Early Theories of Pain
In 1900s, there had been a tendency to consider body as a machine. During then, sensation of pain was explained in a mechanistic view. According to the mechanistic view, if an appropriate stimulus activates a receptor, the signal travels to the spinal cord and then to the brain, and thus sensation results. Specificity theory had been on the basis of this. Specificity theory propounds that body has a separate sensory system for perceiving pain, just as it does for hearing and vision. The system has special receptors for the purpose.
But, according to Pattern theory, there is no separate system. The receptors for pain are shared with other senses, such as touch. The receptors for pain are shared with other senses, such as touch. People feel pain when certain patterns of neural activity occur. These patterns occur only with intense stimulation. Strong and mild stimuli of the same sense modality produce different patterns of neural activity. Therefore being hit hard feels pain, but being caressed does not.
None of these theories adequately explained pain perception. Perhaps the most serious problem with the early theories is that they don't attempt to refer about psychological factors behind it. Research findings indicate that people who are suggested not to feel pain actually do feel less pain when deeply hypnotized than when in normal waking state.
Sensory Decision Theory relied heavily on the psychology of perception. Cognitive processes like perceptual habits, beliefs, expectations, costs and rewards and memory control have influence on how any sensory input is perceived. According to this theory, the experience of pain is increased by attention that is drawn to a specific, painful area of the body or by a conscious decision to focus attention on it. The focusing of attentions may cause even weak or transient sensations to become more noticeable, heightening the experience of pain. However, the theory sticked too much on psychology and neglects biological functions.
Gate Control Theory
In 1965, Ronald Melzack and Peter Wall formulated a new theory of pain, which suggests that pain is not the result of a linear process that begins with sensory stimulation of pain pathways and ends with the experience of pain. Rather pain perception is subject to a number of modulations that can influence the experience of pain. These modulations begin in the spinal cord. The researchers hypothesized that structures in the spinal cord acts as a gate for the sensory input that is interpreted as pain, thus it is called the 'Gate Control Theory'.
Gate control theory views pain as a multidimensional perspective experience.
In pain, ascending psychological inputs and descending psychological inputs are equally involved. The gating mechanism is existed in the dorsal horn of spinal cord that permits or inhibits the transmission of pain impulses.
The dorsal horn receives inputs from nociceptors which it transmits to the brain via neural gate. The dorsal horn also receives information from brain about the psychological and emotional state of the individual. This information can acts as an inhibitory control that closes the neural gate preventing the transmission of nociceptive impulses and thus modifying the perceptions of pain. The mechanism operates based on the relative activity of the peripheral nociceptor fibers and the descending cortical fibers. Pain impulses must reach conscious awareness before pain is experienced. If awareness can be prevented, the experience of the pain is decreased, eliminated or differed.
The gating mechanism is located in the spinal cord, more specifically in substantia gelatinosa of the dorsal horns, which are parts of gray matter that runs the length of the core of the spinal cord. Signal of noxious stimulation enter the gating mechanism (substantia gelatinosa) from the pain fibers. After these signals pass through the gating mechanism, they activate transmission cells, which send impulses to the brain. When the output of signals pass from the transmission cells reaches a critical level, the person perceives pain. The greater the output beyond the level, the greater the pain intensity.
Sensory input is subject to modulation depending on the activity of the large A-beta fibers, the small A-delta fibers and the small C fibers that enter the spinal cord and synapse in the dorsal horns. Certain neurons located in the gray matter of the spinal cord (the gate) have the ability to block or facilitate the transmission of pain impulses passed by A-delta and C fibers. The C fibers are inhibitory to the gate neurons, so impulses traveling along them tend to open the gate, whereas the A-beta fibers are excitatory to the gate neurons, so impulses traveling along them tend to close the gate. If impulses in the C fibers are stronger than those in A beta fibers, the gate opens.
The gateway neurons produce the pain-blocking neurotransmitter enkephalin (an endorphin), which is similar to opiates and can block substance "P" (sharp pain substance). Enkephalins can also be released through electrical stimulation of the brain and in acupuncture, which may be the cause of the reduction in pain.
Gate control theory further emphasizes the importance of specialized nerve impulses that arise in the brain itself and travel down the spinal cord to influence the gate. The central control trigger transmits impulses through large, rapidly conducting efferent fibers to the gate in the spinal cord. (Actually there are many gates located along the spinal cord). The central control trigger can send either inhibitory or excitatory impulses to the gate and sensitized it to either C fibre or A-beta fibre inputs. If the brain sensitizes the gate to C fibre impulses, then it is more likely to open. If it sensitizes the gate to A-beta fibre inputs, then it is more likely to close.
To conclude three factors are involved in GCT.
1. The amount of the activity of the pain fibers: The stronger the noxious stimulation, the more active the pain fibers.
2. The amount of activity in other perpheral fibers (A-beta fibers): ...carries information about harmless stimuli or mild irritation as touching or lightly scratching. Activity in A-beta fibers tends to close the gate inhibiting the perceptions of pain when noxious stimulation exists.
3. Messages that descend from the brain: Efferent pathways to the spinal cord and the impulses they send can opne or close the gate.
