Fig. 1. Immature and mature configurations of the afferent innervation of the cochlea by the intrinsic SGN. (Top) In the early postnatal (P3) rat cochlea, type I and type II SGN innervation is mismatched with target hair cells. (Bottom) By the onset of hearing (around P11), several type I SGN neurites (blue) exclusively innervate individual inner hair cells (IHC) with pruning of the synaptic processes to a few puncta. By contrast, the considerably less numerous type II SGN neurites (red) drop their innervation of the IHC and provide extensive en passant innervation of multiple outer hair cells (OHC) through the outer spiral bundles (osb). rf, radial fibers. Spiral ganglion neurite outgrowth is promoted by neurotrophins, particularly BDNF, which is a paracrine factor secreted by the hair cells. Here we provide evidence that extracellular ATP signaling acts through a P2X_2/3 heteromeric receptor to inhibit this neurotrophic support.