Staggered Starts

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Science  10 Oct 2008:
Vol. 322, Issue 5899, pp. 167
DOI: 10.1126/science.322.5899.167a

If you are a small animal with many predators in your environment, you'd better react quickly to the earliest sign of danger. Startle reflexes have evolved as one way to deal with such situations. Input from sensory organs is rapidly transmitted to efferent systems, usually the muscles that are involved in flight responses. One such reflex is the backward tailflip response in crayfish. The coincident arrival of synaptic inputs is crucial because only when enough synapses fire simultaneously will they be able to activate the giant fiber system; however, the medial giant fibers receive input from widely differing distances along the crayfish antennules. Mellon and Christison-Lagay found that sensillar axons have precisely calibrated conduction velocities. Axon diameters near the flagellum tip were very small, and axon diameters increased continuously toward more proximal locations along the flagellar axis. In nonmyelinated nerve fibers, conduction velocity is proportional to the square root of the axon diameter. This linear adjustment of axonal conduction velocity thus ensures that a train of action potentials from an array of feathered sensilla on the first antennae arrives simultaneously in the brain and can drive the medial giant fibers to produce the tailflip response. — PRS

Proc. Natl. Acad. Sci. U.S.A. 105, 14626 (2008).

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