Pain and aspirin

2 How does it hurt?

This is a useful question because once we know the mechanism of pain sensation we can do something about alleviating it.

When tissue is injured there follows a rapid release of ‘messenger’ chemicals that stimulate the nerve endings. Electrical impulses are relayed through the nerves to the spinal column and to the brain, which registers the sensation of pain. It usually, but not always, also directs our attention to the site where the damaged tissue initiated the pain message.

Drugs to alleviate pain act to interrupt this flow of information. There are three basic types grouped together by the way in which they work.

  1. Drugs such as aspirin act at the site of the injury to stop or at least reduce the production of messenger chemicals that stimulate the nerve endings.
  2. Another class of drugs, the opiates such as codeine and morphine, act on the central nervous system (brain and spinal cord). Sometimes, the aspirin-like and codeine/morphine-like drugs are combined into one remedy, for example co-codamol and co-proxamol.
  3. Local anaesthetics.

For the time being, we will concentrate on aspirin, to illustrate the development of drugs and how they are able to achieve their effects.