Muscles may vary in their response rates even though they look the same. "Fast" muscle fibers contract rapidly in response to a nerve impulse. "Slow" muscle fibers contract several times more slowly. Although both types use the same contractile machinery, the slow muscle cells have less sarcoplasmic reticulum. This means that the calcium ions necessary to initiate contraction are not released as quickly everywhere along the contrac tile fibrils and are not pumped away from them as quickly as they are in the fast muscle cells.
Most invertebrate muscles are synchronous. This means that each contraction is initiated by a nerve impulse. The rate of contraction of synchronous muscles is determined by the rate of passage of nerve impulses. Even in flight, these muscles usually contract only about thirty-five times per second. Synchronous flight muscles are connected directly to the wings and are found in insects such as grasshoppers, moths, butterflies, and dragonflies.
True flies and bees, as well as beetles and true bugs, have a specialized kind of flight muscle called an asynchronous muscle. In asynchronous muscles, every contraction is not initiated by a nerve impulse. The contraction rates, up to one thousand times per second, are so rapid that nerve impulses could not be received and acted on quickly enough to produce that contraction rate. In these muscles, contraction is initiated by a nerve impulse received and transmitted by the T tubules, but there is no sarcoplasmic reticulum. Even fibrils removed from the cell and placed in a solution containing calcium ions and ATP contract and relax in an oscillatory fashion. The rates seem to be regulated by the myofibrils rather than by the calcium concentration. The T tubules seem to signal the contraction to begin and later turn off the cycling behavior of the myofibrils.
Asynchronous muscles are not directly attached to the wings of the insect. Elevator muscles are attached to the roof of the thorax. Their contraction pulls the roof of the thorax down and elevates the wings. Contraction of the wing depressor muscles pulls down the wings, but this stretches the wing elevators, stimulating them to contract and allowing the thorax roof to "pop up." Raising the thorax shortens the wing depressor muscles and terminates the active state of the depressors. They relax until stretched again by the elevation of the wings. The elevator and depressor muscle contractions follow the same sequence of stretch ^ contraction ^ shortening ^ relaxation ^ stretch, but are out of phase with each other. The frequency of wing beats depends upon the mechanical properties of the thorax and wings and not upon the frequency of nerve impulses.
Unlike vertebrate animals, there is a correlation between sarcomere length and speed of contraction. Rapidly contracting arthropod muscle fibers have short sarcomeres and relatively low ratios of thin to thick fibrils. Slowly contracting muscle fibers have long sarcomeres and high ratios of thin to thick fibrils. Intermediate types of fibers can exist within the same muscle.
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