A lateral excitatory network in the escape circuit of crayfish.

A phasic stimulus directed to the rear of a crayfish (Procambarus clarkii) creates mechanosensory input to the lateral giant (LG) interneuron, a command neuron for escape. A single LG spike is necessary and sufficient to produce a highly stereotyped tail flip that thrusts the animal away from the source of stimulation. Here we describe a lateral excitatory network among primary afferent axons in the last abdominal ganglion of crayfish that produces nonlinear amplification of the sensory input to the command circuitry for escape. The lateral excitation is mediated by electrical synapses between central terminals of primary mechanosensory afferents. The network enables stimulated afferents to recruit unstimulated afferents that contribute additional input to LG and to mechanosensory interneurons that also converge on LG. When depolarized, the LG neuron increases its own inputs from primary afferents and primary interneurons by facilitating the recruitment of both. Conversely, hyperpolarization of LG reduces the excitability of primary afferents and primary interneurons. The crayfish's decision to escape, previously thought to lie exclusively in the synaptic integrative properties of LG, is now seen to depend on the interactions between LG dendritic postsynaptic potentials and the responses of primary afferent terminals in the lateral excitatory network.