Neural events underlying escape swimming behaviour in the squat lobster 'Galathea strigosa' (Crustacea, Anomura)

Abstract

1. The anatomy and physiology of escape swimming behaviour in the squat lobster, Galathea strigosa, have been investigated and the results discussed in the context of comparative mechanisms of escape in rela~ed species. 2. In contrast to many other decapods, G. strigosa, does not possess a giant-fibre system which underlies escape. 3. In terms of the number, size and position of neuronal somata, the fast flexor motorneuron pools in Galathea and crayfish are homologous. 4. A neuron, homologous with the crayfish MeG, has been studied. Unlike the crayfish neuron, MoGH is a typical, unspecialized fast flexor motorneuron. 5. The anatomy of afferent and efferent neurons involved in abdominal extension has been investigated. The extensor motorneuron and accessory neuron pools in crayfish and Galathea are largely homologous. 6. A small degree of intersegmental and interspecific variation in abdominal flexor and extensor motorneuron pools is reported. 7. The anatomy and physiology of the abdominal MRO's has been examined. These are found to be homologous in structure and function with other decapod MRO's. 8. The considerable differences between the phasic and tonic MRO sensory dendrites may account for their different response characteristics. 9. The MRO's excite both extensor motorneurons and the flexor inhibitor motorneuron via an apparently monosynaptic pathway. Similar input properties have been described for the crayfish MHO's . 10. The MRO's, which are shown to fire in response to abdominal flexion, produce EPSP's in extensor motorneurons which both summate and facilitate. This feature has not been decribed previously and may be important in the reflex function of the MHO's during escape swimming behaviour. 11. The relative roles of proprioceptive and exteroceptive feedback on the generation of the swimming rhythm have been studied using a variety of preparations involving restraint and deafferenta- tion. 12. Sensory feedback both excites and inhibits swimming. It is deduced that proprioceptive feedback has excitatory effects and extero- ceptive feedback inhibits swimming behaviour. 13. It is suggested that the MHO's may playa role in exciting the neural circuits underlying swimming bewvbur via both direct connections with the thoracic nervous system and a restricted portion of the abdominal motorneuron pool. 14. A deafferented preparation has been used to analyse the motor programme underlying swimming behaviour. The ability to record swimming activity, identical with that recorded in the intact animal, in the absence of sensory feedback from the abdomen, suggests that swimming behaviour is controlled by a central pattern generator (CPG). 15. A method of inducing swimming activity by high frequency electrical- stimulation of abdominal sensori-motor roots is described. 16. The CPG for swimming behaviour is shown to be most likely to reside in the suboesophageal or thoracic ganglion. 17. The activity of flexor and extensor motorneurons in abdominal ganglia has been analysed at the cellular level using both extra- cellular and intracellular recording techniques. 18. Fast flexor motorneurons are driven by a combination of brief unitary synaptic potentials and a large underlying oscillatory slow wave depolarization. 19. Current injection into the somata of fast flexor motorneurons during swimming has dramatic effects on slow wave amplitude and suggests that motorneuron drive results from powerful periodic excitation via chemical synapses. 20. In contrast to the fast flexor motorneurons, fast extensor motorneurons are driven by only brief unitary synaptic potentials and not by an underlying slow wave depolarization. The contrasting mechanisms for excitation in antagonistic sets of motorneurons is documented and a possible explanation presented. 21. Among the fast extensor motorneurons there is an apparent gradation in spike thresholds which can be correlated with a gradation in soma diameter. The largest of the available pool of extensor motorneurons has the highest spike threshold. 22. The activity of the phasic inhibitors of the extensor and flexor muscles has been analysed. The extensor inhibitor, which fires in antiphase with other extensor motorneurons during the flexion phase of the swim cycle, appears to receive the same slow wave depolarization as fast flexor motorneurons. The extensor inibitor motorneuron burst is terminated by a high frequency barrage of IPSP's superimposed upon the membrane slow wave. The flexor inhibitor motorneuron receives complex excitation and inhibition during swimming, involving both unitary events and membrane waves. 23. The coordination of segmented limb structures during swimming has been investigated. The walking legs are physically protracted during flexion while the unmodified male swimmerets are flicked posteriorly. 24. Swimmeret retraction during swimming is controlled by the activity of a single swimmeret motorneuron which appears to be part of the swimming circuit and which may also be a primitive homologue of the Segmental Giant neuron in crayfish. 25. It is concluded that escape swimming behaviour is homologous with non-giant backwards swimming in crayfish and may also be homologous with swimming in certain sand crab species. The evolutionary relationships of a number of decapods is discussed on the basis of escape circuitry and it is suggested that Galathea may represent an ancestral type of swimming decapod.