Question

Explain the molecular regulation and mechanism of muscle contraction. Start with an action potential moving down a T-tubule and end with relaxation following a contraction. Your answer should include details of specific cellular features, molecules, ions, and their interactions to the extent that we covered it in class. Assume that we understand the basic structure of a sarcomere so that you don’t have to describe the overall structure in your response.

Answer 1

The motor neuron sends a signal that reaches the tip of the axons, then the axons release a molecule called acetylcholine. This molecule makes a small trip, reach the muscle surface and attach to specific receptors. This is a signal given to the muscle cells, acetylcholine detection triggers the entrance of sodium ions into the muscle cells cytoplasm, this ion flux causes the release of calcium ions from the sarcoplasmic reticulum. T tubules membrane projections extend all along the muscle, this interconnects the cells and allows to regulate and equalize the calcium ion concentration all along the same muscle. This provides a more controlled and even contraction of the muscle as a unit. The calcium ions reach the muscle fibers, they reach the sarcomere. The calcium binds to troponine molecules, this binding changes the configuration of tropomyosine and this leads to the exposure of the binding site in actin filaments. Once exposed, myosin can attach to it and it bends producing a contraction. ATP comes into play to separate the myosin head from actin, just after this deattachment the myosin head attaches again to actin but now further in into the filament, leading to a behavior compared to "walking". When the signal fades out due to lack of nerve stimulation, calcium concentrations drop and the actin binding sites gt covered again, avoiding myosin from attaching to it. The muscle then relaxes.