Nociception and Acute Pain: Ascending Functional Structure and Descending Modulation

Pain is – a pain in the neck, isn´t it? Yes and no. Yes, it hurts. Yet, it helps. At least do acute and transient pain. Acute pain resulting from an acute event is a fundamental condition for survival, insofar as it warns against imminent or actual tissue damage, a potentially life-endangering threat. The importance of the physiological, protective role of nociceptive pain is underscored by cases, in which a failure to sense pain such as in the case of congenital insensitivity often leads to self-mutilation, bone fractures, joint deformities, amputations, and even early death. While the goal of acute pain based on nociception thus appears to be clear, its implementation is anything but that because to achieve this goal calls for a number of requirements to be fulfilled. The first is the identification of a noxious stimulus, including its intensity and location on the body surface or within the body. The second is the orchestration of counter-measures, including arousal, emotional and various motor reactions. The third is the mobilization of the required energy as well as cardio-vascular and respiratory responses. All this implies multi-dimensional activations of diverse neural and neuro-muscular systems. This review attempts to describe the structures and mechanisms underlying nociception and pain in quite some detail to emphasize their complexity. It starts with a structural description of the nociceptive and pain system in an ascending order, from peripheral nociceptors to supraspinal structures involved in nociceptive and pain processing. This is followed by a description of the systems organizing descending pain modulation. The focus will here be on acute pain. It turns out that even acute nociception and pain and the underlying neural systems are very complex. There are many reasons for this complexity. First, many neuronal nodes receive multifarious inputs and send multiple outputs to other nodes, which often have additional functions other than nociception and pain. This constitutes an extended, multi-functional, multiple input-multiple output network. Second, individual nodes often have an inhomogeneous structure chracterized by diverse neuron groups and inter-connections. Third, sub-cellular processes are complex, but will not be treated here. It comes as no surprise, then, that we face difficulties in dealing with pain and its clinical consequences, and find appropriate treatments. It will not suffice to manipulate a single screw or only a few.