Skeletal Muscle
Skeletal muscle is also known as striated muscle because of its striped appearance under the microscope and as voluntary muscle because it can be controlled at will. These muscles are attached to, and move, the bones, and are arranged in antagonistic pairs that enable movement in opposite directions. For example, the biceps flexes the lower arm while the triceps extends it.
A muscle is like a set of Russian nesting dolls. Each level of structure has a smaller element within it. A muscle such as the biceps is composed of many long fibers running in parallel. Each muscle fiber contains many myofibrils, and each myofibril contains many copies of two types of myofilaments, actin, or thin filaments; and myosin, or thick filaments. These, in turn are composed of aggregates of actin and myosin molecules, respectively.
The actin and myosin filaments are arranged in repeating units called sarcomeres. Each sarcomere is bounded by a Z-line on each end. The actin filaments are attached to the Z-lines on each end of the sarcomere and extend inward toward the center of the sarcomere. Since they do not quite contact each other, they leave a space between them in the center of the sarcomere. There are several rows of these actin filaments parallel to each other in each sarcomere. The pairs of actin filaments alternate with single myosin filaments, which do occupy the center of the sarcomere and extend outward toward, but do not touch, the Z-lines.
A nerve impulse from a motor neuron causes a muscle contraction, in which the myosin filaments "grab" the actin filaments, pulling them closer together and shortening each sarcomere, and therefore the entire muscle. Each myosin molecule has a "head" and a "tail." The tails combine to make the myosin filament, and the heads stick out to the sides of the filament. The myosin heads are capable of hydrolyzing adenosine triphosphate (ATP), and when they do, some of the energy released changes the shape of the head and allows it to bind to a specific site on the actin filament. When it returns to its normal shape, it pulls the actin filament toward the center of the sarcomere and then releases the actin. This attachment-pull-release process can be repeated five times per second. The myosin heads act like the oars of a crew team, but instead of pulling a boat through the water, they pull actin filaments closer together.
Rigor mortis, the stiffness of the body that occurs after death, is a result of lack of ATP and the inability of myosin to release the actin to which it is bound.
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