Choose the correct statements regarding muscle contraction. A. A motor neuron carries a signal sent by the Central Nervous System (CNS) to the sarcolemma of the muscle fibre. B. The neural signal generates an action potential which causes the release of Ca$^{2+}$ into sarcoplasm. C. Increase in Ca$^{2+}$ inactivates the actin for breaking cross bridges. D. Actin binds to the myosin head to form a cross bridge. E. Shortening of sarcomere takes place, by pulling actin filaments towards the centre of 'A' band. Choose the correct answer from the options given below :

Solution & Explanation

Answer: 4 (A, B, D and E only). A is correct: muscle contraction is initiated by a signal sent by the CNS via a motor neuron reaching the sarcolemma. B is correct: the neural signal generates an action potential in the sarcolemma which causes the release of Ca$^{2+}$ into the sarcoplasm. D is correct: utilising energy from ATP hydrolysis, the myosin head binds to the exposed active sites on actin to form a cross bridge. E is correct: this pulls the attached actin filaments towards the centre of the 'A' band, shortening the sarcomere. C is INCORRECT: a rise in Ca$^{2+}$ does NOT inactivate actin; rather, increased Ca$^{2+}$ binds troponin and unmasks (activates) the actin active sites, promoting cross-bridge formation. Actin is inactivated only later, when Ca$^{2+}$ is pumped back, causing relaxation. NCERT Ch 17, p.222, line 14: "causes the release of calcium ions into the sarcoplasm. Increase in"