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Comparative Physiology - Electroreception -
BioElectricity
After Lissmann (1958) had discovered that weakly electric fish can sense disturbances in the field generated by their electric organ, Murray (1962) found that the ampullae of Lorenzini in sharks were sensitive to electrical potential differences in the microvolt range. Because sharks have no electric organ, the question arose what purpose the electrosensory organs should serve? Muscle potentials of other animals were seen as a possible biological adequate stimuli. And although the Laboratory for Comparative Physiology was not involved directly in research on the discharges of weakly electric fish, some specimes of Gnathonemus were always kept for intruductory demonstrations of electroreception.
When Rob C. Peters, as a student of Kalmijn, recorded the activity of a nerve innervating the ampullae of Lorenzini in dogfish, he captured the dogfish's response to the potentials of a plaice buryng itself into the sandy bottom. Remember that muscle potentials were putative stimuli to the ampullae of Lorenzini! Although Dijkgraaf and Kalmijn (1966) accepted Peters' recordings as pure responses to electrical stimuli, Kalmijn had his doubts about this particular recording. He suspected the signal to be "polluted" with responses of mechanosensitive lateral line fibers. In retrospect, most likely the plaice's DC field caused the response.
H.W. Lissmann suggested that the electrosensitive ampullae of Lorenzini in elasmobranchs might serve to detect muscle potentials generated by moving fish. However, the 'muscle potentials' that were recorded from fish bodies were mainly respiratory potentials, which span a lower frequency range than muscle potentials. It was the Nobel laureate Sir John Eccles who during a visit (1969) to Dijkgraaf's lab suggested that fairly strong DC voltages could be measured in the throat and oral cavity of fish. This fact threw a totally new light on the matter. From then on, attention was focussed not on muscle potentials, but on DC voltages. The omnipresent bioelectric DC potentials proved indeed measurable, and were strong enough to serve as a stimulus for the ampullary organs. See Kalmijn 1972.
All aquatic organisms generate weak electrical fields. Usually, a DC voltage with an AC voltage superimposed, associated with the moving parts of the body. Such electrical fields most likely accompany the processes involved in maintaining cellular homeostasis, the equilibrium between the cell interior and the environment. When an electrosensory fish passes a DC voltage source, the electrosensory organs in the skin detect a changing potential, or AC stimulus. Not only aquatic animals produce electrical fields; all kinds of chemical and physical processes in the soil generate electrical fields. If such fields would be stable enough, electrosensitive fish could use them for orientation. Apparently, there exists an electrical world detectable by electrosensitive fish, with features useful for survival.
Inspired by a publication of Anton Roth, Franklin Bretschneider and Rob C. Peters developed a device with rotating electrodes in order to measure DC fields in water. The measuring electrodes span a distance of 30 cm, about the size of an adult catfish. The measurement of DC voltages in water is rather problematic due to uncontrollable electrode potentials. By spinning a pair of electrodes, any DC field is converted into an alternating voltage, which is easier to filter and record .
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