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. 2018 Aug 2;7:e38554. doi: 10.7554/eLife.38554

Figure 9. Model describing the MDN-mediated backward crawling.

Figure 9.

(A) MDN neurons are present in larval stages where they promote backward peristaltic crawling via abdominal premotor and motor neurons; MDNs subsequently persist into the adult fly where they promote backward walking of the six-limbed adult fly, using a different pool of thoracic motor neurons. Solid arrows represent direct excitatory connectivity; solid T-bars represent direct inhibitory connectivity; dashed arrows represent known (larva) or predicted (adult) polysynaptic connectivity. (B,C) Model. (B) During forward locomotion the MDNs, Pair1 and A18b are silent; an unknown neuron (?) may initiate forward locomotion. (C) To initiate backward locomotion, MDNs activate Pair1 descending neuron which inhibits the forward-active A27h premotor neuron in segments A5-A7 to halt forward locomotion. MDNs also activate A18b in A1 and/or in more anterior segments which triggers a backward motor wave.