Atherosclerosis is a form of arteriosclerosis. Atherosclerosis is the hardening of the arteries caused by cholesterol plaque deposits. It can occur in the coronary arteries, the carotid arteries, the aorta, and the leg arteries. Healthy arteries are flexible, strong, and elastic. The inner layer, the tunica interna, is smooth, enabling blood to flow freely. As a person ages, the arteries normally become thicker and less elastic, and their calcium content increases. This natural ' hardening' process occurs throughout the artery system.
Atherosclerosis, by contrast, affects only the larger arteries. As the plaque builds up, the inner layers of the artery walls become thick and irregular. Fat, cholesterol, and other materials accumulate in certain areas. This gradual build-up over a long period of time reduced the circulation of blood and increases the risk of heartattack, stroke, and other serious arterial diseases. A person having atherosclerosis will often experience symptoms of angina, stroke, and claudication. All of the symptoms are caused by insufficient blood flow due to atherosclerosis. Initially, the deposits of fat-containing cells that lead to atherosclerosis are only slight streaks, and are called fatty streaks. As the fatty streaks enlarge, they invade the deeper layers of the artery walls, causing scarring and calcium deposits. Large deposits are called athermas or plaques. The plaques calcify into a chalky substance. The plaque consists of a firm shell that contains calcium with areas of fatty material, and a centre consisting of soft cholesterol. As the plaque deposits grow the cardiac muscle beyond the blockage becomes deprived of blood, a condition known as myocardial ischemia. The healthy elastic wall of the artery changes into dead and unresponsive scartissue. If insufficient blood flow continues, the cardiac muscle dies, causing myocardial infarction. As the degeneration of the endothelial lining of the arteries continues, the lining may be damaged. Bloodplatelets stick to the site of injury, and a chemical signal is activated that promotes an influx of cholesterol.
The symptoms of atherosclerosis develop slowly as the development of the occlusion of the arteries progresses. Symptoms include angina, stroke, and claudication. The specific symptoms depend on which artery or arteries are occluded. If the leg arteries are affected, symptoms usually include numbness, fatigue, or pain in the leg. Occlusion of the coronary arteries may lead to angina or even a heartattack. When atherosclerosis occurs in the coronary arteries, it can lead to myocardial ischemia, an insufficient flow of blood to the heart. If the duration of ischemia is brief, the damage is reversible. However, if the duration of ischemia is longer than 40 to 60 minutes, irreversible damage may occur, and the parts of the heart muscle deprived of blood become permanently damaged, leading to myocardial infarction. Other commonly affected large arteries include the carotid arteries and the abdominal aorta. Initially, the symptoms of atherosclerosis are more likely to occur during exercise or strenuous activity than at rest. The symptoms develop during exercise because the arteries cannot supply the muscles with enough oxygen and nutrients. This process results in the build-up of by-products in the muscle that cannot be removed efficiently because of the occluded blood flow. This build-up of waste products, such as lactic acid, causes pain. It is similar to the build-up of lactic acid in muscles due to overexertion. As the narrowing of the arteries increases, the symptoms become prevalent with less and less exertion. The symptoms generally disappear after a few minutes of rest. However, the occlusion can be so severe that even the resting muscle does not get enough blood flow and the symptoms may be experienced even when sitting still. Research Atherosclerosis
Embedded in the wall of the heart are four structures that conduct impulses through the cardiac muscle to cause first the atria then the ventricles to contract. These structures are the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, and the Purkinje fibres. On the rear wall of the right atrium is a barely visible knot of tissue known as the sinoatrial, or SA node. This tiny area is the control of the heartspacemaker mechanism. Impulse conduction normally starts in the SA node. It generates a brief electrical impulse of low intensity approximately 72 times every minute in a resting adult. From this point the impulse spreads out over the sheets of tissue which make up the two atria, exciting the muscle fibres as it does so. This causes contraction of the two atria and thereby thrusts the blood into the empty ventricles. The impulse quickly reaches another small specialized knot of tissue known as the atrioventricular, or AV node, located between the atria and the ventricles. This node delays the impulse for about 0. 07 seconds, which is exactly enough time to allow the atria to complete their contractions. When the impulses reach the AV node, they are relayed by way of the bundle of His and Purkinje fibres to the ventricles, causing them to contract. The electrical current races across the two ventricles within 0.06 seconds, causing the squeezing, thrusting motion of these powerful pumping chambers. The heart also has its own built in safety factors. The AV node, in an emergency situation, can take over the functions of the SA node by becoming the generator of the impulses. It is not quite as efficient, generating only a rate of 40 or 50 beats per minute. Research Atrioventricular Node
