Axon chemotaxis: Nature's most sensitive gradient detector?

Axonal chemotaxis plays an important role in wiring up the developing and regenerating nervous system, but little is known about the mechanisms by which axons responsd to molecular gradients. We have developed a new assay that allows measurement of the long-term response of axons to gradients of controllable shape in a three-dimensional gel. We show that axons are among the most sensitive chemical gradient detecting devices yet discovered, capable of responding to concentration differences that average less than one molecule across the growth cone, the sensing structure at the tip of the developing axon (Rosoff et al, Nat. Neurosci., 7:678- 682, 2004). We also present a theoretical model of axonal response to gradients in the presence of stochastic receptor binding that quantitatively matches the experimental data.