Asynchronous muscle

Definition

Striated muscle capable of generating multiple contractions from a single nerve impulse, enabled by stretch-activation rather than repeated neural stimulation. Unlike synchronous muscle, which contracts once per action potential, asynchronous muscle achieves high-frequency oscillation through mechanical feedback: stretch during wing recoil triggers the next contraction cycle. This mechanism decouples contraction frequency from motor neuron firing rate, allowing extremely rapid, energetically efficient movement.

Etymology

From Greek a- (not) + synchronos (occurring at the same time), referring to the temporal decoupling of muscle contraction from neural signaling.

Example

The dorsal longitudinal muscles powering the wingbeat of a blow fly (Calliphora) contract at 150 Hz while receiving motor impulses at only 3–5 Hz; the tymbal muscle of periodical cicadas (Magicicada) similarly produces sound pulses exceeding 100 Hz through asynchronous mechanics.

Synonyms

• fibrillar muscle
• myogenic muscle

Related Terms

• synchronous muscle
• Indirect flight muscles
• Stretch-activation
• Wingbeat frequency
• Tymbal
• Flight muscle
• Neurogenic muscle

Usage Notes

The term contrasts strictly with synchronous (neurogenic) muscle, where each action potential produces exactly one contraction. Asynchronous muscle is restricted to certain insect lineages—notably Diptera, Coleoptera, Hymenoptera, and some Hemiptera—and is always associated with high-frequency, oscillatory systems. The older synonym "fibrillar muscle" refers to the distinctive histological appearance (large, fibril-rich fibers with sparse mitochondria and sarcoplasmic reticulum), but "asynchronous" more precisely describes the physiological mechanism. Not all fast muscle is asynchronous; some small insects achieve high wingbeat frequencies through synchronous muscle with specialized neuromuscular properties.