If the input volleys contain a substantial contribution from the C fibres, the cells fire as we have just described but then long-term secondary excitabilities set in. The heavy bombardment, aided by the emission of peptides from the C fibres, sets off a cascade of chemical changes. The big white transmitting cell becomes more excitable and goes on firing after the input has dropped.
If you slip off a kerb and badly twist an ankle, you are likely to feel two quite different pains. First you feel a sharp, quick, intense pain in the ankle that fades in seconds. Then a new type of pain may start up. It is deep, spreading, sickening. Slowly the ankle becomes tender and the tenderness spreads to the whole foot and lower leg. You don't like anyone touching it and you hobble. The first phase of the pain involved the immediate gate control mechanism which permitted the impulses to enter the cord and set the amplification. The second phase involved the slow changes of excitability and connectivity of cells in the region that received the massive input volley. Patients who have been operated on feel only the second phase because they are anaesthetized during the first.