The cercus-to-giant interneuron system in the bushcricketTettigonia cantans: Morphology and response to low-frequency sound

Summary

• 1. The response characteristics of the cercus-to-giant interneuron system in the bushcricketTettigonia cantans to low-frequency airborne sound have been studied using intracellular or quasi-intracellular recording; the morphology of the giant interneurons has been investigated using Lucifer Yellow filling and axonal backfill with cobalt.

• 2. The giant interneurons excited by the cereal sensory afferents can be classified into four types based on their time courses of firing (Fig. 1): ‘quasi’-tonic, tonic burst, phasic and ‘sustained’ burst type, respectively.

• 3. The giant interneurons in the bushcricket are maximally sensitive to air particle displacement at 300 Hz and the lowest threshold is less than 0.1 μm (Fig. 2).

• 4. The intensity characteristics for three tpyes of the giant interneurons (Type I, II and IV) indicate that spike counts for these interneurons increase with increasing stimulus amplitude (Fig. 3). The phasic type of the interneuron (Type III) fired more frequently only one spike to each sound stimulus.

• 5. The cercus-to-giant interneuron system in the bushcricket is composed of seven bilateral pairs of the interneurons, each of which possesses a cell body, neurite and dendrites within the last abdominal ganglion and also has a giant axon which ascends the ventral nerve cord. Compared with other neurons, the axons of these neurons are giant and have diameters of 8–15 μm for GI1 and GI4 to 7, and about 5 μm for GI2 and GI3. The position of the cell body and the locations and orientations of the major processes are characteristic for each giant interneuron (Fig. 5).

• 6. The projections of single cereal afferents are made visible with Lucifer Yellow filling, which restricted within the ipsilateral half of the terminal ganglion. There is a great degree of the overlap between the cereal projections and the dendritic fields of the GIs (Fig. 4).

• 7. The branching details of the investigated giant interneurons within the thoracic ganglia and the brain are disclosed by Lucifer Yellow filling (Figs. 6, 7). The neurons Type I and III send two main collaterals medially directed within the prothoracic ganglion and the neuron Type IV sends three collaterals close to the midline of this ganglion. Within the metathoracic ganglion the neuron Type II sends three medially directed collaterals. At the ends of many branching terminals the buttonlike enlargements with stronger fluorescence can be seen and probably these terminals are the presynaptic endings. The projections of the axons of the GIs also can be found within the brain. The axons run through the tritocerebrum and deutocerebrum and terminate mainly in the mediolateral region of the lateral posterior protocerebrum.