The sympathetic nervous system
In peacetime, these fibres regulate the amount of blood flowing through tissue and regulate the steady activity in such organs as heart, gut, bladder and sweat glands. In emergencies, the fibres change their action when their cell bodies are informed of damage by a reflex loop originating in the increased activity of the sensory nerve fibres. Apart from regulating blood flow, they play a role in triggering some of the inflammation that is characteristic of tissue damage.
Nerve damage
Severe injury, of which amputation is an extreme example, may cut across whole nerves. There are also medical causes of nerve damage, such as the invasion of nerves by viruses, which produces shingles, or the metabolic failures in diabetes that cause a breakdown of nerve fibres. These conditions produce all the standard signs of inflammation we have just described but, in addition, are particularly likely to produce pain that outlasts all signs of inflammation. The cut nerve attempts to regenerate and sends out fine sprouts which have unstable properties, especially if the regeneration fails to send the fibres back to their former target. In addition, the cut nerve absorbs highly unusual chemicals which are transported back to the ganglion cells and to the spinal cord, where they trigger exaggerated sensitivity, as we will discuss later.
Handling sensory messages
Let us take as an example one of the large cells in the dorsal horn in laminae V that responds to sensory messages and sends its order to the brain and to motor cells. The cell is represented as the large open circle in fig. 8 with its input from the large, sensory input fibres, A beta, and from the smaller sensory fibres, A delta and C. No cell is ever found which simply relays the inputs onto the output. There are always small surrounding cells that modulate the action of the input on the output. Some of these small cells, shown in black, exaggerate the effects of the input; others, shown as open circles, diminish the effect. All of these cells are also influenced by fibres descending from the brain