Circuit Homology between Decussating Pathways in the Ciona Larval CNS and the Vertebrate Startle-Response Pathway  Kerrianne Ryan, Zhiyuan Lu, Ian A. Meinertzhagen  Current Biology  Volume 27, Issue 5, Pages 721-728 (March 2017) DOI: 10.1016/j.cub.2017.01.026 Copyright © 2017 Terms and Conditions

Figure 1 The Network of Descending Decussating Neurons (A) Outline of a larva depicting the notochord and the following rostrocaudal regions of the CNS: brain vesicle, neck, motor ganglion, and caudal nerve cord (CNC). (B, D, E, G, and H) Neurons of the ddN network illustrated as cartoons of their cell bodies. (B and C) Paired MG neurons illustrated by the relative positions and sizes of cell bodies (B) and complete reconstructions (C) from a left-lateral view, with a representative neuron of each pair labeled. These include descending decussating neurons (ddN), descending MG interneurons (MGIN1–MGIN3), and motoneurons MN1–MN5. Each pair of the latter has distinctive neuromuscular terminal end plates, MI1’s being frondose [18, 19], and alone reaching down to the medial muscle cells. (D) Enlargement from (A) illustrating cell body, axon pathways, and locations of axon terminals in and around the motor ganglion. (E and F) Dorsal view of paired MG neurons (in B and C) illustrated by relative positions and sizes of cell bodies (E) and complete reconstructions (F). Asterisks in (E) identify previously reported neurons [18]. (G and H) Relative sizes and positions of paired and unpaired neurons of the ddN network (G, dorsal; H, left lateral). Also shown are nuclei of ependymal cells in the neck (black) and the somata of two neck neurons (yellow). Em2: Eminens2; NeckNL: left neck neuron; AMGs: ascending MG neurons 1–7; PMGN1: posterior MG neuron; ACINs: ascending contralateral inhibitory neuron; MTN: midtail neuron; 116, 140, and 157: photoreceptor-AMG relay neurons. Scale bars, 100 μm (A and D) and 10 μm (B–H). See also Table S1 for summary of cells, abbreviations, and cell types and network diagram in Figure S1. Current Biology 2017 27, 721-728DOI: (10.1016/j.cub.2017.01.026) Copyright © 2017 Terms and Conditions

Figure 2 Mechanosensory Interneuron Inputs to Ciona ddNs: ddNL and ddNR (A) Network diagram of ddN synapses from mechanosensory network components: Eminens2 (Em2), ascending motor ganglion interneurons (AMG2–AMG7), and bipolar planate neurons of the tail (BTN1–BTN4). Line width (key) represents cumulative depth of synaptic contacts (μm); see the Experimental Procedures. Synapses between mechanosensory interneurons are omitted for clarity but are shown in Figure S2. (B) Gap junctions complementing synapses in (A). (C) Locations of pre- and postsynaptic sites and gap junctions on ddNs, dorsal view, anterior to the left. Each punctum represents a single contact, color coded by its participating cell (key). Scale bars, 10 μm. (D) Synaptic matrix for ddNL and ddNR and their synaptic partners. Synaptic pathways for presynaptic cells (rows) and their postsynaptic partners (columns) are color coded for their cumulative depth of synaptic contact (∼0.5–7 μm: key, bottom) with the value (μm) given in each intercept. See also Figure S2. Current Biology 2017 27, 721-728DOI: (10.1016/j.cub.2017.01.026) Copyright © 2017 Terms and Conditions

Figure 3 Synaptic Pathways from ddNs to Components of the Motor Pathways (A–E) EMs of synaptic contacts (scale bars, 500 nm). (A) Right ddN (ddNR) provides input (arrows) to left motoneuron, MN2L, and a left bipolar tail neuron BTN3. (B) Gap junction between ddNR and ascending contralateral inhibitory neuron ACIN1L (arrowhead). (C) Eminens2 (Em2) provides synaptic input to ddNL and ddNR (arrows) and forms gap junctions with MGIN1L and ddNR (arrowheads). (D) Ascending motor ganglion interneuron AMG4 provides synaptic input to ddNR (note postsynaptic density, asterisk). (E) Synaptic input from Em2 to ddNL at adjacent sites (arrows). (F and G) Pathway networks for ddNs. (F) Chemical synapses. (G) Gap junction network. In both networks, line width represents cumulative depth of contacts (μm); see the Experimental Procedures (key). Black lines indicate connections at which ddNs are presynaptic. Abbreviations: Em2 (Eminens2), AMG (ascending motor ganglion neuron), ddN (descending decussating neuron), NeckN (neck neuron), MGIN (motor ganglion interneuron), MN (motoneuron), ACIN (ascending contralateral inhibitory neuron), PMGN (posterior motor ganglion interneuron), MTN (midtail neuron), BTN (bipolar tail neuron); 116,140,157: photoreceptor-AMG relay neurons. See also proportions of inputs in Table S2 and network diagrams in Figure S3. Current Biology 2017 27, 721-728DOI: (10.1016/j.cub.2017.01.026) Copyright © 2017 Terms and Conditions

Figure 4 Comparison between the Mauthner Escape Response Pathway and Ciona’s Descending Decussating Pathway Cells, color coded by cell type, shown with lines illustrating gap junctions (electrical), and excitatory or inhibitory synapses, putatively so in Ciona. Anterior is up. (A) Mauthner cell pathway in the lamprey adapted from published reports. (B) Ciona ddN pathway. ddNs lack the characteristic M-cell soma shape but have axon pathways resembling those of M-cells. In both cases, inputs are from peripheral interneurons onto cell bodies and axon hillock regions. Neurons are color coded to match inferred counterparts among vertebrate homologs. Cell types listed in key. Synaptic input to M-cells from the VIIIth nerve, and to motoneurons from descending interneurons is in both cases presumed to be excitatory (+) but mediated by glutamate in vertebrates, whereas available evidence indicates that neurotransmission to ddNs from ascending MG neurons and to motoneurons from MGINs in Ciona is in both cases cholinergic. See also Figure S4. Current Biology 2017 27, 721-728DOI: (10.1016/j.cub.2017.01.026) Copyright © 2017 Terms and Conditions