Somatosensory Afferents and Principal Fiber Tracts

The thalamus is the main relay structure for sensory information destined for the cortex; it is involved in reception, integration, and transfer of nociceptive potentials. WDR neurons project to the ventroposterolateral (VPL) and ventroposteromedial (VPM) nuclei. SN neurons project to the ventroposteroinferior (VPI) nucleus, considered the main somatosensory relay. It receives both noxious and innocuous information of cutaneous, muscular, and articular origin. This nucleus has numerous interconnections with the primary somatosensory (SI) cortex. The VPI participates in the processing of visceral pain, occurring through the postsynaptic dorsal column pathway with nucleus gracilis projections.

The VPM nucleus is likewise involved in sensory-discriminative aspects of thermal, mechanical, and tactile information. Owing to its projections to the prefrontal cortex, the convergence of fibers arising from the parabrachial region within the lateral pons at the locus ceruleus level, as well as to amygdala, hypothalamic, and periaqueductal gray interconnections, the VPM nucleus is likely involved with emotional pain, as well as psychomotor and autonomic reactions to painful stimuli. Posterior division of the ventromedial nucleus (VmPO) and posterior nucleus (PO) are essential parts of the medial nociceptive system establishing insular and cingular cortex connections involved in affective-cognitive aspects of pain. Specific spinothalamic tract projections, originating from lamina I, suggest that these nuclei are noxious information integration centers, especially for cases of freezing and visceral sensations.

The thalamus medial complex receives afferent input from laminae I and V of the spinothalamic tract, interconnecting with the striatum and the cerebellum. This is responsible for the control of attention and motor responses, suggesting that this area may be involved in escape behavior in the presence of harmful stimuli.

Ultimately the nociceptive signal is relayed from the thalamus to a variety of cortical regions. Two systems of nociceptive cortical projection are commonly distinguished: the lateral and medial systems. There are three important cortical regions: primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and the anterior cingulate cortex. The lateral nociceptive system participates directly in the sensory-discriminative ascription of nociception involving specific thalamic nuclei, projecting to SN and WDR neurons of the SI and SII cortices. SN neurons are associated with topographic localization of peripheral stimuli, whereas WDR neurons code the intensity of these stimuli. Nociceptive neurons in the SII cortex code the painful stimulus in temporal terms. Both SI and SII cortices have connections with the posteroparietal area and the insula, responsible for somatosensory input association with learning and memory. This pathway is crucial to assessment of the stimuli features and behavioral decisions in relation to the prefrontal cortex functions. Conversely, the medial nociceptive system has more diffuse projections from the medial thalamus to SI and SII, and limbic structures, such as the insula and the anterior cingulate cortex. Accordingly, it is predominantly responsible for the motivational-affective component of pain.

The insula relays information from the lateral nociceptive system to the limbic system, mainly via the amygdala and prefrontal cortex associated with the emotional and affective component and with memory integral to the painful experience. The anterior cingulate cortex coordinates inputs from parietal areas with frontal cortical regions, integrating the perception of threat with the appropriate pain behavior.

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