Chapter 25 – Pain System

The pain system is the set of neural pathways responsible for mediating the sensations interpreted as pain. Sensory-discriminative aspects of pain originating from the body surface include coding for the precise quality, intensity, duration, and location of the sensory input. Pain arising from visceral organs and other deep structures of the body is usually poorly localized and has a different, dull aching quality. Neurons that signal the sensory-discriminative components of pain sensation encode these aspects of painful stimuli in a manner generally similar to that used by other sensory systems, such as the visual and auditory systems. However, pain more powerfully engages mechanisms of attention and arousal in limbic regions of the brain than is generally the case for vision and audition. The sense of pain is more often accompanied by strong motivational-affective responses, including emotional reactions (suffering, anxiety, depression), somatic and autonomic reflexes, and endocrine changes. This is especially true for visceral pain. The neural pathways that mediate the motivational-affective components of pain overlap to some extent those involved in pain sensation, but also include additional neural structures and pathways. A better understanding of many complex mechanisms and circuitries underlying sensory dysfunction gained through the study of animal models has translated into existing successful therapies. Focus on the neuroplasticity and dysfunctional mechanisms induced in chronic pain models is leading to the development of better future therapeutics.