Muscle Relaxation

As long as action potentials continue to be produced—which is as long as neural simulation of the muscle is maintained—the Ca2+ release channels in the sarcoplasmic reticulum will remain open, Ca2+ will passively diffuse out of the sarcoplasmic reticu-lum, and the Ca2+ concentration of the sarcoplasm will remain high. Thus, Ca2+ will remain attached to troponin, and the cross-bridge cycle will continue to maintain contraction.

To stop this action, the production of action potentials must cease, causing the Ca2+ release channels to close. When this occurs, the effects of other transport proteins in the sarcoplasmic reticulum becomes unmasked. These are active transport pumps for Ca2+—termed Ca2+-ATPase pumps, which move Ca2+ from the sarcoplasm into the sarcoplasmic reticulum.

Since these active transport pumps are powered by the hydrolysis of ATP, ATP is needed for muscle relaxation as well as for muscle contraction.

Test Yourself Before You Continue

1. With reference to the sliding filament theory, explain how the lengths of the A, I, and H bands change during contraction.

2. Describe a cycle of cross-bridge activity during contraction and discuss the role of ATP in this cycle.

3. Draw a sarcomere in a relaxed muscle and a sarcomere in a contracted muscle and label the bands in each. What is the significance of the differences in your drawings?

4. Describe the molecular structure of myosin and actin. How are tropomyosin and troponin positioned in the thin filaments and how do they function in the contraction cycle?

5. Use a flowchart to show the sequence of events from the time ACh is released from a nerve ending to the time Ca2+ is released from the sarcoplasmic reticulum.

6. Explain the requirements for Ca2+ and ATP in muscle contraction and relaxation.

Skeletal Muscle Contraction Calcium

© Nicotinic acetylcholine receptor

) Transverse tubule voltage-gated calcium channel

© Nicotinic acetylcholine receptor

| © Skeletal muscle voltage-gated sodium channel

) Transverse tubule voltage-gated calcium channel

@ Sarcoplasmic reticulum calcium release channel

■ Figure 12.16 The structures involved in excitation-contraction coupling. The acetylcholine released from the axon binds to its nicotinic receptors in the motor end plate. This stimulates the production of a depolarization, which causes the opening of voltage-gated Na+ channels and the resulting production of action potentials along the sarcolemma. The spread of action potentials into the transverse tubules stimulates the opening of their voltage-gated Ca2+ channels, which (directly or indirectly) causes the opening of voltage-gated Ca2+ channels in the sarcoplasmic reticulum. Calcium diffuses out of the sarcoplasmic reticulum, binds to troponin, and stimulates contraction.

Table 12.3 Summary of Events in Excitation-Contraction Coupling

1. Action potentials in a somatic motor neuron cause the release of acetylcholine neurotransmitter at the myoneural junction (one myoneural junction per myofiber).

2. Acetylcholine, through its interaction with receptors in the muscle cell membrane (sarcolemma), produces action potentials that are regenerated across the sarcolemma.

3. The membranes of the transverse tubules (T tubules) are continuous with the sarcolemma and conduct action potentials deep into the muscle fiber.

4. Action potentials in the T tubules, acting through a mechanism that is incompletely understood, stimulate the release of Ca2+ from the terminal cisternae of the sarcoplasmic reticulum.

5. Ca2+ released into the sarcoplasm attaches to troponin, causing a change in its structure.

6. The shape change in troponin causes its attached tropomyosin to shift position in the actin filament, thus exposing binding sites for the myosin cross bridges.

7. Myosin cross bridges, previously activated by the hydrolysis of ATP, attach to actin.

8. Once the previously activated cross bridges attach to actin, they undergo a power stroke and pull the thin filaments over the thick filaments.

9. Attachment of fresh ATP allows the cross bridges to detach from actin and repeat the contraction cycle as long as Ca2+ remains attached to troponin.

10. When action potentials stop being produced, the sarcoplasmic reticulum actively accumulates Ca2+ and tropomyosin returns to its inhibitory position.

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