FUNCTIONAL ORGANIZATION OF TWO TYMPANIC NEURONS IN NOCTUID MOTHS

Abstract

The information sent from the tympanic neuron of noctuid moths, Adris tyranus, Lagoptera juno, and Oraesia excavata, were studied electrophysiologically.
1. The tympanic organ of noctuid moths can send the phasic and tonic information through two tympanic neurons, fast and slowly adapting neurons. The threshold of the former is higher by 20-30dB than that of the latter.
2. The slowly adapting neuron responds to the sound of 1-85kc./sec. in A. tyranus and L. juno and of 1-45kc./sec. in O. excavata. The fast adapting neuron responds to the sound of 3 or 4-30kc./sec. in the former two species and of 3-25kc./sec. in the latter. The most effective frequencies to those neurons are between 10 and 20kc./sec.
3. The response area of the fast adapting neuron is relatively smaller than that of the slowly adapting one and the most effective frequency is the same in both neurons. The discharge patterns of those neurons bear no reference to the frequency of a sound. Therefore the tympanic neurons can not send the information about the frequency.
4. The number of impulses in two neurons changes with the change of sound intensity at the same proportion. With weak sound the slowly adapting neuron can well signal the change of sound intensity. In case of strong sound, the fast adapting neuron discharges too and supplements the information about the sound intensity.
5. The response of the slowly adapting neuron to a short tone burst regularly delivered is first masked by that to a continuous pure tone, but it becomes recognizable in two hysteretic manners, i. e. in one of them, the response to the tone burst becomes clear gradually by suppressing a train of impulses in the response to the continuous pure tone. In the other case, the response appears by augmenting impulse discharges in the response to the continuous pure tone. Such hysteretic responses are evoked on one and the same neuron by the different relation in intensity and frequency between two sounds.
6. The hysteretic responses are only observed when a tone burst with a short duration is delivered with a continuous pure tone. The mechanism is now unknown, but the hysteretic responses seems to show that the tympanic organ is convenient for detecting pulsatory sounds in a background noise.
7. Two neurons in the tympanic organ are very similar to the fast and slow cells in a crayfish stretch receptor. Those organs with fast and slowly adapting neurons may be a prototype of mechanoreceptors.