Coupled sensory interneurons mediate escape neural circuit processing in an aquatic annelid worm, Lumbriculus...

The interneurons associated with rapid escape circuits are adapted for fast pathway activation and rapid conduction. An essential aspect of fast activation is the processing of sensory information with limited delays. Although aquatic annelid worms have some of the fastest escape responses in nature, the sensory networks that mediate their escape behavior are not well defined. Here, we demonstrate that the escape circuit of the mud worm, Lumbriculus variegatus, is a segmentally arranged network of sensory interneurons electrically coupled to the central medial giant fiber (MGF), the command-like interneuron for head withdrawal. Electrical stimulation of the body wall evoked fast, short-duration spikelets in the MGF, which we suggest are the product of intermediate giant fiber activation coupled to MGF collateral dendrites. Since these contact sites have immunoreactivity with a glutamate receptor antibody, and the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dion abolishes evoked MGF responses, we conclude that the afferent pathway for MGF-mediated escape is glutamatergic. This electrically coupled sensory network may facilitate rapid escape activation by enhancing the amplitude of giant axon depolarization.