Structure and function of spiny lobster ligamental nerve plexuses: Evidence for synthesis, storage, and secretion of biogenic amines

Abstract

AbstractThree pairs of ligaments support not only the heart of spiny lobsters but also ligamental nerve plexuses, the complex terminal aborizations of segmental nerves. Segmental nerves 1–4 project from the thoracic ganglia into the pericardial cavity and ultimately ramify along the strands of the anterior, medial, and posterior ligaments. In each branch, a core of large axons sends fibers to terminate in a surrounding cortex of fine and varicose secretory processes. Electron micrographs reveal at least five distinct populations of granule‐filled neuronal profiles, many with vesicles clustered at membrane thickenings adjacent to the epineural sheath.The ligamental nerve plexuses synthesize and accumulate octopamine, dopamine, 5‐HT, and acetylcholine. Octopamine and 5‐HT are predominant, comprising 33% and 65%, respectively, of the synthetic activity devoted to the four amines. Thus, the anatomy, ultrastructure, and neurochemistry of the ligamental nerve plexuses establishes their homology with the pericardial organs of other Crustacea.Octopamine and 5‐HT are released by a Ca++‐dependent mechanism upon electrical stimulation of preterminal nerve trunks, and, in vivo, would be swept immediately through ostia into the heart. These observations, when considered with known effects of octopamine and 5‐HT on crustacean cardiac activity, neuromuscular transmission, muscle tension, and cyclic AMP metabolism provide a strong case for hormonal actions at target sites throughout the animal.Segmental nerve processes in the dorsal nerve trunk ramify into a plexus around the dorsal nerve apparatus, a small muscular bulb that lies recessed in the cardiac surface. The dorsal nerve, carrying excitatory and inhibitory input to the cardiac ganglion directly through the bulb's hollow interior. The apparatus synthesizes and contains acetylcholine and the three amines mentioned above. In situ, it may beat rhythmically out of phase with the heart.