Central processing of pain within the brain- the expression and interpretation of pain

Ultimately, pain can only be appreciated in the brain - although the brain itself is insensitive to pain. As described already, the spinal cord is the central link between our senses and our brain. At least six different pathways carry pain from the spinal cord to the brain. Exactly where, and how, pain is interpreted in the brain are questions that remain unresolved. Obviously, to appreciate pain the brain must first be conscious. If we are rendered unconscious by general anaesthesia, the brain is unaware of any sensory input - including pain (although the body is still able to respond to pain by increasing the blood pressure, skin pallor and sweating). How general anaesthesia causes the brain to lose consciousness is still unknown.

When general anaesthetic gases (e.g. nitrous oxide) are given in low enough concentrations, it is possible to relieve pain without actually losing consciousness. In other words, the dose required to relieve pain is less than that required to produce complete anaesthesia. This is the principle behind inhaling nitrous oxide during labour

Psychological factors are also important in our appreciation of pain. Our response to pain can be modified by past experience, current surroundings and expectations for the future. For instance, past experience of painful events in our lives (e.g. having an injection) has a big influence on our attitude towards future pain. If we are anxious, or afraid, we also feel pain more acutely than if we are calm and relaxed.

Pain relief can be targeted at factors operating within the brain. Psychological techniques such as concentration, auto-suggestion, distraction, and hypnosis have all been used in childbirth with varying results. Their mechanism of action is unknown. Opioid drugs such as morphine, pethidine and fentanyl produce pain relief within the brain by mimicking the effects of endorphins. Opioids given by intramuscular injection or intravenous patient controlled analgesia work in this way. Nitrous oxide diminishes nervous activity within certain parts of the brain but exactly how it causes pain relief is not known.