Aδ and C fibers are afferent nerve fibers of nociceptors that conduct sensory information to the central nerve system. The conduction velocity of Aδ fibers is ~15 m/s, in contrast with the much slower conduction speed of C fibers, at ~1 m/s. As a result of this difference, a single and intense cutaneous nociceptive stimulus will cause a double sensation: an initial Aδ-fiber-related pricking "first" pain that is specific to one area and a C-fiber-related burning "second" pain that is spread out over an unspecific area.  

Compelling evidence now shows that selective stimulation of Aδ-nociceptors elicits robust responses in the primary somatosensory cortex (S1). However, whether the C-fiber nociceptive input eliciting second pain has an organized projection to S1 remained an open question until recently.  

Researchers led by Prof. HU Li from the Institute of Psychology of the Chinese Academy of Sciences, along with collaborators from University College London (Prof. Iannetti Giandomenico) and Shenzhen University (Dr. PENG Weiwei), have now addressed this question.  

They examined whether C-fiber input has a somatotopically organized projection to S1 when laser radiant heat stimuli were delivered to different parts of the body, in both humans using electroencephalography (EEG), and in rodents using electrocorticography (ECoG).  

Electrocortical responses elicited by nociceptive-specific laser stimulation of the four limbs in 202 humans and 12 freely moving rats were recorded. Topographical analysis and source modeling revealed a clear gross somatotopy of unmyelinated C-fiber input within the S1 contralateral to the stimulated side, in both species.  

In the human EEG data, S1 activity could be isolated as an early-latency negative deflection (C-N1 wave at 710-730 ms) following hand stimulation, but not following foot stimulation, because of spatiotemporal overlap with subsequent large-amplitude supramodal vertex waves (C-N2/P2).  

In contrast, in rat ECoG, S1 activity could be isolated as a C-N1 wave following both forepaw and hindpaw stimulation, because of the cross-species difference in the representation of the body surface within S1.  

In addition, Prof. HU and his collaborators observed a functional dissociation between the generators of somatosensory-specific lateralized wave (C-N1) and generators of supramodal vertex waves (C-N2/P2). The results indicate that C-fiber unmyelinated input is processed in functionally distinct somatosensory and multimodal cortical areas.  

These exciting findings demonstrate that C-fiber input conveys information about the spatial location of noxious stimulation across the body surface – a prerequisite for deploying an appropriate defensive motor repertoire. 

This study was supported by grants from the National Natural Science Foundation of China, the Scientific Foundation project of the Institute of Psychology of the Chinese Academy of Sciences, Wellcome Trust, and Consolidator Grant of the European Research Council.  

The paper is now available online in Journal of Neuroscience.