A muscle fiber relaxes when:
a. The concentration of Ca²⁺ in the cytosol returns to resting levels.
b. The supply of ATP is exhausted.
c. Ca²⁺ flood the cytosol.
d. Acetylcholine is released from the axon terminal and the sarcolemma depolarizes.

Verified step by step guidance

1. Understand the process of muscle contraction and relaxation: Muscle contraction is initiated when calcium ions (Ca²⁺) are released into the cytosol, and relaxation occurs when these ions are removed from the cytosol and returned to the sarcoplasmic reticulum.
2. Recall the role of calcium ions (Ca²⁺): Calcium ions bind to troponin, causing a conformational change that allows actin and myosin to interact, leading to contraction. For relaxation, calcium ions must be removed from the cytosol.
3. Consider the role of ATP: ATP is required for both muscle contraction and relaxation. During relaxation, ATP powers the calcium pumps (Ca²⁺-ATPase) that actively transport calcium ions back into the sarcoplasmic reticulum.
4. Analyze the options provided: Option (a) aligns with the physiological process of muscle relaxation, as it involves the return of Ca²⁺ to resting levels. Option (b) is incorrect because ATP depletion would prevent both contraction and relaxation. Option (c) describes a condition that promotes contraction, not relaxation. Option (d) refers to the initiation of contraction, not relaxation.
5. Conclude that the correct answer is the option that describes the return of Ca²⁺ to resting levels, as this is the key event that allows the muscle fiber to relax.

Key Concepts

Calcium Ion Regulation

Calcium ions (Ca²⁺) play a crucial role in muscle contraction and relaxation. When a muscle fiber is stimulated, Ca²⁺ is released from the sarcoplasmic reticulum into the cytosol, initiating contraction. For relaxation to occur, the concentration of Ca²⁺ must return to resting levels, which is achieved by reuptake into the sarcoplasmic reticulum.

ATP and Muscle Function

Adenosine triphosphate (ATP) is essential for muscle contraction and relaxation. It provides the energy required for the myosin heads to detach from actin filaments after a contraction. If ATP supply is exhausted, muscle fibers cannot relax properly, leading to a state of rigor or sustained contraction, known as rigor mortis in post-mortem muscles.

Neuromuscular Transmission

Neuromuscular transmission involves the release of neurotransmitters, such as acetylcholine, from the axon terminal of a motor neuron. This neurotransmitter binds to receptors on the sarcolemma, causing depolarization and initiating an action potential that leads to muscle contraction. The cessation of acetylcholine release is critical for muscle relaxation, as it stops the stimulation of the muscle fiber.