Question

Relaxation of muscle is due to the (a) Pumping of \(\mathrm{Ca}^{+2}\) into sarcoplasmic cisternae (b) Presence of ATP (c) Conformational change in troponin and masking of actin filaments (d) Both (a) and (c)

Short answer

The correct answer is (d) Both (a) and (c)

Step-by-step solution

Understanding Muscle Contraction and Relaxation

Muscle contraction and relaxation involve several biochemical events mediated by specific muscle proteins. When a muscle is stimulated, \(\mathrm{Ca}^{+2}\) is released from the sarcoplasmic cisternae and binds to troponin, a protein that controls the interaction between actin and myosin (two other proteins responsible for muscle contraction).

Role of \(\mathrm{Ca}^{+2}\), ATP, Troponin, and Actin in Muscle Relaxation

During muscle relaxation, the \(\mathrm{Ca}^{+2}\) ions are pumped back into the sarcoplasmic cisternae, which reduces \(\mathrm{Ca}^{+2}\) binding to troponin. This prompts a conformational change in troponin, leading to the masking of actin filaments and preventing their interaction with myosin. While the presence of ATP is crucial for muscle contraction, it is not the primary reason for muscle relaxation.

Choosing the Correct Answer

From the above-discussed process, it can be deduced that muscle relaxation is primarily due to (a) the pumping of \(\mathrm{Ca}^{+2}\) into the sarcoplasmic cisternae, and (c) the conformational change in troponin and the masking of actin filaments, which prevents actin-myosin interaction. Thus, the correct answer is (d) Both (a) and (c).

Key concepts

Calcium Ions in Muscle Physiology

Calcium ions (\(\mathrm{Ca}^{+2}\)) play a pivotal role in muscle physiology, functioning almost like a molecular switch that turns on muscle contraction. When a muscle fiber receives a signal to contract, \(\mathrm{Ca}^{+2}\) is released from internal storage sites. These ions bind to specific proteins that initiate a series of interactions leading to contraction.

Without \(\mathrm{Ca}^{+2}\), the contraction process cannot start, as they are essential for moving the blocking molecules that inhibit the binding of the motor proteins actin and myosin.

Thus, calcium ions are crucial in both triggering and regulating the timing of muscle contractions.

Sarcoplasmic Reticulum and Muscle Relaxation

The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum in muscle cells that serves as a reservoir for calcium ions. During muscle relaxation, the SR plays a fundamental role by reabsorbing calcium ions back into its lumen.

This action reduces the calcium concentration in the muscle cytosol, causing the dissociation of calcium from the proteins that were part of the contraction process.

• This reuptake of \(\mathrm{Ca}^{+2}\) is energy-dependent, using ATP to transport the ions against a gradient.
• As \(\mathrm{Ca}^{+2}\) levels decrease, the muscle fibers transition from a contracted to a relaxed state.

The efficient storage and release of calcium in the sarcoplasmic reticulum are vital for the muscle's ability to contract and relax repeatedly.

Role of Troponin in Muscle Contraction

Troponin is a complex of three proteins that is key in regulating muscle contraction. It is part of the thin filament of muscle tissue and binds calcium ions to control whether the contraction machinery is active.

When \(\mathrm{Ca}^{+2}\) ions bind to troponin, it causes a shift in the troponin complex, leading to the exposure of active sites on actin filaments. This allows the myosin heads to attach to actin, initiating the contraction.

As muscle relaxation occurs, the level of calcium falls, causing troponin to cover the active sites on actin once again, inhibiting interaction with myosin and thus stopping contraction.

• Troponin is a critical regulator because it ensures that muscle contraction is only initiated when \(\mathrm{Ca}^{+2}\) is present.
• This mechanism prevents unwanted contractions and aids in the precise control of muscle activity.