The Blood Supply of the Brain and Spinal Cord The entire blood supply of the brain and spinal cord depends on two sets of branches from the dorsal aorta. The vertebral arteries arise from the subclavian arteries, and the internal carotid arteries are branches of the common carotid arteries. The vertebral arteries and the ten medullary arteries that arise from segmental branches of the aorta provide the primary vascularization of the spinal cord. These medullary arteries join to form the anterior and posterior spinal arteries. If any of the medullary arteries are obstructed or damaged (during abdominal surgery, for example), the blood supply to specific parts of the spinal cord may be compromised. The pattern of resulting neurological damage differs according to whether the supply to the posterior or anterior artery is interrupted. As might be expected from the arrangement of ascending and descending neural pathways in the spinal cord, loss of the posterior supply generally leads to loss of sensory functions, whereas loss of the anterior supply more often causes motor deficits. Blood supply of the spinal cord. View of the ventral (anterior) surface of the spinal cord. At the level of the medulla, the vertebral arteries give off branches that merge to form the anterior spinal artery. Approximately 10 to 12 segmental arterie The brain receives blood from two sources: the internal carotid arteries, which arise at the point in the neck where the common carotid arteries bifurcate, and the vertebral arteries The internal carotid arteries branch to form two major cerebral arteries, the anterior and middle cerebral arteries. The right and left vertebral arteries come together at the level of the pons on the ventral surface of the brainstem to form the midline basilar artery. The basilar artery joins the blood supply from the internal carotids in an arterial ring at the base of the brain (in the vicinity of the hypothalamus and cerebral peduncles) called the circle of Willis. The posterior cerebral arteries arise at this confluence, as do two small bridging arteries, the anterior and posterior communicating arteries. Conjoining the two major sources of cerebral vascular supply via the circle of Willis presumably improves the chances of any region of the brain continuing to receive blood if one of the major arteries becomes occluded The major arteries of the brain. Ventral view. The enlargement of the boxed area shows the circle of Willis. Lateral and midsagittal views showing anterior, middle, and posterior cerebral arteries. Idealized The physiological demands served by the blood supply of the brain are particularly significant because neurons are more sensitive to oxygen deprivation than other kinds of cells with lower rates of metabolism. In addition, the brain is at risk from circulating toxins, and is specifically protected in this respect by the blood-brain barrier . As a result of the high metabolic rate of neurons, brain tissue deprived of oxygen and glucose as a result of compromised blood supply is likely to sustain transient or permanent damage. Brief loss of blood supply (referred to as ischemia) can cause cellular changes, which, if not quickly reversed, can lead to cell death. Sustained loss of blood supply leads much more directly to death and degeneration of the deprived cells. Strokes—an anachronistic term that refers to the death or dysfunction of brain tissue due to vascular disease—often follow the occlusion of (or hemorrhage from) the brains arteries . Historically, studies of the functional consequences of strokes, and their relation to vascular territories in the brain and spinal cord, provided information about the location of various brain functions. The location of the major language functions in the left hemisphere, for instance, was discovered in this way in the latter part of the nineteenth century Now, noninvasive functional imaging techniques based on blood flow have largely supplanted the correlation of clinical signs and symptoms with the location of tissue damage observed at autopsy. The Blood-Brain Barrier. The interface between the walls of capillaries and the surrounding tissue is important throughout the body, as it keeps vascular and extravascular concentrations of ions and molecules at appropriate levels in these two compartments
Posted on: Thu, 16 Jan 2014 02:31:29 +0000