4) The following is a list of the events that occur during a muscle contraction

Question

4) The following is a list of the events that occur during a muscle contraction 1. 2. 3. 4. 5. 6. Myosin cross-bridges bind to the actin. The free myosin head splits ATP Calcium ion is released from the sarcoplasmic reticulum The myosin head pivots toward the center of the sarcomere. Calcium ion binds to troponin. The myosin head binds an ATP molecule and detaches from the actin. The correct sequence of these events is 4) A) 1, 3, 5, 4, 6, 2. B) 1, 4, 6, 2, 3, 5. C) 3, 5, 1, 2, 4, 6 D) 3, 5, 1, 4, 6, 2 E) 5, 1, 4, 6, 2, 3

Explanation / Answer

Muscle contraction is the basis of all skeletal movements. Skeletal muscles are composed of muscle fibres, which are in turn made up of repititive functional units known as sarcomeres. Sarcomeres contain parallel and overlapping, thin actin filaments and thick myosin filaments. The muscles contract when these filaments slide over each other, thus shortening the sarcomere and in turn the muscle. Muscle contraction is brought about by the stimulation by nerve impulse and release of Ca2+ ions. The troponin units on the actin filament are bound by Ca2+ ions. This binding displaces tropomyosin along the filaments which in turn exposes the myosin binding sites. The myosin head is bound to ADP and phosphate molecule. The myosin heads release these phosphate molecules and bind to actin filaments near the newly exposed myosin binding sites. The myosin filaments glide past one another propelled by head first movement of the myosin units, powered by the chemical energy stored in their heads. As the units move, they release ADP molecule bound to their heads. Their gliding motion is halted when ATP molecules bind to the myosin heads, thereby severing the bonds between myosin and actin. The ATP molecules then breakdown into ADP and phosphate. The energy released by this reaction is stored in myosin heads, ready to be used in the next cycle of movements. The myosin heads resume their starting position along the actin filament and can begin a new sequence of actin binding.

Thus, the correct answer to this question is D) 3,5,1,4,6,2.

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