Excitation- contraction coupling process in skeletal muscle and in smooth muscle


  By emi

Excitation- contraction coupling process in skeletal muscle and in smooth muscle



Excitation can be defined as the condition of being stimulated. The excitation-contraction coupling which occur during latent period, the period between action potential initiation and beginning of mechanical activity, is the sequence of events by which an action potential transmit along the sarcolemma leads to mechanical activity of the sliding of myofilaments. Muscle fibers depolarize and initiate contraction. The action potential will ends before any signs of contraction are obvious.

Basically, the skeletal muscle cells are stimulated by motor neurons of the somatic nervous system. The signals transmit via the axon and later divide as it enters the muscle and ends with neuromuscular junction. The rising calcium (Ca2+) is due to the electrical signal that in turns allows the filament sliding. As the Ca2+ concentration of the sarcoplasm rises, this indicates a sufficient amount of Ca2+ bind to troponin thus initiate the muscle contraction.

The acetylcholine (ACh) released from the axons and binds to ACh receptors in the plasma membrane of motor end plate. Release of ACh cause electrical activation of skeletal muscle fibers that will stimulate the production of depolarization. ACh through its interaction with receptors in the sarcolemma, produce action potential that regenerated across the sarcolemma. The action potential will then transmitted through the transverse tubules (T tubules) because the T tubules continuous with the sarcolemma. Action potential will continue into deep of the muscle fiber. In the same time, the production of a depolarization causes the opening of voltage-gated Na+ and thus resulting production of action potentials along the sarcolemma. As the action potential transmit across the T tubules, stimulates the voltage-gate Ca2+ channels which direct or indirectly cause the opening of voltage-gated Ca2+ channels in the sarcoplasmis reticulum (SR). Terminal cisternae of the sarcoplasmic reticulum will release Ca2+ into the sarcoplasm. This process known as excitation-contaction coupling since the T tubules not physically continuous with the sarcoplasmic reticulum. The Ca2+ that release into the sarcoplasm will attach to the troponin, causing a change in structure. This help to remove the blocking action of tropomyosin thus exposed actin active sites. When Ca2+ level is about 10-5 M, myosin heads attach and pull the thin filaments towards the center of the sarcomere thus, activate the muscle cell. Attachment of fresh ATP allow the cross bridges to detach from actin and repeat the contraction cycle as long as Ca2+ remains attach to the troponin. Due to Ca2+ short-lived, it will cause the signal ends. The level of Ca2+ decrease causing ATP-dependent calcium pump to remove Ca2+ back into the sarcoplasmic reticulum after the action potential ends. Then, the tropomyosin happen again and myosin-actin interaction is inhibited thus contraction ends and muscle fiber relaxes.

The contractile mechanism of smooth muscle essentially same as those of skeletal muscle. However, cells of smooth muscles have important structural and functional differences. Unlike the skeletal muscle the regulation of contraction in smooth muscle cells are involuntary. They are self-excitatory in the external stimuli. Neural and chemical stimuli may modify the rate and strength of smooth muscle contraction. They may excite or inhibit the smooth muscle cell but the effect to skeletal muscle only to excite. Both skeletal and smooth muscles involve reaction of actin and myosin. Besides, both also triggered by membrane impulses and release of calcium ions and both use energy from ATP molecules. Neural control in skeletal muscle differ where the skeletal muscle fibers only contain one junction with somatic nerve fiber and the receptors for the neurotransmitter are located only at the neuromuscular junction. Meanwhile, smooth muscles contain many neurotransmitter receptors at its surface. During the stretch of autonomic nerve fibers, neurotransmitter will be released. Bulges of varicosities, region of autonomic nerve fibers that release the transmitters that in turn stimulate the smooth muscle cell
s.

As same as in skeletal muscle, contraction of smooth muscle is triggered by a sharp rise in the Ca2+ concentration within the cytoplasm of the muscle cells. However, the sarcoplasmic reticulum of smooth muscle is less developed than that in skeletal muscle and thus the Ca2+ release may only for the initial phase of the smooth muscle contraction.

The present of gap junction in smooth muscle help the smooth muscle to transmit the action potential from fiber to fiber thus produce synchronized contractions. Smooth muscle in the stomach and intestine contain pacemaker cell which once stretch it cause entire others cell to excite as well. These pacemaker cells depolarize without external stimuli.

The events of Ca2+ entry into the cytoplasm are different in smooth muscle compare to the skeletal muscle. If in skeletal muscle the Ca2+ combine with troponin but in smooth muscle cell the troponin are not present. Therefore, to activate the myosin, the Ca2+ combines with a protein in the cytoplasm called calmodulin. Structully, it is similar to troponin. Calmodulin-Ca2+ complex activate myosin light-chain kinase, an enzyme that catalyzes the phosphorylation which is the addition of phosphate group to the myosin heads. Unlike the skeletal muscle, in smooth muscle the phosphorylation of myosin cross bridges is the regulatory event that permits them to bind to actin and produce contraction. As in the skeletal muscle, the smooth muscle contraction relax when the level of Ca2+ drop.

In comparison, smooth muscle takes longer times to contract and relax due to lack of troponin. It only contains calmodulin which when bound to calcium need to activate enzyme myosin light-chain kinase. However, most smooth muscle contraction is prolonged and staeady. This is probably cause by the slowness attachment and detachment of the cross-bridges.

The cycling of the cross-bridges, which is the attachment to actin, then release from the actin and reattach again for next cycle in the smooth muscle is slow compare to those in the skeletal muscle. This is may be due to the less ATPase activity than in skeletal muscle at the cross-bridge heads. Thus, the use of ATP is greatly reduced with the slow rate of cycling. Therefore, energy requirement to sustain the same tension on contraction in smooth muscle is only little. This is again due to the slow attachment of the cross-bridge cycle. This low energy requirement gives the ability for the organ that contains smooth muscle such as the intestine to maintain the tone of contraction longer. Due to this also the smooth muscles involve mainly the aerobic respirations while skeletal muscle involves aerobic and anaerobic respirations as its high energy needed increase oxygen demand. Aerobic respirations occur in mitochondria where the oxygen is being used while anaerobic respirations are the respirations where the oxygen is at deficit level. Smooth muscle contracts for extended period at low energy cost and without fatigue compare to skeletal muscle.

Although the smooth muscle contain few myosin filaments and have slow cycling time of cross bridge, the smooth muscle however have greater maximum force of contraction than in skeletal muscle. This result from the prolonged period of attachment of myosin cross-bridges to the actin filaments.

Besides that, smooth muscles also differ in the percentage of shortening of smooth muscle during contraction. Smooth muscle able to shorten a far greater percentage of its length and maintain almost full force of contraction. Skeletal muscle can only contract about 1/4 to 1/3 of its stretched length. These special features important for hollow viscera and other internal body structure to change their lumen diameters from large to small size. This is due to the optimal overlapping between actin and myosin filaments and the actin filaments are much longer compare to the skeletal muscle. Besides, skeletal muscle also lack of sarcomeres.

The smooth muscle response to stretch also difference compare to skeletal muscle. Both will contract when being stretch. However, in smooth muscle although the stretch continued, the pressure will return almost exactly back to the original level and the same effect will occur when being stretch again. This phenomenon called as stress-relaxation response, give the smooth muscle ability to return nearly to its original force of contraction second or minutes after it has been elongated or shortened.

Tags & Keywords : excitation-contraction coupling process

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