Question

1. Give a brief detail of the physiologic steps to muscle contraction. Explain with support of the appropriate chapters in your book. Using Patton, k. T; & Thiobodeau, G.A. (2015) Anatomy and physiology (8th ed.) Mosby. To provide pages numbers from the book.

Answer 1

1.First step is a nerve impulse travels to the neuromuscular junction on a muscle cell.

The neuromuscular junction is the point where the axons of the nerve meet with the muscle cell.

2. Ach is released from the terminal knobs axon into neuromuscular junction and Ach bind to receptors located on the sarcolemma .

3. The binding Ach causes depolarization of the sarcolemma by opening ion channels and allowing Na+ ions into the muscle cell.

4. Na+ ions diffuse into the muscle fiber and depolarization occurs.

5. Depolarization creates a wave of action potential (electrical current) across the sarcolemma.

Action potential is transmitted along the sarcolemma of the muscle fiber and then down the T tubules

6)Depolarization of T-tubules causes conformation change in DHP receptors, opening the Ca++ release channels in the terminal cisternae of the sarcoplasmic reticulum.

7) This triggers release of Ca++ from the terminal cisterns of the L-tubule. Ca++ peaks at approximately 20 msec after the Action potential.

8)The released Ca++ binds to troponin-C Troponin-C facilitates movement of the associated tropomyosin molecule toward the actin filament .

9)The tropomyosin ‘moves away’, exposes the myosin binding site on the actin filament This triggers the cross-bridge cycling, including the powerstroke which begins the contraction process.

9. In order to contract, ATP binds to the Myosin.

10. ATP is then hydrolyzed (broken down) to ADP and Pi, which gives the myosin the energy to “cock” its head to the high-energy position.

11. Actin and myosin bind together to form a cross bridge.

12. The myosin heads then pull the actin filaments inward and release the ADP and Pi and return to a low energy position.

Myosin ready for more Atp to bind and continue the contraction .ATP and calcium required for contraction.

When calcium goes into sarcoplasmic reticulum then phase of relaxation get started .