The neuronal basis of behavior in Tritonia. 3. Neuronal mechanism of a fixed action pattern.

We have investigated the roles played by numerous identified brain cells in initiating and controlling the coordinated sequence of movements of an instinctive escape-swimming sequence in an intact animal preparation of the nudibranch mollusc Tritonia diomedia. Intracellular electrical activity in different neurons has been correlated with the various phases of the behavior. We recognized four major stages in the response: (1) reflex local withdrawal; (2) preparation for swimming; (3) swimming; and (4) termination. We have located and studied brain cells whose activity is associated with the following aspects of swimming: withdrawal; elongation; triggering behavior; dorsal flexion; ventral flexion; and neurons which excite both dorsal and ventral flexor neurons simulataneously. We find that specific neurons play clearly defined and invariant roles in control of escape-swimming and that the neuronal circuitry underlying the coordination of the sequence is the same in different individuals of the species. Details of the neuronal circuitry and a number of the general functional attributes of interacting cell groups have been determined directly or inferred from observations of cell to cell interactions. A preliminary model of the neuronal apparatus which controls this behavior is discussed. The principal findings are: (1) a discrete group of electrically coupled neurons determines, by its output, whether or not escapeswimming will be executed; (2) the neuronal elements responsible for execution of the swimming stages of the sequence are maintained in an excited state for the required period, in part by a regenerative feedback system; (3) alternating bursts of impulses in functional antagonists are co-ordinated in part by reciprocal inhibition between them; and (4) termination of the sequence occurs abruptly at a particular phase in the swimming cycle and appears to be an active neural process, rather than a simple running-down.

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