Chapter 6 Vascular Supply of the Brain and Spinal Cord

The brain is a highly vascular organ and its profuse blood supply is characterized by a densely branching arterial network (Fig. 6.1). It has high metabolic activity due in part to the energy requirements of constant neural activity. It demands about 15% of the cardiac output and uses 25% of the total oxygen consumed by the body. The brain is supplied by two internal carotid arteries and two vertebral arteries that form a complex anastomosis (circulus arteriosus, or the circle of Willis) on the base of the brain. Vessels diverge from this anastomosis to supply the various cerebral regions. In general, the internal carotid arteries and the vessels arising from them supply the forebrain, with the exception of the occipital lobe of the cerebral hemisphere; the vertebral arteries and their branches supply the occipital lobe, the brain stem and the cerebellum. Venous blood from the brain drains into sinuses within the dura mater. Acute interruption of the blood supply to the brain for more than a few minutes causes permanent neurological damage. Such ischaemic strokes along with intracranial haemorrhages are major sources of morbidity and mortality.

Fig. 6.1 Resin cast of the arterial supply of the brain.

(Photograph by Sarah-Jane Smith.)

Arterial Supply of the Brain

The arterial supply of the brain is derived from the internal carotid and vertebral arteries, which lie, together with their proximal branches, within the subarachnoid space at the base of the brain.

Internal Carotid Artery

The internal carotid arteries and their major branches (sometimes referred to as the internal carotid system) essentially supply blood to the forebrain, with the exception of the occipital lobe.

The internal carotid artery (Figs 6.2, 6.3) arises from the bifurcation of the common carotid artery, ascends in the neck and enters the carotid canal of the temporal bone. Its subsequent course is said to have petrous, cavernous and cranial parts.

Fig. 6.2 Internal carotid arteriogram. This image is a lateral projection obtained by intra-arterial digital subtraction angiography.

(Courtesy of Professor P. D. Griffiths, Academic Unit of Radiology, University of Sheffield.)

Fig. 6.3 Internal carotid arteriogram. This image is a Towne’s projection obtained by intra-arterial digital subtraction angiography.

(Courtesy of Professor P. D. Griffiths, Academic Unit of Radiology, University of Sheffield.)

Several preterminal vessels leave the cerebral portion of the internal carotid. The ophthalmic artery arises from the internal carotid as it leaves the cavernous sinus, often at the point of piercing the dura, and enters the orbit through the optic canal. The posterior communicating artery (Figs 6.4–6.6) runs back from the internal carotid above the oculomotor nerve and anastomoses with the posterior cerebral artery (which is a terminal branch of the basilar artery), thereby contributing to the circulus arteriosus around the interpeduncular fossa. The posterior communicating artery is usually very small. Sometimes, however, it is so large that the posterior cerebral artery is supplied via the posterior communicating artery rather than the basilar artery (‘fetal posterior communicating artery’). It is often larger on one side. Small branches from its posterior half pierce the posterior perforated substance, together with branches from the posterior cerebral artery. Collectively, they supply the medial thalamic surface and walls of the third ventricle. The anterior choroidal artery leaves the internal carotid near its posterior communicating branch and passes back above the medial part of the uncus. It crosses the optic tract to reach and supply the crus cerebri of the midbrain; it then turns laterally, recrosses the optic tract and gains the lateral side of the lateral geniculate body, which it supplies with several branches. It finally enters the inferior horn of the lateral ventricle via the choroid fissure and ends in the choroid plexus. This small but important vessel also contributes to the blood supply of the globus pallidus, caudate nucleus, amygdala, hypothalamus, tuber cinereum, red nucleus, substantia nigra, posterior limb of the internal capsule, optic radiation, optic tract, hippocampus and fimbria of the fornix.

Fig. 6.4 Arteries at the base of the brain. The anterior part of the right temporal lobe has been removed to display the initial course of the middle cerebral artery within the lateral fissure.

Fig. 6.5 Arteries on the base of the brain injected with resin.

(By permission from Crossman, A.R., Neary, D. 2000. Neuroanatomy, 2nd ed. Edinburgh, Churchill Livingstone.)

