When the action potential reaches the end of the presynaptic cell, calcium channels release calcium, causing vesicles to fuse with the plasma membrane and release neurotransmitter. These diffuse to the postsynaptic cell, bind to receptors, and cause depolarization. In an inhibitory synapse, the binding would lead to hyperpolarization.
Work Step by Step
Check the chapter for the two kinds of synapse and how polarity changes in each; the chapter also gives the steps in transmission of a signal across a synapse.