The Giant Axon Escape System of A Hydrozoan Medusa, Aglantha Digitale

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Abstract
Aglantha digitale (Hydrozoa) has a ring giant axon, up to 35 μm in diameter, which runs all round the margin of the bell in the outer nerve-ring. Running from the margin, up the inside of the bell towards the apex are eight motor giant axons, up to 40 μm in diameter. These synapse with the sub-umbrellar myoepithelium and are therefore motoneurones. The myoepithelial cells line the inside of the bell and have striated muscular tails running circumferentially. Contraction of this circular musculature forces water out of the bell and propels the medusa through the water. A large axon (up to 7 μm in diameter) runs on the aboral side of each tentacle. The tentacles retract during swimming and contain longitudinally aligned striated muscle tails. Intra- and extracellular recordings from the giant axons indicate that they are involved in the rapid escape swimming response of Aglantha. Stimuli which evoke escape swimming lead to a brief burst of two to six impulses (2–3 ms in duration) in the ring giant axon. These propagate round the margin (at up to 2·6 m s−1). The large tentacle axons fire one-to-one with the ring giant axon and the tentacles contract. The motor giant axons are excited at chemical synapses (blocked by divalent cations and with a synaptic delay of 1·6 ± 1 ms), where large, facilitating epsps evoke an impulse which then propagates up the bell at up to 4 m s−1. The overshooting motor giant axon impulses are Na+ dependent, 2–3 ms in duration, and excite the myoepithelium at synapses with a 1·6 ± 1 ms synaptic delay. A regenerative muscle impulse is evoked. It is of long duration (15–70 ms), Ca2+ dependent, overshoots zero, and propagates through the myoepithelium at 0·22 to 0·29 ms−1. A nearly synchronous contraction of the whole subumbrellar circular musculature is evoked. Isolated strips of muscle can reach peak tension in 40 ms. In the absence of sarcoplasmic reticulum, Ca2+ for excitationcontraction coupling probably enters the muscle during the impulse. This preparation has allowed a clear understanding of how the two types of axon co-ordinate escape behaviour. It should also be valuable for the study of synaptic transmission, previously very inaccessible in Cnidarians.