Another common complication of chronic pain is central sensitization. This is a development involving both the peripheral nervous system (PNS) and the central nervous system (CNS). Local tissue injury and inflammation activate the PNS, which sends signals through the spinal cord to the brain. Central sensitization occurs when there is an increase in the excitability of neurons within the CNS at the level of the spinal cord and higher. Eventually normal inputs from the PNS begin to produce abnormal responses. Low-threshold sensory fibers activated by very light touch of the skin activate neurons in the spinal cord that normally only respond to painful stimuli. As a result, an input that would normally produce a harmless sensation now produces significant pain.