Chapter 3: Vasculature of the Central Nervous System and the Cerebrospinal Fluid

Brain vasculature disorders constitute a major class of nervous system disease. The principal source of nourishment for the central nervous system is glucose, and because neither glucose nor oxygen is stored in appreciable amounts, when the blood supply of the central nervous system is interrupted, even briefly, brain functions become severely disrupted.

Much of what is known about the arterial supply to the central nervous system derives from three approaches. First, classical studies in normal postmortem tissue used colored dye injected into a blood vessel to identify the areas it supplies. Second, in postmortem tissue or on radiological examination, the portion of the central nervous system supplied by a particular artery can be inferred by observing the extent of damage that occurred after the artery became occluded. Third, radiological techniques, such as cerebral angiography and magnetic resonance angiography, make it possible to view the arterial and venous circulation in the living brain (see Box 3–1). These important clinical tools also permit localization of a vascular obstruction or other pathology.

As discussed in previous chapters, brain vasculature is closely related to the ventricular system and the watery fluid contained within it, the cerebrospinal fluid. This is because most cerebrospinal fluid is produced by active secretion of ions from blood plasma by the choroid plexus. Moreover, to maintain a constant brain volume, cerebrospinal fluid is returned to the blood through valves between the subarachnoid space and the dural sinuses.

This chapter initially focuses on arterial supply because of the importance of distributing oxygenated blood to the brain and spinal cord for normal function, followed by venous drainage. The blood-brain barrier, which isolates the intravascular compartment from the extracellular compartment of the central nervous system, is considered next. Finally, cerebrospinal fluid production and circulation within the different components of the ventricular system is examined.

Local regions of the central nervous system receive blood from small sets of penetrating arteries that receive their blood from the major arteries (Table 3–1). Cessation or reduction of the arterial supply to an area results in decreased delivery of oxygenated blood to the tissue, a condition termed ischemia. Decreased blood supply typically occurs when an artery becomes occluded or when systemic blood pressure drops substantially, such as during a heart attack. Occlusion commonly occurs because of an acute blockade, such as from an embolus, or the gradual narrowing of the arterial lumen (stenosis), as in atherosclerosis.

A brief reduction in blood flow produces transient neurological signs, attributable to lost functions of the oxygen-deprived area. This event is termed a transient ischemic attack (TIA). If ischemia is persistent and is uncorrected for several minutes, it can cause death of the tissue it normally supplies, termed an infarction. This can result in more enduring or even permanent impairments. These events describe an ischemic stroke. Under special circumstances, the local reduction in arterial blood flow may not produce an ischemic stroke and infarction because the tissue receives a redundant supply from another artery. This is termed collateral circulation, which is covered further in the section on the anterior and posterior circulations.

Hemorrhagic stroke can occur when an artery ruptures, thereby releasing blood into the surrounding tissue. A hemorrhagic stroke not only produces a loss of downstream flow but also can damage brain tissue at the rupture site because of the volume now occupied by the blood outside of the vessel. A common cause of a hemorrhagic stroke is when an aneurysm, or ballooning of an artery due to weakening of the muscular wall, ruptures.

The principal blood supply for the brain comes from two arterial systems that receive blood from different systemic arteries: the anterior circulation, fed by the internal carotid arteries, and the posterior circulation, which receives blood from the vertebral arteries (Figure 3–2, inset; Table 3–1). The vertebral arteries join at the junction of the medulla and pons (or pontomedullary junction) to form the basilar artery, which lies unpaired along the midline (Figure 3–2). The anterior circulation is also called the carotid circulation, and the posterior circulation, the vertebral-basilar circulation. The anterior and posterior circulations are not independent but are connected by networks of arteries on the ventral surface of the diencephalon and midbrain and on the cortical surface (see below).

Whereas the cerebral hemispheres receive blood from both the anterior and posterior circulations, the brain stem receives blood only from the posterior circulation. The arterial supply for the spinal cord is provided by the systemic circulation—which also supplies muscle, skin, and bones—and, to a lesser degree, by the vertebral arteries. Cerebral and spinal arteries drain into veins. Although spinal veins are part of the general systemic circulation and return blood directly to the heart, most cerebral veins drain first into the dural sinuses, a set of large venous collection channels in the dura mater.

