Ultrastructural and physiological studies of the contraction of the trunk musculature of Sagitta setosa (chaetognath).

The relationships between structure and function of the contractile apparatus of the trunk musculature in the chaetognath Sagitta setosa, were studied by electron microscopy and mechanical recordings. We also investigated the nature of the neuromediator at the synaptic level. Contraction, relaxation and stretch can be explained on the sliding model basis. The primary filaments are linked to a large Z line, via C filaments. These C filaments vary in length according to the mechanical states of the contractile apparatus. These muscles are directly excitable by electrical current, high K(+) solutions induced depolarization, or exogenous acetylcholine. The characteristics of the unitary contractile response (twitch) and the effects of the frequency of stimulation set up the contractions of Sagitta among the fastest in the animal kingdom. Acetylcholine appears to be the best candidate for the mediator released at the neuromuscular junction. Its effect on the post-junctional membrane seems to require a receptor of the nicotinic type.