Pain is a complex phenomenon of our perception and has sensory, cognitive, affective, motor and vegetative components.Pain is bound up with the waking consciousness and must be differentiated from nociception, which is the term used for the uptake of noxious (painful) stimuli, their translation into neuronal excitement and the transmission and central processing of nociceptive information. Nociceptors are the sensory endings of slowly conducting afferent nerve fibres ("free nerve endings"), which are specialized fibres for recording noxious stimuli.Most nociceptors are polymodal, which means they can be excited by chemical, thermal and noxious mechanical stimuli.Persistent noxious stimulation (e.g., in the case of inflammation) can lead to sensitization of nociceptors and to primary hyperalgesia.Independen tly of the nociceptors, a nerve injury can cause neurogenic (neuropathic) pain.The nociceptive information taken up by the primary afferent nerves is transmitted via the synapse in the posterior horn of the spinal cord and modulated by long-acting neurotransmitters, a result of which is the central plasticity of the nociceptive system.Sensitization of central nociceptive neurons leads to secondary hyperalgesia. Referred pain probably comes about by way of the convergent faulty switching of the primary afferent nerves coming from various organs to the same central ascending neurons.Different segments of the pain pathway project into different cortical and subcortical regions. Pathways descending from the brain to the spinal cord counteract the ascending nociceptive activation, opiates being effective at several levels of this endogenous pain-inhibiting system.The ontogenetic development of this peripheral and central nociceptive system has hardly been investigated in humans; it has to be assumed that pain perception is already developed prenatally. © Springer-Verlag 2002.
