Neuroarchitecture of the central complex of the desert locust: Intrinsic and columnar neurons

The central complex is a group of neuropils in the center of the insect brain. It consists of four major subunits: the upper and lower divisions of the central body (CBU, CBL), the protocerebral bridge (PB), and the paired noduli. A distinctive feature of the central complex is a modular architecture characterized by rows of 16 columns, intersected in the central body by stacks of layers. Evidence from locusts suggests that the central complex plays a major role in sky compass orientation. To understand signal processing in this brain area further, we have analyzed the morphologies of columnar neurons of the central complex of the locust Schistocerca gregaria. Intracellular dye fills revealed 21 types of columnar neurons that connect columns of different subunits, three types of pontine neurons linking pairs of columns within the CBU, and one amacrine cell. Most neurons appeared to be part of isomorphic sets with cell type-specific heterolateral projection patterns. Evaluation of arborization areas and neuron polarity suggests that these neurons are either intrinsic to the central complex or provide output to the lateral accessory lobes (LALs) or anterior lip region. No direct connections were found between the CBU and CBL. Instead, neurons of either subdivision were connected with the PB, but projected to non-overlapping regions in the LALs and to different layers of the noduli. This study provides novel insights into the functional organization of the central complex, especially with respect to its likely role in right-left signal matching and decision making.