Final answer:
The incorrect step is that Ca2+ causes vesicles to fuse with the postsynaptic membrane; the correct process is Ca2+ causing vesicles to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft.
Step-by-step explanation:
The INCORRECT step for a chemical synapse is: B. Ca2+ causes vesicles to fuse with postsynaptic membrane. In a chemical synapse, the action potential triggers voltage-gated Ca2+ channels to open, allowing Ca2+ to flow into the presynaptic neuron. This influx of calcium causes synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane (not the postsynaptic) and release their contents into the synaptic cleft. The neurotransmitters then cross the synaptic cleft and bind to receptors on the postsynaptic membrane, resulting in the opening or closing of postsynaptic channels, which causes a change in the postsynaptic potential. Finally, the neurotransmitter is removed from the synaptic cleft either by reuptake into the presynaptic neuron, enzymatic degradation, or uptake by surrounding glial cells.