Summary of pathways in crayfish startle behaviour

The startle response of crayfish is a simple behaviour, which is initiated with the least possible delay. Nevertheless, the neuronal circuit underlying this behaviour is quite sophisticated and involves complexities that could hardly be predicted from the behaviour itself. The main pathways involved in linking the response to an adequate stimulus are shown in Fig. 3.7. The initial tail flip is triggered by sensory information, which is fed with minimal processing on to the lateral giant, which acts as a command neuron. The lateral giant produces excitation of the flexor muscles and, at the same time, short-lasting inhibition of the extensors. This is followed by delayed, long-lasting inhibition of the flexors and of sensory input to the lateral giant itself. Finally, excitation of the extensor muscles is produced by sensory feedback from the first flexion. Through the operation of these pathways, the first tail flip is completed (to full re-extension) by about 110 ms from the initial stimulus.

Figure 3.7 A flow diagram summarising the functional relations between the major components of the startle behaviour in crayfish. The usual symbols are employed to represent excitatory (—and inhibitory (—•) relations, but here the labelled boxes do not represent individual neurons, except in the case of the lateral giant. The components of the initial, giant-mediated tail flip are enclosed in the horizontal, dotted rectangle, and those of non-giant swimming are enclosed in the vertical, dotted rectangle. The flexor component has some vertical elements that are not common to both systems, whereas the three separate sensory components may well have some elements in common. (Modified after Wine, 1984.)

Figure 3.7 A flow diagram summarising the functional relations between the major components of the startle behaviour in crayfish. The usual symbols are employed to represent excitatory (—and inhibitory (—•) relations, but here the labelled boxes do not represent individual neurons, except in the case of the lateral giant. The components of the initial, giant-mediated tail flip are enclosed in the horizontal, dotted rectangle, and those of non-giant swimming are enclosed in the vertical, dotted rectangle. The flexor component has some vertical elements that are not common to both systems, whereas the three separate sensory components may well have some elements in common. (Modified after Wine, 1984.)

Sensory information that is adequate to trigger the giant-mediated tail flip also triggers escape swimming by an independent pathway that involves more elaborate sensory processing. This elaborate processing enables the swimming system to take account of directional information in the stimulus that is ignored by the giant-mediated first tail flip and also introduces a considerable delay. The delayed excitation triggers a bout of swimming, in which the extensors lead the flexors in each cycle. The delay in triggering swimming is such that the first movement of escape swimming, an extension, overlaps with or immediately follows the re-extension of the first tail flip. When the crayfish swims, inhibition prevents both activation of the extensors, through sensory feedback from their receptor organs, and activation of the lateral giant system.

As the startle behaviour of crayfish continues to be studied, many additional complexities are coming to light; these include additional circuit elements such as non-spiking interneurons that control abdominal muscles and circuits that control walking legs. Finally, it should be noted that the startle response is not automatic, although it will override and inhibit any conflicting, competing activity once it has been triggered. The readiness with which the startle response can be triggered is modulated by a range of factors, including strong control of elements in the circuit by neurons descending from the brain that can make it almost impossible to elicit a startle response in some circumstances (Krasne and Teshiba, 1995).

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