Skeletal muscle is not just a uniform block of tissue; it is organized in layers wrapped in connective tissue membranes that protect the muscle fibers, support blood vessels, and transmit the force of contraction to the skeleton.
At the ends of the muscle, these three connective tissue layers merge to form a tendon or aponeurosis, which anchors the muscle firmly to bone. Midwife students should note that studying these layers helps explain how local infections or trauma can remain localized or spread along fascial planes in the limb.
An individual skeletal muscle fiber is essentially a long, cylindrical cell with multiple nuclei pushed just beneath the plasma membrane. The cell membrane of a skeletal muscle fiber is called the sarcolemma, and its cytoplasm is known as the sarcoplasm.
Unlike typical cells, skeletal muscle fibers develop by the fusion of many embryonic cells, which accounts for their multinucleate condition. Packed tightly within the sarcoplasm are hundreds to thousands of long, cylindrical organelles called myofibrils, which take up most of the cell's volume. Running parallel to each other, these myofibrils give the muscle fiber its characteristic striated appearance under a light microscope, created by alternating dark and light bands along their length.
For a muscle to contract uniformly, an electrical signal traveling across the surface must reach the deepest parts of the cell almost instantly. This is achieved through a specialized network of membranes.
A T-tubule flanked by two terminal cisternae forms a structural trio called a triad, which sits right at the junction where light and dark bands meet, ensuring rapid coupling of electrical signals to mechanical contraction.
The myofibril is segmented into repeating functional units called sarcomeres, which are the actual generators of muscle tension. Under a microscope, a single sarcomere extends from one Z-line (or Z-disc) to the next.
The banding pattern of the sarcomere gives skeletal muscle its striated look:
When a muscle contracts, the individual sarcomeres shorten, drawing the Z-lines closer together even though the length of the filaments themselves does not change.
Within each sarcomere, two primary types of protein myofilaments interact to generate force: thin filaments made mainly of actin and thick filaments made of myosin.
Thick filaments are composed of hundreds of bundled myosin molecules, each shaped like a golf club with a globular head that projects outward, ready to bind to actin. Thin filaments consist of fibrous actin strands twisted into a helix. Attached to these actin strands are two important regulatory proteins:
When calcium ions bind to troponin during excitation, the troponin-tropomyosin complex shifts its position, exposing the active binding sites on actin so contraction can proceed.
The process by which a muscle shortens is known as the sliding filament theory. Rather than shortening or stretching like elastic bands, the individual protein filaments simply slide past one another, driven by cyclic interactions between myosin and actin.
This cycle repeats rapidly as long as calcium and ATP are abundant in the sarcoplasm, allowing continuous shortening of the muscle until relaxation occurs.