Feedforward inhibitory circuits are involved both in the suppression of excitability and timing of action potential generation in principal cells. In the CA3 hippocampus, a single mossy fiber from a dentate gyrus granule cell forms giant boutons with multiple release sites, which are capable of detonating CA3 principal cells. By contrast, mossy fiber terminals form a larger number of Lilliputian-sized synapses with few release sites onto local circuit interneurons, with distinct presynaptic and postsynaptic properties. This dichotomy between the two synapse types endows the circuit with exquisite control over pyramidal cell discharge. Under pathological conditions where feedforward inhibition is compromised, focal excitation is no longer contained, rendering the circuit susceptible to hyperexcitability.