Question

Can you explain why the amount of force produced by a muscle cell is proportional to intracellular calcium concentration? The number of active crossbridges?

Can you explain the metabolic pathways utilized by slow twitch and fast twitch muscle

fibers generate ATP?

What causes fatigue during muscular activity?

Answer 1

Excitation engenders from the fiber layers inside the filaments by means of the transverse tubular framework prompting arrival of Ca2+ ions from the sarcoplasmic reticulum. With an expanding intracellular Ca2+ concentration and Ca2+ ion– troponin binding along the thin (actin) fibers, the myosin heads set up links (called crossbridges) between the myosin and actin fibers. Muscle contraction starts. The excitation–contractile machinery linkage is mediated by calcium release (the amount of Ca2+ ions released inside the fibers) and its binding to the troponin at the thin (actin) filaments.

Fatigue occurring during exercise can be defined as the inability to maintain the initial force or power output. As fatigue becomes pronounced, force and maximum velocity of shortening are greatly reduced and force relaxation is prolonged. In principle, force loss during fatigue can result from a decrease in the number of cross-bridges generating force or a decrease of the individual cross-bridge force or to both mechanisms. The present experiments were made to investigate this point in single fibres or small fibre bundles isolated from flexor digitorum brevis (FDB) of C57BL/6 mice at 22–24°C. During a series of 105 tetanic contractions, we measured force and fibre stiffness by applying small sinusoidal length oscillations at 2.5 or 4 kHz frequency to the activated preparation and measuring the resulting force changes. Stiffness data were corrected for the influence of compliance in series with the cross-bridge ensemble. The results show that the force decline during the first 20 tetani is due to the reduction of force developed by the individual cross-bridges and thereafter as fatigue becomes more severe, the number of cross-bridges decreases. In spite of the force reduction in the early phase of fatigue, there was an increased rate of tetanic force development and relaxation. In the latter stages of fatigue, the rate of force development and relaxation became slower. Thus, the start of fatigue is characterised by decreased cross-bridge force development and as fatigue becomes more marked, the number of cross-bridges decreases. These findings are discussed in the context of the current hypotheses about fatigue mechanisms.