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Peripheral Nociception(The Sensation of Pain)

Every type of tissue in the body has special detectors that they respond to stimulations such as heat, cold, stretch, pressure, position, pain, and many other things. The most important sensors for pain are actually the simplest; they are free nerve endings nor attracted to any specialized detectors. Any of the specialized detectors can also generate a pain message if they receiive too much stimulation. For example, a temperature sensor usually sends "warm-cold" messages to the brain, but if things are too warm, it sends a pain message.
The free nerve endings, as the name implies, are just the end branches of a nerve fiber. When seen under a microscope, they appear quite similar to the branches of a tree. They generally send no messages to the brain until something damages the tissues. Like the smoke detector in a house, they are quiet as long as all is well. When activated, they fire off an electrical signal that rapidly travel up the nerve to warm the brain of a problem. On treir way to the brain, these nerves connect to other nerves that may trigger certain reflexes to help us move away from the source of danger and damage.
Although the pain signal is electrical when it travels through a pain nerve, the stimulation that actually starts the signal at the free nerve ending is a chemical. A number of different chemicals, many of which can start a pain signal, are released when a tissue is damage by injurny, disease, or inflammation. Pain-stimulating chemicals incluede simple elements, such as potassium or hydrogen, and complex molecules, such as bradykinin, serotonin,histamine, and the prostaglandins.
In addition to causing the free nerve endings to send a pain message, some of these chemicals can "sensitize" the pain receptors, making them more likely to send another pain message if there is any further stimulation. If this increased sensitivity becomes severe, even sensations that are not normally painful, such as the brushing of fabric across the skin, can become intensely painful. The medical name for this hypersensitivity is allodynia. Most people have experienced allodynia after becoming sunburned, when a simple pat on the shoulder can be pretty uncomfortable. The most severe cases of allodynia occur after certain types of nerve injury, such as the damage that may follow an episode of shingles.
The same chemical substances that stimulate a nerve ending to send a pain signal also cuase other changes in the tissues, including changes in the smallest blood vessels. Among other things, the capillaries become dilated(swollen) and "leaky,“ bringing more blood to the area and allowing fluid to escape from the blood vessels into the tissues. This causes the inflammation that you notice after an injury. Swelling can help the body fight infection by allowing specialized white blood cells to travel into the tissues, but it can also contribute to pain and may cause other problems. The different chemicals have other effects, including attracting infection-fighting cells to the damaged area, increasing the ability of blood to clot, and, most importantly for our topic, causing pain.
Understanding the effects of these chemical substances has led to developing a number of medications that target pain and inflammation at the site of injury. For example, a large group of these chemical messagers are all produced from one "parent" chemical found in the walls of the cells of the body. When a cell is injured, this parent compound is released and modified to produce a whole family of chemicals called prostaglandins, thromboxanes, and leukotriences.
All of the anti-inflammatory medications, such as aspirin and ibuprofen(Motrin and others), work in large part by stopping the creation of these chemical messengers. Aspirin (and the other anti-inflammatory medications)blocks the manufacture of these chemicals at the site of injury, reducing the sensitivity of the nerve endings. Tylenol (acetanophen), while aspitin works best on other types.
Finally, the body does not just use one chemical signal to initiate the pain message and cause changes in the tissues. More than a dozen other chemical messagers are used to transfer the pain signal from one nerve to another -- there are no direct electrical connections between nerves. It's not important to know all the chemical names of these substances ir akk the effects they cause. It is important to understant that a large number different chemical substances outside the nerves, as well as the electrical signals within the nerves, must work together to send a pain signal to the brain. In one way or another, almost every treatment for pain works by blocking the chemical and electrical transmission of these signals.