CONTRACTION AND RELAXATION OF CARDIAC FIBRES
Heart, the driving force. The constant and steady contraction and relaxation of the cardiac muscle is what ensures the continuum of life. The heart is essentially responsible for provision of nutritional products from food to cellular level. This is executed by ensuring perfusion of all the organs of the body with blood which is made possible by the seemingly inexhaustible contractions of the heart muscle.editorial proofreading With every beat, the heart pumps blood into the circulation. Each beat is followed by a small period of relaxation during which the heart fills up with blood to be pumped during the next beat. Once in the circulation the blood travels through the arteries into smaller and smaller arteries until it reaches the capillaries where the nutrients in the blood are finally exchanged with the cells for waste products and carbon dioxide produced from cellular metabolism. The blood then carries this waste product from the cells back into circulation via the vessels, now veins. These veins gradually increase in their caliber until they finally reach the heart which first pumps it into the lungs in order to oxygenate the carbon dioxide rich blood and finally pumps it into circulation again. Taking the heart out of the equation would mean imperative death for all the metabolic cycles held responsible to sustain the body and thus mean inevitable death.
From microscopic theory to macroscopic reality. Having established the importance of the cardiac muscle one questions the invariable ability of this fist sized organ to keep working through out a person’s life span. Moreover the synchronized contraction appears to be explicitly innate for the cardiac muscle compared to any other muscle in the body. This specificity of the cardiac musculature has to do with its histological uniqueness. The cardiac muscle fibres are like the skeletal muscle fibres in regards to the sarcoplasm, the sarcoplasmic reticulum and T tubules. However, the ion channels differ from those of skeletal fibres. The cardiac fibres have slow calcium channels in addition to fast sodium channels that are also present in skeletal fibres. Also the cardiac fibres unlike other muscle fibres in the body are connected together by intercalated discs. These discs provide permeable communicating junctions which allow free flow of ions(Guyton and Hall 2006). This ion flow in turn is necessary for generating the action potential responsible for the coordinated cardiac contraction.
As the action potential or electrical current spreads over the muscle sarcolemma and to the T tubules there is a release of calcium ions through voltage gated calcium channels from the muscle sarcoplasm into the reticulum. This allows the ions to bind to troponin which results in actin and myosin, the contractile units of the muscles to slide over one another which becomes apparent as a contraction. Thus when one of the fibres become stimulated the entire musculature behaves as single unit and contracts as one. As the calcium concentration decreases the ions unbind from troponin and the crossbridges formed by actin and myosin are dismantled and both the myofibrils assume their relaxed positions conforming their relaxed position. Afterwards the calcium is moved back into the reticulum with the help of energy dependent pumps called Ca-ATPase(Silverthorn 1988). In addition unlike skeletal and smooth muscle fibres the calcium is also removed via the sodium calcium exchange pump in the cardiac fibres. Macroscopically the action potential manifests as a contraction of the myocardium. The upper part of the heart designated as the atria contract first while the ventricles contract with a slight delay. Unlike contractions elsewhere in the body the cardiac contractions can be graded. The force of contraction can be varied depending upon the amount of calcium mineral discharged.