Different neural pathways coordinate Drosophila flight initiations evoked by visual and olfactory stimuli.

To determine the role played by the giant fiber interneurons (GFs) in coordinating the jumping stages of visually elicited and olfactory-induced fight initiation we have recorded extracellularly from the cervical connective nerve during flight initiation. A spike is recorded from the cervical connective upon brain stimulation that has the same threshold as does activation of the tergotrochanteral muscle (TTM) and dorsal longitudinal muscles (DLMs). A consistent time interval occurs between the spike and activation of the TTM. Thus, the spike probably results from activity in the GFs. The time intervals between the spike and activation of the TTM during GF stimulation and visually elicited flight initiation are similar. These results suggest that the GFs coordinate the activation of the TTM and DLMs during the jumping stage of visually elicited flight initiation. A spike is also recorded from the cervical connective during olfactory-induced flight initiations, but its shape and the time interval between it and activation of the TTM is different from that observed during GF stimulation. Although some olfactory-induced flight initiations exhibit a pattern of muscle activation, olfactory-induced flight initiations exhibit a pattern of muscle activation indistinguishable from that evoked by GF stimulation, our results indicate that regardless of the pattern of muscle activation, olfactory-induced flight initiations are not coordinated by the GF circuit. The sterotypic sequence and timing of activation of TTM and DLMs characteristic of the GF pathway can, therefore, be evoked by neurons other than those constituting the GF pathway.