A skeletal muscle myofibril is a long, contractile fiber found within skeletal muscle cells. These myofibrils are the fundamental contractile units, directly responsible for generating the force that leads to muscle movement. Composed of organized protein filaments, myofibrils give skeletal muscle its characteristic striped appearance under a microscope. Their coordinated action allows for the shortening of muscle cells, which in turn powers body movements.
The Myofibril’s Place in Muscle Anatomy
Myofibrils are located inside skeletal muscle fibers, which are elongated, multi-nucleated cells forming the bulk of muscle tissue. Within each muscle fiber, thousands of myofibrils run parallel along the cell’s length, filling approximately 80% of its volume. These rod-like structures, typically 1 to 2 micrometers in diameter, facilitate muscle contraction and relaxation.
Myofibrils are organized longitudinally within the muscle cell, attaching to the sarcolemma, the muscle cell’s plasma membrane, at their ends. This arrangement ensures that when myofibrils shorten, the entire muscle cell contracts. The number of myofibrils within a single muscle cell can vary significantly, and changes in muscle size, such as growth or atrophy, are linked to the regulation of myofibril numbers per fiber.
Detailed Structure of the Sarcomere
The sarcomere is the basic functional unit of a myofibril, repeating along its length. Each sarcomere is defined as the region between two Z-discs, which are dense protein structures. The characteristic striated appearance of skeletal muscle arises from the organized arrangement of two primary protein filaments within each sarcomere: thick filaments composed of myosin and thin filaments made of actin.
Within a sarcomere, several distinct bands and lines are visible. The A band, a dark region, contains the entire length of the thick myosin filaments and includes areas where thin and thick filaments overlap. In contrast, the I band is a lighter region that contains only the thin actin filaments. The Z-disc bisects the I band and serves as the anchoring point for the thin filaments.
At the center of the A band lies the H zone, which contains only thick myosin filaments. Running through the middle of the H zone is the M line, where thick filaments are anchored by proteins like myomesin.
Beyond actin and myosin, other proteins contribute to the sarcomere’s structure and function. Titin, a large elastic protein, provides elasticity and helps stabilize the position of thick filaments, connecting the M line to the Z-disc. Nebulin spans the length of the thin filament, assisting in binding actin to the Z-disc. Troponin and tropomyosin are regulatory proteins associated with the thin filaments, playing roles in controlling muscle contraction.
How Myofibrils Power Movement
Myofibrils generate force and cause muscle movement through a process explained by the “sliding filament theory”. This theory proposes that muscle contraction occurs as the thin actin filaments slide past the thick myosin filaments, causing the sarcomere to shorten. The lengths of the individual actin and myosin filaments themselves do not change; rather, their overlap increases.
The dynamic interaction begins with myosin heads forming cross-bridges with actin filaments. This interaction is triggered by the release of calcium ions from the sarcoplasmic reticulum, a specialized endoplasmic reticulum within muscle cells. Calcium binds to troponin, a protein on the thin filament, which then causes tropomyosin to shift, exposing the myosin-binding sites on the actin.
Once the binding sites are exposed, the myosin heads attach to the actin, forming a cross-bridge. This attachment is followed by a “power stroke,” where the myosin head pivots, pulling the actin filament towards the center of the sarcomere. Adenosine triphosphate (ATP) serves as the energy source for this cycle.
ATP binds to the myosin head, causing it to detach from actin. ATP is then hydrolyzed into ADP and inorganic phosphate, re-energizing the myosin head to bind to a new site further along the actin filament, continuing the cycle as long as calcium is present. This repetitive binding, pulling, and detachment ultimately shortens the sarcomeres, leading to the overall contraction of the muscle.