What term describes the basic contractile units within muscles that contract in response to nerve impulses?

What term describes the basic contractile units within muscles that contract in response to nerve impulses?

• A. Tendons
• B. Muscle fibers
• C. Bones
• D. Ligaments

Answer: (B)

Muscles contract in response to nerve impulses through their basic functional units called muscle fibers. When a motor neuron fires, acetylcholine is released at the neuromuscular junction, triggering an electrical signal that travels along the muscle fiber. This leads to calcium release inside the fiber and activates the actin–myosin cross-bridge cycles inside the sarcomeres, causing them to shorten and the whole muscle to contract. The term that fits the given options best is muscle fibers, since they are the fundamental contractile units within muscle tissue that respond to nerve impulses. Other options describe structures with different roles: tendons connect muscle to bone, bones provide the skeletal framework, and ligaments connect bones to other bones; none of these contract in response to nerve signals.