Cardiac+Muscle

Cardiac muscle shares characteristics of both skeletal and smooth muscle. It is a striated, involuntary type of muscle that is located in the walls of the heart. Its cells each contain a single central nucleus, and they branch and join one another by the intercalated discs. The discs allow the cells that span the whole length of the muscle to communicate and create a sequential contraction from the bottom of the ventricle to the top. Its role in the body is to propel blood into the heart and through the blood vessels as it contracts. In a person’s lifetime, their heart will beat around 70 times a minute, pumping around 5 liters of blood each minute. Cardiac muscle must constantly work without consciously thinking about it, so it is highly resistant to fatigue. The muscle contains a large number of mitochondria, the “powerhouse” of the cells because it has a high dependence on cellular respiration. This allows for continuous aerobic respiration to take place, as well as the presence of many [|myoglobins], and a good blood supply to provide nutrients and oxygen. Cardiac muscle differs in its stimulation of contraction compared to skeletal muscle. Skeletal muscle contracts in response to nerve stimulation; however, cardiac muscle contracts at a steady rate that is set by the heart’s pacemaker. This means that the muscle is “self-excitable”, or that it contracts without an electrical impulse from the nervous system. A type of muscle found only in the walls of the heart; it is under control of the autonomic nervous system. Cardiac muscle is like skeletal muscle in that it is striated and [|multinucleate], and like smooth muscle in that the nuclei are centrally located and many cells are required to span the length of the muscle. It differs from both skeletal muscle and smooth muscle in that its cells branch and are joined to one another via intercalated disks. Intercalated disks allow communication between the cells such that there is a sequential contraction of the cells from the bottom of the ventricle to the top. This makes possible the maximal ejection of blood from the ventricle during contraction and occurs without nervous innervation to each cell or group of cells. Cardiac muscle also differs from the other two muscle types in that contraction can occur even without an initial nervous input. The cells that produce the stimulation for contraction without nervous input are called the pacemaker cells.
 * Cardiac Muscle **

Specialized pacemaker cells located on the wall of the right atrium determine the rate of contraction of the heart. The average resting pulse of a person is 72 beats a minute. Although the central nervous system is not involved directly with the contractions of the heart, as previously stated above, the[| autonomic nervous system] sends signals which either speed up or slow down the heart rate. The cardiac muscles do not benefit from glycolysis when little oxygen is present. When the flow of blood (which contains oxygenated cells) is limited, damage is done to the heart. Cardiac muscle is [|myogenic], so the pacemaker will only coordinate and alter the cardiac muscle contractions. Therefore, the cardiac muscles would still be able to contract with a deficiency of pacemaker cells, even though it would be unorganized and futile. This is known as fibrillation. Fibrillation is the fast, irregular contraction of the cardiac muscle fibers. Fibrillation can be used after a surgical procedure on the heart in order to stop it from beating while fixing any leaks. Cardiac muscle is made up of intercalated discs and striated cells that are branched. The striations are formed by the alternating thick and thin protein filaments, anchored by the T-tubules which run perpendicular to the muscle fiber at the Z line. This location allows for the excitation-contraction coupling to take place. The two structural proteins of cardiac muscle are actin and myosin. The actin proteins are the thin filaments, which make up the lighter colored I bands. The myosin proteins make up the thick protein filaments that are darker in appearance (also known as the A bands). The constant pattern of dark A bands to light I bands is what gives the striated look to the cardiac muscle when observed under a microscope.
 * Rate of Heart **
 * Appearance **

Excitation-contraction coupling occurs from the generation of tension through the cross-bridge cycling of actin and myosin. Within the T-tubules are L-type calcium channels that activate in response to a stimulus. The calcium channels are opened during this process, allowing calcium to flow down and bind to troponin. The binding to the troponin exposes actin active binding sites, generating the force of myosin cross-bridges attaching and detaching. Because the contraction force requires calcium, the presence of extracellular calcium ions is very important.
 * Excitation-Contraction Coupling **

Useful links which helped contribute to the site: http://en.wikipedia.org/wiki/Cardiac_muscle http://www.uoguelph.ca/zoology/devobio/210labs/muscle1.html http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Muscles.html