Subtitles section Play video Print subtitles Muscle is one of the primary tissue types. Skeletal muscle perform six major functions. Skeletal muscle produces skeletal movement by contracting and pulling on the bones of the skeleton. Skeletal muscles maintain posture and body position by maintaining tension, thereby allowing you to hold your head while sitting and reading a book or balancing your body when you walk or stand. Skeletal muscles support soft tissues, such as abdominal muscles and pelvic floor muscles, supporting the organs of the abdominal-pelvic cavity. Skeletal muscles guard entrances and exits by surrounding the openings of the digestive and urinary tracts. Skeletal muscles maintain body temperature by releasing heat when they are working. And finally, skeletal muscles can store nutrient reserves because the protein in muscles can be broken down into amino acids, which can then be used to produce energy. Each muscle is composed of muscle cells called muscle fibers. These muscle fibers are contained in bundles called fascicles. Muscles have connective tissue that's associated with the entire muscle, with the fascicle or muscle bundle, and with the individual muscle fiber. Epimysium is a dense layer of collagen fibers that surrounds the entire muscle. It separates the muscle from nearby tissues and organs. The perimysium divides the skeletal muscle into a series of compartments. Each compartment contains a muscle fascicle. The perimysium contains blood vessels and nerves. Within each fascicle, the endomysium is more delicate and surrounds each individual muscle fiber. The endomysium contains capillary blood vessels, nerve fibers, and myosatellite cells, which are stem cells, that help to repair damaged muscle tissue. The collagen fibers of the epimysium, the perimysium and the endomysium come together to form either a bundle, known as the tendon, or a broad sheet, called an aponeurosis. Tendons and aponeuroses usually attach skeletal muscles to bones. Skeletal muscle cells or fibers are very different from the typical cells we've seen so far. One obvious difference is that these skeletal muscle fibers are much larger than other cells. A muscle fiber from the thigh muscle could have a length up to 12 inches. A second difference is that skeletal muscle contains hundreds of nuclei just internal to the cell membrane. These nuclei are needed to produce enzymes and structural proteins that are required for normal muscle contractions. The cell membrane of a muscle fiber is known as the sarcolemma and the cytoplasm is known as the sarcoplasm. Inside each muscle fiber are hundreds to thousands of structures called myofibrils. These structures are cylindrical in shape. They can actively shorten in shape and are responsible for skeletal muscle fiber contraction. Myofibrils consist of protein filaments called myofilaments. These myofilaments are either thin filaments composed primarily of actin or thick filaments composed primarily of myosin. The Myofibrils are anchored to each end of the muscle fiber, which is connected to its tendon. As a result, when the myofibrils contract, the entire cell shortens and pulls on the tendon. Transverse tubules or T-tubules are narrow tubes that are continuous with the sarcolemma and extend deep into the sarcoplasm. They are filled with extra cellular fluid and form passageways through the muscle fiber, like a network of tunnels through a mountain. Electrical impulses called action potentials travel along the T-tubules into the cell interior. These action potentials trigger muscle fiber contraction. Branches of the T-tubules surround each myofibril. The sarcoplasmic reticulum is similar to the smooth endoplasmic reticulum of other cells. The sarcoplasmic reticulum fits over each individual myofibril like a lacy shirt sleeve. Where a T-tubule encircles the myofibril, the sarcoplasmic tubule expands, and these chambers are called terminal cisternae. The terminal cisternae contains stored calcium. And sometimes the concentration of calcium inside the cisternae is a thousand times higher than the levels inside the sarcoplasm. The thick and thin filaments of the myofibril are organized into repeating functional units called sarcomeres. Sarcomeres are the smallest functional units of the muscle fiber. Interactions between the thick and thin filaments of sarcomeres are responsible for muscle contraction. One myofibril consists of approximately 10,000 sarcomeres end to end. A sarcomere contains thick filaments, thin filaments, proteins that stabilize the positions of the thick and thin filaments, and proteins that regulate the interactions between the thick and thin filaments. The thick and thin filaments are different in size, density, and distribution. These differences account for the banded appearance of each myofibril. The thick filaments are at the center of each sarcomere. Proteins of the M-line connect the central portion of each thick filament to neighboring thick filaments. M stands for middle. The thin filaments are located between the thick filaments in an area called the zone of overlap. A single thin filament contains two rows of 300 to 400 individual globular molecules. Each of these molecules contains an active site that can bind to myosin. Under resting conditions, a complex called the tropomyosin-troponin molecule covers the active sites and prevents the binding of myosin to the active site. Tropomyosin is a double-stranded, rope-like structure that covers the active sites on the actin molecules. Troponin is a molecule that locks the tropomyosin molecule to the actin molecule, thereby preventing the exposure of the actin molecule's active site. A thick filament contains about 300 myosin molecules. The myosin molecules are twisted around each other. Each myosin molecule has a long tail and a head which projects outward toward the nearest thin filament. When the myosin heads interact with thin filaments during a contraction, they are known as cross-bridges. The connection between the head and the tail of the myosin acts as a hinge that lets the head pivot. When they head pivots, it swings toward the M-line or the center of the sarcomere. All the myosin molecules are arranged with their tails pointing towards the M-line. When the myosin head forms a cross-bridge with the actin molecule's active site and the head pivots, causing the thin filaments to slide toward the center of each sarcomere, this is known as the sliding filament theory. During a contraction, sliding occurs in every sarcomere along the myofibril. As a result, the myofibril gets shorter. When myofibrils get shorter, so does the muscle fiber.