Whatever the cause of the stimulus, sudden pain has three effects. Firstly, it triggers an avoidance response by reflexes which are controlled in the spinal cord, e.g. we take our hand out of the fire before we even feel the pain; secondly, the impulse travels to the brain, where it causes activation of a particular area in the brain which localizes and clarifies the pain; and thirdly, the whole brain is activated and thrown into a state of readiness.Long-standing (chronic) pain differs from acute pain in that the avoidance response does not occur; the specific response with appreciation of pain still takes place but the activation of the brain is variable, possibly because the persistent stress of chronic pain produces chemical changes in the body.
The pain thresholdIn pain appreciation, another important factor is attention. It is common experience that a toothache is worse at night on going to bed; as soon as the mind is occupied and engrossed, the pain subsides into the background. Experiments show that this applies to other sensory inputs, e.g. a cat will cease to show its usual electrical response to sound when it is shown a mouse. (This point is taken to extremes by certain forms of meditation and by hypnosis, where the conscious mind can be distracted or ‘shut out’ entirely.) This aspect is important, as the appreciation of chronic pain varies not only in the same person, but from person to person, depending on the state of mind. The person who is depressed, either because of the debilitating effects of chronic recurrent pain or for other reasons, is less likely to put up resistance to the continuous attack on his sensorium.People have different responses to pain, not only in their conscious reaction, but also in the amount of pain required to impinge on consciousness. This pain threshold can easily be tested by, for instance, asking the patient when a certain temperature applied to the skin becomes painful. One such experiment was done as follows: a 100 watt lamp was focused on to the blackened forehead of a subject. The intensity of the lamp could be varied and the forehead was illuminated for three seconds by a shutter. The lowest level of illumination producing a minimal pain (prickling sensation on the forehead) was taken as the pain threshold. There was a remarkable similarity in this value as long as the subjects concentrated on the task in hand. Distraction, suggestion, and hypnosis could raise this threshold by 35 per cent as could pain-killing (analgesic) drugs. The pain thresholds vary greatly and become unpredictable in subjects who are tired or anxious. Scientific, objective methods of measurement and analysis are very necessary in the study of what is, in the final analysis, a subjective complaint.People tend to suppress the memory of unpleasant events and, for this reason; it is often difficult to give an accurate account of a previous pain. The suppression is partly due to unconscious forgetting, but partly because the chemicals produced by the pain dulls memory.Response to pain is also to some extent dependent on sociological factors. People in social classes I and II are more likely to complain of headache, a tendency that has given the false impression that they are more prone to get headaches.Brain substance itself is insensitive to pain; neurosurgeons can operate on it without general anesthesia. The structures within the head that feel pain are the blood vessels and the coverings of the brain (the meninges). The pain felt in migranous headaches is due to pressure of swollen blood vessels against the sensitive membranes that sheath the skull.

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