Fig. 6.6 Circulus arteriosus at the base of the brain showing the distribution of central (perforating or ganglionic) branches. The anteromedial, posteromedial, posterolateral and anterolateral (lateral striate) vessels are shown. The medial striate and anterior choroidal arteries are also shown.

Anterior Cerebral Artery

The anterior cerebral artery is the smaller of the two terminal branches of the internal carotid artery (see Figs 6.5, 6.6).

The surgical nomenclature divides the vessel into three parts: A1, from the termination of the internal carotid artery to the junction with the anterior communicating artery; A2, from the junction with the anterior communicating artery to the origin of the callosomarginal artery; A3, distal to the origin of the callosomarginal artery. This segment is also known as the pericallosal artery.

The anterior cerebral artery starts at the medial end of the stem of the lateral cerebral fissure and passes anteromedially above the optic nerve to the great longitudinal fissure, where it connects with its fellow by a short transverse anterior communicating artery. The anterior communicating artery is approximately 4 mm long and may be double. It gives off numerous anteromedial central branches that supply the optic chiasma, lamina terminalis, hypothalamus, para-olfactory areas, anterior columns of the fornix and cingulate gyrus.

The two anterior cerebral arteries travel together in the great longitudinal fissure. They pass around the curve of the genu of the corpus callosum (Fig. 6.7) and then along its upper surface to its posterior end, where they anastomose with posterior cerebral arteries. They give off cortical and central branches.

Fig. 6.7 Major arteries of the brain. A, Medial aspect. B, Lateral aspect.

The cortical branches of the anterior cerebral artery are named by distribution. Two or three orbital branches ramify on the orbital surface of the frontal lobe and supply the olfactory cortex, gyrus rectus and medial orbital gyrus. Frontal branches supply the corpus callosum, cingulate gyrus, medial frontal gyrus and paracentral lobule. Parietal branches supply the precuneus, and the frontal and parietal branches both send twigs over the superomedial border of the hemisphere to supply a strip of territory on the superolateral surface (Fig. 6.8). Cortical branches of the anterior cerebral artery, therefore, supply the areas of the motor and somatosensory cortices that represent the lower limb.

Fig. 6.8 A, Lateral surface of the left cerebral hemisphere, showing the areas supplied by the cerebral arteries. B, Medial surface of the left cerebral hemisphere, showing the areas supplied by the cerebral arteries. In these figures, the area supplied by the anterior cerebral artery is coloured blue, that by the middle cerebral artery is pink and that by the posterior cerebral artery is yellow.

Central branches of the anterior cerebral artery arise from its proximal portion and enter the anterior perforated substance (see Fig. 6.6) and lamina terminalis. Collectively, they supply the rostrum of the corpus callosum, the septum pellucidum, the anterior part of the putamen, the head of the caudate nucleus and adjacent parts of the internal capsule. Immediately proximal or distal to its junction with the anterior communicating artery, the anterior cerebral artery gives rise to the medial striate artery, which supplies the anterior part of the head of the caudate nucleus and adjacent regions of the putamen and internal capsule.

Middle Cerebral Artery

The middle cerebral artery is the larger terminal branch of the internal carotid.

The surgical nomenclature identifies four subdivisions: M1, from the termination of the internal carotid artery to the bi- or trifurcation (also known as the sphenoidal segment); M2, the segment running in the lateral (Sylvian) fissure (also known as the insular segment); M3, coming out of the lateral fissure (also known as the operator segment); and M4, the cortical portions.

The middle cerebral artery runs first in the lateral cerebral fissure, then posterosuperiorly on the insula; it divides into branches distributed to this and the adjacent lateral cerebral surface (see Figs 6.5, 6.6, 6.8). Like the anterior cerebral artery, it has cortical and central branches.

Cortical branches send orbital vessels to the inferior frontal gyrus and the lateral orbital surface of the frontal lobe. Frontal branches supply the precentral, middle and inferior frontal gyri. Two parietal branches are distributed to the postcentral gyrus, the lower part of the superior parietal lobule and the whole inferior parietal lobule. Two or three temporal branches supply the lateral surface of the temporal lobe. Cortical branches of the middle cerebral artery, therefore, supply the motor and somatosensory cortices representing the whole body, with the exception of the lower limb, the auditory area and the insula.

