Skeletal Muscle Cells
Skeletal muscle cells are a form of striated muscle cells and one of the three major muscle cell types besides cardiac muscle cells and smooth muscle cells. Skeletal Muscle Cells are under the voluntary control of the somatic nervous system. Most skeletal muscles are attached to the bones by bundles of collagen fibers (tendons). A skeletal muscle refers to multiple bundles of cells called muscle fibers. Muscle fibers, or muscle cells, are formed from the fusion of developmental myoblasts during the myogenesis process. Skeletal Muscle Cells are cylindrical and have more than one nucleus. They also have multiple mitochondria to meet energy needs. Skeletal Muscle Cells are in turn composed of myofibrils. The myofibrils are composed of Actin and Myosin filaments repeated in units called sarcomeres. The sarcomere is responsible for the striated appearance of skeletal muscle and forms the basic machinery necessary for muscle contraction.
Individual Skeletal Muscle Cells are formed via the fusion of several undifferentiated and immature cells known as myoblasts into long, cylindrical, multi-nucleated cells. Differentiation into this state is primarily completed before birth with the cells continuing to grow in size thereafter. Every organelle and macromolecule of a skeletal muscle cells is arranged to ensure that form meets function. The cell membrane is called sarcolemma with the cytoplasm known as the sarcoplasm. The sarcoplasm contains the myofibrils, the contractile apparatus. Myofibrils are long protein bundles myofilaments. While the muscle fiber does not have some smooth endoplasmic cisternae, it contains a sarcoplasmic reticulum. The sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction.
In addition to Actin and Myosin components that constitute the Sarcomere, skeletal muscle cells also contain two other important regulatory protein: Troponin and Tropomyosin. These proteins are associated with actin and cooperate to prevent its interaction with myosin. Skeletal muscle cells are excitable and are subject to depolarization by the neurotransmitter acetylcholine, released at the neuromuscular junction by motor neurons. Once a cell is sufficiently stimulated, the cell's sarcoplasmic reticulum releases Ca++, which then interacts with the regulatory protein Troponin. Calcium-bound Troponin undergoes a conformational change that leads to the movement of tropomyosin, subsequently exposing the Myosin-binding sites to Actin. This allows for Myosin and Actin ATP-dependent cross-bridge cycling and shortening of the muscle.
PPARGC1A, a transcriptional coactivator of nuclear receptors important to the regulation of a number of mitochondrial genes involved in oxidative metabolism, directly interacts with MEF2 to synergistically activate selective slow twitch (ST) muscle genes and also serves as a target for calcineurin signaling. A PPAR δ-mediated transcriptional pathway is involved in the regulation of the skeletal muscle fiber phenotype. Calcineurin, calmodulin-dependent kinase, PPARGC1A, and activated PPAR δ form the basis of a signaling network that controls skeletal muscle fiber-type transformation and metabolic profiles that protect against Insulin resistance and Obesity.