PAIN TRANSMISSION AND MODULATION MECHANISMS

Abstract

Pain is a protective sensation, but the neural processing that produces it is far from simple. This chapter reviews how noxious information is carried from the tissues to the brain and how that information is adjusted along the way. It follows the four processes of nociception — transduction, transmission, perception and modulation — and identifies the primary afferent fibres, the Aδ and C fibres, that carry the signal inward. The chapter considers the organisation of the spinal dorsal horn, the ascending spinothalamic tract and the cortical regions involved in the experience of pain. Modulation receives particular attention, covering the gate control theory, the descending pathways from the periaqueductal grey and rostral ventromedial medulla, and the endogenous opioid system. Peripheral and central sensitisation are thendiscussed as the changes that underlie many chronic pain states. Taken together, these mechanisms explain how current analgesics work and where newer treatments might eventually act.