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Explain the chemical changes taking place during muscle contraction. Plzz explain (3marks) .

The physiological process of generating a tension in a muscle fibre through a proper stimulation is called muscle contraction.
When a muscle fibre is stimulated, a chemical or neurotransmitter is released from the neuromuscular junction. This chemical increase the permeability of sarcolemma to Na+ ions. Now Na + ions move inside, through the concentration and electrical gradient. This entry of Na + ions inside the sarcolemma makes it positive with respect to outside. The sarcolemma of muscle is now called depolarised and the change is known as action potential.
  The action potential cause calcium ions to release into the sarcoplasm.These calcium ions can now bind to the subunit of troponin on actin filament. It will allow the binding of ATP containing myosin head to the exposed site on the actin filament (F- actin) forming a actomyosin complex, also known as cross bridge. It will pull the actin filament towards the sarcomere centre which in turn will cause bringing of Z lines of actin filaments closer. Sarcomere now becomes short. This is known as contraction. Energy for the whole mechanism is provided by the hydrolysis of ATP by the enzyme ATPase.

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Whenmusclecontracts certainchemical changesof distinct nature take place simul­taneously.

The knowledge about the study of chemical changes invol­ved in the musclecontractionbegins with von Helmholtz (1845) idea that in the liberation of energy in the form of mechanical work as it occurs in muscle the energy must come from some pre-existing energy.

In the year of 1907 Fletcher and Hopkins declared that thelactic acidis formed during anaerobic contraction of vertebrate mus­cle.

A few years latter Parnas showed that the lactic acid originates from the glycogen. This discovery led to an enormous source of acti­vity, both biophysical and biochemical.

On the biophysical side A.N. Hill began his studies on the energetics of muscular contraction, while on the biochemical side Otto Meyerhof began the studies of glycolysis which ultimately led to our present rather complete under­standing of this complex and important process.

The immediate energy for the contraction of the muscle is deri­ved, whether directly or indirectly is uncertain, from the breakdown of the ATP to ADP, while the ultimate source is the combustion of the carbohydrates (glucose, glycogen and others) present in the mus­cle.

However, there is strong evidence that lipids are also utilized at times. These lipids exist in the form of freefatty acidswhich can diffuse rapidly from the blood stream into the muscle or derive from the muscles fat stores.

All the enzymes required for the oxidative metabolism of the free fatty acids are found in the muscle. The avail­ability of oxygen determines the degree to which free fatty acids con­stitute a source of energy.

When ATP is not available in sufficient amount during the mus­cle contraction, creatine phosphate (phosphocreatine) serves as a sup­plemental source of energy which through the action of a transferase enzyme restores the depleted ATP.

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