Electroreceptive representation and its dynamics in the cerebellum of the catfish,Ictalurus nebulosus (Ictaluridae, Siluriformes)

Abstract

1. Extracellular unit spike responses have been recorded in the cerebellum of catfish during electroreceptive as well as several other kinds of stimulation. 2. Electric pulses in the water adequate to stimulate ampullary receptors activate some large cells of the cortex of the lobus caudalis pars lateralis and a few in the adjacent eminentia granularis and corpus cerebelli. These cells generally do not respond to vibration, sound, water movement, fin bending, or light, i.e., most are unimodal electrosense units, but some are moderately sensitive to drops of water falling on the water surface up to 15 cm away. 3. Some units have irregular background activity, others are regular and some are silent until stimulated. Presumably some are Purkinje cells and others are not; the latter may in part be equivalent to cells of cerebellar deep nuclei in amniotes. 4. Response characteristics are described, including the following: dipole axis and polarity discrimination, phasic, tonic and phasic-tonic units, and the effects of duration, intensity, pulse stimulus repetition, sine wave stimulus frequency, and movement of the stimulating current source on the response magnitude, form and latency. Low threshold cerebellar units respond to 0.4 μV/cm. In a small sample of units tested, tuning curves showed best frequencies between 2 and 8 Hz. Frequency following may extend only up to 8–15 Hz sine waves. The ON response can markedly alter the following of the first few cycles. Some units are responsive to a moving current source that is subthreshold when stationary. 5. Most units have a receptive field to a small (3 mm) dipole held close to the skin, of < 1/4 of the body length. The center of many receptive fields is excitatory when the current direction is inward; in other receptive fields, when it is outward. Many units have receptive fields on the ispsi-lateral side, some on both left and right sides. Of the latter some are symmetrical, others are asymmetrical either in position or in excitatory polarity. 6. Within the lobus caudalis pars lateralis, receptive fields classified as mainly on the head, on the body or on the tail are systematically distributed in a topographic map. The distribution of best dipole orientations appears random.