What is the morphology of muscle
• Skeletal muscle consists of cells called fibers. Fibers are grouped into fascicles.
• Muscle fibers are part of motor units that consist 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.
• Muscle fibers are divided into three types based on their metabolism and response to stimuli: types 1, 2a, and 2b. Fiber type can be altered by reinnervation with a different motor neuron type, physical training (controversial), or disease processes. However, heredity is the most important determinant of fiber type distribution. On average, muscle contains 40% type 1 and 60% 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.