In the jellyfish Aglantha digitale two forms of swimming arise from two separate propagating axonal impulses; a fast, overshooting action potential that depends on TTX-resistant Na+ channels, and a low amplitude spike that depends on T-type Ca2+ channels. While the Na+ action potential is propagated simply and without distortion, the shape of the Ca2+ spike depends on the past history of the axon; it is processed as well as propagated.

Patch- and voltage-clamp experiments show how three classes of K+ channels contribute to this apparently unique system. Dual Na+/Ca2+ impulses may increase the bandwidth of an axonal line of communication but it also places restrictions on the form of the synaptic input needed for spike initiation.