Effect of electric organ discharge on ampullary receptors in a mormyrid

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(1) Afferents from ampullary receptors were shown to be strongly affected by the electric organ discharge (EOD) in the mormyrid Gnathonemus petersii. (2) Over a broad range of resistivities (4–60 kΩcm) the response to the EOD was similar to the response to a brief (50–200 μsec) outside positive pulse, i.e. an intitial acceleration of the discharge rate followed by a deceleration. (3) Brief biphasic positive-negative or negative-positive pulses where both phases were of equal amplitude and duration had no effect on ampullary afferents. Each phase had an effect, however, when given in isolation. These results suggest that a DC component in the EOD may cause the response in the ampullary afferent. (4) The response of ampullary afferents decreased sharply as resistivity was lowered below 10 kΩcm. Responses to the EOD in mormyromast afferents also decreased. These effects were probably due to loading of the EOD at low resistivities and to a more rapid spatial decay of EOD voltage. (5) Responses of ampullary afferents to the EOD were much less affected by external non-conducting objects than were the responses of mormyromasts. These observations plus other considerations indicate that mormyromasts must still be held to play the major role in active electrolocation. Unresponsiveness of ampullary afferents to the EOD can not be taken as a reason for this, however.