Interneurons of the thoracic nerve cord activated by tympanic nerve fibres in noctuid moths

Many noctuid moths turn in free flight and assume a course away from a source of faint ultrasonic pulses. This avoidance behaviour appears to be steered primarily by a difference in the afferent signals delivered by one acoustic (A1) sense cell in each tympanic organ. This paper reports the first or ‘afferent’ stage of an attempt to trace functional interneuronal connexions made by the A fibres in the moth's central nervous system.The A fibres distribute signals ipsilaterally within the three thoracic ganglia. They connect with relay (R) neurons that convey the afferent signal with minor transformation to other ipsi- and contralateral regions of the meso- and prothoracic ganglia and to the ipsilateral cervical connectives. The posterior medial part of the mesothoracic ganglion appears to be a site for various types of integration of the afferent signal. This takes the form of discarding certain parameters of the stimulus and retaining others. Ultrasonic frequency is discarded in the neural signal generated by the A cells. Their signal is in turn transformed by pulse-marker (PM) neurons so that pulse intensity and duration are discarded, leaving only pulse repetition rate in the PM response. A, R, and PM units respond unilaterally, that is, only to the stimulation of one ear. Other neural units summate signals from right and left ears, while still others respond with a tonic discharge unphased by the repetition rate of the stimulus. Inhibitory phenomena have not yet been encountered.