Neurology and psychiatry are related fields, because neurological disorders often manifest as, and get misdiagnosed as, psychiatric disease. What is the borderline between neurology and psychiatry? What makes one disease neurological and another mental? Are some psychiatric disorders more neurological than others?
This is a philosophical question and you could discuss it for as long as you wanted. Rather than doing that, try to see which disorders are, at the moment, considered to fall into each category.
To mention a few, schizophrenia is considered the most neurological psychiatric disorder and is in fact the least talked about in Neurology. Depression is top amongst the psychiatric disease. Autism occupies a middle ground, discussed by psychiatrists and neurologists, but ADHD is almost as neurological as autism. Mental retardation is also intermediate, though it is 30:70 in favour of neurology. Whether autism is really less neurological than mental retardation, is a good question.
Out of the disorders with a known neuropathology, Alzheimer’s disease, Huntington’s disease and dementia overlap with Alzheimer’s are a bit psychiatric while headache and epilepsy is almost one hundred percent neurological. Why this is, is not entirely clear, since both dementia and epilepsy are caused by neurological damage, and they can both cause psychiatric symptoms.
I think the difference is that it is just much harder to treat Alzheimer’s, Huntington’s and dementia. With epilepsy or meningitis, neurologists have a very good chance of controlling the symptoms and few patients will be left with ongoing psychiatric problems, but with the neurodegenerative disorders, neurologists can’t really do much, leaving a large pool of people for psychiatrists to study it.
Someone once said that neurologists take all of the curable diseases and leave psychiatrists with the ones they cannot help. These observations suggest that there may be truth in this.