Modification of the motor reflex responses due to repetition of the peripheral stimulus in the cockroach. I. Habituation at the level of an isolated abdominal ganglion.

INTRODUCTION All living organisms react to modifications of the external environment in their own generally quite precise manner, the best known example being the reflex reactions of animals to a given stimulus. If the stimulus is repeated, however, the reflex generally undergoes a change which allows the animal to adapt itself to the new environmental conditions. This change can be manifested by an increase or a decrease of the successive reflex responses. If the stimulus is repeated at a frequency lower than about 1 /sec, the former can be referred to as a 'sensitization' (cf. e.g. Kandel & Spencer, 1968) and the latter as a 'habituation' (e.g. Horn, 1967), provided the receptors for the reflexogenic stimulus have not changed and the capacity for the response to occur is unaltered. These phenomena are very general and can be found in the whole animal kingdom, from the least-organized beings—protozoa are habituable (Applewhite, 1968)—to the superior organisms. The final aim of our work was to try to explain observations of this type of behavioural plasticity by the transfer properties of the different synapses involved. The chosen reflex was the forward escape reflex of the cockroach to an air puff applied to small abdominal appendices, the cerci. It is described (Roeder, 1948) as anatomically simple, i.e. having only two central non-cephalic synaptic levels successively involved: the 6th abdominal ganglion (A.G.), followed by the thoracic ganglion innervating the legs of the corresponding segment. In the intact animal receiving successive air puffs a particularly rapid and durable habituation was observed by Roeder (1948); the escape reflex decreases and even disappears after a few stimuli. Occasionally we could observe a sensitization preceding the response diminution; the reflex reactions to the first three or four stimuli then increase progressively before habituation sets in. Some authors had, before us, tried to relate this habituation to the transfer properties of the synaptic relays participating in the reflex, but they had come to partial and contradictory results which can be summarized as follows: stimulation of the cerci by an air puff or electrical stimulation of a cereal nerve always elicits a response in the abdominal cord, while in the motor nerves the response is much more labile