Neurophysiology of Electric Fishing

An explanation of the neurophysiological mechanisms leading to the galvanotaxy of fish with direct current electrofishing was developed by observing reactions of the fish to different potential gradients and by sectioning or otherwise interfering with the nervous system at various points. With fish facing the positive electrode the first galvanotaxic response toward the positive pole (at 14 volts under the experimental conditions) is an inhibited swimming through effect on the long motor axons. Next (at 18 volts) there is intense, coordinated swimming resulting from intense negative charges on the sensory somas introducing a brain reflex, overcoming the inhibition of the motor axons. Third (at 40 volts) the fish becomes limp because the brain reflex cannot break through the increased inhibition of the motor somas. Fourth (90-100 volts) pseudoforced-swimming in which equilibrium is lost occurs through reflexes in the spinal nerves. Finally (at 150 volts) tetany is brought on by direct excitation of the muscle fibers. With fish facing the negative pole, the first galvanotaxic response (at 14 volts) is a tetanylike swimming toward the negative pole, due to facilitation of the central motor nerves, and only partially controlled by the brain. As the voltage increases, this brain control fails, and tetany appears through more excitation of (1) the motor somas in the brain, (2) the motor spinal nerves, and (3) the muscle fibers. When fish are placed across the electric field, the bodies curve toward the positive pole because the motor spinal nerves facing the negative pole are inhibited while those facing the positive pole are stimulated. Normal ecological behavior of the fish may significantly affect the response to electric current. Flatfishes, for example, may burrow or remain on the bottom resisting the swimming response until narcosis or tetany take over.