The Blood Supply of the Brain and Spinal Cord (2024)

The entire blood supply of the brain and spinal cord depends on two sets of branchesfrom the dorsal aorta. The vertebral arteries arise from the subclavianarteries, and the internal carotid arteries are branches of the commoncarotid arteries. The vertebral arteries and the ten medullary arteriesthat arise from segmental branches of the aorta provide the primary vascularizationof the spinal cord. These medullary arteries join to form the anterior andposterior spinal arteries (Figure1.19). If any of the medullary arteries are obstructed or damaged (duringabdominal surgery, for example), the blood supply to specific parts of the spinalcord may be compromised. The pattern of resulting neurological damage differsaccording to whether the supply to the posterior or anterior artery is interrupted.As might be expected from the arrangement of ascending and descending neuralpathways in the spinal cord, loss of the posterior supply generally leads to loss ofsensory functions, whereas loss of the anterior supply more often causes motordeficits.

The brain receives blood from two sources: the internal carotidarteries, which arise at the point in the neck where the common carotidarteries bifurcate, and the vertebral arteries (Figure 1.20). The internal carotid arteries branch to form twomajor cerebral arteries, the anterior and middle cerebral arteries. Theright and left vertebral arteries come together at the level of the pons on theventral surface of the brainstem to form the midline basilar artery.The basilar artery joins the blood supply from the internal carotids in an arterialring at the base of the brain (in the vicinity of the hypothalamus and cerebral peduncles) called the circle of Willis. The posterior cerebral arteriesarise at this confluence, as do two small bridging arteries, the anterior andposterior communicating arteries. Conjoining the two major sources ofcerebral vascular supply via the circle of Willis presumably improves the chances ofany region of the brain continuing to receive blood if one of the major arteriesbecomes occluded (see Box D).

The major branches that arise from the internal carotid artery—the anteriorand middle cerebral arteries—form the anterior circulationthat supplies the forebrain (Figure 1.20B).These arteries also originate from the circle of Willis. Each gives rise to branchesthat supply the cortex and branches that penetrate the basal surface of the brain,supplying deep structures such as the basal ganglia, thalamus, and internal capsule.Particularly prominent are the lenticulostriate arteries that branch from the middlecerebral artery. These arteries supply the basal ganglia and thalamus. Theposterior circulation of the brain supplies the posterior cortex,the midbrain, and the brainstem; it comprises arterial branches arising from theposterior cerebral, basilar, and vertebralarteries. The pattern of arterial distribution is similar for all thesubdivisions of the brainstem: Midline arteries supply medial structures, lateralarteries supply the lateral brainstem, and dorsal-lateral arteries supplydorsal-lateral brainstem structures and the cerebellum (Figures 1.20 and 1.21).Among the most important dorsal-lateral arteries (also called longcircumferential arteries) are the posterior inferior cerebellarartery (PICA) and the anterior inferior cerebellarartery (AICA), which supply distinct regions of the medullaand pons. These arteries, as well as branches of the basilar artery that penetratethe brainstem from its ventral and lateral surfaces (called paramedianand short circumferential arteries), are especially common sites ofocclusion and result in specific functional deficits of cranial nerve, somaticsensory, and motor function (see BoxesA and D).

The physiological demands served by the blood supply of the brain are particularlysignificant because neurons are more sensitive to oxygen deprivation than otherkinds of cells with lower rates of metabolism. In addition, the brain is at riskfrom circulating toxins, and is specifically protected in this respect by theblood-brain barrier (Box E). As aresult of the high metabolic rate of neurons, brain tissue deprived of oxygen andglucose as a result of compromised blood supply is likely to sustain transient orpermanent damage. Brief loss of blood supply (referred to as ischemia) can causecellular changes, which, if not quickly reversed, can lead to cell death. Sustainedloss of blood supply leads much more directly to death and degeneration of thedeprived cells. Strokes—an anachronistic term that refers to the death ordysfunction of brain tissue due to vascular disease—often follow theocclusion of (or hemorrhage from) the brain's arteries (see Box D). Historically, studies of thefunctional consequences of strokes, and their relation to vascular territories inthe brain and spinal cord, provided information about the location of various brainfunctions. The location of the major language functions in the left hemisphere, forinstance, was discovered in this way in the latter part of the nineteenth century(see Chapter 27). Now, noninvasivefunctional imaging techniques based on blood flow (see Box C) have largely supplanted the correlation of clinicalsigns and symptoms with the location of tissue damage observed at autopsy.