RECEPTOR CELL HABITUATION IN THE A(1) AUDITORY RECEPTOR OF 4 NOCTUOIDMOTHS

Moths of both sexes of Empyreuma affinis (=pugione) and Syntomeida epi lais (Arctiidae, Ctenuchinae), Maenas jussiae (Arctiidae, Arctiinae) a nd Sponoptera frugiperda (Noctuidae, Amphipyrinae) were studied. Spike activity in the A(1) cell was recorded using a stainless-steel hook e lectrode from the tympanic nerve in the mesothorax. Acoustic stimuli c onsisting of 25 and 100 ms pulses at the best frequency for the specie s and at intensities that evoke Al cell saturation response were used at repetition rates of 0.5 and 5 Hz for 100 ms stimuli, and between 2 and 20 Hz for 25 ms stimuli. Stimuli at a repetition rate correspondin g to a duty cycle of 5 %, (25 ms at 2 Hz and 100 ms at 0.5 Hz) did not evoke monotonic changes in the responses of the A(1) cell. With 25 ms pulses, rates above 5 Hz evoked an exponential decrease in the number of spikes and an increase in the latency of the responses of all the 37 specimens tested. The response duration showed no apparent change w ith stimulus repetition rates even at the highest duty cycle used (50 %), i.e. 25 ms at 20 Hz and 100 ms at 5 Hz. The higher the rate of sti mulus repetition, the more marked were the changes in the A(1) cell re sponses. In 16 of 17 preparations from two species, habituation had no effect on the adaptation rate in each response, while in seven of eig ht specimens of another species, the adaptation rate decreased with st imulus repetition. These results, and those from another mechanorecept or cell, indicate that receptor cell adaptation (changes evoked in the response by a stimulus of constant intensity) and habituation (change s in the responses due to stimulus repetition rate) are two distinctiv e phenomena. The A(1) cell in its habituated state showed an increase in its response to incremental increases in stimulus intensity of 10 d B. This result supports the idea that receptor cell habituation does n ot seem to be due to fatigue, i,e, to a temporary loss of the ability to respond to stimulation induced in a sensory receptor by continued s timulation.