Small central branches of the middle cerebral artery—the lateral striate or lenticulostriate arteries—arise at its commencement and enter the anterior perforated substance together with the medial striate artery. Lateral striate arteries ascend in the external capsule over the lower lateral aspect of the lentiform complex; they then turn medially, traverse the lentiform complex and the internal capsule and extend as far as the caudate nucleus.

Vertebral Artery

The vertebral arteries and their major branches (sometimes referred to as the vertebrobasilar system) essentially supply blood to the upper spinal cord, brain stem, cerebellum and occipital lobe of the cerebrum (Figs 6.9, 6.10). In addition, other branches have a wider distribution.

Fig. 6.9 Vertebral arteriogram. This image is a lateral projection obtained by intra-arterial digital subtraction angiography.

(Courtesy of Professor P. D. Griffiths, Academic Unit of Radiology, University of Sheffield.)

Fig. 6.10 Vertebral arteriogram. This image is a Towne’s projection obtained by intra-arterial digital subtraction angiography.

(Courtesy of Professor P. D. Griffiths, Academic Unit of Radiology, University of Sheffield.)

The vertebral arteries are derived from the subclavian arteries. They ascend through the neck in the foramina transversaria of the upper six cervical vertebrae and enter the cranial cavity through the foramen magnum, close to the anterolateral aspect of the medulla (see Fig. 6.5). They converge medially as they ascend the medulla and unite to form the midline basilar artery at approximately the level of the junction between the medulla and the pons.

One or two meningeal branches arise from the vertebral artery near the foramen magnum. These ramify between the bone and dura mater in the posterior cranial fossa and supply bone, diploë and the falx cerebelli.

A small anterior spinal artery arises near the end of the vertebral artery and descends anterior to the medulla oblongata to unite with its fellow from the opposite side at the mid-medullary level. The single trunk then descends on the ventral midline of the spinal cord and is reinforced sequentially by small spinal rami from the vertebral, ascending cervical, posterior intercostal and first lumbar arteries, which all enter the vertebral canal via intervertebral foramina. Branches from the anterior spinal arteries and the beginning of their common trunk are distributed to the medulla oblongata.

The largest branch of the vertebral artery is the posterior inferior cerebellar artery. It arises near the lower end of the olive, which it curves back around, and then ascends behind the roots of the glossopharyngeal and vagus nerves to reach the inferior border of the pons. There it curves and descends along the inferolateral border of the fourth ventricle before it turns laterally into the cerebellar vallecula between the hemispheres and divides into medial and lateral branches. The medial branch runs back between the cerebellar hemisphere and inferior vermis and supplies both. The lateral branch supplies the inferior cerebellar surface as far as its lateral border and anastomoses with the anterior inferior and superior cerebellar arteries (from the basilar artery). The trunk of the posterior inferior cerebellar artery supplies the medulla oblongata dorsal to the olivary nucleus and lateral to the hypoglossal nucleus and its emerging nerve roots. It also supplies the choroid plexus of the fourth ventricle and sends a branch lateral to the cerebellar tonsil to supply the dentate nucleus. The posterior inferior cerebellar artery is sometimes absent.

A posterior spinal artery usually arises from the posterior inferior cerebellar artery, but it may come directly from the vertebral artery near the medulla oblongata. It passes posteriorly and descends as two branches that lie anterior and posterior to the dorsal roots of the spinal nerves. These are reinforced by spinal twigs from the vertebral, ascending cervical, posterior intercostal and first lumbar arteries, all of which reach the vertebral canal by the intervertebral foramina, thereby sustaining the posterior spinal arteries to the lower spinal levels.

Minute medullary arteries arise from the vertebral artery and its branches and are distributed widely to the medulla oblongata.

