The painful consequences of peripheral injury

Peripheral damage is immediately assessed by the central nervous system by way of a gate control system so that the sensory outcome depends not only on the fact of the injury and the injury signals but also on other convergent impulses from the periphery and on descending controls from brain to spinal cord. However peripheral injury, particularly when nerves are affected, sets off a chain of slow reactions which start in the area of damage but spread centrally. There are the local inflammatory reactions which change the sensitivity of nerves or of sprouts growing from cut nerves. There are changes which move over the entire length of the damaged axon changing the dorsal root ganglia and the terminals of afferents within the spinal cord. The arrival of injury produced impulses in the spinal cord triggers changes with a latency of many minutes which persist for hours even if no further impulses arrive. These increases of excitability and expansion of receptive fields which are triggered by C fibres may be the basis of the secondary hyperalgesias and reflex changes associated with injury.