Physiological properties of an identified giant interneuron (GI4) as related to the escape and flight circuitries of the cockroach Periplaneta americana

The cockroach's giant interneurons (GIs) are known to mediate escape and flight behaviors. The ventral group of giant interneurons (vGIs) (numbered 1, 2 and 3) mediates ground escape and the dorsal group (dGIs) (numbered 5, 6 and 7) mediates flight. Giant interneuron number 4 is the only one of the seven abdominal identified gaint interneurons of the cockroach whose function in escape or flight has been hardly investigated. It shows some morphological characteristics of a vental giant interneuron while others are those of a dorsal giant interneuron. Likewise, in this paper, I show that interneuron 4 presents some intermediate physiological properties, some of which are characteristic of a vGI and others of a dGI. For instance: like the ventrals, interneuron 4 does not fire strongly at the onset of flight and does not initiate flight. Like the dorsals, interneuron 4 is not inhibited during flight and its wind sensitivity is unchanged during flight. However, during flight, interneuron 4 fires spontaneously, unlike the dorsals which fire rhythmically or the ventrals which are silent. To summarize then, the wind sensitive giant interneuron 4, which has so far been included in the ventral giant interneurons group, shows intermediate morphological and physiological properties of both the ventral and the dorsal giant interneurons.