Cutaneous nociceptors

The cutaneous nociceptor, the subject of this chapter, has been the centrepiece of research on peripheral nociceptive mechanisms. This is for three reasons: it is relatively easy to apply temperature, chemical, and mechanical stimuli to the skin; the skin is accessible both in animals and man for psychophysical studies; the nerve fibres that lead to the skin may be recorded with facility. This chapter focuses on knowledge gleaned from single-fibre recordings in normal skin. The properties discussed pertain to nociceptor studies in the primate, specifically the monkey. Injury may sensitize nociceptors, and this plays a fundamental role in the development of hyperalgesia. Cutaneous sensory receptors may be subdivided into four categories. First, there are the predominantly myelinated afferents that respond to gentle deformations of the skin referred to as low-threshold mechanoreceptors (LTMs) and provide information to the brain regarding texture and shape. A second class of afferents are those that are sensitive selectively to gentle cooling stimuli. The third category, ‘warm fibres’, are C fibres that respond to mild heat stimuli and the fourth group, the subject of this chapter, is a heterogeneous population of high-threshold receptors responsible for the sensations of pain, itch, and prickle.