Neuroanatomy An Illustrated Colour Text, 4 ed.

Chapter 7. Blood supply of the central nervous system

 

Three longitudinal arterial vessels run the length of the spinal cord (Fig. 7.1). These are the single anterior spinal artery and the paired posterior spinal arteries. The anterior spinal artery arises in a Y-shaped configuration from the two vertebral arteries at the level of the medulla (Fig. 7.2) and descends along the ventral surface of the cord in the midline. The posterior spinal arteries arise from either the vertebral arteries or the posterior inferior cerebellar arteries and run caudally on the posterolateral surface of the cord.

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Figure 7.1 The arterial supply and venous drainage of the spinal cord.

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Figure 7.2 The arrangement of arterial vessels on the base of the brain. The diagram shows the circulus arteriosus (circle of Willis).

The anterior and posterior spinal arteries alone are insufficient to supply the cord below cervical levels and, therefore, they receive serial reinforcement by anastomosis with radicular arteries derived from segmental vessels, including the ascending cervical, intercostal and lumbar arteries. Radicular arteries pass through the intervertebral foramina and divide into anterior and posterior branches, which run with the dorsal and ventral spinal nerve roots, respectively. One particularly large radicular artery (the great radicular artery, or artery of Adamkiewicz) may arise from a lateral intercostal or lumbar artery at any level from T8–L3.

imageDisorders of blood supply of the spinal cord

The blood supply of the spinal cord is most vulnerable in the thoracic region and in the anterior portion of the cord. Occlusion of the anterior spinal artery leads to an acute thoracic cord syndrome with paraplegia and incontinence. The spinothalamic modalities of pain and temperature are preferentially lost, whereas the proprioceptive functions of the dorsal columns are relatively preserved.

Venous drainage of the spinal cord

The venous drainage of the cord follows a basically similar pattern to the arterial supply (Fig. 7.1). Six longitudinal interconnecting venous channels exist. These consist primarily of anterior and posterior spinal veins, which run in the midline. More irregular, sometimes incomplete, bilaterally paired anterolateral and posterolateral veins are situated near the lines of attachment of the ventral and dorsal nerve roots, respectively. All of these vessels drain via anterior and posterior radicular veins into the internal vertebral venous plexus (epidural venous plexus), which is situated between the dura mater and the vertebral periosteum. The internal venous plexus communicates with an external vertebral venous plexus and thence with the ascending lumbar veins, the azygos and hemiazygos veins.

Blood supply of the spinal cord

image The spinal cord is supplied by the anterior and posterior spinal arteries, supplemented by radicular arteries.

image Venous drainage is by anterior and posterior spinal veins which drain, via radicular veins, into the internal vertebral venous plexus.

Blood supply of the brain

Arterial supply of the brain

The brain is supplied with blood by two pairs of vessels, the internal carotid arteries and the vertebral arteries (Figs 7.27.3 and see Fig. 7.7). The internal carotid artery arises from the common carotid artery and enters the middle fossa of the cranial cavity through the carotid canal. Its course then follows a series of characteristic bends, known as the carotid syphon (Fig. 7.4), after which it passes forwards through the cavernous sinus and then upwards on the medial aspect of the anterior clinoid process, reaching the surface of the brain lateral to the optic chiasma. Along its course, the internal carotid artery gives rise to a number of preterminal branches.

image Hypophyseal arteries arise from the intracavernous section of the internal carotid to supply the neurohypophysis. They also form the pituitary portal system of vessels by which releasing factors are carried from the hypothalamus to the adenohypophysis (Ch. 16).

image The ophthalmic artery passes into the orbit through the optic foramen. It supplies the structures of the orbit, the frontal and ethmoidal sinuses, the frontal part of the scalp and dorsum of the nose.

image The anterior choroidal artery supplies the optic tract, the choroid plexus of the lateral ventricle, the hippocampus and some of the deep structures of the hemisphere, including the internal capsule and globus pallidus.

image The posterior communicating artery passes backwards to join the posterior cerebral artery, thus forming part of the circle of Willis.

