Speaking and swallowing are highly-automatic behaviors. When we speak, we don’t think about taking air into our lungs, contracting our diaphragm, or moving our vocal folds, tongue or lips; it all happens automatically. It’s the same for swallowing; it’s an automatic behavior.
Dopamine facilitates automatic movements. However, in Parkinson's, there is a considerable loss of dopamine. With less dopamine, automatic movements are disrupted, which can contribute to speech and swallowing difficulties.