How neuronal processes get positioned in the neuropile
The position of neural projections in the neuropile essentially determines where potential synapses can be formed by these cells. Therefore, decisions during the neuronal growth phase have essential impact on the later established neuronal circuitry. Neurons in Drosophila (like other insects) show reproducible spatial arrangements of their projections (see >>>). A variety of mechanisms have been described which guide neuronal outgrowth within the different dimensions of the neuropile.
Images show ventral nerve cords of Drosophila in dorsolateral (A; compare >>>; arrow points anterior), horizontal (B) and frontal view (C; compare >>>). The projections of the red and blue neurons illustrate typical positions in the neuropile: a, anterior commissure; co, contralateral (= on the opposite side of cell body); dl, dorso-lateral; dm, dorso-medial; ip, ipsilateral (= on the same side as cell body); p, posterior commissure; vm, ventro-medial. Mechanisms regulating the positioning in the four different dimensions of the neuropile are addressed below:
ipsilateral versus contralateral positioning
lateral versus median positioning
positioning in anterior versus posterior commissures
dorsal versus ventral positioning