Porth's Essentials of Pathophysiology, 4e

930

Nervous System

U N I T 1 0

inferior frontal gyrus, the motor and premotor frontal cortex concerned with delicate face and hand control (Fig. 37-13). It is the major vascular source for the lan- guage cortices, the primary and association auditory cortices, and the primary and association somatosen- sory cortices for the face and hand. Because the middle cerebral artery is a continuation of the internal carotid, emboli arising in the internal carotid most frequently become lodged in branches of the middle cerebral artery. The consequences of ischemia of these areas may be the most devastating, resulting in damage to the fine manipulative skills of the face and upper limbs and to receptive and expressive communication functions (e.g., aphasia). The two vertebral arteries arise from the subclavian artery and enter the foramina (opening) in the trans- verse spinal processes at the level of the sixth cervical vertebra and continue upward through the foramina of the upper six vertebrae. They wind behind the atlas and enter the skull through the foramen magnum and unite to form the basilar artery, which then diverges to termi- nate in the posterior cerebral arteries. Branches of the basilar and vertebral arteries supply the medulla, pons, cerebellum, midbrain, and caudal part of the diencepha- lon. The posterior cerebral arteries supply the remaining occipital and inferior regions of the temporal lobes and the thalamus. The distal branches of the internal carotid and ver- tebral arteries communicate at the base of the brain through the circle of Willis; this anastomosis of arteries can provide continued circulation if blood flow through one of the main vessels is disrupted (see Fig. 37-12B). For instance, occlusion of one middle cerebral artery may have limited consequences if the anterior and pos- terior communicating arteries are patent, allowing col- lateral flow from the ipsilateral posterior cerebral and opposite carotid arteries. There are many normal vari- ants across individuals in the completeness of the circle of Willis, such that collateral supply may be limited. Without collateral input, disruption of blood flow in a cerebral artery results in ischemic neural damage as metabolic needs of electrically active cells exceed nutri- ent supply. The cerebral circulation is drained by two sets of veins that empty into the dural venous sinuses: the deep (great) cerebral venous system and the superficial venous system. In contrast to the superficial cerebral veins that travel through the pia mater on the surface of the cere- bral cortex, the deep venous system is well protected. These vessels are directly connected to the sagittal sinuses in the falx cerebri by bridging veins. They travel through the CSF-filled subarachnoid space and pene- trate the arachnoid and then the dura to reach the dural venous sinuses. This system of sinuses returns blood to the heart primarily through the internal jugular veins. The intracranial veins do not have valves. The direction of flow depends on gravity and pressures in the venous sinuses as compared with those of the extracranial veins. Increases in intrathoracic pressure, as can occur with coughing or performance of the Valsalva maneuver (i.e., exhaling against a closed glottis), produce a rise in

SUMMARY CONCEPTS (continued)

Cerebrovascular Disease Cerebrovascular disease encompasses a number of dis- orders involving vessels in the cerebral circulation. As elsewhere in the circulation, cerebrovascular disor- ders can involve vessel occlusion or rupture that leads to either focal or localized brain damage or to global hypoxia-ischemia that causes widespread brain injury. The Cerebral Circulation The blood flow to the brain is supplied by the two inter- nal carotid arteries anteriorly and the vertebral arter- ies posteriorly 17 (Fig. 37-12A). The internal carotid artery, a terminal branch of the common carotid artery, branches into several arteries—the ophthalmic, poste- rior communicating, anterior choroidal, anterior cere- bral, and middle cerebral arteries. Most of the arterial blood in the internal carotid arteries is distributed through the anterior and middle cerebral arteries (see Fig. 37-12B). The anterior cerebral arteries supply the medial surface of the frontal and parietal lobes and the anterior half of the thalamus, the corpus striatum, part of the corpus callosum, and the anterior limb of the internal capsule. The genu and posterior limb of the internal capsule and medial globus pallidus are fed by the anterior choroidal branch of the internal carotid artery. The middle cerebral artery passes laterally, supply- ing the lateral basal ganglia and the insula, and then emerges on the lateral cortical surface, supplying the ■■ Deterioration of brain function is manifested by alterations in sensory and motor function and changes in the level of consciousness. In progressive brain injury, coma usually follows a rostral-to-caudal progression with characteristic changes in levels of consciousness, the pupillary light response and eye movements, posturing, and respiratory responses. ■■ Consciousness is a state of awareness of self and environment. It exists on a continuum from normal wakefulness and sleep to the pathologic continuum of stupor and coma. Any deficit in level of consciousness, from mild confusion to stupor or coma, indicates injury to either the ascending reticular activating (RAS) system or to both cerebral hemispheres concurrently. Consciousness may decline due to severe systemic metabolic derangements that affect both hemispheres, or from head trauma causing shear injuries to white matter of both the RAS and the cerebral hemispheres.