Neuroarchitecture of the central complex of the desert locust: Tangential neurons

Abstract

The central complex (CX) comprises a group of midline neuropils in the insect brain, consisting of the protocerebral bridge (PB), the upper (CBU) and lower division (CBL) of the central body and a pair of globular noduli. It receives prominent input from the visual system and plays a major role in spatial orientation of the animals. Vertical slices and horizontal layers of the CX are formed by columnar, tangential, and pontine neurons.While pontine and columnar neurons have been analyzed in detail, especially in the fruit fly and desert locust, understanding of the organization of tangential cells is still rudimentary. As a basis for future functional studies, we have studied the morphologies of tangential neurons of the CX of the desert locust Schistocerca gregaria. Intracellular dye injections revealed 43 different types of tangential neuron, 8 of the PB, 5 of the CBL, 24 of the CBU, 2 of the noduli, and 4 innervating multiple substructures. Cell bodies of these neurons were located in 11 different clusters in the cell body rind. Judging from the presence of fine versus beaded terminals, the vast majority of these neurons provide input into the CX, especially from the lateral complex (LX), the superior protocerebrum, the posterior slope, and other surrounding brain areas, but not directly from the mushroom bodies. Connections are largely subunit-and partly layer-specific. No direct connections were found between the CBU and the CBL. Instead, both subdivisions are connected in parallel with the PB and distinct layers of the noduli.

A visual pathway for skylight polarization processing inDrosophila

“…The question remains open as to whether a sun position system and skylight polarization system are independent in the bulb. Unlike the TuLAL neurons in locusts (homologous to TuBu), where there is convergence on the dendrites of postsynaptic neurons (Hadeln et al, 2020;Pegel et al, 2018;Pfeiffer et al, 2005), TuBu neurons appear to form one-to-one contact with individual ring neurons (Omoto et al, 2017). Hence, we posit that the site of integration of celestial cues is not at the synapse between TuBu and ring neurons.…”

The head direction circuit of two insect species

“…These presynaptic terminal domains are separated by seven glomeruli ( Figure 2A and Figure 2D ). In Drosophila , the Delta7 neurons have postsynaptic terminals across all remaining glomeruli of the PB ( Wolff and Rubin, 2018 ; Franconville et al, 2018 ) while in locusts the Delta7 neurons have postsynaptic terminal domains only in specific glomeruli ( Heinze and Homberg, 2007 ; Beetz et al, 2015 ; Hadeln et al, 2020 ).…”

“…However, in principle, there could be any degree of uniformity of the inhibition between these two extremes. So far, the locust inhibition has been modelled as a summation of two Gaussian functions that approximates the synaptic density across the PB glomeruli, as derived from estimates of dendritic density along the PB in dye-filled Delta7 neurons (Heinze and Homberg, 2007;Beetz et al, 2015;von Hadeln et al, 2020).…”

Sequence heterochrony led to a gain of functionality in an immature stage of the central complex: A fly–beetle insight

“…Decussations occur at different positions subdividing the future central body into columns (B iii ). PB omitted for simplicity; based on [15,22,32,33]. (C) The Drosophila (C i ) and Tribolium (C ii ) brains differ in their orientation within the head (lateral views).…”

“…The CX consists mainly of columnar and tangential neurons [16,19]. Tangential neurons connect other brain areas with one CX neuropil ( Fig 1A) [16,[21][22][23]. The focus of this work lies on columnar neurons, which connect the different neuropils of the CX with each other by projecting as 4 prominent tracts (called the WXYZ tracts) from the PB into FB, EB, NO, and other brain structures ( Fig 1A) [15,19,[24][25][26][27].…”