms a U-shaped bend (hair-pin bend) with convexity forward called carotid siphon. The siphon dampens the arterial pulsations to provide a more steady regular stream of blood flow to the brain.

The carotid siphon is an important part of the cerebral angiogram to inspect, for masses in the pituitary region will open out (widen) the curl in the artery.

Branches of the cerebral part of the internal carotid artery (Fig. 15.3)

• Ophthalmic artery arises from the ventral convexity of the carotid siphon and enters the optic canal to reach the orbital cavity to supply the structures of the orbit including eyeball.

• Posterior communicating artery arises close to the termination of the internal carotid artery. It runs backwards and anastomoses with the proximal part of the posterior cerebral artery.

• Anterior choroidal artery is a long slender branch, which arises just distal to the origin of the posterior communicating artery. It courses backwards above and along the optic tract, to enter the inferior horn of the lateral ventricle through the choroid fissure to end in the choroid plexus.

Due to its long subarachnoid course and a relatively small lumen, the anterior choroidal artery is most susceptible to thrombosis and is often referred to as artery of cerebral thrombosis.

• Anterior cerebral artery is a smaller terminal branch of the internal carotid artery. It runs forwards and medially above the optic nerve to the commencement of the median longitudinal cerebral fissure, where it comes very close to its fellow of the opposite side and gets joined with it by a short transverse anterior communicating artery. The anterior cerebral artery then curves around the genu of corpus callosum.

The branches given off just distal to the anterior communicating artery supply the medial part of the orbital surface of the frontal lobe.

The artery continues along the upper surface of the corpus collosum as the pericallosal artery and gives a large branch, the callosomarginal artery which runs in the cingulate sulcus. Near the splenium of corpus callosum, the artery ends by anastomosing with the branches of the posterior cerebral artery.

Clinical Correlation

The arched course of the anterior cerebral artery around the genu of corpus callosum makes it easy to identify it in a carotid angiogram.

• Middle cerebral artery is the larger terminal branch of the internal carotid artery. It appears to be the direct continuation of the internal carotid artery and carries about 30% of the carotid blood flow.

The middle cerebral artery first runs laterally in the stem of the lateral sulcus (Fig. 15.5) and then turns backwards and upwards in the posterior ramus of the lateral sulcus, where it breaks up into frontal, parietal and temporal branches which emerge from the lateral sulcus and run towards the areas of their supply. Some of these branches are named (Fig. 15.7).

Fig. 15.7 Arteries on the superolateral surface of the left cerebral hemisphere.

The branches of main arteries supplying the brain are summarized in Table 15.1

Table 15.1

Branches of the main arteries of the brain

Cerebral part of internal carotid artery	Fourth part of vertebral artery	Basilar artery
• Ophthalmic artery	• Meningeal arteries	• Anterior inferior cerebellar artery
• Anterior cerebral artery	• Anterior spinal artery	• Labyrinthine artery
• Middle cerebral artery	• Posterior spinal artery	• Pontine arteries
• Posterior communicating artery	• Posterior inferior cerebellar artery	• Superior cerebellar artery
• Anterior choroidal artery	• Medullary arteries	• Posterior cerebral artery

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Tags: Textbook of Clinical Neuroanatomy

Jan 2, 2017 | Posted by admin in NEUROLOGY | Comments Off

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