ABSTRACT
Pain is a subjective phenomenon, which is usually described as unpleasant experience associated with exposure to potentially damaging stimuli. Such pain signaling is termed acute pain and is of clear functional significance: it allows an organism to avoid dangerous situations (Millan, 1999). Nevertheless, in some cases pain sensation can occur without any clear external stimuli and can persist for months. Such pain is called chronic pain and it is not clear if it has any functional meaning (Woolf & Doubell, 1994). It is likely that chronic pain simply reflects a malfunction in pain signaling. There is the intermediate case when pain lasts for hours or days. This type of pain, called prolonged pain, occurs during injury or inflammation and prevents unnecessary disturbance of the tissue while undergoing healing. Normally, acute pain does not induce long-lasting (>5 min) changes in the sensory perception. Repeated application of painful stimuli and injury can produce long-lasting changes in pain perception (Sandkuhler & Liu, 1998). The main focus of this review will be synaptic and cellular plasticity in the spinal cord evoked by acute or prolonged pain or an analogous stimulus.
