Neuronal mechanisms of pain with special emphasis on visceral and deep somatic pain.

Abstract

Pain has several dimensions: the sensory-discriminative, the motivational-affective, the cognitive and the motor and autonomic dimension. Each dimension can be roughly identified with certain brain areas. The sensory-discriminative dimension is also called the "nociceptive dimension" or, briefly, "nociception". 1. Noxious stimuli applied to the skin appear to be encoded quite specifically by certain types of nociceptive afferent units with group III (A) and IV (C) fibers. The impulse activity of these cutaneous primary afferents converges on spinal "nociceptive-specific" neurones, most of which seem to be located in lamina I of the dorsal horn, and together with the nonnociceptive information from the skin on "wide-dynamic range" (multisensory) neurones in the grey matter, most of which are situated in lamina V, but some also in adjacent laminae and lamina I. 2. Many of these "nociceptive-specific" and "wide-dynamic range" neurones project with their axons through the anterolateral tract to the nucleus ventralis posterolateralis (VPL) of the thalamus and also to other thalamic nuclei, to the mesencephalon and to the reticular formation of the brain stem. In the VPL of the thalamus, most neurones with nociceptive input have a wide-dynamic range property, very few are nociceptive-specific. 3. Noxious events leading to deep somatic pain are encoded by thin myelinated (A delta) and unmyelinated afferent fibers (e.g., from skeletal muscle, tendon and joint capsule). Besides these deep somatic "nociceptive" afferent units, other nonnociceptive deep somatic afferent units with fine afferents have been claimed to exist and it is believed that these are involved in functions other than nociception. The specificity of responses of these afferents, with respect to the natural stimuli, is only relative. 4. For the viscera nociceptive spinal visceral afferents, which are only activated when injurious or potentially injurious events in the visceral domain (which may lead to pain) occur, cannot be unambiguously shown to exist. It seems more likely that the activity in the same population of spinal visceral afferents is involved in nociceptive as well as in nonnociceptive sensory functions, in the regulation of visceral organs and in various types of reflexes. 5. No neurones in the spinal grey matter have been found which specifically transmit and process information from fine deep somatic and spinal visceral afferents. This information seems to converge not only on many "wide-dynamic range" (multisensory) spinal neurones but also on some "nociceptive-specific" neurones.(ABSTRACT TRUNCATED AT 400 WORDS)