system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: WhenⅠget a headache, what is actually hurting? Is it my skull, my brain, tissue? What??
gpt
The brain itself does not have pain receptors. It's the tissues surrounding the brain that are hurting. There are pain receptors in the periosteum (which surrounds the bones in the skull), the muscles, arteries, veins, and subcutaneous tissues. 

There are several different types of headaches as well. Sinus headaches are brought on by inflammation of the mucous membranes in the sinuses, and the resultant pressure against the nerves surrounding them. Migraines have a speculative cause, the most popular being that they're brought on by constriction of the blood vessels in the brain followed by rapid expansion of the same vessels. Tension headaches are caused by constriction of the muscles that attach to the skull and as well as muscles in the neck.