Question

Skeletal

​A) In skeletal muscle what do we mean by excitation-contraction coupling?

​B) What causes the excitation? Be specific as to the events and ions and channels involved.

​C) How is it linked to the contraction events? What ion is involved and how does it work to initiate contraction? (include the proteins activated)

​D) What causes relaxation? (you do not have to describe the complete contraction event, just what allows it to start and what ends it)

Answer 1

A.1. Excitation-contraction Coupling
Excitation-contraction coupling is the process that occurs in between the excitation and contraction of the muscle. This process involves series of activities, which are responsible for the contraction of excited muscle.
Stages of excitation-contraction coupling
When a muscle is excited (stimulated) by the impulses passing through motor nerve and neuromuscular junc­ tion, action potential is generated in the muscle fiber.
Action potential spreads over sarcolemma and also into the muscle fiber through the ‘T’ tubules. The ‘T’ tubules are responsible for the rapid spread of action potential into the muscle fiber. When the action potential reaches the cisternae of ‘L’ tubules, these cisternae are excited(B). Now, the calcium ions stored in the cisternae are released into the sarcoplasm . The calcium ions from the sarcoplasm move towards the actin filaments to produce the contraction(c).Thus, the calcium ion forms the link or coupling material between the excitation and the contraction of muscle. Hence, the calcium ions are said to form the basis of excitation-contraction coupling.

D.Relaxation of the muscle occurs when the calcium ions are pumped back into the L tubules. When calcium ions enter the L tubules, calcium content in sarcoplasm decreases leading to the release of calcium ions from the troponin. It causes detachment of myosin from actin followed by relaxation of the muscle . The detachment of myosin from actin obtains energy from breakdown of ATP. Thus, the chemical process of muscular relaxation is an active process although the physical process is said to be passive.