Summary

1. In the cockroach Blabera cranüfer repeated air puffs to the cerci induce a habituation of the corresponding escape reflex, which we have tried to relate to the transfer properties of the two central relays of the reflex. In this paper transmission through the first relay, the 6th abdominal ganglion (A.G.), was studied both by electrical stimulation of the sensory cereal nerve and by application of natural stimuli (air puffs) on the cerci, while recording simultaneously on the cereal nerve and the abdominal cord, i.e. respectively before and beyond the synaptic relay.

2. The electrical stimulation study shows transmission to be relatively labile. This property is still more apparent if air puffs are delivered. If they are separated by less than about 10 sec the cereal nerve response decreases, which is a sign of a receptor adaptation. For longer intervals, although this adaptation disappears, the response nevertheless decreases on the cord, implicating the ganglionic relay as the centre of a habituation phenomenon.

3. The characteristics of this habituation are the following: with repetition of the stimuli, the response decreases down to a non-zero plateau (decrease of about 50% if a puff is delivered every 20 sec); it recovers spontaneously if stimulations are suppressed ; habituation is potentiated when successive series of habituations followed by spontaneous restorations are accumulated; it is more pronounced as the stimulation frequency is increased; finally, it does not show generalization. These results confirm that the 6th A.G. independently exhibits a real habituation process.

4. The mechanism of this habituation process is discussed. A post-excitatory presynaptic depression intrinsic to the reflex's excitatory synapses themselves is tentatively proposed.