The spinal cord receives blood from two sources. First are the anterior and posterior spinal arteries (Figure 3–2), branches of the vertebral arteries. Second are the radicular arteries, which are branches of segmental vessels, such as the cervical, intercostal, and lumbar arteries. Neither anterior nor posterior spinal arteries typically form a single continuous vessel along the entire length of the ventral or dorsal spinal cord. Rather, each forms a network of communicating channels oriented along the rostrocaudal axis of the spinal cord. The radicular arteries feed into this network along the entire length of the spinal cord.

Although the spinal and radicular arteries supply blood to all spinal cord levels, different spinal cord segments are preferentially supplied by one or the other set of arteries. The cervical spinal cord is supplied by both the vertebral and radicular arteries (in particular, the ascending cervical artery). In contrast, the thoracic, lumbar, and sacral segments are nourished primarily by the radicular arteries (the intercostal and lumbar arteries). When spinal cord segments are supplied by a single artery, they are particularly susceptible to injury after arterial occlusion. In contrast, segments that receive a redundant (or collateral) blood supply tend to fare better following single vessel occlusion. For example, individual rostral thoracic segments are supplied by fewer radicular arteries than are more caudal segments. When a radicular artery that serves the rostral thoracic segments becomes occluded, serious damage is more likely to occur because there is no backup system for perfusion of oxygenated blood. Interruption of the blood supply to critical areas of the spinal cord can produce sensory and motor control impairments similar to those produced by traumatic mechanical injury, such as that resulting from an automobile accident.

Each of the three divisions of the brain stem and the cerebellum receives its arterial supply from the posterior circulation (Figure 3–3A). In contrast to the spinal arteries, which are located both ventrally and dorsally, arteries supplying most of the brain stem arise from the ventral surface only. Branches emerge from these ventral arteries and either penetrate directly or run around the circumference of the brain stem to supply dorsal brain stem structures and the cerebellum. Three groups of branches arise from the vertebral and basilar arteries: (1) paramedian, (2) short circumferential, and (3) long circumferential (Figure 2–3B). The paramedian branches supply regions close to the midline. The short circumferential branches supply lateral, often wedge-shaped regions, and the long circumferential branches supply the dorsolateral portions of the brain stem and cerebellum.

Even though the spinal arteries primarily supply the spinal cord, they also supply a small portion of the caudal medulla. The spinal arteries lie close to the dorsal and ventral midline and nourish the most medial areas (Figure 3–3B4). The more lateral area is served by direct branches of the vertebral arteries, which are equivalent to the more rostral short circumferential branches.

The rest of the medulla is supplied by the vertebral arteries. Small (unnamed) branches that exit from the main arteries supply the medial medulla (ie, paramedian and short circumferential branches). Because these arteries supply axons of the corticospinal tract and the medial lemniscus (see Figure 2–2), when the arteries become occluded, patients develop impairments in voluntary limb movement and mechanosensation. The major laterally emerging (long circumferential) branch from the vertebral artery, the posterior inferior cerebellar artery (PICA), nourishes the most dorsolateral region (Figure 3–3B3). This region of the medulla does not receive blood from any other artery. The absence of a collateral arterial supply makes the posterior inferior cerebellar artery particularly important because occlusion almost always results in significant tissue damage. When this occurs, patients commonly develop characteristic sensory and motor impairments due to destruction of nuclei and tracts in the dorsolateral medulla. Common neurological signs include loss of facial pain sensation and uncoordinated limb movements, both on the side of the occlusion, and loss of limb and trunk pain on the opposite side. An understanding of this complex pattern of sensory and motor loss will be achieved when pain and motor control circuits are described in later chapters.

The two vertebral arteries join to form the basilar artery at the pontomedullary junction (Figure 3–3A), from which paramedian and short circumferential arteries supply the base of the pons, where corticospinal fibers are located. The dorsolateral portion of the caudal pons is supplied by a long circumferential branch of the basilar artery, termed the anterior inferior cerebellar artery (AICA). The region in the pons rostral to that supplied by the anterior inferior cerebellar artery is nourished by the superior cerebellar artery, another long circumferential branch of the basilar artery (Figure 3–3A).

Long circumferential branches of the vertebral and basilar arteries supply the cerebellum. The posterior inferior cerebellar artery supplies the caudal portion of the cerebellum. More rostral portions are supplied by the anterior inferior cerebellar artery and the superior cerebellar artery (Figure 3–2 and Figure 3–3A).

