SENSITIVITY AND RESPONSE DYNAMICS OF ELASMOBRANCH ELECTROSENSORY PRIMARY AFFERENT NEURONS TO NEAR-THRESHOLD FIELDS

Elasmobranch fishes localize weak electric sources at field intensitie s of < 5 eta V cm(-1), but the response dynamics of electrosensory pri mary afferent neurons to near threshold stimuli in situ are not well c haracterized. Electrosensory primary afferents in the round stingray, Urolophus halleri, have a relatively high discharge rate, a regular di scharge pattern and entrain to l-Hz sinusoidal peak electric field gra dients of less than or equal to 20 eta V cm(-1). Peak neural discharge for units increases as a non-linear function of stimulus intensity, a nd unit sensitivity (gain) decreases as stimulus intensity increases. Average peak rate-intensity encoding is commonly lost when peak spike rate approximately doubles that of resting, and for many units occurs at intensities <1 mu V cm(-1). Best neural sensitivity for nearly all units is at 1-2 Hz with a low-frequency slope of 8 dB/decade and a hig h-frequency slope of -23 dB/decade. The response characteristics of st ingray electrosensory primary afferents indicate sensory adaptations f or detection of extremely weak phasic fields near 1-2. Hz. We argue th at these properties reflect evolutionary adaptations in elasmobranch f ishes to enhance detection of prey, communication and social interacti ons, and possibly electric-mediated geomagnetic orientation.