A tumor that forms from cells that release hormones in response to a signal from the nervous system. Some examples of neuroendocrine tumors are carcinoid tumors, islet cell tumors, medullary thyroid carcinomas, pheochromocytomas, and neuroendocrine carcinomas of the skin (Merkel cell cancer). These tumors may secrete higher-than-normal amounts of hormones, which can cause many different symptoms.