Labeled lines meet and talk: population coding of somatic sensations

Abstract

The somatic sensory system responds to stimuli of distinct modalities, including touch, pain, itch, and temperature sensitivity. In the past century, great progress has been made in understanding the coding of these sensory modalities. From this work, two major features have emerged. First, there are specific neuronal circuits or labeled lines transmitting specific sensory information from the skin to the brain. Second, the generation of specific sensations often involves crosstalk among distinct labeled lines. These features suggest that population coding is the mechanism underlying somatic sensation.

Figure 1. Crosstalk among warm, cold, and pain labeled lines.

C-warm fibers mediate a neuronal pathway or labeled line for warmth sensation; A-cold fibers are part of the labeled line for innocuous cold sensation; and C-cold fibers respond to innocuous cold but evoke hot or burning pain sensation. The activity of C-cold fibers can be centrally inhibited by the activation of A-cold fibers, and the activity of A-cold fibers can be masked by the activation of C-warm fibers. Simultaneous application of cold and warm stimuli by thermal grill allows C-cold fibers to activate the hot/burning labeled line, thereby leading to the thermal grill illusion.

Figure 2. Population coding of thermal perceptions.

The model is modified from ref. . (A) Functions and interaction of thermosensitive fibers. Note that CH, CMH, C2, and CMHC fibers all connect with neural pathways involved with hot or burning pain sensation. Activation of Aδ-cold fibers is able to suppress the sense of hot or burning pain. Aδ-cold fiber activity can be masked by the activation of C-warm fibers. (B–F) Fiber activity (within each rectangle) when a specific sensation (above each rectangle) is evoked. “on” means that the fiber is activated, and “off” means that the fiber is silent or inactivated. CH, C-fibers responding only to noxious heat; CMH, C-fibers responding to noxious mechanical stimuli and heat; C2, C-fibers responding cold and warmth/heat; CMHC, C-fibers responding to noxious mechanical stimuli, heat, and noxious cold; Aδ-cold, A-fibers responding to innocuous cold; C-warm, C-fibers responding to warm temperatures; Aδ-MH, A-type fibers responding to mechanical stimuli and heat.

Figure 3. Population coding of pain versus itch.

Pain and itch are processed along two different labeled lines. Activation of pain-sensing neurons may activate Bhlhb5-expressing inhibitory neurons in the spinal cord, which in turn suppress itch-sensing GRPR-expressing spinal neurons.