Question

1. ist the steps in muscle contraction. For each step, you should

   1. Describe what is happening to actin and myosin

   2. Describe what is happening to ATP

   3. Include “power stroke” and “cocked”/high energy conformation steps

Answer 1

Steps involved in muscle contraction:

step1 :The distal end of motor neuron releases acetylcholine (neurotransmitter)

2. Acetylcholine diffuses across the gap at neuromuscular junction binding to receptors on the muscle fibre membranes.

3. action potential is generated in muscle fibres.

4. Inward propagation of depolarization along T tubules.

5. Release of calcium ions from sarcoplasmic reticulum and bind to troponin C.

6. Troponin and tropomyosin interact to expose myosin binding sites on actin.

7. Formation of linkages between actin and myosin and sliding of thin and thick filaments.

Coupling of actin-myosin and ATP hydrolysis:

Myosin head without ATP locks tightly on to the actin filament. Binding of ATP to myosin head causes a slight changes in the confirmation of the actin binding site. This reduces the affinity of the head for actin and allows it to move along the filament. Hydrolysis of ATP causes the head to be displaced along the filament by a distance of about 5nm. However, the product of ATP hydrolysis, ADP and Pi remain tightly bound to the myosin head. Release of Pi causes binding of myosin head to a new site on the actin filament. This release triggers the power stroke ( the force generating change in shape during which the head regains its original confirmation). In the course of power stroke, the head loses its bound ADP and returns to start a new cycle.

The skeletal muscle contraction is regulated by tropomyosin- troponin complex bound to the actin thin filament leads to contraction in the presence of calcium and relaxed state in its absence. . In contrast , contraction of vertebrae smooth muscle is regulated by a pathway in which myosin regulatory light chain is assosciated with myosin 2 undergoes phosphorylation and dephosphorylation. The smooth muscle contracts when the regulatory chain is phosphorylated by the enzyme myosin light chain kinase. Because this enzyme is activated by calcium, the cytosolic calcium level indirectly regulates the extent of light chain phosphorylation and leads to contraction.The calcium dependent regulation of myosin light chain kinase activity is mediated by calcium binding protein called calmodulin. calcium first binds to calmodulin and forms calcium calmodulin complex . then it binds to myosin light chain kinase and activates it. because this regulation involves myosin is known as thick filament regulation. smooth muscle contraction also occurs in calcium independent manner. In this rho kinase activares the regulatory light chain by phosphorylation.

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