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).