Two types of electroreceptive lateral lemniscal neurons of the lateral line lobe of the catfishIctalurus nebulosus; connections from the lateral line nerve and steady-state frequency response characteristics

Abstract

1. The lateral line lobe of the bullhead catfishIctalurus nebulosus was found to contain two types of higher-order neurons which can be activated antidromically from the mesencephalon and which respond to weak (2.5×10−5 μa/mm2) current stimulation of the lateral line electroreceptors. 2. The response of these neurons to weak current stimulation is reciprocal, one type responding with an increase in spike frequency to cathodal receptor stimulation, the other with an increase in response to an anodal current. One type was found to receive a monosynaptic excitation from the ipsilateral lateral line nerve, whereas the second type was found to receive a disynaptic inhibition. This difference in synaptic inputs probably accounts for the difference in the response of the two types to receptor stimulation of a particular polarity. 3. The receptive fields of both types were found to be distributed over the entire body and head with greatest density on the head and on the caudal peduncle. 4. The steady state frequency response of these neurons indicates that both types have characteristic (best response) frequencies which are lower than those of the electroreceptive primary afferents. 5. The spike activity of lemniscal neurons was strongly modulated by the respiratory potentials generated by a small fish 7 cm away. A Fourier analysis of the potentials recorded from small fish and from one species of aquatic arthropod indicates that there is a good match between their frequency spectra and the steady-state bandpass characteristics of the lemniscal units.