There are approximately 640 muscles in human body and muscles constitute up to 40 percent of the adult body mass.
Skeletal muscle consists of cells called fibers. Fibers are grouped into fascicles.
Skeletal fibers are part of motor unit. Motor unit consists of a lower motor neuron originating from a spinal cord anterior horn cell and all the muscle fibers it innervates. All muscle fibers within a motor unit are of the same type. Different fibers within a single fascicle are innervated by different motor neurons.
Skeletal System & Structure of Muscle Fiber. Credit:Wikipedia
Muscle fibers are divided into three types based on their metabolism and response to stimuli: type 1, 2a, and 2b. Fiber type can be altered by reinnervation with a different motor neuron type, physical training or disease processes.
Heredity is the most important determinant of fiber type distribution. On average, muscle contains 40 percent type 1 and 60 percent type 2 fibers.
Each muscle fiber is surrounded by a plasma membrane called a sarcolemma.
Fibers contain myofilaments called actin, troponin, tropomyosin, and myosin, which are contractile proteins. The myofilaments are bathed in sarcoplasm and organized into fibrils, which are enveloped by the sarcoplasmic reticulum. Communication between the sarcolemma and sarcoplasmic reticulum occurs through a channel network called the T-tubule system.
