Question

Why and how do humans get goosebumps, and how does the brain known when to send signals to the nervous system?

Answer 1

When you feel certain powerful emotions, a part of your brain called the hypothalamus sends a message via your nerves to the muscles in your skin to tighten up. When the skin on your body gets tight, your hairs stand on end and goose bumps form

Answer 2

when you feel cold or scared you body sends that info to your brain and it causes goosebumps.