. Which list is in the correct order of DECREASING size?
A. Muscle fibre, sarcomere, myofilament, myofibril
B. Muscle, fasciculus, muscle fibre, myofibril
C. Sarcomere, fasciculus, myofibril, myofilament
D. Muscle, muscle fibre, myosin, myofibril
The Correct Answer and Explanation is:
Correct Answer: B. Muscle, fasciculus, muscle fibre, myofibril
Explanation
Understanding the organization of skeletal muscle is essential in comprehending how muscle contraction and force generation occur. Skeletal muscle is structured hierarchically, from the largest structure to the smallest components that are directly responsible for contraction. The correct order of decreasing size is:
1. Muscle → 2. Fasciculus → 3. Muscle fibre → 4. Myofibril
Let’s break down each component:
- Muscle: This is the entire organ, such as the biceps brachii or quadriceps. It is composed of bundles of fasciculi and surrounded by a connective tissue sheath called the epimysium.
- Fasciculus (or fascicle): A muscle is made up of multiple fascicles. Each fascicle is a bundle of muscle fibers and is surrounded by another connective tissue layer called the perimysium.
- Muscle fibre (or muscle cell): Each muscle fiber is a single, elongated, multinucleated cell. Muscle fibers are the basic units that contract to produce movement. Each fiber is wrapped in the endomysium.
- Myofibril: Within each muscle fiber are numerous myofibrils—long, thread-like structures that run parallel along the length of the fiber. Myofibrils are composed of repeating units called sarcomeres, which are the actual contractile units.
Smaller than the myofibrils are myofilaments, which include actin (thin filament) and myosin (thick filament). These filaments slide past each other during muscle contraction, shortening the sarcomeres and thus the muscle.
Options A, C, and D are incorrect because they place structures like myosin and sarcomere out of proper hierarchical order. For instance, sarcomeres and myofilaments are subunits of myofibrils, not larger than them.
Hence, Option B correctly represents the sequence from the largest to the smallest structural units in skeletal muscle.
