The pharmacological therapy for Parkinson's disease is aimed at replenishing dopamine levels, mimicking dopamine's action, or antagonizing the excitatory effects of cholinergic neurons.
Some neurotransmitters bind to the receptors and tell the cell to open up the ion channels and relay an electrical message, and these are called excitatory neurotransmitters.
Ultimately this imbalance between inhibitory and excitatory activities leads to the manifestation of typical clinical symptoms that include resting tremor, rigidity, postural instability, and slowed movement.
Now, these signals can be either excitatory, which is that they raise the likelihood the neuron will fire, or inhibitory in that they lower the likelihood that the neuron will fire.
Excitatory neurotransmitters depolarize the postsynaptic neuron by making the inside of it more positive and bringing it closer to its action potential threshold, making it more likely to fire that message on to the next neuron.