Introduction
There are 3 types of muscle categories in humans.
Skeletal muscles, they have a strong, quick discontinues voluntary contraction.
Cardiac muscles, have strong, quick continues involuntary contraction.
Smooth muscles, they have weak, slow involuntary contraction
Morphology
Skeletal muscle is a complex and highly organized tissue specialized for force production and movement. At the cellular level, skeletal muscle is composed of myofibers, which are elongated, multinucleated cells formed by the fusion of many precursor cells 10-12cm . Each myofiber is encased by the sarcolemma, a cell membrane that not only protects the inner contents but also plays a crucial role in transmitting signals for muscle contraction. Just beneath the sarcolemma lies the basal lamina, an extracellular matrix that provides structural support and anchors the fibers in place. Within the myofiber, the sarcoplasm—the equivalent of the cytoplasm in other cells—houses various organelles and molecular components critical for muscle function. Among these are glycogen granules, which serve as readily available energy stores for muscle contraction. The sarcoplasmic reticulum (SR), an elaborate network similar to the endoplasmic reticulum, plays a pivotal role in regulating intracellular calcium levels, essential for muscle contraction dynamics. Adjacent to the SR are the T-tubules, or transverse tubules, that help transmit action potentials deep into the muscle cells, ensuring a coordinated contraction. Key to this contraction mechanism are the sarcomeres, the basic contractile units of muscle tissue. These structures are defined at their boundaries by Z-lines, which anchor the actin and titin filaments, maintaining the alignment of the sarcomere. Within each sarcomere, the thick filaments composed of myosin—a protein with two heavy chains and four light chains—interact with the thin filaments made of actin. This interaction is finely regulated by troponin and tropomyosin. Troponin controls the exposure of binding sites on actin when calcium is present, while tropomyosin blocks these sites in the absence of calcium, preventing contraction.
Moreover, the motor end plates at the neuromuscular junction are critical for the initiation of a contraction, as they receive and transmit neural signals via the sarcolemma. Structural integrity during contraction and relaxation is supported by the M lines in the center of the sarcomere and intermediate filaments, which provide mechanical stability by linking myofibrils to each other and to the sarcolemma.