Definition: A lamina monopolar neuron with short collaterals that project radially from the main projection in the lamina and that arborizes in medulla layer M2. In the lamina it receives input from photoreceptors R1-R6, lamina monopolar neuron L4 (collaterals from the same and adjacent columns), lamina intrinsic (amacrine) cells, centrifugal neurons C2 and C3 and lamina wide-field neurons Lawf. It forms postsynaptic connections in the lamina with photoreceptors R1 and R2 and lamina monopolar cells L1, L4 and L5 (Meinertzhagen and O'Neil, 1991; Rivera-Alba et al., 2011). It has both pre- and postsynaptic connections in medulla layer M2, with synaptic input coming from centrifugal neurons C2 and C3 (Takemura et al., 2008) and output to transmedullary neurons Tm1 and Tm2. It is a cholinergic neuron (Takemura et al., 2011) and is electrically synapsed to lamina monopolar neuron L1 (Joesch et al., 2010).
Comment: Cell connectivity in the lamina was determined by analysis of semi-automated EM reconstruction of 749 serial sections of 21 complete lamina cartridges. The number of synaptic connections for L2 (presynaptic/postsynaptic) in a single lamina cartridge with each cell type was the following: R1 (1/46), R2 (1/45), R3 (-/39), R4 (-/41), R5 (-/39), R6 (-/47), L1 (3/-), L4 (10/7), L5 (1/-), Lai (-/1), C2 (-/3), C3 (-/5) and Lwf (-/7) (Rivera-Alba et al., 2011). Connectivity in the medulla was extrapolated from serial EM of 3 columns from a single sample. Note that for many of the output synapses in M2, Tm1 and Tm2 share the same sites (Takemura et al., 2008; Takemura et al., 2011). Electrical connectivity was shown by dye labelling (Joesch et al., 2010). The neurotransmitter was assessed by single cell RT-PCR measurement of choline acetyltransferase (FBgn0000303) expression (Takemura et al., 2011).