Question

Correct order of stages of muscle contraction is

• A. Stimuli Neurotransmitter secretion$\to$Release of calcium$\to$Cross-bridges formation$\to$Excitation of T-system$\to$Sliding of actin filament
• B. Stimuli Neurotransmitter secretion$\to$Excitation of T-system$\to$Release of C$a^{2+}$$\to$Cross-bridges formation$\to$Sliding of actin filaments$\to$H band diminishes
• C. Stimuli Excitation of T-system$\to$Neurotransmitter secretion$\to$Crossbridges formation$\to$Sliding of actin filaments$\to$H band diminishes
• D. Stimuli$\to$Neurotransmitter secretion$\to$Cross-bridges formation$\to$Excitation of T-system$\to$Sliding of actin filaments

Answer 1

The correct option is B Stimuli Neurotransmitter secretion$\to$Excitation of T-system$\to$Release of C$a^{2+}$$\to$Cross-bridges formation$\to$Sliding of actin filaments$\to$H band diminishes

• Active sites for myosin(Thick filament) are present on actin which are masked by troponin-C in resting state. when Ca++ level increases it leads to the binding of calcium with a subunit of troponin i.e Troponin-C on actin(thin) filaments and thereby remove the masking of active sites for myosin.
• Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
• This pulls the attached actin filaments towards the center of ‘A’ band.
• The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. Hence the ‘I’ band and 'H' band get reduced, whereas the ‘A’ bands retain the length.

So, Correct answer is 'Stimuli Neurotransmitter secretion →→ Excitation of T-system →→ Release of $C{a}^2+$ →→ Cross-bridges formation →→ Sliding of actin filaments →→ H band diminishes'