Myomere

Myomeres are blocks of skeletal muscle tissue arranged in sequence, commonly found in aquatic chordates.

Generally, cyclostome myomeres are arranged in vertical strips while those of jawed fishes are folded in a complex matter due to swimming capability evolution.

[1] Since myomeres are composed of multinucleated myofibers (contractile cells), force can be generated via muscle contraction that gets transmitted by the intricate connective tissue (myosepta) network.

The folded shape of each myomere as V- or W-shaped extends over various axial segments, allowing fibers control over a large amount of the body.

Further, myomeres of fish are divided by a horizontal septum into dorsal (epaxial) and ventral (hypaxial) sections as mentioned in previous paragraphs.

For example, high loading situations such as fast-starts and turning require almost maximal myomere activation in teleost fish.

For example, this microdissection and polarized light microscopy research suggests that anterior myomeres have elongated and reinforced dorsal posterior cones that allow epaxial muscle force to be transmitted to the neurocranium for elevation during suction feeding.

Published information on Pikaia gracilens (a well-known Cambrian fossil) explains evolution of swimming ability in chordates related to myomere shape and function.

Specifically, myomeres in this species possessed minimal overlap between successive ones and myosepta dividing them were gently curved.

The tail-bending maneuver generated by myomeres in zebrafish requires innervation from motor neurons for both the hypaxial and epaxial muscle regions.

This process is mediated by a circuit that controls motor neuron activation during swimming behaviors, which, in turn, affects force generation.

Thus, function in Chondrichthyes is similar to that of bony fish, where myomeres contribute to propulsive force for locomotion.