Acute pain feels different than persistent pain. When pain is acute the brain helps modify the pain by diminishing it. When an acute pain occurs the brain's modifier chemical's come into play and work to decrease the acute pain signal that is suddenly bombarding the brain. This is why pain diminishes quickly after an injury. This is actually a highly adaptive mechanism that has many more chemical and anatomical reactions than shown in this simple animation. On the other hand persistent pain is quite different. When persistent pain sets in the spinal cord and brain have been bombarded by pain signals at a high intensity for a long period of time. Due to a complex series of chemical reactions and an adaptation by the spinal cord, the incoming pain signal begins to be amplified in a phenomenon known as windup pain. The pain signal that reaches the brain is much stronger and the ability of modifier pathways to change it are greatly decreased.