… in conjunction with local inflammatory events
…is arguably the commonest of the human ills, and perhaps, in proportion to its impact, one of the least well understood.
With respect to the cause of the pain the mechanisms are much less well understood for the primary than for the secondary headaches. Whereas the pain due to injury to the skin, for example, is well understood as being due to stimulation of specific nerve endings in conjunction with local inflammatory events, it is not clear in primary head pain whether the nerves are firing normally or abnormally in response to various stimuli. Much work is to be to done, especially in regard to understanding tension-type headache.
[Both excerpts are from the Oxford Companion to the Body]
