<prompt> Contraction of muscle can be caused by -Release of noradrenalineRelease of acetylcholineRelease of serotoninRelease of histamine  <response>: Ans. is b i.e., Release of acetylcholine o The process by which excitation depolarization of muscle fiber initiates its contraction is known as excitation contraction coupling.o The skeletal muscle fiber is innervated by A-a motor neuron.o When A-a motor neuron is stimulated, it carries the impulse action potential to neuromuscular junction where release of acetylcholineQ from presynaptic vesicle occurs. Acetylcholine binds to specific receptors NmnicotinicQ on motor-end plate post-synaptic membrane.o This results in production of end-plate potential. If end-plate potential depolarizes the motor-end plate to firing threshold level, action potential is produced.o Once the Ca2 concentration outside the reticulum has been lowered sufficiently, chemical interaction between myosin and actin ceases and the muscle relaxes,o This is due to reuptake of Ca2 by sarcoplasmic reticulum,o Thus during relaxation :- Cytoplasmic concentration of Ca decreases.Sarcoplasmic reticulum concentration of Ca2 increases.About option ao Ca2ion does not move out of cytoplasm i.e. extracellular. Ca2is taken up by sarcoplasmic reticulum.Mechanism of muscle contractiono The Ca2 released from the terminal cisterns bind with troponin C and this triggers further steps in muscle contractionAI 08 Ca2- Troponin - C complex induces changes in troponin I and Troponin-T, which in turn brings about a lateral shift of tropomyosin awray from active sites of actin. As a result, the binding sites active sites on the actin filament are exposed.o ATP binds to myosin and myosin splits this ATP into ADP due to its ATPase activity. The energy released activates the myosin and the activated myosin head binds to active sites of actin filament, i.e.. Cross-bridging of myosin with actin. This cross-bridging immediately produces bending flexion of myosin head from 90deg to 45deg. Bending of myosin head produces power stroke that causes sliding of actin on myosin and muscle contraction,o As the myosin head flexes to 45,: during power stroke, the ADP present on it is cast off making way for a fresh molecule of ATP. When fresh ATP binds to myosin ATPase, the myosin head detaches from actin and the cross-bridge repeats all over-again till the contraction is complete - So, this is called cross-bridge cycle.o During contraction: -Z lines come closerQ thereby shortenining sarcomereH-band disappearsQ.Width of I band decreasesQ.Width of A band is unchaged.Discharge in motor neuronRelease of acetylcholine at NM junction from presynaptic vesiclesBinding of acetylcholine to nicotinic Ach receptorsOpening of Na channels in Motor end plateGeneration of end plate potentialGeneration of action potential or sarcolemma adjacent to NM junctionSpread of AP muscle fibers in both directionInward spread of depolarization along T-tubulesInteraction of dihydropyridien receptor and Ryanodine receptorRelease of Ca2 ions from terminal cisterns of sarcoplasmic reticulum through Ryanodine receptor channelsIncreased cytoplasmic sarcoplasmic Ca2 concentrationBinding of Troponic C and CaLateral movement of tropomyosin with uncovering of active sites of actinCross-linkage of myosin with actin and muscle contractionMechanism of relaxationo Relaxation is due to removal of calcium ions from sarcoplasm into terminal cisterns by active transport Ca2 ATPase. Due to this calcium pump Ca2 ATPase the concentration of Ca2 ions in terminal cisterns of sarcoplasmic reticulum is very high and very low in sarcoplasmQCa2 pumped back into terminal cisterns of sarcoplasmic reticulum by Ca2 ATPaseRelease of Ca2 from troponinTropomyosin covering active sites of actinCessation of interaction between myosin and actin with resultant muscle relaxation 