Question

The excitation-contraction coupling of the cardiac contracting cell is similar to that of skeletal muscle in all of the following except ONE. Pick the one that is NOT CORRECT.

A. In both muscle types, Ca++ binds to troponin
B. In both muscle types, Ca++ enters the cell from outside
C. In both muscle types, Ca++ - troponin combination moves tropomyosin off the actin binding sites
D. In both muscle types, contraction ends when the Ca++ is removed

Answer 1

The excitation-contraction coupling of the cardiac contracting cell is similar to that of skeletal muscle in all of the following except for when contraction ends when the Ca++ is removed.

Step-by-step explanation:

The excitation-contraction coupling of the cardiac contracting cell is similar to that of skeletal muscle in all of the following except ONE:

1. In both muscle types, Ca++ binds to troponin
2. In both muscle types, Ca++ enters the cell from outside
3. In both muscle types, Ca++ - troponin combination moves tropomyosin off the actin binding sites
4. In both muscle types, contraction ends when the Ca++ is removed

The correct answer is D. In both muscle types, contraction ends when the Ca++ is removed. In skeletal muscle, contraction ends when Ca++ is removed and the troponin-tropomyosin complex re-shields the actin-binding sites. However, in cardiac muscle, the troponin-tropomyosin complex does not re-shield the actin-binding sites. Instead, the presence of Ca++ keeps the binding sites exposed, allowing the contraction to continue.