Dendritic spines and filopodia display actin-based morphological plasticity. The function of this rapid motility is unknown. Its ubiquitous expression during development has led to the hypothesis that motility plays a role in synaptogenesis. We investigated this by simultaneously imaging presynaptic boutons and dendritic protrusions in acute hippocampal slices from GFP-M transgenic mice loaded with FM 1-43 followed by immunostaining. Postsynaptic motility was inversely correlated with the presence of stable synaptic contacts. Filopodia were highly motile and made transient interactions, whereas spines were less motile and had stable contacts, although they could still move together with a synaptic terminal. "Head morphing" of spines was associated with interactions with more than one presynaptic terminal. Our data indicate that filopodia motility could serve to transiently sample the surrounding neuropil, while the motility of established spines could mediate interactions with two axonal terminals. Spine "morphing" could therefore be the morphological signature for synaptic input competition in central synapses.