Giant axons and control of jetting in the squid Loligo and the jellyfish Aglantha

Abstract

Both Loligo and Aglantha have sets of giant axons arranged in a cascading series, which transmit impulses at high velocity and with short synaptic delays to the swimming muscles. The layout of the axons is such as to guarantee symmetrical, near-simultaneous contractions of all parts of the swimming muscles, necessary for effective jetting. The giant axons in both animals are multinucleate, syncytial structures. In both animals, the first-order giants receive a rich afferent innervation. Despite these similarities in the layout and properties of their giant axon systems, Loligo and Aglantha control their swimming muscles in very different ways. The most striking difference is that Loligo has separate fast and slow innervations as well as fast and slow muscles, whereas Aglantha has only one innervation and one set of muscles, but can produce two types of contraction by switching between rapidly propagated sodium spikes and slowly propagated calcium spikes in its giant motor axons. A second major difference emerging from recent investigations is that whereas Aglantha uses its fast impulse propagation system for a conventional, short-latency escape response, squid appear (at least in laboratory studies) to use their fast pathway merely to augment the power of contractions brought about by their slow innervation, making no use of the potential the giant fibres hold for short-latency, reflex escape behaviour.