Theories of Pain

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Introduction

Several theoretical frameworks have been proposed to explain the physiological basis of pain, although none yet completely accounts for all aspects of pain perception. A number of theories have been postulated to describe mechanisms underlying pain perception. These theories date back several centuries and even millennia (Kenins 1988; Perl 2007; Rey 1995)

1. Specificity Theory 

This theory considers pain as an independent sensation with specialised peripheral sensory receptors [nociceptors], which respond to damage and send signals through pathways (along nerve fibres) in the nervous system to target centres in the brain. These brain centres process the signals to produce the experience of pain.

Von Frey (1895) argued that the body has a separate sensory system for perceiving pain—just as it does for hearing and vision and this system contains its own special receptors for de:ecting pain stimuli, its own peripheral nerves and pathway to the brain, and its own area of the brain for processing pain signals. But this structure is not correct. 

2.  Pattern Theory

This theory consider that peripheral sensory receptors, responding to touch, warmth and other non-damaging as well as to damaging stimuli, give rise to non-painful or painful experiences as a result of differences in the patterns [in time] of the signals sent through the nervous system.

Goldschneider (1920) proposed that there is no separate system for perceiving pain, and the receptors for pain are shared with other senses, such as of touch. According to this view, people feel pain when certain patterns of neural ctivity occur, such as when appropriate types of activity reach excessively high levels in the brain. These patterns occur only with intense stimulation. Because strong and mild stimuli of the same sense modality produce different patterns of neural activity, being hit hard feels painful, but being caressed does not.

3. Gate Control Theory

Melzack has proposed a theory of pain that has stimulated considerable interest and debate and has certainly been a vasy improvement on the early theories of pain. According to his theory, pain stimulation is carried by small, slow fibers that enter the dorsal horn of the spinal cord; then other cells transmit the impulses from the spinal cord up to the brain. These fibers are called T-cells. The T-cells can be located in a specific area of the spinal cord, known as the substantial gelatinosa. These fibers can have an impact on the smaller fibers that carry the pain stimulation. In some cases they can inhibit the communication of stimulation, while in other cases they can allow stimulation to be communicated into the central nervous system. For example, large fibers can prohibit the impulses from the small fibers from ever communicating with the brain. In this way, the large fibers create a hypothetical "gate" that can open or close the system to pain stimulation. According to the theory, the gate can sometimes be overwhelmed by a large number of small activated fibers. In other words, the greater the level of pain stimulation, the less adequate the gate in blocking the communication of this information.

There are 3 factors which influence the 'opening and closing' of the gate:

• The amount of activity in the pain fibers. Activity in these fibers tends to open the gate. The stronger the noxious stimulation, the more active the pain fibers.
• The amount of activity in other peripheral fibers—that is, those fibers that carry information about harmless stimuli or mild irritation, such as touching, rubbing, or lightly scratching the skin. These are large-diameter fibers called A-beta fibers.Activity in A-beta fibers tends to close the gate, inhibiting the perception of pain when noxious stimulation exists. This would explain why gently massaging or applying heat to sore muscles decreases the pain.
• Messages that descend from the brain. Neurons in the brainstem and cortex have efferent pathways to the spinal cord, and the impulses they send can open or close the gate. The effects of some brain processes, such as those in anxiety or excitement, probably have a general impact, opening or closing the gate for all inputs from any areas of the body. But the impact of other brain processes may be very specific, applying to only some inputs from certain parts of the body. The idea that brain impulses influence the gating mechanism helps to explain why peopie who are hypnotized or distracted by competing environmental stimuli may not notice the pain of an injury.

References