Locomotion and Digestion

At very long lengths, there is no overlap between the thick and thin filaments, and no cross-bridges can form. At very short lengths, the thick filaments collide with the Z lines, stopping further shortening.
             The muscle fibers in a vertebrate skeletal muscle do not contract at the same time under normal conditions. If they are to contract, they must be stimulated by the nervous system. There are several steps involved in the initiation of contraction in a skeletal muscle fiber by the nervous system. First, the motor neuron produced electrical impulses that are conducted to the ends of its axons, where the neuron forms synapses (neuromuscular junctions) with one or more muscle fibers. At these synapses, the neuron releases acetylcholine as a chemical neurotransmitter. Acetylcholine excites each muscle fiber, stimulating it to produce impulses that are similar to those in the neuron. Second, the muscle fiber's impulses are conducted along its sarcolemma (plasma membrane), and along the infoldings of the membrane are called transverse tubules. The transverse tubules extend deep into the interior of the muscle fiber, where they are closely apposed to the sacroplasmic reticulum (a specialized smooth endoplasmic reticulum that surrounds the myofibrils). Then, the impulses conducted along the transverse tubules stimulate the release of Ca++ ions into the cytoplasm from the sarcoplasmic reticulum, where they are stored. Afterwards, 2 regulatory proteins associated with the thin filament, troponin and tropomyosin, are involved. Tropomyosin lies against the thin filament, and troponin is bound to tropomyosin. In a resting muscle fiber, the concentration of Ca++ in the cytoplasm is very low, and tropomyosin is near the myosin-binding sites on the thin filament. In this position, tropomyosin physically blocks the myosin heads from binding to actin, thus preventing contraction. In a stimulated muscle fiber, the Ca++ released by the sarcoplasmic reticulum binds to troponin.