14.3: Microscopic Anatomy of Skeletal Muscles
Skeletal muscle cells, also called muscle fibers, are distinctly elongated, multi-nucleated, slender biological units. They are packed with specialized structures designed to facilitate their primary function, which is contraction.
The muscle sarcolemma is a plasma membrane enclosing each muscle cell that conducts electrical signals called action potentials. The sarcolemma extends into the cell to form T-tubules, ensuring the neural impulses are uniformly distributed across the entire muscle fiber, triggering a coordinated contraction. The sarcolemma holds the sarcoplasm, the equivalent of the cellular cytoplasm. Inside the sarcoplasm are several organelles, such as mitochondria and glycosomes. It also contains proteins, such as myoglobin, which releases oxygen when needed. Muscle cells have more mitochondria compared to other cells in the body to be able to meet the extensive ATP demand during muscle contraction. This adaptation allows muscles to be active for extended periods without getting tired.
Additionally, the sarcoplasm is densely packed with myofibrils, the contractile units of the muscle cell. Each myofibril comprises repeated sections called sarcomeres, containing contractile proteins: actin and myosin. Sarcomeres are aligned end to end within each myofibril, and their patterned arrangement gives skeletal muscle its characteristic striated appearance.
The sarcoplasmic reticulum, a specialized endoplasmic reticulum, wraps around each myofibril and stores calcium ions, which are crucial for triggering muscle contraction. In this way, each part of the muscle fiber, from the sarcolemma to the myofibrils, plays a vital role in the complex symphony of muscle function.