The basilar artery splits at the pons-midbrain border into the two posterior cerebral arteries. While the posterior cerebral artery is part of the vertebral-basilar system, it develops from the anterior system, thereby receiving blood from the carotid arteries. However, later in development much more blood comes from the basilar artery, making the posterior cerebral artery functionally part of the posterior circulation in maturity. The posterior cerebral artery nourishes most of the midbrain (Figure 3–3B1). Paramedian and short circumferential branches supply the base and tegmentum, whereas long circumferential branches supply the tectum. The colliculi, the principal portion of the tectum, also receive a small supply by the superior cerebellar artery.

The internal carotid artery consists of four segments (Figure 3–4A): (1) The cervical segment extends from the bifurcation of the common carotid (into the external and internal carotid arteries; see Figure 3–2) to where it enters the carotid canal; (2) the intrapetrosal segment courses through the petrous portion of the temporal bone; (3) the intracavernous segment courses through the cavernous sinus, a venous structure overlying the sphenoid bone (see Figure 3–15); and (4) the cerebral segment extends to where the internal carotid artery bifurcates into the anterior and middle cerebral arteries. The intracavernous and cerebral portions form the carotid siphon, an important radiological landmark.

Branches emerging directly from the cerebral segment of the internal carotid artery supply deep cerebral and other cranial structures. The major branches of this artery (Figure 3–4B), in caudal to rostral order, are (1) the ophthalmic artery, which supplies the optic nerve and the inner portion of the retina, (2) the posterior communicating artery, which primarily nourishes diencephalic structures, and (3) the anterior choroidal artery, which supplies diencephalic and subcortical telencephalic structures.

The internal carotid artery divides near the basal surface of the cerebral hemisphere to form the anterior cerebral and middle cerebral arteries (Figure 3–2). Thus, the anterior and middle cerebral arteries receive their blood from the anterior circulation and, as described, the posterior cerebral artery receives blood from the posterior circulation. The three cerebral arteries each comprise deep and cortical branches. The deep branches come off the arteries proximally. The deep branches of the three cerebral arteries, together with the arteries that branch from the cerebral segment of the internal carotid artery, supply deep brain gray matter and white matter regions. The cortical branches are the distal, or terminal, endings of the cerebral arteries; they supply the various neuronal laminae of the cerebral cortex.

Collateral Circulation Can Rescue Brain Regions Deprived of Blood

There are two sites where the anterior and posterior circulations communicate, on the ventral and dorsal brain surfaces. Communication between the circulations is clinically important because decreased flow in one system can be compensated by increased flow in the other. At the ventral site, the proximal portions of the cerebral arteries and the communicating arteries form the circle of Willis. This is an example of a network of interconnected arteries, or an anastomosis. The two posterior communicating arteries allow blood to flow between the middle and posterior cerebral arteries on each side, and the anterior communicating artery allows blood to flow between the anterior cerebral arteries (Figure 3–2). When either the posterior or the anterior arterial circulation becomes occluded, collateral circulation may occur through the circle of Willis to rescue the region deprived of blood. Many individuals, however, lack one of the components of the circle of Willis. In these individuals, a functional "circle" may not be achieved, resulting in incomplete cerebral perfusion by the surviving system.

The second site for communication is where the terminal ends of the cerebral arteries anastomose on the dorsal convexity of the cerebral hemisphere (Figure 3–5). These interconnections occur between branches only when they are located on the cortical surface, not when the artery has penetrated the brain. When a major artery becomes compromised, these anastomoses limit the extent of damage. For example, if a branch of the posterior cerebral artery becomes occluded, tissue with compromised blood supply in the occipital lobe may be rescued by collateral circulation from the middle cerebral artery that connects anastomotically with the blocked vessel. This collateral circulation can rescue the gray matter of the cerebral cortex. In contrast, little collateral circulation exists in the white matter.

Although collateral circulation provides the cerebral cortex with a safety margin during arterial occlusion, the dorsal anastomotic network that provides such insurance also creates a vulnerability. When systemic blood pressure is reduced, the region served by this network is particularly susceptible to ischemia because such anastomoses occur at the terminal ends of the arteries, regions where perfusion is lowest. The peripheral borders of the territory supplied by major vessels are termed border zones, and an infarction occurring in these regions is termed a border zone infarct.

