The Connectome of the Larval Brain of Ciona intestinalis (L.).
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The anatomical substrate for animal behaviour is founded on the circuits of synaptic connections formed by the brain’s neurons. Small nervous systems allow us to examine the entirety of a nervous system in comprehensive detail, revealing details of both the neurons’ ultrastructure and their synaptic networks. This study uses serial-section EM (ssEM) as an anatomical connectomic approach to analyze the number and distribution of synapses within the networks of the larval brain of the ascidian Ciona intestinalis. There are 166 neurons in a single larva studied. These form at least 32-50 cell types, and 8,390 synapses. Instances of sidedness include stronger connections for left-side neurons and the lack of left-side neurons corresponding to those on the right. Neurons are mostly monopolar with <25% having dendrites, and their axons usually form an obvious terminal. Dense reconstruction of the entire CNS reveals all cells, unlike studies that use reporter genes to reveal morphological forms of selected neurons in their entirety, but fail to identify unlabelled neighbours, which are a majority. The completed larval connectome reveals that Cajal’s Dynamic Law of Polarization is widely violated, many synapses forming between axons (axo-axonic synapses) and terminals (axo-terminal synapses), especially among relay interneurons, rather than onto dendrites or somata. Thus, presynaptic sites are most commonly located over axons or their terminals, while dendrites and somata are heterodoxically presynaptic. In contrast, terminals of sensory neurons bear most of the neurons’ presynaptic sites. Synaptic structure is sometimes unpolarised, with synaptic vesicles situated opposite each other, neurons so connected forming both reciprocal and serial synapses. Larval responses to environmental cues rely on the network of underlying neuronal connections that translates these cues into motion. The anatomical connectome reveals the components and connections of pathways for sensory integration of visual, gravity, and peripheral information both to and from pathways of other sensory systems. These sensory pathways feed into motor networks reported here that are involved in central pattern generation and a putative escape response. The network complexity of the larval brain of Ciona intestinalis is considerable compared with the reported simplicity of larval behaviour and the number of neurons.