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| Muscular Contraction |
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| Muscle is composed of a large number of muscle fibres. The muscle fibres are arranged in groups. Each group is under the control of a single motor neuron, the axon of which sends a terminal branch to each fibre of the group. All the muscle fibres of a group contract when a nerve impulse travels down their motor neruon. A motor neuron and the group of muscle fibres innervated by its axon constitute a functional unit, called the motor unit. The number of muscle fibre and motor unit is variable and depends on the fineness of the control, a motor unit has to exercise. |
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| When a muscle is stimulated, a short latent period follows, during which it is taking up the stimulus. It then contracts, where it becomes short and thick, and finally it relaxes and elongates. |
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| In case of a striped muscle fibre the contraction lasts for only a fraction of a second and each contraction occurs in response to a single nerve impulse. The force with which a whole muscle contracts is adjusted by varying the number of fibres contracting and the frequency with which each fibre contracts. |
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| Mechanism of Muscle contraction |
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| During muscle contraction, the laterally projecting heads (cross bridges) of the thick myosin myofilaments come in contact with the thin actin myofilaments and rotate on them. This pulls the thin myofilaments towards the middle of the sarcomere past the thick myofilaments. The Z lines come closer together and the sarcomere becomes shorter. Length of the A band remains constant. Myofilaments stay the same length. Free end of actin myofilaments move closer to the centre of the sarcomere, bringing Z lines closer together. I bands shorten and H zone narrows. A similar action in all the sarcomeres results in shortening of the entire myofibril, and thereby of the whole fibre and the whole muscle. A contracted muscle becomes shorter and thicker and its volume remains the same. |
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| Events durings Muscle Contraction |
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| This theory which is called the sliding filament theory proposed by A.F. Huxley and J. Hansen is the most satisfactory and accepted one. |
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| The energy for the muscular contraction |
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| The energy for the muscular contraction is provided by the conversion of adenosine triphosphate (ATP) into adenosine diphosphate(ADP) and inorganic phosphate, releasing energy. A enzyme myosin ATPase catalyse the reaction in the presence of Ca2+ and Mg2+ ions. |
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| The used up ATP has to be restored for additional contractions, So phosphocreatine now comes into picture. It donates its high energy-phospate bond to ADP, producing ATP. This reaction is catalysed by an enzyme creatine kinase |
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| When creatine phospate is used up, new ATP is generated by aerobic respiration in the muscle cells. If the ATP is used up faster than the muscle fibres can produce aerobically, then the muscle fibres start anaerobic respiration to provide ATP. This produces lactic acid. This lactic acid is diffused in to the blood leaving a small part to accumulate in the muscle fibre. Major part of lactic acid is passed into the liver where it is oxidised to CO2 and H2O. The energy released from this oxidation is used for changing the remaining lactic acid to glycogen. |
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| Physical changes during muscle contraction: |
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| Following physical changes are observed during a muscle contraction. |
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| Shortening of muscle fibre |
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| During muscle contraction the sarcomeres shorten to about 60-70% of the rest length. This depends on the strength of the stimulus and the number of motor units involved. |
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| Viscosity |
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| Viscosity during muscle contraction and density of the sarcoplasm increases. |
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| Tonicity |
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| The partial contraction of a relaxed muscle is called tonus. During muscle contraction, the tonicity increases as more and more motor units come into the phase of contraction. |
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| Heat production |
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| The energy released by ATP during muscle contraction is partially converted to heat which helps in maintaining homeothermy. |
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| Electrical changes |
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| The sarcolemma is with a rest potential of 90mV. During contraction an action potential of +45 to +50mV is developed. |
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| Biochemical changes during muscle contraction |
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| During muscle contraction a number of biochemical changes occurs, which were studied by Albert Szent-Gyorgyi in 1942 and later by R.E. Davis in 1963. In a resting muscle fibre, sarcolemma is electropositive outside and electronegative inside. This potential difference across a membrane is called resting potential. A membrane with such a resting potential is said to be polarised. |
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| Na+ ions predominate the outside of the sarcolemma and potassuim ions predominate the inside. Due to the difference in concentration on the two sides of the sarcolemma, potassium ions leave and sodium ions enter the muscle fibre. |
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| Sarcolemma is more permeable to K+ions than to sodium ions. Hence potassium ions leave the muscle fibre faster than sodium ions enter, and this builds a positive charge outside. |
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| When the motor nerve impulse reaches the neuro-muscular junction, then the vesicles present in the motor end plate secrete a neurotransmitter chemical called acetylcholine(ach). |
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| Ach binds to the receptors on the sarcolemma makes it more permeabe to Na+ than K+ so that, sodium rapidly diffuse along the concentration gradient and the electrical gradient. Now sarcolemma becomes electropositive inside and electronegative outside. This new potential difference is called action potential. Such sarcolemma is called depolarised. |
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| Action potential stimulates the sarcoplasmic reticulum to release calcium ions which initiate the biochemical changes in muscle contraction. |
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| Calcium and megnesium ions act as co-factors for myosine ATPase enzyme which hydrolyses ATP into ADP and inorganic phosphate releasing energy. |
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