Deep Branches of the Anterior and Posterior Circulations Supply Subcortical Structures

The arterial supply of the diencephalon, basal ganglia, and internal capsule derives from both the anterior and posterior circulations (Figures 3–6 and 3–7; Table 3–1). This supply is complex, and there are many individual variations. As discussed, the branches supplying these structures emerge from the proximal portions of the cerebral arteries or directly from the internal carotid artery. The superior half of the internal capsule is supplied primarily by branches of the middle cerebral artery (Figure 3–7). The inferior half of the anterior limb and genu of the internal capsule is supplied primarily by the anterior cerebral artery and the posterior limb, by the anterior choroidal artery (see Figures 4–5 and 4–6). The basal ganglia receive their arterial blood supply from the anterior and middle cerebral arteries and the anterior choroidal artery (Figure 3–6). Many of the proximal branches of the anterior and middle cerebral arteries are also termed the lenticulostriate arteries. The thalamus is nourished by branches of the posterior cerebral and posterior communicating arteries. The hypothalamus is fed by branches of the anterior and posterior cerebral arteries and the two communicating arteries.

Different Functional Areas of the Cerebral Cortex Are Supplied by Different Cerebral Arteries

The cerebral cortex is supplied by the distal, or cortical, branches of the anterior, middle, and posterior cerebral arteries (Figure 3–8 and Figure 3–4). The anterior cerebral artery is C-shaped, like many parts of the cerebral hemispheres (see Box 1–2). It originates where the internal carotid artery bifurcates and courses within the sagittal fissure and around the rostral end (termed the genu; see Figure 1–11B) of the corpus callosum (Figure 3–4B). Knowledge of the approximate boundaries of the cortical regions supplied by the different cerebral arteries helps explain the functional disturbances that follow vascular obstruction, or other pathology, of the cerebral vessels. As its gross distribution would suggest, the anterior cerebral artery supplies the dorsal and medial portions of the frontal and parietal lobes (Figure 3–4B).

The middle cerebral artery supplies blood to the lateral convexity of the cortex (Figure 3–4A). The middle cerebral artery begins at the bifurcation of the internal carotid artery and takes an indirect course through the lateral sulcus (Figure 3–8), along the surface of the insular cortex, and over the inner opercular surfaces of the frontal, temporal, and parietal lobes. It finally emerges on the lateral convexity. This complex configuration of the middle cerebral artery can be seen on radiological images of brain vasculature (Box 3–1).

The posterior cerebral artery, originating where the basilar artery bifurcates (Figure 3–2 and Figure 3–3A), courses around the lateral margin of the midbrain (Figure 3–4B). This artery supplies the occipital lobe and portions of the medial and inferior temporal lobes (Figure 3–4B).

Venous drainage of the cerebral hemispheres is provided by superficial and deep cerebral veins (Figure 3–13). Superficial veins, arising from the cerebral cortex and underlying white matter, are variable in distribution. Among the more prominent and consistent are the superior anastomotic vein, lying across the parietal lobe, and the inferior anastomotic vein, on the surface of the temporal lobe. The deep cerebral veins, such as the internal cerebral vein (Figure 3–13, inset), drain the more interior portions of the white matter, including the basal ganglia and parts of the diencephalon. Many deep cerebral veins drain into the great cerebral vein (of Galen) (Figure 3–13, inset, and Figure 3–15).

Drainage of blood from the central nervous system into the major vessels emptying into the heart—the systemic circulation—is achieved through either a direct or an indirect path. Spinal cord and caudal medullary veins drain directly, through a network of veins and plexuses, into the systemic circulation. By contrast, the rest of the central nervous system drains by an indirect path: The veins first empty into the dural sinuses before returning blood to the systemic circulation. The dural sinuses function as low-pressure channels for venous blood flow back to the systemic circulation. They are located within layers of the dura. The portion of the dura mater overlying the cerebral hemispheres and brain stem contains two separate layers: (1) an outer periosteal layer, which is attached to the bone, and (2) an inner meningeal layer, which apposes the arachnoid (Figure 3–14A, B). The dural sinuses are located between the periosteal and meningeal layers of the dura (Figure 3–14A, B).

The superficial cerebral veins drain into the superior and inferior sagittal sinuses (Figure 3–15A). The superior sagittal sinus runs along the midline at the superior margin of the falx cerebri. The inferior sagittal sinus courses along the inferior margin of the falx cerebri just above the corpus callosum. The inferior sagittal sinus, together with the great cerebral vein (of Galen), returns venous blood to the straight (sometimes called rectus) sinus (Figure 3–15). At the occipital pole, the superior sagittal sinus and the straight sinus join to form the two transverse sinuses. Finally, these sinuses drain into the sigmoid sinuses (Figure 3–15B), which return blood to the internal jugular veins. The cavernous sinus, into which drain the ophthalmic and facial veins, is also illustrated in Figure 3–15B.

