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Question
Define sliding filament theory of muscle contraction.
Solution 1
Huxley:
By studying striated muscle fibres under an electron microscope, he observed the specific arrangement of actin and myosin rods present in them. Considering this configuration, he proposed the snake fibre or rod creep theory of muscle fibre contraction.
Mechanism of contraction of striated muscles:
Contraction in striated muscles occurs as a result of nerve stimulation. Actin rods slide over myosin rods and enter inside them (towards the centre of the spindle), due to which contraction of the muscle fibre takes place.
Fibre or rod creep theory of muscle contraction:
Under normal conditions, the sarcomere contains ATP and magnesium ions; Calcium ions are also present in minute quantities. Actin rods remain attached to tropomyosin in a way that they cannot attach to myosin rods. When a muscle fibre receives threshold stimulation by a nerve impulse, Ca++ (calcium ions) are released from the endoplasmic reticulum (ER) of the muscle fibre into the sarcomere. These calcium ions combine with tropomyosin and the actin rods become free. At the same time, energy is released as a result of the decomposition of ATP in water. In the presence of this energy, actin and myosin become active and new bridge bonds are formed. As a result, the actin rods slide over the myosin rods and move towards the centre of the sacmeer. Actin and myosin, together, form actomyosin.
rotation of head and breaking of cross bridge
In this process, the tension of the muscle fibres reduces, which means relaxation takes place. When the stimulation ends, active pumping of calcium ions is pumped into the endothelium. Tropomyosin becomes free, due to which the bridge bonds between actin and myosin are broken. Actin then combines with tropomyosin. The muscle fibre returns to its old state. In cases of death, Ca++ cannot be transported back into the sarcomechanical trap due to the non-formation of ATP. Basically, the muscles remain pale and the body remains motionless.
Energy Supply:
Energy for muscle contraction is supplied by ATP. ATP is produced in muscles as a result of the catabolism of glycogen.
During muscle contraction, energy is obtained from the water decomposition of ATP.
\[\ce{ATP -> [hydrolysis]ADP + Pi + \text{energy}}\]
Another high-energy compound present in muscles is called creatine phosphate. It is also used in ATP production.
\[\ce{ADP + PCr -> ATP + Cr}\]
At rest, creatine phosphate is produced again by ATP.
ATP + Cr → PCr + ADP
In this way, a store of creatine phosphate remains in the muscle, which can provide ATP when needed.
Solution 2
According to the sliding filament theory of muscle contraction, the actin and myosin filaments slide past each other using cross-bridges to shorten the length of the sarcomeres.
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