Question

Regarding the neuromuscular junction...

a. Depolarization of the presynaptic membrane directly causes voltage-gated calcium channels in the membrane to open.

b. Action potentials in the sarcolemma directly cause calcium channels to open.

c. Action potentials in the sarcoplasmic reticulum indirectly cause the calcium ATPase pump to function, if ATP is present.

d. both a) and b) are correct, and c) is incorrect

e. a), b) and c) are all correct

2)

Following ATP hydrolysis by myosin, which steps occur during one cross-bridge cycle?

	a.	Myosin is activated; if calcium is present in the sarcoplasm, then myosin binds to actin; inorganic phosphate dissociates from myosin.
	b.	Myosin is activated; if calcium is present in the sarcoplasm, then the conformation of troponin and tropomyosin changes; calcium ATPase pumps calcium into sarcoplasmic reticulum.
	c.	Myosin is activated; if calcium is not present in the sarcoplasm, then ADP and inorganic phosphate dissociate from myosin; troponin and tropomyosin remain bound to actin, covering the myosin binding site.
	d.	both a) and b) are correct, and c) is incorrect
	e.	a), b) and c) are all correct

5 points   

Answer 1

1 Ans: The correct option is E (All a, b and c are correct options)

The plasma membrane of the muscle cell is sarcolemma. Excitation of the sarcolemma induces the release of Ca2+ ion from the sarcoplasmic reticulum. Sarcoplasmic reticulum have ATP pump in its membrane and calcium ion is pumped back in the sarcoplasmic reticulum in presence of ATP, causing muscle to relax.