Figure 2.

Comparing principal features of neuronal organisation and growth in Drosophila (larva) and vertebrates (human). (a) Saggital section (one body half; dotted line is midline) through the larval brain and ventral nerve cord (compare Figure 1a,b). (b) Saggital section through the adult human brain and one half of the spinal cord. Symbols are explained in the box below. Whereas axons of unipolar inter- and motorneurons in Drosophila have to grow into the synaptic area where they form dendrites, comparable neurons in vertebrates are multipolar and locate themselves in the synaptic area. All Drosophila motorneurons locate their dendrites in the dorsal neuropile, regardless of their soma position (see 'i' versus 'ii') [236]. Vice versa, sensory somata in Drosophila are located next to their dendrites, whereas cell bodies of most sensory neurons in vertebrates are grouped together in the dorsal root ganglia. Sensory output (dark grey) and motor input areas (bright grey) are inverted in both phyla, which might be explained through a general dorsoventral body axis inversion between vertebrates and arthropods [328], that is, not represent an organisational difference between their CNS. Ascending/descending axons in Drosophila are non-myelinated and project through the synaptic area (compare neurons 2 and 13 in Figure 1) where they take on characteristic positions [236,329]. In vertebrates, ascending/descending axons are myelinated and positioned outside the synaptic area, grouping into characteristic tracts in defined positions of the white matter; examples named here: fasciculus cuneatus (1), tractus corticospinalis lateralis (2; pyramidal tract; only descending), and tractus spinothalamicus lateralis (3).