Recent experimental evidence shows that cellular connectivity in the brain is not random. More specifically, bidirectional connections between pairs of excitatory neurons are predominantly found when neurons connect by short-term facilitating synapses. For this type of synapse, excitatory postsynaptic potentials (EPSPs) transiently increase upon repeated presynaptic activation. Unidirectional connections between pairs of excitatory neurons, however, are predominantly found when neurons are connected by short-term depressing synapses. For these synapses, EPSPs transiently attenuate upon repeated activation. Here we present a simple model that combines Short-Term Plasticity (STP) and Spike-Timing Dependent Plasticity (STDP) that might explain the correlation between specific synaptic dynamics and, unidirectional or bidirectional, connectivity patterns.