In the future we shall need to modify the classical view that nerve impulses which signal the presence of injury are reliably transmitted by specified and automatic relay cells. We must investigate at least four different modifying mechanisms that are likely to generate chronic intractable pains. With a latency of milliseconds, combinations of afferent signals and of descending controls operate a rapid and powerful gate control. With a latency of minutes, impulses in C fibres change the excitability of peripheral endings and of spinal cord circuits. With a latency of days, chemical transport in C fibres from areas of damage further modifies cord connectivity with a disappearance of inhibitions and an expansion of receptive fields. With a latency of weeks and months, anatomical degeneration produces secondary changes in deafferented cells with atrophy, sprouting and abnormal firing patterns.