giant axons
Extraordinarily large unmyelinated axons found in invertebrates. Some, like the squid giant axon, can approach 1mm diameter. Large axons have high conduction speeds; the giant axons are invariably involved in panic or escape responses, and may (eg. crayfish) have electrical synapses to further increase speed. Vertebrate axons with high conduction velocites are much narrower: they have a myelin sheath, allowing saltatory conduction.

Dictionary of molecular biology. 2004.

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