Neurons which are part of a pathway, such as a reflex pathway, will receive stimuli from many other neurons. Here we see first one such neuron then another. The first neuron produces an EPSP, an excitatory post synaptic potential. Excitatory potentials are depolarizations, and produce an upward deflection in the electrical recording, a deflection toward the threshold which is -55 mv. It is called excitatory because, by depolarizing the neuron a little, it makes it easier for summation from other EPSPs to produce threshold depolarization and produce an action potential. An IPSP produces a hyperpolarization, an increase in polarity, which is illustrated by a downward deflection in the recording. This inhibits the neuron by making it more difficult for summation to proudce threshold depolarization.