How can illness affect the brain?
Because the human brain is so complicated and has little capacity to regenerate, it is vulnerable to the effects of damage and disease. Losing part of the vast network of cells, or changing the level of a neurotransmitter, can have devastating results. Alzheimer's and Parkinson's diseases are examples of disorders of the nervous system in which brain cells gradually die. In disorders of the mind, such as schizophrenia and depression, the symptoms are caused by more subtle changes in the brain that are as yet poorly understood.