The first thing we need to know is why pain occurs, any pain. It is caused by a series of substances called algic. Generally, it is the inflammatory processes that cause the release of these substances in the body, for example prostaglandins or leukotrienes, which stimulate receptors that we call nociceptors that are in the tissues, in the viscera and in the glands. When these nociceptors are stimulated, they travel through the marrow, reach the brain and from there that feeling, pain occurs.
There are many types of pain. The acute, for example, which is that intense pain usually very localized and that can be a pain of visceral type. There is another type of diffuse or referred pain that is one that does not have the exact perception of where it comes from. But we must understand that the mechanism that causes it is the same for all types of pain: liberation of a algid substance produced by tissues that break, by fever or inflammation.
As for chronic pain, it is a pain that persists even if the cause that originated it has passed. The origin is the same as for the other types of pain, that mechanism of stimulation of nociceptors that go to the central nervous system, to the brain, and there generate painful sensations. Nowadays, chronic pain is already typified as a disease and not as a symptom, as is the case with the rest of the pain. What happens with chronic pain is that it can last a lifetime and can often lead to situations of depression, to low moods because it is very persistent and always remains in time. While acute pain disappears when the cause that causes it is eliminated, chronic pain is a sensation that is established. There are people who confuse chronic pain with fibromyalgia, but they are different diseases. In the case of fibromyalgia, it is a rheumatoid pain caused by generalized inflammation with total discomfort that sometimes causes paralysis.
What causes chronic pain can be the same as causing acute pain, for example, inflammation. But what happens with the first one is that when the cause ends, for example, an abdominal or renal inflation or a vasodilator process like varicose veins, that is to say a cardiovascular type that is very painful, as I say, when these processes have been completed, the pain continues. The reason may be that these nerve endings are already permanently stimulated. There is something similar, a type of pain called the phantom limb syndrome suffered by some people who have had a limb amputated. Some of these people still feel the pain because the endings continue to receive stimuli, sometimes even from the skin.
The amount of pain also depends on the people, on that threshold necessary for the pain to be felt. But in general, for most, the joints are very painful, and the head, too
To treat this type of pain, and the rest, we have analgesia. And it's interesting to know how it works. Analgesia reduces sensation because it increases the need for stimulation. As we are talking about the nervous system and the nervous system works through neurons, let's say that the neuron is excited when there is a stimulus of a certain intensity. What does analgesia do? In general, it increases the level of intensity needed by neurons to provoke the sensation of pain, therefore stimuli that without analgesia could cause pain sensation, with analgesia do not produce it.
It is also important to know that any part of the body can hurt but the amount of pain felt depends on the number of those receptors, called nociceptors, that each of those areas or organs have. There are parts that have many more nociceptors, that is, they receive much more stimulation than others. The amount of pain also depends on the people, on that threshold necessary for the pain to be felt and that in each person is different. But in general, for most, the joints are very painful and the head too, through the processes of vasoconstriction and vasodilation; then there are the visceral pains, for example, of stomach that may be more sensitive than intestinal pains, or gonadal pain. And there are areas that are less sensitive, for example, the area of the sole of the foot has a lower density of receptors and, therefore, it is usual to generate less pain.
Pilar Aranda is Professor of Physiology and President of the University of Granada.
Question asked via email by J Carlos Gómez
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