![]() These tubules pass down into the muscle cell and go around the MYOFIBRILS. These "holes" lead into tubes called TRANSVERSE TUBULES (or T-TUBULES for short). The SARCOLEMMA has a unique feature: it has holes in it. Contraction of the sarcomere occurs when the Z-lines move closer together, making the myofibrils contract, and therefore the whole muscle cell and then the entire muscle contracts (Source: Davies and Nowak 2006). The Z-line (also known as the Z-disk or Z-band) defines the lateral boundary of each sarcomeric unit. The light band, (known as the I-band) is made up of thin filaments, whereas the dark band (known as the A-band) is made up of thick filaments. The characteristic 'striations' of skeletal and cardiac muscle are readily observable by light microscopy as alternating light and dark bands on longitudinal sections. Myofilaments can be either thick filaments (comprised of myosin) or thin filaments (comprised primarily of actin). Myofibrils are contractile units that consist of an ordered arrangement of longitudinal myofilaments. As shown in the figure, the SR forms a network around the myofibrils, storing and providing the Ca 2+ that is required for muscle contraction. Transverse (T)-tubules invaginate the sarcolemma, allowing impulses to penetrate the cell and activate the SR. The sarcoplasm is the specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria. Muscle cells are long, cylindrical structures that are bound by a plasma membrane (the sarcolemma). Hundreds or thousands of muscle fibers (cells) bundle together to make up an individual skeletal muscle. Skeletal muscle is the muscle attached to the skeleton. To understand how a muscle contracts, you need to know a bit about the structure of muscle cells. However, the 'function' of impulses in muscle cells is to bring about contraction. So, impulses travel along muscle cell membranes just as they do along nerve cell membranes. The cell membrane of a muscle cell is called the sarcolemma, and this membrane, like that of neurons, maintains a membrane potential. Branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more minute blood vessels called capillaries (Source: ). ![]() Generally, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Before a skeletal muscle fiber can contract, it has to receive an impulse from a neuron. Skeletal muscles have an abundant supply of blood vessels and nerves. Within the fasciculus, each individual muscle cell, called a muscle fiber, is surrounded by connective tissue called the endomysium. Each bundle of muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. Each compartment contains a bundle of muscle fibers. Portions of the epimysium project inward to divide the muscle into compartments. ![]() Fascia, connective tissue outside the epimysium, surrounds and separates the muscles. Each muscle is surrounded by a connective tissue sheath called the epimysium. Skeletal muscles may be made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering. They range from extremely tiny strands such as the stapedium muscle of the middle ear to large masses such as the muscles of the thigh. Skeletal muscles vary considerably in size, shape, and arrangement of fibers. ![]()
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