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According to this theory, muscle contraction is a cycle of molecular events in which thick myosin filaments repeatedly attach to and pull on thin actin filaments, so they slide over one another. The actin filaments are attached to Z discs, each of which marks the end of a sarcomere.
The type of contraction is called fixed-end if the muscle-tendon length does not change. A truly isometric contraction occurs without muscle fibre length change. However, these two terms ( fixed end contraction and isometric contraction) are sometimes used interchangeably.
The fundamental repeat unit within muscle that is responsible for contraction is the sarcomere. The sarcomere consists of a bundle of myosin-containing thick filaments flanked and interdigitated with bundles of actin-containing thin filaments (Fig. 1).
The process of muscle contraction (excitation-contraction coupling) is as follows: An action potential arrives at a neuromuscular junction . As a result of the action potential reaching the end of the axon and depolarizing it, acetylcholine (ACh) is released from the neuron and binds to receptors on the muscle fiber, opening sodium ion channels.
During the contraction phase, Ca 2+ ions in the sarcoplasm bind to troponin, tropomyosin moves from actin-binding sites, cross-bridges form, and sarcomeres shorten. During the relaxation phase, tension decreases as Ca 2+ ions are pumped out of the sarcoplasm and cross-bridge cycling stops.
An example of this is the contraction of the biceps brachii muscle when a hand weight is brought upward toward the body. An eccentric contraction occurs when the muscle tension produced is less than the load and a muscle lengthens while under tension.
Muscle contraction occurs when sarcomeres shorten, as thick and thin filaments slide past each other, which is called the sliding filament model of muscle contraction. ATP provides the energy for cross-bridge formation and filament sliding.
