Asynchronous muscles

Asynchronous muscles are muscles in which there is no one-to-one relationship between electrical stimulation and mechanical contraction. The majority of flying insects use asynchronous muscles to generate wingbeat frequencies faster than what most neuromuscular systems can sustain. Insects that use asynchronous muscles evolved from synchronous ancestors ~400 million years ago (despite previous reports of 7–10 independent events). Unlike synchronous muscles that contract once per neural signal, asynchronous muscles can be triggered through mechanical oscillations. The rate of mechanical contraction of asynchronous muscles can reach an order of magnitude faster than electrical signals. Although they achieve greater force output and higher efficiency at high frequencies, they have limited applications because of their dependence on mechanical stretch.

Structure

Molecular

The exact molecular mechanisms used by asynchronous muscles are unknown, but it is believed that asynchronous muscles have no unique molecular structures as compared to their synchronous counterparts. A study investigating the asynchronous power muscles in bumblebees with X-ray diffraction videos showed that actin and myosin alone are sufficient for generating asynchronous behavior. This finding helps explain how asynchronous muscles independently evolved across insect taxa. More recent work using similar X-ray diffraction techniques in Lethocerus discovered that troponin bridges may play a critical role in stretch activation.