A comprehensive mechanosensory connectome reveals a somatotopically organized neural circuit architecture controlling stimulus-aimed grooming of the head.

Animals respond to tactile stimulations of the body with location-appropriate behavior, such as aimed grooming. These responses are mediated by mechanosensory neurons distributed across the body, whose axons project into somatotopically organized brain regions corresponding to body location. How mechanosensory neurons interface with brain circuits to transform mechanical stimulations into location-appropriate behavior is unclear. We previously described the somatotopic organization of bristle mechanosensory neurons (BMNs) around the head that elicit a sequence of location-aimed grooming movements (Eichler et al., 2024). Here, we use a serial section electron microscopy reconstruction of a full adult fly brain to identify nearly all of BMN pre-and postsynaptic partners, uncovering circuit pathways that control head grooming. Postsynaptic partners dominate the connectome, and are both excitatory and inhibitory. We identified an excitatory hemilineage of cholinergic interneurons (hemilineage 23b) that elicit aimed head grooming and exhibit varied connectivity to BMNs from different head locations, revealing lineage-based development of a somatotopic parallel circuit architecture. Presynaptic partners provide extensive BMN presynaptic inhibition, consistent with models of sensory gain control as a mechanism of suppressing grooming movements and controlling the sequence. This work provides the first comprehensive map of a somatotopically organized connectome, and reveals how this organization could shape grooming. It also reveals the mechanosensory interface with the brain, illuminating fundamental features of mechanosensory processing, including feedforward excitation and inhibition, feedback inhibition, somatotopic circuit organization, and developmental origins.