Coordination of lateral giant and non-giant systems in crayfish escape behavior

Summary

• 1. The modal escape response of a wellrested crayfish to a tap on the abdomen consists of an initial lateral giant-mediated flexion, a movement-triggered reflex extension, and then a series of centrally patterned, non-giant mediated, extension-flexion cycles (Figs. 1 and 2).

• 2. We investigated three models to explain how the non-giant system is triggered (Fig. 3). Direct stimulation of the giant axons in unrestrained animals showed that neither their central activity nor the sensory consequences of the tailflip they produce are sufficient to trigger the non-giant system (Fig. 4).

• 3. In well-rested animals, weak taps often triggered the non-giant system alone, i.e., without activating the giant axons (Fig. 5). This may have occurred rarely in earlier studies since the non-giant system habituates more rapidly than the lateral giant system (Fig. 6).

• 4. Although firing the giant axons does not trigger the non-giant system, the giant axons do cause subthreshold excitation which can summate with sensory input to trigger the non-giant system (Fig. 7).

• 5. The combined results show that the giant and non-giant systems are usually activated in parallel. The orderly transition from giant to non-giant escape behavior is in large part the result of marked differences in the latencies of each system: the median latency for the lateral giant axon response is 6 ms compared to approximately 140 ms for the non-giant response (Fig. 8).

• 6. The long latency of the non-giant system may be inherent to the processing time required to set up the nervous system for an optimal response. Direct observations establish that the non-giant system uses at least one bit of information that the giant axon system ignores: it reacts to the laterality of the stimulus as well as to its anterior-posterior location (Fig. 9).