Veins of the midbrain drain into the great cerebral vein (Figure 3–13, Figure 3–15A), which empties into the straight sinus, whereas the pons and rostral medulla drain into the superior petrosal sinus (Figure 3–15B). Cerebellar veins drain into the great cerebral vein and the superior petrosal sinus.

The intravascular compartment is isolated from the extracellular compartment of the central nervous system (Figure 3–16A). This feature, the blood-brain barrier, was discovered when intravenous dye injection stained most tissues and organs of the body but not the brain. This permeability barrier protects the brain from neuroactive compounds in the blood as well as rapid changes in the ionic constituents of the blood that can affect neuronal excitability.

The blood-brain barrier results from two unique characteristics of endothelial cells in the capillaries of the brain and spinal cord (Figure 3–16A). First, in peripheral capillaries, endothelial cells have fenestrations (pores) that allow large molecules to flow into the extracellular space. Moreover, the intercellular spaces between adjacent endothelial cells are leaky. In contrast, in central nervous system capillaries, adjacent endothelial cells are tightly joined, preventing movement of compounds into the extracellular compartment of the central nervous system (Figure 3–16A). Second, there is little transcellular movement of compounds from intravascular to extracellular compartments in the central nervous system because the endothelial cells lack the required transport mechanisms. Moreover, relatively nonselective transport may occur by pinocytosis in peripheral but not central nervous system capillaries.

Although most of the central nervous system is protected by the blood-brain barrier, eight brain structures lack a blood-brain barrier. These structures are close to the midline, and because they are closely associated with the ventricular system, they are collectively termed circumventricular organs (Figure 3–16B). At each of these structures, either neurosecretory products are secreted into the blood or local neurons detect blood-borne compounds as part of a mechanism for regulating the body's internal environment. One circumventricular organ, the area postrema (Figure 3–16B), is important for trigger vomiting in response to circulating blood-borne chemicals. A band of specialized glial cells with tight junctions is present that forms a blood-brain barrier between the area postrema and the rest of the medulla.

Cerebrospinal fluid fills the ventricles. It also fills the subarachnoid space and thus bathes the external brain surface. Together, the ventricles and subarachnoid space contain approximately 140 mL of cerebrospinal fluid, of which 25 mL are in the ventricles and the remaining in the subarachnoid space. Intraventricular pressure is normally around 10–15 mm Hg.

Cerebrospinal fluid serves at least three essential functions. First, it provides physical support for the brain, which floats within the fluid. Cerebrospinal fluid cushions the brain from physical shocks. Second, it serves an excretory function and regulates the chemical environment of the central nervous system. Because the brain has no lymphatic system, water-soluble metabolites, which have limited ability to cross the blood-brain barrier, diffuse from the brain into the cerebrospinal fluid. Third, it acts as a channel for chemical communication within the central nervous system. Neurochemicals released by neurons can enter the cerebrospinal fluid and be taken up by cells on the ventricular floor and walls. Moreover, once in the cerebrospinal fluid, these compounds also have relatively free access to neural tissue adjacent to the ventricles because, in contrast to the blood-brain barrier, most of the ventricular lining presents no barrier between the cerebrospinal fluid compartment and the extracellular compartment of the brain.

Most of the Cerebrospinal Fluid Is Produced by the Choroid Plexus

Cerebrospinal fluid is secreted mainly by the choroid plexus. The cellular constituents of choroid plexus are blood vessels and pia, which form the core of the choroid plexus, and the choroid epithelium, which is specialized to secrete cerebrospinal fluid. The choroid plexus is present only in the ventricles; in the roof of the third and fourth ventricles. In the lateral ventricles, the choroid plexus is located in the ventricular roof and floor. A barrier imposed by the choroidal epithelium prevents the transport of materials from blood into the cerebrospinal fluid. This is the blood–cerebrospinal fluid barrier, analogous to the blood-brain barrier. The choroidal epithelium is innervated by autonomic fibers, which serve a regulatory function. For example, denervation of the sympathetic fibers produces hydrocephalus in animals. A second blood–cerebrospinal fluid barrier exists between the arachnoid and the dural blood vessels.

