Ionic currents in the myoepithelium of Aglantha digitale

When “fishing” for prey the jellyfish Aglantha digitale undergoes a series of “slow swims” driven by pacemaker neurones at the base of its bell‐shaped body wall. To avoid predators Aglantha generates an altogether stronger form of “escape” swimming. During an escape swim, the striated muscle sheet that lines the inside of the body wall gives a strong contraction and water is forced from the bell opening. Neuromuscular synapses are distributed widely within this myoepithelium. An overshooting Na + ‐dependent action potential in each motor axon sets off a large depolarising post‐synaptic potential (psp) with a 1 ms synaptic delay. A spike‐like component on its rising phase initiates contraction. During slow swimming, low amplitude impulses in the motor axons set off a more slowly rising psp and spike in the myoepithelium. Although the properties and functions of different excitable epithelia have been studied extensively, the basis of their epithelial impulses is not well understood. Using the loose patch clamp technique to study the muscle spike in Aglantha , we find that the voltage‐gated inward (Na + and Ca 2+ ) and outward (K + ) currents that form its ionic basis are inactivated within 10 ms. We suggest that differences in the strength of muscle contraction during swimming arise from differences in the rate of rise of the psp. The more slowly rising psp during slow swimming partially inactivates the voltage‐gated currents, producing a reduced Ca 2+ influx and a weaker contraction. This flexible response to synaptic input accommodates the two forms of swimming in Aglantha but may be absent in other medusae, which depend on myoid‐propagated spikes in their striated muscle sheets rather than the presence of distributed synapses.