Muscle Structure: Skeletal, Smooth, and Cardiac Muscle

Muscle Structure

Skeletal Muscle

Skeletal muscle, also known as striated muscle, is responsible for controlling body movements. Its structure includes muscle fibers, myofibrils, and sarcomeres. Muscle fibers are surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. Each muscle fiber is composed of many myofibrils, which contain sarcomeres with light and dark regions that give the cell its striated appearance. The sarcomere is the basic unit of muscle contraction, consisting of thin actin filaments and thick myosin filaments arranged in a geometric pattern. Actin filaments are anchored to a structure known as the Z disc, while myosin is visible as the A band of the sarcomere. The arrangement of actin and myosin filaments within each sarcomere forms stripes of dark A bands and light I bands when viewed under a light microscope.

During contraction, the lengths of the sarcomeres decrease due to the 'sliding filament theory', whereby myosin filaments walk along actin towards each Z disc, pulling them centrally. This reduces the size of the H zone, the central region of the A band that does not contain both myofilaments. Skeletal muscle contains multiple layers of connective tissue, including the epimysium, perimysium, and endomysium, which provide support and facilitate force transmission.

Smooth Muscle

Smooth muscle is responsible for controlling visceral processes and regulating internal organ functions. Unlike skeletal muscle, smooth muscle cells lack striations and have a less organized structure. They are composed of elongated, spindle-shaped cells with a predominantly cellular appearance, meaning there are more nuclei present compared to the extracellular collagen fibers. The contractility of smooth muscle is regulated by autonomic nerves and hormones, rather than voluntary controls. Smooth muscle is found within the walls of hollow organs like blood vessels, glands, and the digestive system.

Cardiac Muscle

Cardiac muscle, also known as heart muscle, is responsible for pumping blood throughout the body. It shares structural similarities with skeletal muscle but has specialized features that allow it to function efficiently without conscious control. The cells are striated, meaning they have a regular arrangement of sarcomeres. However, cardiac muscle cells differ from skeletal muscle cells due to their branching shape and an intercalated disc structure, which allows them to form electrically excitable junctions called gap junctions. These gap junctions facilitate communication between adjacent cells, allowing coordinated contraction and relaxation.

Muscle Fibers, Sarcomeres, and Histological Structure

Muscle fibers are the functional units of muscle tissue, composed of myofibrils within each cell. Myofibrils contain repeating regions of thick and thin filaments arranged in a specific pattern, forming the basic unit of contractile activity known as a sarcomere. During contraction, the sarcomeres shorten through the sliding filament mechanism, whereby actin filaments slide along myosin filaments towards a central region. This shortening leads to changes in the appearance of the A bands and I bands under light microscopy.

The histological structure of muscles varies depending on the type. Skeletal muscle appears cross-sectioned with darker A bands and lighter I bands in longitudinal sections. Smooth muscle lacks these alternating bands and has a less ordered arrangement of myofibrils. Cardiac muscle shows some degree of striation but differs from skeletal muscle due to its branching shape and intercalated disc structures.