Chapter 14 Peptidergic afferents: physiological aspects

Abstract

This chapter discusses the physiological aspects of peptidergic afferents. Primary afferent neurons express a wide array of neuropeptides in varying proportions, which may differ according to their target tissues and response to nerve injury. Therefore, it is not easy to relate peptide content to a functional modality of the primary afferent. Many of the peptidergic fibers contain a variety of neuropeptides. However, there are some physiological characteristics and certain modalities that predict the peptide content of the primary afferent. Thus, small-diameter myelinated and unmyelinated afferent fibers with small-diameter cell bodies are excited by noxious thermal and mechanical stimuli. Considerable evidence indicates that some of these nociceptive fibers contain the neurokinins, substance P (SP) and neurokinin A (NKA). The nature of all of these stimuli implies that these fibers are involved in nociception. This chapter examines the behavioral and physiological characteristics of fine primary afferent fibers that conduct at low velocity, and have a high threshold for electrical stimulation, are activated by algogenic substances, express tetrodotoxin (TTX)-resistant Nachannels, and are the most likely ones, to release SP and NKA from their terminals in the periphery and in the spinal cord (.