Muscle Cells
Muscle cells are elongated and are therefore also called muscle fibers. There are three types of muscle tissue, each with distinct morphological and functional characteristics. All muscle cells have contractile properties, meaning that they can shorten along their longitudinal axis during contraction and subsequently return toward their original length.
Common characteristics of muscle fibers
All types of muscle cells share several important characteristics:
Contractile proteins: Their cytoplasm contains large amounts of contractile proteins, primarily myosin and actin, together with several associated proteins that regulate and support contraction. The organization of these proteins differs among the three types of muscle tissue. In striated muscle, they are arranged in a highly organized pattern that produces the characteristic striations seen when one observes muscle sections with a mocroscope, whereas their organization is different in smooth muscle.
Generation of force and movement: Contraction of muscle cells generates force (tension) that can produce movement of other body structures or maintain their position. Because muscle cells are elongated, they can undergo substantial shortening along their longitudinal axis. Muscle contraction can therefore produce movement when the muscle shortens, or generate tension without significant shortening when the muscle contracts against a load.
Striated and smooth muscle
Two types of muscle tissue — skeletal muscle and cardiac muscle — are classified as striated muscle because of the characteristic organization of their contractile proteins.
In longitudinal sections examined under a microscope, striated muscle fibers show a regular pattern of transverse bands (striations) across their cytoplasm.
In smooth muscle, the contractile proteins are organized differently. Consequently, smooth muscle cells do not exhibit the transverse striations characteristic of skeletal and cardiac muscle.
Main types of muscle tissue
Skeletal striated muscle: Skeletal muscle fibers are responsible for voluntary movements of the body. Most are attached to bones, but skeletal muscle also occurs in structures that are not directly attached to bones. Examples include skeletal muscle fibers in the tongue, skin, facial (mimetic) muscles, and orbicularis oculi.
Cardiac striated muscle: Cardiac muscle contracts involuntarily and forms the myocardium, the muscular wall of the heart. Cardiac muscle is also found in the proximal portion of the aorta.
Smooth muscle: Smooth muscle contracts involuntarily and is found primarily in the walls of the viscera and blood vessels, as well as in many other structures throughout the body.