Principles of coincidence detection
Fig. 1: Two EPSP's innervated in rapid succession sum to produce a larger EPSP or even an action potential in the postsynaptic cell.
Coincidence detection relies on separate inputs converging on a common target. Consider a basic neural circuit with two input neurons, A and B, that have excitatory synaptic terminals converging on a single output neuron, C (Fig. 1). If each input neuron's EPSP is subthreshold for anaction potential at C, then C will not fire unless the two inputs from A and B are temporally close together. Synchronous arrival of these two inputs may push the membrane potential of a target neuron over the threshold required to create an action potential. If the two inputs arrive too far apart, the depolarization of the first input may have time to drop significantly, preventing the membrane potential of the target neuron from reaching the action potential threshold. This example incorporates the principles ofspatial and temporal summation. Furthermore, coincidence detection can reduce the jitter formed by spontaneous activity. While random sub-threshold stimulations by neuronal cells may not often fire coincidentally, coincident synaptic inputs derived from a unitary external stimulus will ensure that a target neuron fires as a result of the stimulus.