Why are some muscles striated? A structural mechanism that amplifies shortening velocity

Muscles can be categorized broadly into striated and smooth muscle types. Physiology textbooks consistently describe how the repeating sarcomeric organization of vertebrate skeletal and cardiac muscle generates a striated appearance. Unfortunately, very few texts discuss the common functional role of this sarcomeric organization. Briefly, the repeating sarcomeres in striated muscles provide a mechanism to amplify the nanometer scale steps of myosin cross bridges into the dynamic large-scale movements embodied by diverse animals. That is, the sarcomeres contracting in unison dramatically expand the speed of muscle shortening. This functional role was recognized by the proponents of the sliding filament theory in the mid-1950’s. Striated muscles are found not only in vertebrate skeletal and cardiac muscles but are used to power movement in animals as disparate as jellyfish, clams, swimming crabs, and flying insects. Historically, several different invertebrate muscles have been studied in great detail. Recent studies based on whole sequenced genomes and transcriptome sequencing are providing new evidence that striated muscles evolved at the dawn of animal life. Although there is a deep evolutionary divergence of striated and smooth muscle myosins, striated muscles have likely arisen multiple times through convergent evolution. Several well studied examples of these striated muscles from diverse animals are discussed to illustrate their common functional roles. The central thesis presented here is that striations provide a structural mechanism to amplify the speed of muscle shortening. Comprehending this principle is foundational to the teaching of basic muscle structure and function relationships.