Here you can see the entire G-protein receptor process, from the activation of the receptor by the neurotransmitter dopamine to the opening of the sodium ion channel. Now an anti-psychotic drug comes in from the left and one binds to the receptor. It binds better than dopamine, but can't activate the G-protein. Dopamine tries to bind, but its binding site is occupied by the drug. This is how antipsychotic drugs prevent sodium ions from entering the postsynaptic cell.