1.1Muscle

Most cells possess cytoskeletal elements that are capable of lengthening or shortening and so enable the cell to change its shape.This capacity is important in a variety of cellular functions,e.g.,locomotion,phagocytosis,mitosis and extension of processes.Proteins referred to as molecular motors can effect changes of length much more rapidly than systems that are dependent on polymerization-depolymerization mechanisms(actin,tubulin),by using energy from the hydrolysis of adenosine 5'-triphosphate(ATP),Of these ATP-dependent systems,one of the most widespread is based on the interation of actin and myosin. In muscle cells the filaments of actin and myosin and their associated proteins are so abundant that they almost fill the interior of the cell.Moreover they align predominanty in one direction,so that interactions at the molecular level are translated into linear contraction of the whole cell. The ability of these specialized cells to change shape has thus become their most important property.Assemblies of contractile muscle cells,the muscles,are machines for converting chemical energy into mechanical work.The forces generated move limbs,inflate the lungs,pump blood,close and open tubes,etc.In man,muscle tissue constitutes 40-50% of the body mass. Muscle cells(fibers) are also known as myocytes(the prefixes myo- and sarco- are frequently used in naming structures associated with muscle).They diffentiate along one of three main pathways to form skeletal,cardiac or smooth muscle.Both skeletal and cardiac muscle may be called striated muscle,because their myosin and actin filaments are organized into regular,repeating elements which give the cells a finely cross-striated appearance when they are viewed microscopically. Smooth muscle,in contrast,lacks such repeating elements and thus has no striations. Other contractile cells,including myofibroblasts and myoepithelial cells,are different in character and developmental origin.They contain smooth muscle like contractile proteins and are found singly or in small groups. skeletal muscle forms the bulk of the muscular tissue of the body and consists of parallel bundles of long, multinucleate fibers.This type of muscle is capable of powerful contractions(about 100 watts per kilogram)because of the regular organization of its contractile proteins.However, the price paid for this organization is a limited contractile range:wherever a larger range of movement is required, it is achieved through the amplification provided by the lever systems of the skeleton to which the muscle is attached(hence the name skeletal muscle)(see below). skeletal muscle is innervated by somatic motor nerves.It is sometimes referred to as voluntary muscle, because the movements in which it participates are often initiated under conscious control.However, this is a misleading term:skeletal muscle is involved in many movements,such as breathing, blinking,swallowing,and the actions of the muscles of the perineum and in the middle ear, which are usually or exclusively driven at an unconscious level. Cardiac muscle is found only in the heart, and in the walls of large veins where they enter the heart. It consists of a branching network of individual cells that are linked electrically and mechanically to function as a unit.Compared with skeletal muscle,cardiac muscle is much less powerful(3-5 watts per kilogram)but far more resistant to fatigue.It is provided with a continuous supply of energy by numerous blood vessels around the fibers, and abundant mitochondria within them.Cardiac muscle differs structurally and functionally from skeletal muscle in some important respects.It is,for example,intrinsically capable of rhythmic contraction,with a rate and strength which is nevertheless responsive to hormonal and automomic nervous control. Smooth muscle is found in all systems of the body,in the walls of the vicera,including most of the gastrointestinal,repiratory,urinary and reproductive tracts,in the tunica media of blood vessels,in the dermis(as the arrector pili muscles),in the intrinsic muscles of the eye,and the dartos muscular layer of the scrotum,In some places,smooth muscle fasciculi are associated with those of skeletal muscle,e.g., the sphincters of the anus and the urinary bladder, the tarsal muscles of the upper and lower eyelids,the suspensory muscle of the duodenum,a transitional zone in the esophagus,and fasciae and ligaments on the pelvic aspect of the pelvic diaphragm. Smooth muscle contains actin and myosin,but they are not organized into repeating units,and its microscopic appearance is therefore unstriated(smooth),the elongated cells are smaller than those of striated muscle, and taper at the ends.They are capable of slow but sustained contraction,and although this type of muscleis less powerful than striated muscle, the amount of shortening can be much greater. These functional attributes are well illustrated by its role in the walls of tubes and sacs,where its action regulates the size of the enclosed lumen and,in some cases,the consequent movement of luminal contents. A smooth muscle cell may be excited in several ways, most commonly by an autonomic nerve fiber, a blood-borne neurohormone,or conduction from a neighboring smooth muscle cell.Since none of these routes is under conscious control, smooth muscle is sometimes referred to as involuntary muscle.