The subunit arrangement of elements in the protocerebral bridge (PB), the fan-shaped body (FB) and the ellipsoid body (EB).

The PB comprises discrete dendritic clusters that intersect arrangements of parallel fibers. The parallel fibers (ASC) ascend from the protocerebrum where they receive information from ascending plurisegmental neurons relaying data from segmented motor neuropils. Subunits in the bridge (1-8, left, right) send axons to the FB and EB. There, axons terminate in columnar subunits such that the linear arrangement of elements in the left half of the bridge overlap the linear arrangement of elements in the right half. Thus, element 1 left meets axons from 8 right, 2 left from 7 right etc. This organization may compare the timing of signals carried by ASC fibers representing left and right leg motors and thus serve to detect left-right asymmetry. Mutants of Drosophila that show defects in the protocerebral bridge result in the inability of a fly to properly adjust leg movements during turns (Wannek and Strauss 1997; Strauss et al., 1995 [J. Neurogenetics 8:125-155)]. The FB and EB also receive fan-shaped terminals of afferent neurons (AFF) that relay information from multimodal afferents (MM) to the central complex. It is proposed that these afferents serve to relay the weight of sensory signals from the left and right sides of sensory fields and thereby contribute to the strengths of outputs from the central complex carried by efferent neurons (EFF). These end in the lateral accessory lobe (LAL) where they impinge on collaterals of descending neurons (DN) that extend to the leg motor.