THE BODY DETECTS, THE BRAIN REACTS: TISSUE DAMAGE
Following tissue damage, a three-act opera is played out, in the form of immediate, secondary and tertiary responses.
Immediate
When tissue damage occurs suddenly, there is no overture, although more commonly there is slow accumulation of abnormality until the effects are overt. Three changes can immediately provoke pain by way of impulse activity in the sensory nerve fibres. Pressure is one obvious cause, and it is obvious that sudden pressure, as in a slap, is more effective than slow-onset pressure. Heat and cold may trigger pain, which is signalled by impulse activity in fine sensory nerve fibres. Chemicals may seem a less obvious cause of pain but we have all experienced the effect. Mustard, curry and chilli if overdone produce burning pain. Insect bites and nettle stings inject pain-producing substances under the skin. The CS spray used by the police produces pain where it is easily absorbed in the eyes, nose, mouth and skin. Much more importantly, the breakdown of cells following damage produces an array of pain-producing substances.
Secondary
When the fine sensory nerve fibres are stimulated, they emit into the tissue substances called peptides, which in turn cause dilatation of the blood vessels and pain. When cells are smashed, their contents spill out into the tissue and some of these chemicals cause pain. With the disruption of cells, compounds not normally present find their way into contact with enzymes, biological catalysts that break down the debris into smaller molecules, some of which produce pain.
Tertiary
The classical cardinal signs of inflammation were tumor, rubor, calor and dolor (swelling, redness, heat and pain). Now we can describe in detail what is happening. The swelling is produced by the leaking of fluid into the tissue from the dilated blood vessels. In addition to fluid, there is a massive invasion of white cells attracted from the blood by substances released from the broken cells and their breakdown products. The redness is simply due to the widening of the usually narrow blood vessels. The heat is produced by the large increase of hot blood now rushing through the dilated blood vessels. The pain is produced by direct action of the original destructive event and is amplified by the soup of pain-producing chemicals made by the breakdown products. The area is also tender: that is, pain is now produced by gentle pressure that would previously have caused only the sensation of touch. The tenderness comes partly from the sensitization of the nerve ends, as they are soaked in pain producers, and partly by changes in the central nervous system, which we will discuss later. Some C fibres become active only in the presence of inflammation, otherwise, they are sleeping.
The finale of the opera occurs as these reactive processes reach their climax, once dead cells have been swept away by the white cells, and reparative processes begin. Cells capable of producing dense, fibrous tissue, called fibroblasts, move in and pack together to produce the familiar white scar. They may be accompanied by growing blood vessels and nerve fibres to produce the tender, angry, red scar. During all this time, the C fibres have been absorbing the unusual chemicals present in the inflamed tissue and transporting them centrally to change central excitability.
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