Primary auditory neurons in crickets: Physiology and central projections

Summary1.The tibial tympanal organs ofGryllus campestris L. andGryllus bimaculatus DeGeer were stimulated with sound signals simulating the natural calling song, produced by sinusoidal amplitude envelope, using carrier frequencies (CF) in the range of 2–20 kHz at various intensities.2.Glass microelectrodes (filled with 3 M potassium acetate, or with 1 M cobalt nitrate for marking the fibers) were used to record extracellularly or quasi intracellularly the spike activity of single sensory fibers in the leg nerve (Fig. 1) responding to acoustic stimuli and to stain their central projections within the prothoracic ganglion (Figs. 6–9).3.The temporal pattern of the sound stimulus —syllable and verse structure — is reflected in the discharge pattern of the fibers studied (Figs. 1, 6, 7).4.Five types of fibers tuned to specific CFs were found (Fig. 2). These had peak sensitivities (a) at 4–5 kHz (CF of the species calling song), (b) at both 4–5 and 10–12 kHz, (c) below 2 kHZ, (d) at 12 kHz, and (e) at 17 kHz (CF of the species courtship song).5.The tuning is also evident in the responsiveness (spikes/syllable) to stimuli of different frequencies at suprathreshold intensities (Fig. 3).6.In the range of their best frequencies (BF) the fibers tuned to 4–5 and to 10–12 kHz are similar in intensity characteristics and in relationship of latencies to sound intensity, with a linear relationship between responsiveness and latency (Fig. 4). Within a single calling-song verse, the spike response decreases, and the latency increases, from the first to the fourth syllable (Fig. 5). Even at the highest intensities the discharge rate nerver exceeds 350 Hz.7.The central arborization and projection areas of all the marked fibers are restricted to the ipsilateral half of the prothoracic ganglion. Assuming that fibers marked include those recorded physiologically, it appears that those tuned to 4–5 kHz terminate exclusively in the “crescent-shaped” neuropile, localized and termed the “auditory neuropile” (Figs. 6, 7). Fibers with a peak sensitivity below 2 kHz differ distinctly from the 4–5 kHz group in that they have additional arborizations outside the auditory neuropile (Fig. 8). As yet no unequivocal data are available regarding the spatial arrangement of the fibers with two sensitivity peaks (both 4–5 and 10–12 kHz) and those tuned to 12 kHz or 17 kHz (Fig. 9).