An ultrastructural map of a spinal sensorimotor circuit reveals the potential of astroglia modulation.

Information flow through circuits is dictated by the precise connectivity of neurons and glia. However, how glial-synaptic interactions are arranged within a single circuit to impact information flow remains understudied. Here, we use the local spinal sensorimotor circuit in zebrafish as a model to understand how neurons and astroglia are connected in a vertebrate circuit. With semi-automated cellular reconstructions and automated synaptic mapping of all cells, we identify the precise synaptic connections and the astroglia, including bona fide astrocytes, that physically associate with those synapses. Tripartite synapses are equally distributed across the circuit, and singular astroglia interact multiple times within a single circuit and across circuits. Using this ultrastructural map, we model synaptic thresholding and glial modulation on information flow. We validate this mapping by imaging neurons and glia with GCaMP6s. This work provides a foundational map that offers insights in how glia could influence information flow in complex neural networks.