Question

Muscle contraction is dependent on an elevation of intracellular Ca++ concentration. As the twitch frequency increases, the initiation of a subsequent twitch can occur before the previous twitch has subsided. As a result, the amplitude of the individual twitches is summed. At very high twitch frequencies, the muscle exhibits tetanic contraction. Under these conditions, intracellular Ca++ accumulates and supports sustained maximal contraction. What is the role of intracellular Ca++ in muscle contraction?

1) It inhibits muscle contraction
2) It supports muscle contraction
3) It has no effect on muscle contraction
4) It causes muscle relaxation

Answer 1

Intracellular Ca++ supports muscle contraction by binding to troponin, which subsequently leads to cross-bridge formation and thus muscle contraction. Summation and tetanic contraction occur with increased twitch frequency, leading to a sustained maximal contraction facilitated by the accumulation of intracellular Ca++.

Step-by-step explanation:

The role of intracellular Ca++ in muscle contraction is to support and facilitate it. When an action potential reaches a muscle cell, it causes the release of Ca++ ions from the sarcoplasmic reticulum into the sarcoplasm. These Ca++ ions bind to troponin, causing tropomyosin to move away from actin-binding sites, which allows cross-bridge formation between actin and myosin, leading to muscle contraction. Summation and tetanic contraction are phenomena that occur with increased twitch frequency. During summation, subsequent twitches overlap, and the tension increases because the second stimulus causes the release of additional Ca++, activating more sarcomeres. In a tetanic contraction, high stimulation frequency eliminates the relaxation phase, allowing for sustained maximal contraction due to the accumulation of intracellular Ca++. When the contraction ceases, Ca++ ions are actively transported back into the sarcoplasmic reticulum through ATP-dependent calcium pumps, leading to muscle relaxation. However, a basal level of calcium remains in the sarcoplasm to maintain muscle tone.