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Figure 7.3 Arteries on the base of the brain. The arterial system has been injected with a red resin.

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Figure 7.4 Carotid angiograms. Radio-opaque material has been introduced into the internal carotid artery in order to display its intracranial course and the distribution of its branches. (A) Left carotid, lateral view; (B) Right carotid, frontal view.

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

Lateral to the optic chiasma, the internal carotid artery divides into its two terminal branches, the anterior and middle cerebral arteries. The anterior cerebral artery courses medially above the optic nerve and then passes into the great longitudinal fissure, between the frontal lobes of the cerebral hemispheres. As it does so, it is joined to the corresponding vessel of the opposite side by the short anterior communicating artery. Within the great longitudinal fissure, the anterior cerebral artery follows the dorsal curvature of the corpus callosum (Fig. 13.24), branches ramifying over the medial surface of the frontal and parietal lobes, which it supplies (Fig. 7.5). The territory supplied by the anterior cerebral artery, therefore, includes the motor and sensory cortices for the lower limb. Fine terminal branches also extend out of the great longitudinal fissure to supply a narrow lateral band of frontal and parietal cortices.

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Figure 7.5 The cerebral cortical distribution of the anterior, middle and posterior cerebral arteries. (A) Lateral aspect; (B) Medial aspect.

The middle cerebral artery is the largest of the three cerebral arteries and its cortical territory is the most extensive (Fig. 7.5). It passes laterally from its origin to enter the lateral fissure within which it subdivides, its branches supplying virtually the whole of the lateral surface of the frontal, parietal and temporal lobes. This territory includes the primary motor and sensory cortices for the whole of the body, excluding the lower limb. It also serves the auditory cortex and the insula within the depths of the lateral fissure.

Since the structures supplied by branches of the internal carotid artery are normally perfused almost entirely from this source, they are often referred to as being supplied by the ‘internal carotid system’.

The vertebral artery arises from the subclavian artery, ascends through the foramina transversaria of the cervical vertebrae and enters the cranial cavity through the foramen magnum, alongside the ventrolateral aspect of the medulla (Figs 7.27.37.67.7). As they pass rostrally, the two vertebral arteries converge, uniting at the junction of the medulla and pons to form the midline basilar artery. Along its course, the vertebral artery gives rise to a number of branches, including the anterior and posterior spinal arteries, which supply the medulla and spinal cord. Its largest branch is the posterior inferior cerebellar artery, which supplies the inferior aspect of the cerebellum.

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Figure 7.6 Vertebral angiograms. Radio-opaque material has been introduced into the vertebral artery in order to display its intracranial course and the distribution of its branches. (A) Lateral view; (B)Frontal view.

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

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Figure 7.7 ‘Time-of-flight’ MR arteriograms. The scans were performed on a 3.0T MR scanner. This method does not require the injection of contrast media into the patient; rather it relies on complex MR sequences to produce signal from structures with flow, while suppressing signal from stationary tissues. (A) Lateral view; (B) Frontal view.

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

The basilar artery runs the length of the pons, which it supplies by means of many small pontine branches. It also gives rise to the anterior inferior cerebellar artery, which supplies the anterior and inferior portion of the cerebellum, and the labyrinthine artery, which passes into the internal acoustic meatus to supply the inner ear. At the junction of the pons and midbrain, the basilar artery divides into two pairs of vessels, the superior cerebellar arteries and the posterior cerebral arteries. The superior cerebellar artery supplies the superior aspect of the cerebellum. The posterior cerebral artery curves around the midbrain to supply the visual cortex of the occipital lobe and the inferomedial aspect of the temporal lobe (Fig. 7.5).

The brain regions (brain stem, cerebellum and occipital lobe) served by the vertebral and basilar arteries and their branches are described as being supplied by the ‘vertebrobasilar system’.

The internal carotid and vertebrobasilar systems are joined by two thin vessels, the posterior communicating arteries, which pass rostrocaudally between the ends of the internal carotid arteries and the posterior cerebral arteries.

