Question

Muscles relax when: a. calcium is present b. nerve activity ends c. sarcomeres shorten d. Z-lines draw closer

Short answer

Option b: nerve activity ends.

Step-by-step solution

Identify the key concept

Understand that muscle relaxation involves the cessation of contraction mechanisms in muscle fibers.

Analyze the options

Review each option and relate it to the process of muscle contraction and relaxation: - Option a (calcium is present): Calcium presence usually indicates muscle contraction. - Option b (nerve activity ends): Muscle activity ceases when nerve signals stop, leading to relaxation. - Option c (sarcomeres shorten): Sarcomeres shorten during muscle contraction, not relaxation. - Option d (Z-lines draw closer): Z-lines drawing closer is also a characteristic of muscle contraction.

Select the correct answer

Since muscle relaxation occurs when nerve signals stop, option b (nerve activity ends) is correct.

Key concepts

calcium in muscle contraction

Calcium plays a crucial role in muscle contraction. When a nerve signal reaches a muscle fiber, it triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum, a specialized type of endoplasmic reticulum in muscle cells. These calcium ions then bind to a protein called troponin on the thin filaments of the muscle fiber.
This binding causes a conformational change that moves tropomyosin, another regulatory protein, away from the binding sites for myosin on the actin filaments. This action exposes the binding sites, allowing myosin heads to bind to actin and initiate the cross-bridge cycle, leading to muscle contraction.
The presence of calcium is therefore essential for initiating and sustaining muscle contractions.

nerve activity and muscle response

The role of nerve activity in muscle response is a fundamental aspect of how muscles work. Neurons communicate with muscle fibers at a junction called the neuromuscular junction. When a nerve signal, or action potential, reaches the end of a motor neuron, it triggers the release of acetylcholine (ACh), a neurotransmitter.
ACh crosses the synaptic cleft and binds to receptors on the muscle fiber's membrane, causing an action potential in the muscle fiber. This electrical signal travels down the transverse tubules (T-tubules) and stimulates the sarcoplasmic reticulum to release calcium ions, initiating contraction.
When nerve activity ceases, ACh is broken down by acetylcholinesterase, and calcium ions are pumped back into the sarcoplasmic reticulum. This cessation of nerve signals and removal of calcium leads to muscle relaxation.

sarcomere function in muscles

Sarcomeres are the fundamental units of muscle contraction within myofibrils, the elongated threads found in striated muscle cells. Each sarcomere is defined by the region between two Z-lines (or Z-discs). When muscles contract, the sarcomeres shorten, pulling the Z-lines closer together.
During relaxation, these Z-lines move further apart as the muscle returns to its resting state. The main proteins involved in sarcomere function are actin (thin filaments) and myosin (thick filaments).
The interaction between these proteins is driven by the binding of myosin heads to actin, forming cross-bridges. ATP is then utilized to detach myosin heads and reset them for a new cycle of binding and pulling, a process regulated by the presence of calcium ions and ATP.
Proper sarcomere function is essential for the controlled and precise contractions of muscle needed for movement.