The rest of the cerebrospinal fluid is secreted by brain capillaries. This extrachoroidal source of cerebrospinal fluid enters the ventricular system through ependymal cells, the ciliated cuboidal epithelial cells that line the ventricles. Total cerebrospinal fluid production by both sources is approximately 500 mL per day. Although the principal function of the choroid plexus is cerebrospinal fluid secretion, the plexus also has a reabsorptive function. The choroid plexus can eliminate from the cerebrospinal fluid a variety of compounds introduced into the ventricles.

Cerebrospinal Fluid Circulates Throughout the Ventricles and Subarachnoid Space

Cerebrospinal fluid produced by the choroid plexus in the lateral ventricles (Figure 3–17) flows through the interventricular foramina and mixes with cerebrospinal fluid produced in the third ventricle. The lateral ventricle is a C-shaped structure, as are many deep neuronal regions of the cerebral hemispheres (see Box 1–2). From the lateral ventricles, CSF flows through the cerebral aqueduct and into the fourth ventricle, another major site for cerebrospinal fluid production because the choroid plexus is also located there. Three apertures in the roof of the fourth ventricle drain cerebrospinal fluid from the ventricular system into the subarachnoid space: the foramen of Magendie, located on the midline, and the two foramina of Luschka, located at the lateral margins of the fourth ventricle (Figure 3–17, inset).

The subarachnoid space is dilated in certain locations, termed cisterns; cerebrospinal fluid pools here. Five prominent cisterns are located on the midline: (1) the interpeduncular cistern, between the basis pedunculi on the ventral midbrain surface, (2) the quadrigeminal cistern, dorsal to the superior and inferior colliculi (which are also called the quadrigeminal bodies), (3) the pontine cistern, at the ventral portion of the pontomedullary junction, (4) the cisterna magna, dorsal to the medulla, and (5) the lumbar cistern, in the caudal vertebral canal. The subarachnoid space also contains the blood vessels of the central nervous system (Figure 3–18B). Blood vessels penetrate the brain together with the pia, creating a perivascular space between the vessels and pia for a short distance, and a path for CSF flow from the subarachnoid space to interstitial spaces within the brain and spinal cord. These spaces, termed the Virchow-Robin spaces, contain cerebrospinal fluid (Figure 3–17B).

Cerebrospinal Fluid Is Drawn From the Lumbar Cistern

Cerebrospinal fluid can be safely withdrawn from the lumbar cistern, without risking spinal cord damage. This can be understood by considering how the caudal vertebral column and spinal cord develop. Throughout the first 3 months of development, the spinal cord grows at about the same rate as the vertebral column (Figure 3–18A). During this period, the spinal cord occupies the entire vertebral canal, the space within the vertebral column. The dorsal and ventral roots associated with each segment pass directly through the intervertebral foramina to reach their target structures. Later, the growth of the vertebral column exceeds that of the spinal cord. In the adult, the most caudal spinal cord segment is located at the level of the first lumbar vertebra. This differential growth produces the lumbar cistern, an enlargement of the subarachnoid space in the caudal portion of the spinal canal (Figure 3–18B). The dorsal and ventral roots from the lumbar and sacral segments, which subserve sensation and movement of the legs, travel within the lumbar cistern before exiting the vertebral canal (Figure 3–18B). These roots resemble a horse's tail in gross dissection, hence the name cauda equina. Cerebrospinal fluid can be withdrawn from the lumbar cistern without risk of damaging the spinal cord by inserting a needle through the intervertebral space between the third and fourth (or fourth and fifth) vertebrae (Figure 3–18B). The roots are displaced by the needle rather than being pierced. This procedure is known as a spinal or lumbar tap.

The Dural Sinuses Provide the Return Path for Cerebrospinal Fluid

Cerebrospinal fluid passes from the subarachnoid space to the venous blood through specialized structures termed arachnoid villi. Arachnoid villi are microscopic evaginations of the arachnoid mater that protrude into the dural sinuses as well as directly into certain veins. The cerebrospinal fluid flows through a system of large vacuoles in the arachnoid cells of the villi and through an extracellular path between cells of the villi; they are not actually valves. Numerous clusters of arachnoid villi are present over the dorsal (superior) convexity of the cerebral hemispheres in the superior sagittal sinus, where they form a macroscopic structure called the arachnoid granulations (Figures 4–13B and 4–16). The arachnoid granulations can be imaged on MRI (Figure 3–17C). The arachnoid villi are also present where the spinal nerves exit the spinal dural sac. These villi direct the flow of cerebrospinal fluid into the radicular veins.