Cardiac muscle is red-coloured involuntary muscle that contracts automatically and rhythmically, like a smooth muscle, but is striated and multinucleated, like skeletal muscle. The muscle is fast- acting and powerful. It is under the control of the autonomic nervous system and continuously contracts and relaxes throughout life. Research Cardiac Muscle
Skeletal muscles contract rapidly in response to messages from the central nervous system. Each group of several fibres receives a nerve supply that allows voluntary contraction of the muscle. Muscles can move some body parts in several directions and others in only two directions. The direction the body part is moved depends largely on the shapes of the bones at the joints. The stimulus for the muscle contraction begins in the cerebral cortex and passes down the spinal cord and the nerveroot to the junction between the nerve fiber and the muscle surface. This gap, called the end plate, acts as a kind of amplifier, increasing the effect of the tiny current coming down the nerve fiber to stimulate the much larger muscle fiber. On the arrival of the nerve impulse, a chemical called acetylcholine is released from the motor nerve ending and passes across the gap to stimulate the membrane of the muscle fiber. This stimulation is in the form of an electric current which passes along the surface of the muscle, causing it to contract. It takes one millisecond (1/ 1000th of a second) for the current to pass along the surface of the muscular fiber. Cardiac muscle differs slightly from skeletal muscle because it has a built-in mechanism to maintain the necessary rhythmical contraction independently of any nervous connections. Smooth muscles react much more slowly to stimulation than skeletal muscles. The nerves, when present, alter the activity of the muscle rather than initiating it. This action is somewhat similar to cardiac muscle. The contractions take place rhythmically without direct control from the central nervous system. The impulses for contraction come from within the muscle itself. Research Control of Muscles
Muscles are the organs of motion and consist of bundles or fasciculi of parallel reddish contractile fibres. Toward the end of the muscle the fibres are so modified that they form a white cord or tendon of great density and strength. This tendon is attached to the bone so strongly that it is almost impossible to detach it. Sometimes a tendon spreads out like a flat sheet or thick membrane; it is then called fascia or aponeurosis. The fibres of a muscle have the power of contracting and relaxing under different stimuli sent to them by nerves. Muscular contractions cause them to move the bones, and consequently the limbs and body, in such direction as the brain and nerves command. Contraction is the special function of the muscles; all movements are performed by them. There are three kinds of muscle tissue: striated muscle, smooth muscle, and cardiac muscle. Most of the body' s muscle consists of striated muscle which is the skeletal muscle. It is also called voluntary muscle because it can be consciously controlled via the central nervous system.
Smooth muscle is the muscle of the internal organs and is called involuntary because it is not under voluntary control. Cardiac muscle is a special type of muscle found only in the heart. It consists of linked fibres that contract in unison producing the heartbeat. Research Muscles
The wall of the heart is composed of cardiac muscle tissue, referred to as
myocardium. This tissue is similar to the voluntary muscles of the body. In the cardiac tissue, however, the elongated muscle cells are highly interconnected to assure the maintenance of a coordinated and orderly heartbeat. The medical term myocarditis refers to inflammation of the heart muscle. A very smooth tissue, endocardium, lines the interior chambers of the heart. This lining helps to reduce any friction that may be caused by blood that is constantly moving through the four hollow chambers of the heart. Endocarditis is an inflammation of the heart lining. This condition can cause rough spots to develop in the endocardium, which may lead to thrombosis. Research Myocardium
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Atherosclerosis
