Pain Pathways (continued)
Paleospinothalamic Pathway
The paleospinothalamic tract is phylogenetically old. The majority of the first-order nociceptive neurons make synaptic connections in Rexed layer II (substantia gelatinosa) and the second-order neurons make synaptic connections in laminae IV-VIII. The second-order neurons also receive input from mechanoreceptors and thermoreceptors. The nerve cells that furnish the paleospinothalamic tract are multireceptive or wide dynamic range nociceptors. Most of their axons cross and ascend in the spinal cord primarily in the anterior region and thus called the anterior spinal thalamic tract (AST). These fibers contain several tracts. Each of them makes a synaptic connection in different locations: 1) in the mesencephalic reticular formation (MFR) and in the periaqueductal gray (PAG), and they are also called spinoreticular tract; 2) in the tectum, and these fibers are known as the spinotectal or spinomedullary tract; 3) in the PF-CM complex (IL) and they are known as the spinothalamic tract (Figure 7.3). The above three fiber tracts are known also as the paleospinothalamic tract. The innervation of these three tracts is bilateral because some of the ascending fibers do not cross to the opposite side of the cord. From the PF and CM complex, these fibers synapse bilaterally in the somatosensory cortex (SC II-Brodman area 3). The paleospinothalamic pathway also activates brain stem nuclei which are the origin of descending pain suppression pathway regulating noxious input at the spinal cord level (see next chapter).
Figure 7.3 |
The multisynaptic tracts which course via the reticular formation also project to the PF-CM complex [the CM-PF complex are also called intralaminar (IL) thalamic nuclei]. There are extensive connections between the IL and the limbic areas such as the cingulate gyrus and the insular cortex, which is thought to be involved in processing the emotional components of pain. That is to say, the insular cortex integrates the sensory input with the cortical cognitive components to elicit the response to the sensation. The limbic structures, in turn, project to the hypothalamus and initiate visceral responses to pain. The intralaminar nuclei also projects to the frontal cortex, which in turn projects to the limbic structures where the emotional response to pain is mediated.
Archispinothalamic Pathway
The archispinothalamic tract is a multisynaptic diffuse tract or pathway and is phylogenetically the oldest tract that carries noxious information. The first-order nociceptive neurons make synaptic connections in Rexed layer II (substantia gelatinosa) and ascend to laminae IV to VII. From lamina IV to VII, fibers ascend and descend in the spinal cord via the multisynaptic propriospinal pathway (Figure 7.4) surrounding the grey matter to synapse with cells in the MRF-PAG area. Further multisynaptic diffuse pathways ascend to the intralaminar (IL) areas of the thalamus (i.e., PF-CM complex) and also send collaterals to the hypothalamus and to the limbic system nuclei. These fibers mediate visceral, emotional and autonomic reactions to pain.
Figure 7.4 |
Figure 7.5 summarizes the three major spinal thalamic pathways. Information about bodily events is conveyed by primary sensory fibers to higher brain centers through the dorsal column medial lemniscal pathways. This route is considered a "touch pathway," separate from the spinal thalamic pathways. However, recent reports indicate that the dorsal column can also carry noxious information from the viscera and widespread skin regions.
Figure 7.5 |