Arterial Supply of the Spinal Cord and Brain Stem

The arterial supply of the spinal cord is provided by the vertebral (Figure 3–3A) and radicular arteries. The brain is supplied by the internal carotid arteries (the anterior circulation) and the vertebral arteries, which join at the pontomedullary junction to form the basilar artery (collectively termed the posterior circulation) (Figure 3–2). The brain stem and cerebellum receive blood only from the posterior system (Figure 3–3 and Figure 3–4B; Table 3–1). The medulla receives blood directly from small branches of the vertebral arteries and from the spinal arteries and the posterior inferior cerebellar artery (PICA) (Figure 3–3B3,4). The pons is supplied by paramedian and short circumferential branches of the basilar artery. Two major long circumferential branches are the anterior inferior cerebellar artery (AICA) and the superior cerebellar artery (Figure 3–3B2). The midbrain receives its arterial supply primarily from the posterior cerebral artery as well as from the basilar artery (Figure 3–3B1). The PICA supplies the caudal cerebellum, and the AICA and superior cerebellar artery supply the rostral cerebellum (Figure 3–4B).

Stroke and Collateral Circulation

Neural tissue depends on a continuous supply of arterial blood. Cessation or reduction of the arterial supply to an area can produce ischemia and an infarction. A brief interruption in blood flow produces a transient ischemic attack (TIA), or temporary loss of function of the affected region. Persistent loss of blood flow by arterial occlusion produces an ischemic stroke. When an artery ruptures, a hemorrhagic stroke occurs. An aneurysm, which is a ballooning of an artery, can rupture to produce a hemorrhagic stroke. The anterior and posterior systems are interconnected by two networks of arteries that can provide redundant arterial supply, or collateral circulation: (1) the circle of Willis—which is formed by the anterior, middle, and posterior cerebral arteries; the posterior communicating arteries; and the anterior communicating artery (Figure 3–2); and (2) terminal branches of the cerebral arteries, which anastomose on the superior convexity of the cerebral cortex (Figure 3–5).

Arterial Supply of the Diencephalon and Cerebral Hemispheres

The diencephalon and cerebral hemispheres are supplied by the anterior and posterior circulations (Figure 3–2 and Figures 3–4, 3–5, 3–6, 3–7, 3–8). The cerebral cortex receives its blood supply from the three cerebral arteries: the anterior and middle cerebral arteries, which are part of the anterior circulation, and the posterior cerebral artery, which is part of the posterior circulation (Figure 3–2). The diencephalon, basal ganglia, and internal capsule receive blood from branches of the internal carotid artery, the three cerebral arteries, and the posterior communicating artery (Figure 3–2; Table 4–1).

Venous Drainage

The venous drainage of the spinal cord and caudal medulla is direct to the systemic circulation. By contrast, veins draining the cerebral hemispheres, diencephalon, midbrain, pons, cerebellum, and rostral medulla (Figure 3–13) drain into the dural sinuses (Figures 3–14 and 3–15). The major dural sinuses are as follows: superior sagittal, inferior sagittal, straight, transverse, sigmoid, superior, and inferior petrosal.

Blood-Brain Barrier

The internal environment of most of the central nervous system is protected from circulating neuroactive agents in blood by the blood-brain barrier (Figure 3–16A). This barrier is formed by a number of specializations in the capillary endothelium of the central nervous system. Brain regions without a blood-brain barrier, termed the circumventricular organs (Figure 3–16B), include the (1) area postrema in the medulla, (2) subcommissural organ, (3) subfornical organ, (4) vascular organ of the lamina terminalis, (5) median eminence, (6) neurohypophysis, (7) choroid plexus, and (8) pineal gland.

Production and Circulation of Cerebrospinal Fluid

Most of the cerebrospinal fluid is produced by the choroid plexus, which is located in the ventricles (Figure 3–17). It exits from the ventricular system, through foramina in the fourth ventricle—the two foramina of Luschka (located laterally) and the foramen of Magendie (located on the midline)—directly into the subarachnoid space. Cerebrospinal fluid pools in cisterns of the subarachnoid space over the brain and caudal to the spinal cord (Figure 3–18). Cerebrospinal fluid passes into the dural sinuses (Figure 3–14) through unidirectional valves termed arachnoid villi clustered in the arachnoid granulations (Figure 3–17).