Smallest contractile unit (functional unit) of muscle fiber, contains A band with half of an I band at each end, consists of area between Z discs and align end to end along myofibril, like boxcars of train.
The Correct Answer and Explanation is:
The smallest contractile unit (functional unit) of muscle fiber is the sarcomere.
A sarcomere is the segment of a myofibril that is bounded by two Z discs (also called Z lines). It is the fundamental unit responsible for muscle contraction. The sarcomere contains specific structural features, such as the A band, I band, H zone, and M line, each of which plays a role in muscle contraction.
Key Components:
- Z discs: These structures mark the boundaries of each sarcomere. The sarcomeres align end to end along the myofibril, much like the boxcars of a train, forming a long chain that runs the length of the muscle fiber.
- A band: The A band is the region where the thick myosin filaments are located. It spans the entire length of the thick filaments and remains unchanged during muscle contraction.
- I band: This is the lighter region on either side of the A band, which contains only the thin actin filaments. The I band is divided into two halves by the Z disc. The length of the I band decreases during contraction as the actin filaments slide over the myosin filaments.
- H zone: This is a part of the A band where there is no overlap between thick myosin filaments and thin actin filaments. The H zone also shortens during muscle contraction.
- M line: Located at the center of the sarcomere, the M line helps anchor the thick myosin filaments in place.
Contraction Mechanism:
During muscle contraction, the actin filaments slide over the myosin filaments, pulling the Z discs closer together. This shortens the sarcomere, and thus the entire muscle contracts. This process is known as the sliding filament theory, which explains how the sarcomeres function as the contractile units of muscle fibers.
The sarcomere’s structure and function are essential for efficient and powerful muscle contractions.
