Pain

Pain is perhaps the most common human fear. A knowledge of the nature of pain can inform good nursing practice.

How is pain detected?

The skin is sensitive to touch, temperature, pressure and pain, each stimuli being detected by specific peripheral receptors. Pain is detected by a type of receptor called a nociceptor. Two basic types of pain receptor exist, they are mechano-nociceptors and polymodal nociceptors. The mechano type are activated by heavy pressure or pinching. Polymodal receptors respond to a wide range of noxious stimuli such as heat or cold and pain producing chemicals. The intensity of the stimulus determines the frequency of action potentials. This is termed frequency coding and is a major factor in how severe the pain feels.

How are painful stimuli transmitted to the brain?

Nociceptors are afferent sensory neurones. The dendrites of such fibres come together in the spinal nerves and enter the spinal cord via the dorsal horn. Their cell bodies are collected in the dorsal root ganglia. The dorsal horn is the first site of modulation. This process can effect the likelihood of an in-coming impulse being transmitted on to the brain.

From the dorsal horn some sensory impulses cross over the spinal cord to the anterior horn from where they ascend towards the thalmic region of the brain via the spinothalamic tract. Other impulses travel upwards in the dorsal column pathway on the same side of the cord. Pain may be generated in the thalamus, and pathways between the thalamus and the sensory cortex facilitate the localisation of the pain to a particular part of the body.

What types of pain exist?

Pain is commonly classified as fast or slow. Fast pain is immediate, sharp and well localised. This may occur after a needle prick and is also referred to as the first pain. Slightly later a duller or second pain is experienced, this tends to be less well localised. Fast pain is transmitted via rapid saltatory conduction in myelinated A fibres, (5 -36 ms-1) while slow pain is transmitted slowly in unmyelinated C fibres, (0.2 - 1 ms-1). Fast A fibres may initiate withdrawal reflexes while C fibres promote "guarding" and immobilising of injured parts of the body. Both mechanism therefore are designed to prevent further tissue damage.

How is pain generated?

Pain is usually classified according to its cause. These causes may include inflammation, spasm, ischaemia and irritation of serous membranes.

Inflammation

This causes pain via the mechanism of hyperalgesia which occurs as a result of nociceptor sensitization. This lowers the firing threshold of the receptors and causes previously non painful stimuli to cause pain. Insulted tissue releases so called inflammatory agents and two of these, prostaglandins and bradykinin, are known to increase nociceptor sensitivity.