Figure 4.—

Schematic of the GF neural circuit in Drosophila and transmission defects in the GF-TTM pathway of cpo adults. (A) The GF is a large interneuron that drives the escape jump in Drosophila in response to visual stimuli. The GF has a cell body in the brain and projects its axon down the cervical connective to the thoracic ganglion where it makes two major synaptic connections. One connection is with the tergotrochanteral motoneuron (TTMn), which in turn drives the tergotrochanteral muscle (TTM) that enables jumping. The second connection is with the peripherally synapsing interneuron (PSI), which contacts the five dorsal longitudinal motoneurons (DLMn's). These five DLMn's drive the six dorsal longitudinal muscle (DLM) fibers that are responsible for wing depression during flight. Each DLMn innervates a single DLM fiber except for DLMn5, which drives DLM fibers 5 and 6. For simplicity, DLMn's 1–3 and DLM fibers 1–3 are not included. Transmission through the monosynaptic GF-TTM pathway is fast, eliciting a response in the TTM in ∼0.8 msec. The disynaptic GF-DLM pathway produces a DLM response ∼1.2 msec following brain stimulation. In addition to producing responses with very short latencies, both pathways can follow stimuli at high frequencies. The GF-TTM pathway can follow stimulation exceeding 200 Hz, while the GF-DLM pathway exhibits following frequencies >100 Hz (Ikeda et al. 1980; Tanouye and Wyman 1980). The small “e” designations in the figure denote electrical synapses. All other synapses are chemical. The GF-TTMn synapse has been shown to have both electrical and chemical components (Blagburn et al. 1999). (B) Response to brain stimulation in a control fly (cpo^EG1/+). The DLM and TTM always fire together when the brain is stimulated at voltages above the GF threshold and always differ in latency by ∼0.4 msec (Tanouye and Wyman 1980) (calibrations, 10 mV and 1 msec). (C) Response to brain stimulation in a cpo fly (cpo^EG1/cpo^cp1). Shown are responses to three consecutive 0.2-sec pulses given at 0.5 Hz. The DLM latency never deviates while the TTM latency progressively increases before the muscle eventually fails. This indicates that the DLM response is capable of following at this frequency (0.5 Hz), but the TTM response is not (calibrations, 10 mV and 1 msec).