Acta Neurológica Colombiana (Jul 2007)
Pain pathways and mechanisms of neuropathic pain
Abstract
Several neural steps are involved in the processing of noxious signals that can lead to the experience of pain. Nociceptors readily respond to different noxious modalities such as thermal, mechanical or chemical stimuli, but nociceptors do not respond to non-noxious stimuli. Transduction, is the process by which noxious stimuli are converted to electrical signals in the nociceptors and transmission, the second stage process of noxious signals, in which information from the periphery is relayed to the thalamus and then to the cortex. A-delta fibers are thinly myelinated fibers which conduct in the range of 2 m/s to 20 m/s. These fibers respond to high intensity mechanical stimulation and are therefore termed high threshold mechanoreceptors, C-fibers are non-myelinated fibers that conduct in the range of 0.5 m/s to 2 m/s and transmit noxious information from a variety of modalities including mechanical, thermal, and chemical stimuli for this reason, and they are termed polymodal nociceptors. Modulation is a third and critically important aspect of the processing of noxious stimuli that occurs. This process represents the changes which occur in the nervous system in response to a noxious stimulus and allows the noxious signal received at the dorsal horn of the spinal cord to be selectively inhibited, so that the transmission of the signal to higher centers is modified. Endorphins activate the descending modulatory systems through specific receptors called “opioid receptors”. When physicians gives morphine, and others opiates, they are activating the opioid receptors in the midbrain and “turning on” the descending systems (through disinhibition), activating opioid receptors on the second-order pain transmission cells to prevent the ascending transmission of the pain signal, activating opioid receptors at the central terminals of C-fibers in the spinal cord, and activating opioid receptors at the peripheral terminals of the nociceptive C-fibers. It is the activation of these four systems that allows opiates such as morphine to produce clinically useful pain relief.
