Chapter 10 (Muscular Tissue VI)

Question 1

The Sliding Filament Mechanism

Answer 1

Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere
Progressively pulling the thin filaments toward the center of the sarcomere
Z discs come closer together and the sarcomere shortens
Leading to shortening of the entire muscle

Question 2

Contraction Cycle Preparation

Answer 2

The onset of contraction begins with the SR releasing calcium ions into the muscle cell
Calcium ions bind to troponin, causing it to change its shape
The shape change causes tropomyosin to uncover the myosin binding sites on actin

Question 3

The contraction cycle consists of 4 steps

Answer 3

ATP hydrolysis
Formation of cross-bridges
Power stroke
Detachment of myosin from actin

Question 4

ATP hydrolysis

Answer 4

Hydrolysis of ATP reorients and energizes the myosin head

Question 5

Formation of cross-bridges

Answer 5

Myosin head attaches to the myosin-binding site on actin

Question 6

Power stroke

Answer 6

During the power stroke the crossbridge rotates, sliding the filaments

Question 7

Detachment of myosin from actin

Answer 7

As the next ATP binds to the myosin head, the myosin head detaches from actin
The contraction cycle repeats as long as ATP is available and the Ca++ level is sufficiently high
Continuing cycles applies the force that shortens the sarcomere

Question 8

Excitation–Contraction Coupling

Answer 8

An increase in Ca++ concentration in the muscle starts contraction, whereas a decrease stops it
Action potentials cause Ca++ to be released from the SR into the muscle cell
Ca++ moves tropomyosin away from the myosin-binding sites on actin allowing cross-bridges to form
The muscle cell membrane contains Ca++ pumps to return Ca++ back to the SR quickly, decreasing the calcium ion levels
As the Ca++ level in the cell drops, myosin-binding sites are covered and the muscle relaxes