Question

In cardiac muscle tissue, which of the following is correct?

1) Ca++ release from the sarcoplasmic reticulum initiates contraction
2) Ca++ enters the cell from the extracellular fluid, directly initiating contraction
3) Ca++ enters the cell from the extracellular fluid, which causes more Ca++ to be released from the sarcoplasmic reticulum
4) both A and C are correct

Answer 1

In cardiac muscle tissue, both the release of Ca++ from the sarcoplasmic reticulum and the entry of Ca++ from extracellular fluid, which triggers more release of Ca++, initiate contraction. Thus, option 4) both A and C are correct.

Step-by-step explanation:

In cardiac muscle tissue, the process of contraction begins with an action potential traveling down the T-tubules, triggering the release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR). However, in cardiac muscle, calcium-induced calcium release also occurs, where the initial influx of Ca++ from the extracellular fluid into the cell through voltage-gated calcium channels itself triggers the release of more Ca++ from the SR. Therefore, option 4) both A and C are correct.

This dual mechanism, where Ca++ enters from outside the cell and also causes additional release from the SR, uniquely facilitates the contraction of cardiac muscle fibers. This process contributes to the sustained contraction essential for heart function.