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)

The essential contractile unit is the sarcomere, the repeating segment inside each muscle fiber. A muscle fiber is a single cell that contains many myofibrils, and along those myofibrils are numerous sarcomeres—the actual units that shorten to produce contraction. When a nerve impulse reaches the neuromuscular junction, calcium is released inside the muscle fiber, allowing actin and myosin within each sarcomere to form cross-bridges and slide past one another. This sliding shortens the sarcomere, and since a muscle fiber contains thousands of sarcomeres arranged in parallel, the overall contraction of the muscle fiber results. So the term for the basic contractile unit is the sarcomere, while muscle fibers are the cells that house many of these units.