Biological Systems MCAT Practice Exam 2026 – The Comprehensive All-in-One Guide for Exam Success!

The structure within a myofiber responsible for muscle contraction is the sarcomere. The sarcomere is the fundamental unit of muscle contraction, composed of overlapping filaments of actin and myosin. When a muscle fiber receives a signal to contract, the sarcomeres shorten through a process called the sliding filament model. This occurs as the myosin filaments pull the actin filaments closer together, resulting in the shortening of the entire muscle fiber.

While myofibrils play a crucial role as they contain numerous sarcomeres arranged in series, they are not the basic unit responsible for contraction; rather, they are the bundles of sarcomeres. The sarcolemma refers to the cell membrane of the muscle fiber, which plays a role in transmitting the action potential, and fascicles are bundles of muscle fibers, which do not directly contract themselves. Thus, the sarcomere is the correct structure to highlight when discussing the mechanism of muscle contraction.