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Structure and Actions of Skeletal Muscles

An individual skeletal muscle is composed of thousands of muscle fibers, and each muscle fiber is actually a single, specialized muscle cell shaped like a long cylinder, up to several centimeters in length! Muscle cells contain multiple nuclei and mitochondria, and a specialized plasma membrane called the sarcolemma, which can become depolarized and carry an action potential similar to the action potential of a neuron. 


A muscle fiber


Perhaps the most striking feature of the muscle fiber is the presence of thousands of long subunits called myofibrils, surrounded by a tubular membrane system called the sarcoplasmic reticulum (equivalent to the endoplasmic reticulum of a non-muscle cell). Another membranous network of tubes, the transverse tubules, are invaginations of the sarcolemma that permeate the fiber, allowing an action potential to quickly spread to interior regions of each muscle cell. The functional unit of the myofibril is the sarcomere, each of which is composed of thin filaments made of the protein actin associated with thick filaments made up of the protein myosin. Repeating sarcomeres cause a skeletal muscle to take on its characteristic striated appearance. The sarcomere contains light regions (called I bands), composed exclusively of thin filaments anchored directly to a proteinaceous Z line, and darker regions (called A bands), which consist of thick filaments which overlap thin filaments. The thick filaments are indirectly connected to the Z lines, and the region from one Z line to the next is a complete sarcomere.

The sarcomere


Each muscle is surrounded by layers of fibrous connective tissue referred to as fascia. Extensions of the fascia form tendons, the structures that connect skeletal muscles to bones. In order to do its job, a skeletal muscle must be attached to at least two bones. Furthermore, if the contraction (shortening) of the muscle is to move a designated bone in a particular direction, one attachment must be immovable and the other movable. The immovable point of attachment is referred to as the origin, and the movable attachment is called the insertion. Thus when the muscle contracts, it produces a movement by “pulling on” the bone to which it is inserted. Muscles generally work in opposing pairs referred to as antagonistic; this means that since the only action a muscle can perform is contraction, one muscle acts to move a bone in a particular direction, while its antagonistic partner must contract to move it in the opposite direction. 

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