(A) Synaptic transmission begins when an electro-chemical impulse (= action potential) reaches the end bulbs of a presynaptic neuron. (B) Ca2+ channels open in response, and Ca2+ ions diffuse into the end bulb. The influx of Ca2+ causes many of the synaptic vesicles to fuse with the neuron membrane. Thousands of acetylcholine (ACh) molecules are released into the synaptic cleft. A few diffuse across the synaptic cleft and bind to the acetylcholine receptors that are embedded in the membrane of the postsynaptic neuron. (C) When both ACh binding sites are occupied, the acetylcholine receptors open. Both Na+ (= sodium) and K+ (= potassium) ions diffuse through the open receptors: Na+ moves in and K+ moves out. Because both the diffusion (= chemical) and electrical gradients are favorable to Na+, more Na+ ions move into the cell than K+ ions move out. As a result, the postsynaptic membrane depolarizes, slightly from a resting potential of -70mV. The change in polarity, which is called an excitatory postsynaptic potential (= EPSP), can be recorded and measured. EPSP's vary in magnitude depending on how many ions cross the postsynaptic membrane. (D) The depolarization of the postsynaptic neuron quickly reaches a peak because acetylcholine is hydrolyzed (= broken down) by the acetylcholinesterase (= AChE) in the synaptic junction, the acetycholine receptors close, and Na+-K+ pumps move Na+ and K+ back across the membrane. As they continue, these events also repolarize the postsynaptic neuron to its resting potential.