Microscopic+Anatomy+of+a+Skeletal+Muscle+Fiber

__**Fibers**__ Skeletal muscles are made up of many cells, also called fibers. Each of these cylindrical skeletal muscle fibers have multiple nuclei just beneath the sarcoplasm. The fibers can be ten to one hundred micrometers in diameter and can be hundreds of centimeters long. In each cell are the usual organelles: myofibrils, sarcomplasmic reticulum and t- tubules. Myofibrils are rodlike elements responsible for a major part of muscle contraction. These structures are packed very closely together and make up the bulk of the muscle. The myofibrils are surrounded by the sarcoplasmic reticulum. (See below to learn more about the sarcoplasmic reticulum.) These packed myofibrils are organized with a fiber in a pattern. This pattern involves the dark A bands and the light I bands. __**Sarcomeres**__ The next smallest structure of the muscle fiber is the sarcomere. It is the smallest unit of the muscle invloved in contraction. It is composed of myofilaments which are made of contractile proteins. __**Myofilaments**__ There are two types of myofilaments: thick and thin. The thick myofilaments are made of a protein called myosin. Each bit of myosin has two heads and a rod-like tail. The tails are actually to polypeptide chains woven together. The head are two smaller polypeptide chains also known as crossbridges. The thin filaments are composed of actin, also a protein. Each part of the actin is a helical polymer of subunits called G-actin. The G-actin contain the active sites which the myosin heads from the thick filaments attach to during muscle contraction. The thick and the thin filaments connect in a particular banding pattern. The thin filaments extend across the I-band and partway into the A-band, where the thick filaments extend across. The thin filaments are supported by the z- disc, (a sheet of connective tissue proteins) which connects myofibrils together. In this banding pattern, the thick and thin filaments do not overlap in the h-zone. __**Sarcoplasmic Reticulum**__ The sarcoplasmic reticulum is a more complicated form of the smooth endoplasmic reticulum. It runs longitudinally and surrounds each myofibril. Its main functions are the regulation of intracellular calcuim levels. Around this terminal cisternae form perpendicular cross channels. Also around the sarcoplasmic reticulum are the elongated tubes called T-tubules. T-tubules penetrate into the cell's interior at each A-band and I-band junction and are continous with the sarcolemma. The T-tubules conduct impulses to signal the release of calcium from the terminal cisternae. These impulses can reach the deepest part of the muscle.
 * __Myofibrils__**
 * __T-tubules__**