Latency of gustatory neural impulses initiated in frog tongue

Abstract

Latencies of gustatory neural impulses evoked by stimulation of the bullfrog tongue with the 4 basic taste substances (NaCl, acetic acid, quinine-HCl (Q-HCl), sucrose), CaCl₂ and water were studied by recording antidromic impulses conducted to the fungiform papillae. Mean latencies of the impulses ranged from 58 to 107 ms when very strong stimuli, such as 2 M NaCl, 0.1 M acetic acid, 0.1 M Q-HCl, 2 M sucrose and 1 M CaCl₂, were applied. Mean latency in response to water was 2.41 s. The time required for arrival of an applied taste stimulus on the taste receptor membrane was a mean of 20.1 ms. The time required for antidromic conduction from the impulse initiation site to the recording site was a mean of 2.4 ms. Electrical stimulation of the fungiform papilla with a strong intensity produced the impulse with a long and fluctuating latency. The mean minimum latency of the fluctuating impulse, from which the conduction time was substracted, was 5.3 ms. Mechanical destruction of the taste disk situated at the top of the fungiform papilla resulted in a disappearance of the fluctuating impulse, suggesting that this was initiated synaptically via a depolarization of taste cells by electrical current. The minimum 5.3-ms latency was likely to be the time required from the onset of taste cell depolarization to the initiation of an impulse at the first node of Ranvier of myelinated gustatory fiber. These results indicate that the latencies of 58–107 ms by strong taste stimulation were composed of the 30- to 79-ms latency of taste cell receptor potential and the remaining 28 ms latency, which was the sum of the time of the stimulant diffusion, the time from taste cell depolarization to the first impulse and the time of impulse conduction.