Upon stimulation of skeletal muscles, calcium is immediately made available for binding to troponin from

Options

A. bone
B. blood
C. lymph
D. sarcoplasmic reticulum

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Stimulation**: When skeletal muscles are stimulated, a signal is sent from the central nervous system to the muscle fibers. This is the initial step in muscle contraction.

2. **Role of Sarcolemma**: The signal travels to the sarcolemma, which is the plasma membrane of the muscle fiber. The sarcolemma is crucial for transmitting the electrical impulses necessary for muscle contraction.

3. **Transmission to T-tubules**: The impulse then moves from the sarcolemma into the T-tubules (transverse tubules). These T-tubules are extensions of the sarcolemma that penetrate into the muscle fiber and help in conducting the impulse deeper into the muscle.

4. **Interaction with Sarcoplasmic Reticulum**: The T-tubules are closely associated with the sarcoplasmic reticulum, which is a specialized endoplasmic reticulum in muscle cells. The sarcoplasmic reticulum serves as a storage site for calcium ions.

5. **Release of Calcium Ions**: Upon receiving the electrical impulse, the sarcoplasmic reticulum releases calcium ions into the sarcoplasm (the cytoplasm of muscle cells). This release is crucial for muscle contraction.

6. **Binding to Troponin**: The released calcium ions bind to troponin, a regulatory protein found on the actin filaments. This binding causes a conformational change that allows the muscle contraction process to occur.

7. **Conclusion**: Therefore, the source of calcium ions that bind to troponin upon stimulation of skeletal muscles is the sarcoplasmic reticulum.

**Final Answer**: The correct answer is the fourth option: **sarcoplasmic reticulum**.

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