What is sarcoplasmic reticulum (SR)? Choose the correct option.
A. System of tubules that bring the action potential deep inside the muscle fiber
B. Voltage-sensitive channels
C. Excitable cell membrane that encloses the muscle fiber
D. Extensive intracellular sac containing calcium ions
Answer (2)
The sarcoplasmic reticulum (SR) is an extensive intracellular structure in muscle cells that stores calcium ions essential for muscle contraction. The correct answer is D: Extensive intracellular sac containing calcium ions. It plays a critical role in initiating the contraction process by releasing calcium into the cytoplasm of the muscle cell.
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The sarcoplasmic reticulum (SR) is best described by option d: 'Extensive intracellular sac containing calcium ions.'
The sarcoplasmic reticulum is a specialized form of the endoplasmic reticulum found in muscle cells. It plays a crucial role in muscle contraction by storing and releasing calcium ions (Ca²⁺). Here's a step-by-step explanation of its function:
Structure : The SR is made up of a network of tubules and sacs surrounding the myofibrils, which are the contractile elements of muscle cells.
Function in Muscle Contraction :
When a muscle is stimulated by a nerve impulse, an action potential travels along the muscle fiber's membrane (sarcolemma) and propagates into the muscle fiber through the T-tubules.
This signal triggers the release of calcium ions from the SR into the cytoplasm of the muscle fiber.
Role of Calcium Ions :
The released calcium ions bind to troponin, a regulatory protein on the actin filaments of the myofibrils. This binding causes a conformational change in the protein tropomyosin, exposing binding sites for myosin on the actin filaments.
Myosin heads can then attach to these binding sites on actin, leading to muscle contraction via the sliding filament theory.
Relaxation :
After contraction, calcium ions are pumped back into the SR using ATP-driven calcium pumps, which decreases the calcium concentration in the cytoplasm and ends the contraction.
The proper function of the sarcoplasmic reticulum is essential for muscle health and performance, as any disruptions can lead to muscle disorders or diseases. Understanding the SR's role in storing and regulating calcium ions is vital in studying muscle physiology and diseases.
