When an action potential arrives at a synaptic knob it causes calcium channels to open in the pre-synaptic membrane. Calcium ions rush into the synaptic bulb down its concentration gradient from the cleft. This influx of calcium ions causes vesicles containing acetylcholine to move towards the presynaptic membrane. The vesicles fuse with the membrane releasing the neurotransmitter into the synaptic cleft. The acetylcholine diffuses across the cleft and attaches to specific receptor sites on the postsynaptic membrane.