SIMPLE SCIENCE

Pain transmission (or nociception)is the series of events that takes place in our bodies which allows us to feel and react to pain. It is a very complex process.

WHEN PAIN HELPS US OUT

• We are exposed to something that is painful (a painful stimulus). For example, we walk barefoot onto a hot sandy beach.
• Specialised structures on the soles of our feet called pain receptors detect painful stimulus.
• The pain receptors send messages to the brain via the spinal cord (time frame - fraction of a second).
• The brain receives the pain message and interprets it (time frame - fraction of a second).
• The brain co-ordinates a response to the pain (eg. get off the beach, run into the sea, put shoes on).

Pain receptors are locted throughout our bodies. Different types of pain receptor detect different types of stimuli (like temperature, pressure andd chemicals).

WHEN PAIN 'GETS ON OUR NERVES'
Pain becomes a problem to us when it affects the way in which we live our lives (i.e becomes chronic). For example, if we injure our backs or are suffering from a disease like arthritis, we can experience pain that is persistent. But when does pain become 'a pain'? When our pain machinery becomes sensitised and is activated when it shouldn't be. Pain can be triggered by the irritation of nerve endings (one cause of which is inflammation) or by damage to the nerves.

Pain is generally divided into three types:

• Acute nociceptive pain usually originates from the site of injury.
• Inflammatory pain involves the activation of the immune system.
• Neuropathic pain is usually caused by damage to the peripheral or central nervous systems.

Type of pain
Acute nociceptive - injury to muscle, soft tissue, bones, joints or skin. It feels as sharp, stabbing, aching, throbbing. Can be excruciating but is not usually long lasting. Examples: twisted ankle, bee sting , childbirth.

Inflammatory - generation of inflammatory mediators following a painful stimulus. It feels as burning, dull ache. Can be excruciating, can come and go, or be virtually permanent. Example: rheumatoid arthritis.

Neuropathic - damage to nerve tissue. It feels as aching, tingling, numbness. Can be excruciating, can come and go, or be virtually permanent. Example: trapped or compressed nerve, nerve damage caused by diabetes.

HOW DO PAINKILLERS WORK
All painkiller work by interfering with the pain-transmission process. Although there are many different types, painkillers - or analgesics usually relieve pain in one of two ways:
- by predominantly reducing inflammation or
- by predominantly affecting the central nervous system.
The central nervous system is made up of the brain and the spinal cord.

PAINKILLERS THAT ACT BY REDUCING INFLAMMATION
Inflammation is the body's way of responding to injury, infection or invasion by foreign bodies. Inflammation is controlled by inflammatory mediators, substances that are made by the body and which may make inflammation worse by sensitising pain receptors.

Drugs like aspirin and ibuprofen are called non-steroidal anti-inflammatory drugs NSAIDs). As their name suggests, they work by preventing or limiting inflammation- specifically by blocking the manufacture of prostaglandins. Although prostaglandins do not cause pain themselves, they sensitise nociceptive nerve endings to other inflammatory mediators (like bradykinin and histamine) and thereby amplify the basic pain message.

PAINKILLERS THAT ACT ON THE CENTRAL NERVOUS SYSTEM
The opioids (eg. morphine and codeine) are the major group of analgesic drugs that work in the brain and spinal cord. These are substances that are derived from the opium poppy and their effects closely resemble those of the endorphins - home made pain-killing chemicals produced by the body itself.

The effects of the opioids are dediated by opioid receptors, pain-sensing structures which are located both inside and outside the central nervous system. When a drug like morphines acts on an opioid receptor, it blocks the firing of the nerve cell it is attached to, thereby blocking the basic pain message and preventing it from reaching the brain.

Unfortunately, it is very easy to develop tolerance to the pain-relieving effects of opioids. This means that after using these drugs for a prolonged period of time, you will require a larger amount of opioid to give you the same amount of pain relief. Put simply, opioids dampen down pain transmission.