The conduction of diffuse, poorly localized pain arising from deep and visceral structures (gut, periosteum, peritoneum) has been ascribed to these slow-conducting, indirect pathways. Melzack and Casey have proposed that this fiber system (which they referred to as paramedian), with its diffuse projection via brainstem and thalamus to the limbic and frontal lobes, subserves the affective aspects of pain, that is, the unpleasant feelings engendered by pain. It is evident that these spinoreticulothalamic pathways continue to evoke the psychic experience of pain even when the direct spinothalamic pathways have been interrupted. Also, the pathways for visceral pain from the esophagus, stomach, small bowel, and proximal colon are carried largely in the vagus nerve and terminate in the nucleus of the solitary tract (nucleus tractus solitarius [NTS]) before projecting to the thalamus, as described below. Other abdominal viscera still activate the NTS when the vagus is severed in animals, probably transmitting impulses through the splanchnic plexus. However, it is the direct spinothalamic pathway, which projects to the ventroposterolateral (VPL) nucleus of the thalamus and thence to discrete areas of the sensory cortex, that subserves the sensory-discriminative detection of the location, quality, and possibly the intensity of the noxious stimulus.
It should be emphasized that the foregoing data concerning the cells of termination of cutaneous nociceptive stimuli and the cells of origin of ascending spinal afferent pathways have all been obtained from studies in animals (including monkeys). In humans, the specific cells of origin of the direct spinothalamic tract fibers have not been fully identified. Information about this pathway in humans has been derived from the study of postmortem material and from the examination of patients subjected to anterolateral cordotomy for intractable pain. What can be stated of clinical importance is that unilateral section of the anterolateral funiculus produces a relatively complete loss of pain and thermal sense on the opposite side of the body, extending to a level two or three segments below the lesion as noted earlier. After a variable period of time, partial sensibility for pain usually returns, probably being conducted by pathways that lie outside the anterolateral quadrants of the spinal cord that gradually increase their capacity to conduct pain impulses. One of these is a longitudinal polysynaptic bundle of small myelinated fibers in the center of the dorsal horn (the dorsal intracornual tract); another consists of axons of lamina I cells that travel in the dorsal part of the lateral funiculus.
Thalamic Terminus of Pain Fibers
The spinothalamic fibers terminate in the ventral posterior nuclei, the most important of which are the ventroposterolateral (VPL) and ventroposteromedial (VPM) nuclei. These are the nuclei that project to the parietal and other areas of the cortex and subserve discriminative functions relating to pain. A contingent of medially projecting direct spinothalamic fibers terminates in the intralaminar complex of nuclei and in the nucleus submedius. These pertain to less-specific aspects of pain such as alerting and arousal. The indirect spinoreticulothalamic fibers also project onto the intralaminar thalamic nuclei and overlap with the terminations of the direct spinothalamic pathway. Projections from the dorsal column nuclei, which have a modulating influence on pain transmission, are mainly to the ventroposterior group of thalamic nuclei. Each of the thalamic nuclear groups has a distinct cortical projection and each is thought to play a different role in pain sensation (see below).