Neurobiology of Pain

Abstract

Pain is considered a public health problem, and it is defined as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage." Pain is categorized according to its different dimensions and mechanisms. It is classified clinically into acute and chronic pain, and according to its etiology (e.g., postoperative or cancer pain), or the affected area (e.g., low back pain). Also, clinicians classify pain pathophysiologically into nociceptive and neuropathic pain.

At the simplest level, the transmission of information relating to pain from the periphery to the cortex is critically dependent upon integration at three levels within the CNS; the spinal cord, brain stem, and forebrain. Pain signals ascend via different routes (e.g., spinothalamic tract, spinoreticular tract, spina-mesencephalic tract, and postsynaptic dorsal co1umn fibers). There are two primary basic ascending nociceptive pathways namely the spinoparabrachial pathway, feeds areas of the brain concerned with affect (motivational pathway), and the spinothalamic pathway, which probably distributes nociceptive information to areas of the cortex that are concerned with pain discrimination. Pain is carried by the small myelinated A8 fibers and the polymodal small unmyelinated C fibers. Following stimulation, . A8 fibers are recruited initially and transmit (first pain). More intense stimuli activate polymodal nociceptors (second pain). Also there exists a "silent nociceptor" which is very difficult to activate under normal circumstances but can be activated very easily when sensitized.