Segmental giant: evidence for a driver neuron interposed between command and motor neurons in the crayfish escape system.

1. The giant command neurons for tailflip escape behavior in crayfish have been thought to excite the nongiant fast flexor (tailflip producing) motor neurons (FFs) via monosynaptic connections. We show here that excitation of FFs instead occurs via a bilateral pair of segmental giant neurons (SGs) interposed between the command axons and FFs in each segment. 2. Anatomically, the SGs appear to make numerous contacts with ipsilateral command axons and FFs and fewer contacts contralaterally. In contrast, the command axons have only sparse direct connections to the FFs. An SG has an axon in the ipsilateral first ganglionic root and may be a modified swimmeret motor neuron. 3. Each SG is depolarized well beyond threshold by the firing of an ipsilateral command axon and is depolarized to near threshold by the firing of a contralateral command axon. The synapses between command axons and SGs are electrical and probably rectifying. 4. Each FF is excited to a level near firing threshold by the SG ipsilateral to its axon and is excited weakly by the contralateral SG. The synapses between SGs and FFs are electrical and nonrectifying. 5. Variations in excitatory postsynaptic potentials (EPSPs) recorded in FFs during prolonged, high-frequency firing of the command axons can be accounted for by refractoriness of SG spikes, as opposed to refractoriness of dendritic branch spikes as had previously been delivered. 6. These findings illustrate the limitations of conventional tests for monosynapticity. 7. The functional significance of having driver neurons interposed between command neurons and motor neurons is discussed.