Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Vascular supply and drainage of the brain
The brain is a highly vascular organ, its profuse blood supply characterized by a densely branching arterial network ( ). It has a high metabolic rate that reflects the energy requirements of constant neural activity. It receives about 15% of the cardiac output and utilizes 25% of the total oxygen consumption of the body. The brain is supplied by two internal carotid arteries and two vertebral arteries that form a complex anastomosis (circulus arteriosus cerebri, 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, and the vertebral arteries and their branches supply the occipital lobe, the brainstem 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 haemorrhage, are major contemporary sources of morbidity and mortality.
Arteries 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 ( Fig. 26.1 ) and their major branches (the internal carotid system or ‘anterior’ circulation) supply blood to the majority of the forebrain ( ). Some parts of the occipital and temporal lobes are supplied by branches of the vertebrobasilar system (see Fig. 26.5 ).
The internal carotid artery arises from the bifurcation of the common carotid artery, approximately at the level of C4, ascends in the neck (cervical segment) and enters the carotid canal of the temporal bone. Its subsequent course is said to have petrous, cavernous and intracranial parts.
Petrous part
The petrous part of the internal carotid artery ascends in the carotid canal, curves anteromedially and then superomedially above the cartilage that fills the foramen lacerum, and enters the cranial cavity. It lies at first anterior to the cochlea and tympanic cavity, and is separated from the latter and the pharyngotympanic tube by a thin, bony lamella that is cribriform in the young and partly absorbed in old age. Further anteriorly, it is separated from the trigeminal ganglion by the thin roof of the carotid canal, although this is often deficient. The artery is surrounded by a venous plexus and by the carotid autonomic plexus, derived from the internal carotid branch of the superior cervical ganglion. The petrous part of the artery gives rise to two branches. The caroticotympanic artery is a small, occasionally double, vessel that enters the tympanic cavity by a foramen in the carotid canal and anastomoses with the anterior tympanic branch of the maxillary artery and the stylomastoid artery. The pterygoid artery is inconsistent; when present, it enters the pterygoid canal with the nerve of the same name, and anastomoses with a (recurrent) branch of the greater palatine artery.
Cavernous part
The cavernous part of the internal carotid artery ascends to the posterior clinoid process. It turns anteriorly to the side of the body of the sphenoid within the cavernous sinus and then curves superiorly and medial to the anterior clinoid process, to emerge through the dural roof of the sinus. The oculomotor, trochlear, ophthalmic and abducens nerves are lateral to it within the cavernous sinus. The abducens nerve is closely related to the lateral wall of the internal carotid artery, whilst the oculomotor and trochlear nerves are situated in the lateral wall of the cavernous sinus (see fig. 25.8, fig. 25.9 ). This explains the higher risk of abducens nerve injury secondary to pathology such as aneurysms of the cavernous part of the carotid artery. Occasionally, the caroticoclinoid ligament between the anterior and middle clinoid processes becomes ossified, forming a bony ring (caroticoclinoid foramen) around the artery ( ).
The cavernous part of the artery gives off a number of small vessels. Branches supply the trigeminal ganglion, the walls of the cavernous and inferior petrosal sinuses, and the nerves contained therein. A minute meningeal branch passes over the lesser wing of the sphenoid to supply the dura mater and bone in the anterior cranial fossa, and also anastomoses with a meningeal branch of the posterior ethmoidal artery. Numerous small hypophysial branches supply the neurohypophysis, and are of particular importance because they form the pituitary portal system (see Fig. 30.11 ).
Intracranial part
After piercing the dura mater, the internal carotid artery turns back below the optic nerve to run between it and the oculomotor nerve. It reaches the anterior perforated substance at the medial end of the lateral fissure and terminates by dividing into the anterior and middle cerebral arteries ( Fig. 26.2 ).
Several preterminal vessels leave the cerebral portion of the internal carotid. The ophthalmic artery arises from the anterior part of 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 ( Fig. 26.3 ) runs back from the internal carotid above the oculomotor nerve, and anastomoses with the posterior cerebral artery (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. However, sometimes it is so large that the posterior cerebral artery is supplied via the posterior communicating artery rather than from the basilar artery (‘fetal posterior communicating artery’, ); it is often larger on one side only. 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 the walls of the third ventricle. The anterior choroidal artery leaves the internal carotid just distal to 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, 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 temporal (inferior) horn of the lateral ventricle via the choroidal 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 the fimbria of the fornix.
The combination of the petrous, cavernous and intracranial parts of the internal carotid artery is called the ‘carotid siphon’ because of its sigmoid course (see Fig. 26.1 and ). However, in infants, the parasellar region of the internal carotid artery does not form a siphon but takes a relatively straight course ( ). The cranial and sympathetic nerves therefore have different topographical relationships with the artery in the infant compared with that found in older children and adults.
