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Parkinson’s disease is a neurodegenerative disorder. Dopamine-generating cells situated in the midbrain decay, which produces the symptoms (related to movements at first, it can lead to behavioral issues).
I don’t know a lot about Parkinson’s disease. I have read that people with very advanced Parkinson’s disease are not good candidates for surgery, probably because of the nature of the disease and also because with age come different problems which render surgery difficult (heart problems, etc). There are a few surgical options to either stimulate a part of the brain or on the contrary destroy a small part of the brain which is causing symptoms. Though it is a fact that there is no cure for Parkinson’s disease yet, and the surgery is used to alleviate symptoms to relieve patients of some medication and side effects.
As for your other questions, it depends ! Depends on the tumor (see here for hereditary brain tumors), depends on the gene involved. Cancer is very complicated so of course I won’t go into great detail here, but basically it can be caused by a great number of things. Genetic risk factors can be involved, and they’re generally called “risk factors” because most of them won’t directly cause a cancer. Only a small subset of genes involved in very critical cellular processes will cause cancer if mutated. Otherwise, a risk factor will increase the chance of developing a cancer. On the other hand, living a healthy lifestyle will decrease that chance, although random mutations do happen whether we watch out for them or not.
No one really knows if spiders feel pain, though in recent years it has come to light that animals like crabs and shrimps that also rely on a ganglia control, might feel pain. Pain doesn’t require a full brain, just a viable circuit of nerves and the right receptors and chemicals. For one thing, spiders are sensible to drugs. So why not pain ?
Intelligence is another debate, surely it would seem like ganglia are more primitive, though we know that a great deal of an octopus’ neurons are inside its arms and I wouldn’t call this creature dumb. In all cases an automaton won’t really be able to react to its environment, which is an amazing feature shared by spiders, octopuses and humans alike.