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A sarcomere (Greek σάρξ sarx "flesh", μέρος meros "part") is the smallest functional unit of striated muscle tissue. [1] It is the repeating unit between two Z-lines. Skeletal muscles are composed of tubular muscle cells (called muscle fibers or myofibers) which are formed during embryonic myogenesis.
In biology, a motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibres. [1] Groups of motor units often work together as a motor pool to coordinate the contractions of a single muscle.
General depiction of a motor unit, consisting of a motor neuron innervating a group of muscle fibers. Motor unit recruitment is the activation of additional motor units to accomplish an increase in contractile strength in a muscle. [1] A motor unit consists of one motor neuron and all of the muscle fibers it stimulates.
Skeletal muscle cells are the individual contractile cells within a muscle, and are often termed as muscle fibers. [3] A single muscle such as the biceps in a young adult male contains around 253,000 muscle fibers.
In mammals, triads are typically located at the A-I junction; [1] that is, the junction between the A and I bands of the sarcomere, which is the smallest unit of a muscle fiber. Triads form the anatomical basis of excitation-contraction coupling , whereby a stimulus excites the muscle and causes it to contract.
Each muscle cell contains myofibrils composed of actin and myosin myofilaments repeated as a sarcomere. [3] Many nuclei are present in each muscle cell placed at regular intervals beneath the sarcolemma. Based on their contractile and metabolic phenotypes, skeletal muscle can be classified as slow-oxidative (Type I) or fast-oxidative (Type II). [1]
The smallest contractile unit in the fiber is called the sarcomere which is a repeating unit within two Z bands. The sarcoplasm also contains glycogen which provides energy to the cell during heightened exercise, and myoglobin , the red pigment that stores oxygen until needed for muscular activity.
A myofibril (also known as a muscle fibril or sarcostyle) [1] is a basic rod-like organelle of a muscle cell. [2] Skeletal muscles are composed of long, tubular cells known as muscle fibers, and these cells contain many chains of myofibrils. [3] Each myofibril has a diameter of 1–2 micrometres. [3]