Figure 1. The peripheral sensory receptors of some mormyrid species fire spikes, whereas the receptors of other species produce spontaneously oscillating potentials.
(A) An extracellular recording of spontaneous spikes from a receptor of Petrocephalus microphthalmus. (B) Box plots of spontaneous interspike intervals in the receptors of three species. (C) An extracellular recording of spontaneously oscillating receptor potentials in Petrocephalus tenuicauda. (D) Box plots of spontaneous oscillation periods in the receptors of two species. In P. tenuicauda, the amplitude of oscillatory activity varied from 0.04 to 6.1 mV. We only measured the frequency of spontaneous activity if the oscillation amplitude was at least 2.5 times baseline noise (≥0.1 mV). 49 of 69 (71%) receptors met these criteria. The receptors of Petrocephalus soudanensis had spontaneous oscillation amplitudes of 0.09, 0.4, and 1.9 mV and frequencies of 2.1, 2.6, and 2.5 kHz, respectively.