Can you describe the neural basis for the symptoms associated to Parkinson's Disease?
Only a qualified person with a medical degree can answer that, what you will get here are lots of people who think they know but don't have a clue.
Seek professional help.
The underlying cause of Parkinson's disease is a lack of dopamine caused by the death of dopamine producing cells. A large percentage of cells must die before symptoms start to show. exactly how it manifests varies widely from person to person depending on exactly where in the brain the lack of transmitter occurs and what the level is.
Usually problems are a combination of tremor and rigidity which can be alleviated by drugs which either supply a replacement for dopamine or slow down the turnover of dopamine. as the drugs take over more and more of the responsibility for the transmitter, control gets more difficult and symptoms become more difficult to control.
One problem which is rarely mentioned is that dopamine also gives you the 'pleasure rush'. in Parkinsons patients this too can be diminished so patients experienced the lows but the corresponding highs are reduced or lacking.
If you need more detail then you will have to slog through the biochemistry of neurological textbooks. But patient to patient variation is high.