Skeletal Muscles Health Article

Definition

Skeletal muscles have transverse striations and are under conscious or voluntary control by the somatic nervous system.

Macroscopic

Skeletal muscles are often attached to bone, although this is not always the case. Tendons are a common means of attaching skeletal muscle to bone; they are composed of collagen, a structurally strong yet flexible substance. A muscle's origin is the end that moves least on contraction; the other end is referred to as the insertion. There is a large range of muscle sizes, types, and functions. Most human muscles have muscle fibers arranged parallel to a tendon. A unipinnate muscle, however, has all of its muscle fibers inserted at an angle into one side of a tendon.

Microscopic

Skeletal muscles are made up of bundles (called fascicles) of individual muscle fibers lined with connective tissue. Each muscle fiber is a giant multinucleated cell, formed by the fusion of myoblasts (muscle cell precursors) during development. Muscle fibers contain approximately one thousand myofibrils, tubular organelles that appear striated under a microscope; each myofibril is surrounded by a system of vesicles called the sarcoplasmic reticulum. The striations are due to alternating bands of light and dark regions called bands. The light regions are called I-bands, while the dark regions are called A-bands. A dividing line that runs through the A-band is called the Z-line, and the region between successive Z-lines is called the sarcomere.

The sarcomere is the functional unit of skeletal muscle and is associated with muscle contraction. It is composed primarily of two different contractile proteins: actin (or thin filaments) and myosin (or thick filaments). The filaments are arranged in an organized array so that their overlapping pattern produces the striations visible from under a microscope. The light I-bands are formed by actin filaments that are rooted at the Z-line, while the dark A-bands are composed of myosin filaments that overlap the actin filaments to varying degrees based on the extent of muscle contraction. The region in which there is no overlap (i.e., groups of actin filaments) in the center part of the A-band is called the H-band.