Somatosensory System

C-type fibers (unmyelinated) slowly propagate dull, burning (secondary) pain sensation information. Some C fibers are modality-specific and respond only to thermal, mechanical, or chemical noxious stimuli. However, the majority of C fibers are polymodal, which means that they respond to both thermal and mechanical noxious stimuli, as well as to chemical algesiogenic substances (e.g., potassium ions, prostaglandins, substance P, and histamine). A unique C-fiber subtype responds to high-intensity thermal stimuli and also mediates flare responses after tissue damage. Some C-type nociceptors, designated silent receptors, are primarily activated by inflammation.

The primary afferent fibers travel through dorsal nerve roots entering the dorsal horn of the spinal cord, where they divide in a “T” pattern traveling two to three spinal segments within the Lissauer tract in both rostral and caudal directions, and sending collateral projections to the gray matter along the entire four- to six-segment length, thus transmitting pain signals over a broad spinal cord area.

Both myelinated and unmyelinated primary afferent fibers project predominantly to the superficial laminae of the dorsal horn. Although there is considerable overlap in the projection of fibers, signaling innocuous and noxious stimuli, there exists some degree of functional segregation at the postsynaptic level in the superficial laminae. Dorsal horn neurons are classified into three distinct groups. The specific nociceptive neurons that respond exclusively to noxious stimuli are found in Rexed laminae I, II, V, and VI. Their receptive fields in lamina I are punctiform and display somatotopic organization.

Lamina I neurons are classified into several modality-selective classes, relaying information from particular subsets of small-diameter fibers and relating the current physiologic status of body tissues. The two nociceptive cell types, nociceptive-specific (NS) and polymodal nociceptive (HPC, for heat, pinch, and cold) have different characteristics. NS neurons receive mainly Aδ inputs associated with first-pain, and relay information about noxious stimuli localization and physical quality. HPC cells receive polymodal C-nociceptor information and are associated with second pain. Lamina I cells relate current physiologic conditions of all body tissues and regulate spinal cord excitability, and therefore pain behavior, through the activation of descending inhibitory and excitatory pathways from the brainstem.

Lamina V neurons are large cells with dendrites extending across the dorsal horn, receiving myelinated primary afferent input from Aβ, Aδ, and C fibers. According to gate control theory, this fiber group is important for segmental suppression of pain; however, their inhibitory role is not yet confirmed. Almost all of lamina V consists of wide dynamic range (WDR) cells, which have large receptive fields and high-frequency ongoing discharges. WDR neurons demonstrate graded responses to pressure and noxious stimuli, including heat, cold, and deep and visceral stimulation. Their activity represents integration of all dorsal horn afferent inputs. In contrast to lamina I neurons, WDR cells are not somatotopically organized; their complex excitatory and inhibitory receptive fields are musculotopically organized. Their main characteristic is to code stimulus intensity; they demonstrate increasing frequencies of response from innocuous to noxious stimulation.

Intrinsic dorsal horn neurons promote interaction of afferent and efferent nociceptive stimuli and are also responsible for their transfer to supraspinal structures. These are classified as (1) projection neurons directly transmitting information to supraspinal centers, (2) intersegmental propriospinal neurons integrating several spinal levels, and (3) interneurons having inhibitory or excitatory features. Nociceptive projection neurons relay information to various brainstem and diencephalon regions, including the thalamus, periaqueductal gray, bulbar reticular formation, and limbic structures within the hypothalamus, amygdala, and other sites. There is also a visceral nociceptive pathway within the postsynaptic posterior column pathway.

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