Detailed explanations for these questions would be really helpful. I previously

Question

Detailed explanations for these questions would be really helpful. I previously posted this question, but the answer I received did not cover the second half of the question. Thank you.

- Discuss the role of calcium in the contraction of striated skeletal muscle contraction and smooth muscle. Clearly discuss how Ca2+ release is triggered and the effects of the Ca2+ inside the cell in each case.

- Also discuss why sometimes a person may experience some involuntary “twitching” of the skeletal muscle after consuming large amounts of caffeinated drinks.

Explanation / Answer

Question1: Calcium causes contraction of both skeletal and smooth muscles. How calcium release is triggered ? The action potential due to depolarisation of muscle membrane due to release of the transmitter from nerve activates Ryanodine receptors which are present in Endoplasmic reticulum tubules are activated by this and release calcium in the cytoplasm. Now,

In skeletal muscle : This calcium binds to troponin C . This unmasks the myosin binding sites on the actin. ATP then binds to myosin head and energy obtained from its hydrolysis is is used for attachment to Actin. This attached myosin head then pulls the actin filaments to slide over myosin filaments , thereby causing contraction.

In smooth muscles, This calcium binds to calmodulin , this activates calmodulin dependent myosin light chain kinase which phosphorylates the myosin light head Thereby causing the contraction of smooth muscles.

Now , caffeine , a methylxanthine , causes muscle contraction by two following mechanism :

1- It blocks adenosine receptors which prevents the uptake of calcium ions

2- It blocks phosphodiesterase enzyme , therefore cGMP accumulates , this cause activation of protein kinases which phosphorylates the downstream substrates.

Thats how caffeine intake in high quantities can cause involuntary muscle twitching.

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