Electrical activity plays a crucial role in neuronal circuit assembly. Activation of NMDA receptors induces the elevation of intracellular calcium, resulting in the modulation of calcium-calmodulin-dependent protein kinases (CaMKs). The CaMK pathway regulates synaptogenesis by driving the formation of dendritic spines. However, the molecular effectors downstream of this pathway have remained poorly defined. In this issue of Neuron, Saneyoshi et al. identify a new signaling complex containing CaMKK/CaMKI/betaPIX/Rac that regulates the morphogenesis of spines in an activity-dependent manner.