Introduction to Normal Skeletal Muscle: Anatomy, Physiology, Histology, and Ultrastructure

The word “muscle” is derived from the Latin word musculus meaning “little mouse.” Skeletal muscle constitutes ~30–45% of the total body mass in an average adult and ~25% in a neonate. The skeletal muscle mass is influenced by various factors such as the genetic makeup of the individual, physical activity, nutrition, hormones, and associated comorbidities [1]. About 50–75% of the total body protein mass is made up of skeletal muscle, thereby making it the primary site for amino acid metabolism [2]. In human beings, rhabdomyogenesis starts at around 3 weeks of intrauterine life immediately following gastrulation initiated by the formation of the primitive streak [3]. The epiblasts migrate on either side of the primitive streak giving rise to paraxial mesoderm. The paraxial mesoderm develops further to form a pair of somitomeres [4]. Except for the first seven somitomeres, the rest undergo segmentation giving rise to block-shaped structures called somites. As the somites form, primitive streak simultaneously involutes. The entire sequence of events is referred to as somitogenesis [5].