Fiber Ultrastructure and Contraction Kinetics in Insect Fast Muscles

SYNOPSIS. Isometric contraction kinetics were measured and fiber structure was quantified in tymbal muscles from different cicada species. Twitch duration is directly correlated with the size of the myofibrils and with the ratio of the fraction of fiber volume which is myofibril to that which is sarcoplasmic reticulum (SR) and T-tubules (fast muscles have small myofibrils and a relatively large volume of SR and T-tubules). Twitch duration is not significantly correlated with fiber size, with sarcomere length, nor with the fractional volume of the fibers which is mitochondria, indicating that these structural features are not strongly involved in the determination of isometric contraction kinetics. In the tettigoniid Neoconocephalus robustus, twitches from forewing muscles of male animals become progressively shorter over the first five days following the adult molt. This change in contraction kinetics is associated with an increase in the relative volume of SR and T-tubules. Denervation blocks the acquisition of rapid kinetics, indicating that neural input is necessary for this transformation.