The afferent auditory pathway in the ventral cord oflocusta migratoria (Acrididae)

Abstract

The afferent auditory pathway in the region of the ventral cord ofLocusta migratoria has been studied electrophysiologically. Various recording, stimulating and surgical techniques have been used to infer the circuitry underlying the response patterns of the ventral-cord neurons ascending to the supraesophageal ganglion. The stimuli used were artificial sounds (white noise and pure tones varying in frequency, intensity, duration, source direction and repetition rate). The responses of the auditory ventralcord neurons over the range of frequencies and intensities are described. Presentation of two sound stimuli occurring simultaneously or displaced in time permits the demonstration of inhibitory and subthreshold facilitatory influences which, with extracellular recording, cannot be observed directlyvia changes in membrane potential (Figs. 3 and 4). On the basis of comparisons of responses before and after inactivation of a tympanal organ additional inferences are made concerning the relative contributions of the two tympanal organs to the formation of the response-patterns (Figs. 6, 11, 16, 21, 25). The geometry, location and course of certain auditory ventral-cord neurons have been revealed by a combined recording and staining technique (Fig. 7); transection of connectives has made it possible to determine the origin of certain response-patterns and whether these change at various stations of the ventral cord as a result of corresponding synaptic input. The results of such experiments, together with the knowledge of the course and location of endings of the tympanal receptor fibers in the ventral cord, have been used to construct sufficient connectivity diagrams for the ascending ventral-cord neurons (Figs. 8, 12, 17, 22, 26). The following general properties have been revealed: 1. Between receptor cells and ascending auditory ventral-cord neurons, as a rule, interneurons are interposed; this accounts for the observation that the postsynaptic structures of the ventral-cord neurons are for the most part situated outside the auditory neuropile. 2. Six basic connectivity patterns underlie the responses of the 14 ventral-cord neurons described; that is, the same response patterns are produced in sets of two or three ventral-cord neurons, owing to synaptic connectivities involving the same interneurons. The distinctions between the response patterns of the 14 neurons are interpretable as stemming from the addition of another interneuron to, or by elimination of a facilitatory or inhibitory connection from, the basic connectivity patterns.