| Toward understanding a possible gyroscopic function for the antennae in the hawk moth, Manduca sexta, we performed detailed neurophysiological analyses of scolopidial neurons (SNs) in the antennal Johnston's organ. We recorded intracellularly from the axons of 96 SNs while stimulating the flagellum-pedicet joint with step, sinusoidal, frequency sweep and amplitude sweep stimuli. We iontophoretically filled 43 of these SNs with fluorescent dyes. All of them were identified as scolopidial neurons from the Johnston's organ and projected in the antennal mechanosensory and motor center (AMMC). SNs responded with short latencies to step stimuli in the order of 3--5 msec with phaso-tonic firing patterns. Most neurons were phase locked to sinusoidal motions and frequency sweep stimuli in the 0.1--100 Hz frequency range with high vector strength (>0.9). Some SNs had a linear relationship between their firing frequency and the stimulus frequency at double wingbeat frequencies (∼50 Hz). Finally, they were sensitive to small amplitudes <0.05°.;We investigated the encoding properties of SNs using band limited Gaussian noise (BLGN) stimulus and applying first and second-order reverse correlation methods. Scolopidial neurons have 1-dimensional and 2-dimensional feature selectivity tuned to double wing beat frequency. Some SNs were unimodal while other responded with bimodal 2-dimensional functions. From the BLGN data, we generated a cascade model of SNs and computed the information captured during mechanical transduction. The 2-dimensional model was more successful, encoding ∼1 bit/spike. Finally, we compared the information captured by SN spike trains during stimulation with BLGN to natural antennal motion. In both cases SNs encoded auround 100 bits/sec, 3 times greater than the information encoding rate of visual neurons. These results show that SNs can serve as the antennal encoders of gyroscopic forces with a high rate of information transmission, wide frequency range, and high sensitivity at key behavioral frequencies and motions.;Finally, we showed that rapid antennal information is conveyed down the neck connectives to flight motor centers with short latency (7--20 msec), well within the duration of a single wing stroke (∼40 ms). |
