Contractility and Muscles

1. Contractility allows a muscle to shorten in length, and extensibility allows it to stretch, while excitability allows the muscle to react to stimuli, such as nervous impulses (Human, 2004). The property of elasticity allows it to return to its original length and shape after contraction or extension. Contraction and extension are possible because of the structure of muscle fibers. Fibers are composed of thick (myosin) and thin (actin) myofibrils. In skeletal muscle, the microfilaments in these myofibrils are arranged in a regular pattern, with each myosin thick myofilament surrounded by six thin actin myofilaments. Myofibrils are composed of many subunits lined up end-to-end, known as sarcomeres. Within each sarcomere, thin actin myofilaments extend from end to end, but thick myosin myofilaments do not reach the ends of the sarcomeres, and this is why, when skeletal muscle is viewed under the microscope, it appears striated, the lighter sections being the ends of the sarcomere where only thin myofilaments are found. The central sections of the sarcomeres appear darker because they contain both actin and myosin myofilaments. The light areas are known as I-Bands, the dark areas as A-Bands. In the center of each I-Band is a thin dark line known as the Z-band, representing the area where adjacent sarcomeres overlap slightly.

During muscular contraction, an impulse is transferred from a neuron to the sarcolemma (cell membrane of a muscle), down the T-tubules to the s

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