Organization of Skeletal Muscles

Individual muscles are the organs of the muscular system. Each skeletal muscle is composed of individual muscle cells called muscle fibers. Muscle fibers are arranged into a bundle called a fascicle that is surrounded by connective tissue. Many fascicles are bundled together to make up the muscle.

Different kinds of connective tissue aid this organization of muscle fibers into muscles. Each individual muscle fiber is surrounded by a thin, wispy connective tissue called endomysium. Endomysium is made of areolar connective tissues containing the elastic fiber collagen. Endomysium provides a thin, loose structure that connects the muscle fibers together within the fascicle and provides a supportive environment for the muscle fibers (individual muscle cells). Capillaries that serve the muscle fibers run through the endomysium. The motor neurons that activate muscle fibers also run through this layer of tissue.

Each individual fascicle is encased by a layer of tissue called perimysium⎯a connective tissue composed of collagen and elastin fibers. And these bundles of fascicles together compose the whole muscle, which is encased with epimysium. Epimysium is a thick, elastic tissue made of collagen that surrounds whole muscles and prevents them from rubbing against other muscles or bones. Finally, the epimysium is surrounded by fascia—a loose connective tissue that surrounds and supports organs in the body.

The orientation of the fascicles within a muscle determines the strength and directionality of that muscle's contraction. There are seven main ways that fascicles tend to orient in the human body.

Most of the muscles in the body are parallel muscles. As the name suggests, in parallel muscles, the fascicles are arranged along the length of the muscle, running in parallel between attachment points (usually tendons). Tendons are flexible collagen tissue that connects muscles to bone, such as the hamstring in the upper leg. This means that the muscle also generates force along the parallel axis. There are two types of parallel muscles: fusiform and nonfusiform.

• Fusiform parallel muscles are spindle shaped. These muscles bulge in the middle and taper toward each end. In fusiform parallel muscles, the bulge increases when the muscle contracts, such as the biceps brachii in the upper arm.
• Nonfusiform parallel muscles do not bulge in the center, but have a relatively constant diameter, such as the sartorius muscle allowing hip and thigh rotation.

Convergent muscles, such as the pectoralis major in the chest, are fan-shaped muscles that have a broad area of attachment at one end and that taper to a point at the other. Because of this broad area of attachment, convergent muscles do not exert the same amount of force on the tendon as a parallel muscle would, but the broad distribution increases the versatility of the muscle, allowing it to modify the direction of pull.

Another major type of muscle⎯pennate muscle⎯contains tendons that run along its entire length. Fascicles extend from this tendon at an angle; the word pennate comes from penna, the Latin word for feather. In pennate muscles, shorter fascicles are arranged like barbs of a feather extending from the quill. There are three types of pennate muscles: unipennate, bipennate, and multipennate. In unipennate muscles, such as the extensor digitorum in the forearm, fascicles are located on only one side of the tendon. In bipennate muscles, such as the rectus femoris in the leg, fascicles are located on both sides of the tendon. In multipennate muscles, such as the deltoids in the shoulder, the fascicles wrap around the tendon. Because these muscles pull the tendon at an angle, they cannot move it very far; however, all types of pennate muscles generally have more individual muscle fibers, and thus produce more tension for their size.

Finally, circular muscles, such as the orbicularis oris surrounding the mouth, are bundles of muscle fibers arranged concentrically around an opening. Also called sphincters, these muscles shrink or close the opening when they contract, and open it when they relax.

Muscle fibers⎯the cells that make up muscles⎯contain specialized structures that allow them to produce movement. Muscle fibers are packed with rod-like structures, each called a myofibril. Myofibrils are thin fibers that contain ultramicroscopic threads of protein, each of which is called a myofilament. The myofilaments create alternating light and dark bands in the myofibril. Specifically, myofibrils contain the proteins actin, the thin protein filaments that connect to the ends of the sarcomere, and myosin, which is a protein on the thick filaments that sit in the center of each sarcomere that form the bands. Each unit (one light band and one dark band) is called a sarcomere. The sarcomere is a regular repeated structures consisting of thick and thin filaments, present in the myofibrils of muscle fibers. The repeated pattern of sarcomeres gives skeletal muscle its striated, or striped, appearance.

Thick myosin myofilaments sit in the center of each sarcomere, in a region called the A-band. Thin actin myofilaments sit on the end of the sarcomere, in regions called the I-bands. These actin filaments are bound to Z-discs at the borders of each sarcomere. Z-discs are made of several stabilizing proteins and serve as an anchoring point for actin molecules. Actin is bound to the Z-disc by the giant protein titin, or connectin. Finally, in the center of the sarcomere is the H-zone⎯an area that contains just myosin. When stimulated by a motor neuron at the neuromuscular junction, actin and myosin work together to slide past each other and pull the Z-discs closer together. This contracts the muscle, shrinking the H-zone and the I-band.

Muscle fibers have other specialized cellular structures, as well. The cell membrane of muscle fibers is called the sarcolemma. This membrane has specialized features called transverse tubules or T-tubules. Each transverse tubule provides an extension of the sarcolemma that runs as a channel through the bundles of myofibrils. This extension of the membrane allows the signal that activates the muscle to quickly reach all of the myofibrils in the muscle fiber. On either side of the transverse tubule are structures called the terminal cisternae of the sarcoplasmic reticulum. The sarcoplasmic reticulum is a muscle-specific type of smooth endoplasmic reticulum. It forms a network around each myofibril that contains a calcium ion reserve, ready to release calcium ions when stimulated by a motor neuron. The terminal cisternae sit at the edges of this network, next to the transverse tubules, ready to release their calcium reserves should an action potential arrive.

Striated muscle fibers are produced when multiple myocytes—the progenitor cells that give rise to muscle cells—fuse together. Progenitor cells are similar to stem cells, but they develop into specific cells with targeted functions, such as a bone or muscle cell. Because of this, they contain multiple nuclei. These nuclei are flattened and located near the cell membrane so they do not impede the action of the myofibrils. Muscle fibers contain other organelles as well. These specialized cell parts include mitochondria, which in muscle fibers are sometimes called sarcosomes.