Question

Before stimulating the release of calcium ions that causes muscle contraction in skeletal muscle, an action potential must pass through the:
A) Sarcoplasmic reticulum
B) T-tubules and Sarcoplasmic reticulum
C) Plasma membrane and T-tubules
D) Plasma membrane, T-tubules, and Sarcoplasmic reticulum

Answer 1

Hint:
The signal from the nerve triggers an action potential in the sarcolemma. T-tubules(transverse tubules) mediate the propagation of action potential. Voltage-sensitive proteins inactivate due to incoming action potential which is contained in T-tubules.

Complete step by step solution:
The action potential initiates from the plasma membrane at the neuromuscular junction. Around each myofibril, from the sarcolemma T-tubules are extended inward. The opening of calcium release channels in the sarcoplasmic reticulum is triggered by the voltage-sensitive proteins that are activated. Sarcoplasm is the specialized cytoplasm of a muscle fibre that contains a modified ER known as sarcoplasmic reticulum. Transverse tubules invaginate the sarcolemma, allowing impulses to penetrate the cell and activate the sarcoplasmic reticulum. Sarcoplasm contains fine thread-like contractile structures called myofibrils.
The contraction of each myofibril is initiated by the increase in cytosolic calcium. Because the signal from the sarcolemma is passed within a millisecond to every sarcomere in the cell, all the myofibrils in the cell contract at the same time. The increase in calcium concentration is transient because calcium is rapidly pumped back into the sarcoplasmic reticulum by an abundant, calcium ATPase, in its membrane.

Option ‘C’ is correct

Note:
The calcium concentration is restored to resting levels within 30 milliseconds, allowing the myofibrils to relax. The calcium dependence of skeletal muscle contraction is due to tropomyosin and troponin.