This completes an anastomosis of vessels on the base of the brain, known as the circulus arteriosus or circle of Willis (Figs 7.27.3), which encircles the optic chiasma and the floor of the hypothalamus and midbrain. The anastomotic arrangement of vessels provides the possibility that obstruction or narrowing of the proximal parts of the cerebral arteries, which would be expected to lead to insufficiency in the perfusion of their territories, might be compensated by circulation of blood through the communicating arteries. The actual significance of this arrangement is dependent upon the size of the communicating arteries, which is highly variable between individuals. From the arteries that constitute the circle of Willis, numerous small vessels penetrate the surface of the brain. These are known as perforating arteries(central or ganglionic arteries) and consist of two main groups:

1. Anterior perforating arteries, which arise from the anterior cerebral artery, anterior communicating artery and the region of origin of the middle cerebral artery. They enter the brain in the region between the optic chiasma and the termination of the olfactory tract, known as the anterior perforated substance (Fig. 16.13). These vessels supply large parts of the basal ganglia, the optic chiasma, the internal capsule and the hypothalamus.

2. Posterior perforating arteries, which arise from the posterior cerebral and posterior communicating arteries. They enter the brain in the region between the two crura cerebri of the midbrain, known as the posterior perforated substance (Fig. 16.13), to supply the ventral portion of the midbrain and parts of the subthalamus and hypothalamus.

imageDisorders of blood supply of the brain

One of the most common causes of neurological disability is stroke. The sudden occlusion of a cerebral artery leads to death of brain tissue (infarction). Rupture of a blood vessel causes bleeding into the brain (cerebral haemorrhage). These events lead to the rapid development of a focal neurological syndrome. Strokes related to the carotid artery and its cerebral branches are associated with focal epilepsy, a contralateral sensory/motor deficit and a psychological deficit (e.g. aphasia). Strokes involving the vertebrobasilar circulation lead to a focal brain stem syndrome. Recovery of function can occur but may take up to 2 years and can be incomplete.

An aneurysm is an abnormal, balloon-like, swelling of an artery. A surgical emergency arises when an aneurysm ruptures and blood projects around the brain in the subarachnoid space (subarachnoid haemorrhage) and into the brain (intracerebral haemorrhage). A sudden severe headache and neck stiffness are followed by coma and neurological deficits. Neurosurgery or intra-arterial ‘coiling’ are required to seal the aneurysm to prevent further bleeding and allow recovery.

An angioma, or arteriovenous malformation, is a congenital collection of swollen blood vessels that can rupture, causing cerebral haemorrhage, or ‘steal’ blood from adjacent brain regions, leading to epilepsy and a focal cerebral syndrome.

Arterial supply of the brain

image The brain is supplied by paired internal carotid and vertebral arteries.

image The internal carotid artery terminates lateral to the optic chiasma, giving rise to the anterior and middle cerebral arteries.

image The anterior cerebral artery passes into the great longitudinal fissure and supplies the medial aspect of the cerebral hemisphere.

image The middle cerebral artery passes into the lateral fissure and supplies the lateral aspect of the cerebral hemisphere.

image The vertebral arteries run on the ventrolateral aspect of the medulla, uniting to form the midline basilar artery, which extends the length of the pons. Along their course the vertebral and basilar arteries give rise to branches that supply the cerebellum and brain stem.

image The principal terminal branch of the basilar artery is the posterior cerebral artery, which supplies the occipital lobe of the cerebral hemisphere.

image The anterior communicating artery links together the two anterior cerebral arteries. Posterior communicating arteries pass between the internal carotid artery and the posterior cerebral artery, on each side. This anastomosis of vessels constitutes the circle of Willis.

image Small perforating arteries arise from the circle of Willis to supply the hypothalamic area, the basal ganglia and the internal capsule.

Venous drainage of the brain

Three sorts of vessels contribute to the venous drainage of the brain (Figs 7.8-7.10): deep veins, superficial veins and the dural venous sinuses. None of these vessels contains valves.

