Insects can beat their wings rapidly; some insects up to 1000 times per second. Every beat of the wings requires each set of opposing flight muscles to contract and stretch. In the 1960s, it was discovered that the maximum frequency of the nervous system signaling is slower than the muscle contraction cycle. Thus, the cycle of muscle contraction and relaxation is asynchronous with each nerve impulse. It is still a mystery how insect flight muscles can contract and relax in the rapid rhythm.
Muscles contract as a result of the movement of the protein Myosin pulling the actin filaments of the cytoskeleton toward each other, shortening cells. Other proteins, troponin and tropomyosin physically block the actin from interacting with the myosin.
In a synchronous contraction:
1. A nerve impulse releases calcium into the muscle.
2. The calcium binds to the troponin, dislodging the tropomyosin and allowing the actin to interact with the myosin.
3. The myosin pulls the cytoskeleton actin fibers toward each other.
4. As calcium is removed, the tropomyosin once again blocks interaction between the actin and myosin.
5. The next nerve impulse starts the next contraction.
Research by Perz-Edwards and colleagues* has provided strong evidence supporting a model of muscle cycle control for asynchronous muscle contraction:
1. The nerve impulse releases calcium into the muscle.
2. The calcium binds to the troponin. However, unlike the synchronous system, the tropomyosin continues to block the actin from interacting with myosin until the muscle is stretched.
3. Stretching the muscle dislodges the tropomyosin from the actin binding sites (as long as calcium is present) and the myosin pulls the actin cytoskeleton together. The muscle contracts.
4. The contraction of one set of flight muscles stretches the opposing muscles. The myosin binding sites in the contracted muscle will be covered and stop contracting until stretched. The binding sites in the stretched muscle will be uncovered and contraction will begin.
5. This sets in motion a cycle of muscle contraction and stretching that will resonate according to the properties of the wings and the thorax.
6. An occasional nerve impulse will keep enough calcium in the muscle fibers to continue the cycle.
7. When the nerve impulses stop, calcium is not available and the cycle of muscle contractions stops.
* Perz-Edwards, et al. X-ray diffraction evidence for myosin-troponin connections and tropomyosin movement during stretch activation of insect flight muscle. PNAS. January 4, 2011 vol. 108 no. 1 120-125