Effect of sound signals on spontaneous activity in grasshopper interneurons (Orthoptera, Tettigonidae)

According to one of hypotheses proposed for explaining mechanisms of sound signal recognition in insects, their CNS contains a group of rhythmically active neurons that function as a reference standard for comparison with perceived acoustic information. To check this hypothesis, the spontaneous neuronal activity and its changes in perception of conspecific and heterospecific signals (CS and HS) were analyzed in the CNS of two sympatric grasshopper species Tettigonia cantans and Metrioptera roeselii. The activity of individual neurons was assayed in fixed and freely moving insects. The results of the experiment have shown that in the thoracic part of the CNS there is a group of rhythmically active neurons that do not directly respond to sound signal but readjust their impulses under effect of its action. On presentation of CS the following reactions were observed: attenuation or enhancement of impulses; stabilization or destabilization of rhythm; regular increase or decrease in interspike intervals; phasic readjustments leading to synchronization of impulses with sound stimuli (pulses). No similar alterations were usually produced by HS; still, if they did appear, they were less pronounced or of opposite direction. These data indicate that the grasshopper auditory system affects markedly the rhythmically active neurons, their reaction depending considerably on temporal organization of sound signals. Selectivity of these reactions allows us to suggest that the rhythmically active neurons are directly related to the neuronal networks providing the sound signal recognition.[PUBLICATION ABSTRACT]