Basilar Artery

This large median vessel is formed by the union of the vertebral arteries at the mid-medullary level and extends to the upper border of the pons (see Figs 6.5, 6.6). It lies in the pontine cistern and follows a shallow median groove on the ventral pontine surface. The basilar artery terminates by dividing into two posterior cerebral arteries at a variable level, but most frequently in the interpeduncular cistern, behind the dorsum sellae.

Numerous small pontine branches arise from the front and sides of the basilar artery along its course and supply the pons. The long and slender labyrinthine (internal auditory) artery has a variable origin. It usually arises from the anterior inferior cerebellar artery, but it may originate from the lower part of the basilar artery, the superior cerebellar artery or, occasionally, the posterior inferior cerebellar artery. The labyrinthine artery accompanies the facial and vestibulocochlear nerves into the internal acoustic meatus and is distributed to the internal ear.

The anterior inferior cerebellar artery (see Fig. 6.6) is given off from the lower part of the basilar artery and runs posterolaterally, usually ventral to the abducens, facial and vestibulocochlear nerves. It commonly exhibits a loop into the internal acoustic meatus below the nerves, and when this occurs, the labyrinthine artery may arise from the loop. The anterior inferior cerebellar artery supplies the inferior cerebellar surface anterolaterally and anastomoses with the posterior inferior cerebellar branch of the vertebral artery. A few branches supply the inferolateral parts of the pons and occasionally also supply the upper medulla oblongata.

The superior cerebellar artery (see Fig. 6.6) arises near the distal portion of the basilar artery, immediately before the formation of the posterior cerebral arteries. It passes laterally below the oculomotor nerve, which separates it from the posterior cerebral artery, and curves around the cerebral peduncle below the trochlear nerve to gain the superior cerebellar surface. There it divides into branches that ramify in the pia mater and supply this aspect of the cerebellum and also anastomose with branches of the inferior cerebellar arteries. The superior cerebellar artery supplies the pons, pineal body, superior medullary velum and tela choroidea of the third ventricle.

Posterior Cerebral Artery

The posterior cerebral artery (see Figs 6.5, 6.6) is a terminal branch of the basilar artery.

The surgical nomenclature identifies three segments: P1, from the basilar bifurcation to the junction with the posterior communicating artery; P2, from the junction with the posterior communicating artery to the portion in the perimesencephalic cistern; and P3, the portion running in the calcarine fissure.

The posterior cerebral artery is larger than the superior cerebellar artery, from which it is separated near its origin by the oculomotor nerve and, lateral to the midbrain, by the trochlear nerve. It passes laterally, parallel with the superior cerebellar artery, and receives the posterior communicating artery. It then winds around the cerebral peduncle and reaches the tentorial cerebral surface, where it supplies the temporal and occipital lobes. Like the anterior and middle cerebral arteries, the posterior cerebral artery has cortical and central branches.

The cortical branches of the posterior cerebral artery are named by distribution. Temporal branches, usually two, are distributed to the uncus and the parahippocampal, medial and lateral occipitotemporal gyri. Occipital branches supply the cuneus, lingual gyrus and posterolateral surface of the occipital lobe. Parieto-occipital branches supply the cuneus and precuneus. The posterior cerebral artery supplies the visual areas of the cerebral cortex and other structures in the visual pathway.

The central branches supply subcortical structures. Several small posteromedial central branches arise from the beginning of the posterior cerebral artery (see Fig. 6.6) and, together with similar branches from the posterior communicating artery, pierce the posterior perforated substance and supply the anterior thalamus, subthalamus, lateral wall of the third ventricle and globus pallidus. One or more posterior choroidal branches pass over the lateral geniculate body and supply it before entering the posterior part of the inferior horn of the lateral ventricle via the lower part of the choroid fissure. Branches also curl around the posterior end of the thalamus and pass through the transverse fissure, go to the choroid plexus of the third ventricle or traverse the upper choroid fissure. Collectively, these supply the choroid plexuses of the third and lateral ventricles and the fornix. Small posterolateral central branches arise from the posterior cerebral artery beyond the cerebral peduncle and supply the peduncle and the posterior thalamus, superior and inferior colliculi, pineal gland and medial geniculate body.