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Figure 7.8 The deep cerebral veins. The brain is viewed from above and the corpus callosum has been removed to reveal the third and lateral ventricles.

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Figure 7.9 Venous drainage of the brain. (A) Lateral view; (B) Sagittal view.

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Figure 7.10 Phase contrast MR venograms. The scans were performed on a 3.0T MR scanner. (A) Lateral view; (B) Frontal view.

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

Deep cerebral veins drain the internal structures of the forebrain (Fig. 7.8). Of particular note are the thalamostriate vein and the choroidal vein, which drain the basal ganglia, thalamus, internal capsule, choroid plexus and hippocampus. Within each cerebral hemisphere, these vessels merge to form the internal cerebral vein. The two internal cerebral veins then unite in the midline to form the great cerebral vein (of Galen), which lies beneath the splenium of the corpus callosum. This short vessel is continuous with the straight sinus, which lies in the midline of the tentorium cerebelli.

Superficial veins lie within the subarachnoid space (Fig. 7.9). Superior cerebral veins primarily drain the lateral surface of the cerebral hemispheres and empty into the superior sagittal sinus. The superficial middle cerebral vein runs along the line of the lateral fissure and empties into the cavernous sinus. In addition, two major anastomotic channels exist, the superior (great) anastomotic vein and the inferior anastomotic vein, which drain into the superior sagittal sinus and the transverse sinus, respectively.

The deep and superficial cerebral veins drain into the dural venous sinuses (Figs 7.97.10; see also Ch. 5), which are channels formed between the two layers of dura mater. Major venous sinuses are located in the attached borders of the falx cerebri and the tentorium cerebelli, and on the floor of the cranial cavity.

Along the line where the falx cerebri attaches to the interior of the cranium lies the superior sagittal sinus. This receives blood primarily from the superior cerebral veins, which ramify over the lateral surface of the cerebral hemispheres. The free border of the falx encloses the smaller inferior sagittal sinus, into which flow veins on the medial aspect of the hemisphere. Within the tentorium cerebelli, along the line of its attachment to the falx, lies the large straight sinus. Into this run both the great cerebral vein, which drains the deep structures of the forebrain, and the inferior sagittal sinus.

The superior sagittal sinus and the straight sinus converge at the confluence of the sinuses, which lies adjacent to the internal occipital protuberance. From here, blood flows laterally on either side in the transverse sinus, which lies along the line of attachment of the tentorium to the occipital bone. The transverse sinus is continuous with the sigmoid sinus, which, in turn, joins the internal jugular vein at the level of the jugular foramen.

The cavernous sinus (Fig. 7.11) lies lateral to the body of the sphenoid bone. It receives blood from the middle cerebral vein and drains into the internal jugular vein (via the inferior petrosal sinus) and into the transverse sinus (via the superior petrosal sinus). The two cavernous sinuses are connected by intercavernous sinuses that lie anterior and posterior to the hypophysis, forming a venous circle around it (the circular sinus). The dural venous sinuses are connected to extracranial veins via emissary veins.

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Figure 7.11 Transverse section through the cavernous sinus.

imageDiseases of the venous sinuses

Thrombosis of the sagittal sinuses is a rare complication of childbirth, blood-clotting disorders and ear infection. Obstruction of the venous drainage of the brain leads to cerebral swelling (oedema) and the syndrome of raised intracranial pressure (see p. 47). Cerebral damage caused by venous infarction manifests as epileptic seizures and focal paralysis of the limbs.

In cavernous sinus thrombosis there is acute pain and swelling of the orbit and contents, with ophthalmoplegia, ptosis and numbness of the face.

Venous drainage of the brain

image Venous drainage of the brain involves deep veins, superficial veins and dural venous sinuses.

image Deep cerebral veins drain into the great cerebral vein, which is continuous with the straight sinus.

image Superficial veins empty principally into the superior sagittal sinus and the cavernous sinus.

image The superior sagittal sinus and straight sinuses meet at the confluence of the sinuses.

image Venous blood flows, via the transverse sinus and sigmoid sinus, into the internal jugular vein.