Electrosensitivity of canal and free neuromast organs in a gymnotid electric fish

The posterior lateral line nerve was studied by recording singel unit activity with glass miocropipette electrodes to examine the electrosensitivity of the canal and free neuromast organs. The fibers innervating these organs has no or irregular spontaneous discharges. For a 50 msec electric pulse applied through a pair of electrodes at the two ends of the fish, almost all fibers discharged impulses during the pulse, irrespective of its polarity. The electrosensitivity for a 50 msec electric pulse was 37 mV/cm on the average. When a 0.5–1.0 msec electric pulse was used, these fibers showed no responses to it, even if it was stronger than 40 mV/cm. Responses to organ discharges were not found at all. These organs do not seem to function as electroreceptors. The canal organ was excited by water ripple and by weak water currents allied to the trunk and can be considered a mechanoreceptor. The free neuromast of the trunk was mechanically excitable, but it did not respond to moderately strong water current, and its function remains unclear.