Skeletal Muscle 2

Question 1

Membrane depolarization opens what channels in the t-tubule membrane?

Answer 1

-L-type Ca2+ channels

Question 2

What is the source of calcium that causes contraction in skeletal muscle?

Answer 2

Calcium released from Ryanodine receptors in the SR; NOT extracellular Ca2+

Question 3

3 pumps that remove intracellular calcium after an AP?

Answer 3

Ca2+ pump in PM, Ca/Na exchanger in PM, and SERCA in SR (main route)

Question 4

To further facilitate uptake of Ca2+ into the SR, what is it bound to within the SR?

Answer 4

Calrecticulin and Calsequestrin

Question 5

3 states muscle can be found in

Answer 5

Relaxation: regulatory proteins inhibit A-M interactions. Few heads are bound to actin. Sarcomere can be passively stretched

Contraction: muscle is activated by Ca2+, thousands of sarcomeres shorten in series causing muscle to shorten. ATP is hydrolyzed and force is produced

Rigor (dead): ATP is depleted, all heads are tightly bound to actin, strong A-M interaction prevents stretching

Question 6

Troponin is a 3 protein complex: what 3 things does it bind?

Answer 6

-Calcium, actin, and tropomyosin

Question 7

Muscle contraction is regulated by what 2 thin-filament proteins?

Answer 7

Tropomyosin and troponin

Question 8

How does Ca2+ regulate skeletal muscle contraction?

Answer 8

Ca2+ binds troponin which moves tropomyosin out of the myosin-binding site on actin

Question 9

In absence of calcium, myosin dwells in which state?

Answer 9

Weak binding states

- Pi release is inhibited if you cannot bind actin

Question 10

In relaxation with ATP, but no Ca2+, which state is myosin stuck in?

Answer 10

AM.ADP.Pi state
not in rigor state
-as soon as calcium binds and reveals actin binding, it can release Pi. ATPase is so slow without actin and not bound to actin tightly

Question 11

Nerve stimulation determines the contractile force in 2 ways:

Answer 11

1. number of active motor units determines how many muscle cells produce force
2. Rate of stimulation adjusts the force produced by active cells

Question 12

Force-Velocity relationship

Answer 12

• As one decreases the load on the muscle, more myosin heads are available to shorten. Vmax when load =0, V=0 when isometric, maximum tension. In this case, all of the myosin heads are being used to resist the load and none are available to shorten the muscle length
• Shortening muscles use more ATP; muscles not moving or moving more slowly spend less time cycling through the ATPase cycle and thus using less ATP