The initial millimetres of the intracranial part lie between two rings (proximal and distal rings) of dense connective tissue that surround the carotid artery where the ophthalmic artery usually branches. In cases of surgical clipping of ophthalmic artery aneurysms it is of surgical relevance for neurosurgeons to expose this part of the artery by removing the anterior clinoid process and exposing this segment of the carotid, where there is no venous flow ( ).
Anterior cerebral artery
The anterior cerebral artery is the smaller of the two terminal branches of the internal carotid (see Fig. 26.3 ).
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; and A3, distal to the origin of the callosomarginal artery. The A3 segment is also known as the pericallosal artery.
The anterior cerebral artery starts at the medial end of the stem of the lateral fissure. It passes anteromedially above the optic nerve to the great longitudinal fissure where it connects with its contralateral fellow by a short transverse anterior communicating artery. The latter is about 4 mm in length 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 the cingulate gyrus ( Fig. 26.2C ).
The two anterior cerebral arteries travel together in the great longitudinal fissure. They pass around the curve of the genu of the corpus callosum and then along its upper surface to its posterior end, where they anastomose with posterior cerebral arteries.
The anterior cerebral artery has cortical and central branches. Cortical branches are named according to their 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 and parietal branches supply the precuneus. 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 (see Figs 26.2B , 26.4 ). Cortical branches of the anterior cerebral artery, therefore, supply the areas of the motor and somatosensory cortices that represent the lower limb.
Central branches of the anterior cerebral artery arise from its proximal portion and enter the anterior perforated substance (see Fig. 26.3B ) and lamina terminalis. Collectively, they supply the rostrum of the corpus callosum, septum pellucidum, anterior part of the putamen, 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. Surgical nomenclature divides the vessel into four parts: M1 (sphenoidal), from the termination of the internal carotid artery to the bi/trifurcation; M2 (insular), running in the lateral (Sylvian) fissure; M3 (opercular), coming out of the lateral fissure; M4, cortical portions. The middle cerebral artery runs at first in the lateral fissure, then posterosuperiorly on the insula, and divides into branches distributed to the insula and the adjacent lateral cerebral surface ( Fig. 26.4 , see Fig. 26.2 ).
The middle cerebral artery 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 that represent the whole of the body (other than 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 origin 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, 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, the brainstem and cerebellum, and a significant but variable part of the posterior cerebral hemispheres ( Fig. 26.5 ).
The vertebral arteries are derived from the subclavian arteries, 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. 35.11 ). 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 pons.
One or two meningeal branches arise from the vertebral artery near the foramen magnum, 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, descends anterior to the medulla oblongata and unites with its fellow from the opposite side at mid-medullary level. The common trunk 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 (see Fig. 26.5A ). It arises near the lower end of the olive and then ascends behind the roots of the glossopharyngeal and vagus nerves to reach the inferior border of the pons. Here it curves and descends along the inferolateral border of the fourth ventricle, then 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, supplying 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 (branches of 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 may originate 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 via intervertebral foramina and sustain the posterior spinal arteries to the lower spinal levels.
Very small medullary arteries arise from the vertebral artery and its branches and are distributed widely to the medulla oblongata.
Basilar artery
The basilar artery is a large median vessel formed by the union of the vertebral arteries at the junction of the medulla and pons (see Figs 26.3 , 26.5 ). It lies in the pontine cistern and follows a shallow median groove on the ventral pontine surface, extending to the upper border of the pons. It ends by dividing into two posterior cerebral arteries at a variable level behind the dorsum sellae, usually in the interpeduncular cistern.
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 may arise 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 is given off from the distal 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; 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 Figs 26.3 , 26.5 ) 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 round the cerebral peduncle below the trochlear nerve to gain the superior cerebellar surface. Here 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 paired posterior cerebral arteries are the terminal branches of the basilar artery (see Figs 26.2–26.4 ). Surgical nomenclature divides the vessel into three parts: 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 that runs 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 round the cerebral peduncle and reaches the tentorial cerebral surface, where it supplies the temporal and occipital lobes.
The posterior cerebral artery has cortical and central branches. Cortical branches are named according to their distribution. Temporal branches, usually two, are distributed to the uncus and parahippocampal, medial occipitotemporal 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. 26.3B ) and, together with similar branches from the posterior communicating artery, pierce the posterior perforated substance to 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 temporal horn of the lateral ventricle via the lower part of the choroidal fissure. Branches also curl round the posterior end of the thalamus and pass through the transverse fissure, or go to the choroid plexus of the third ventricle, or traverse the upper choroidal fissure: collectively, these branches 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.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here