Introduction to Section II

Abstract

The anatomical basis of the dorsal root afferent input to the superficial dorsal horn has been laid in the previous chapters of this book. Essentially the majority of the axons are small myelinated (Aδ) or unmyelinated (C). The larger myelinated axons or their collaterals pass through lamina I on their way to terminate in deeper laminae. It is, therefore, reasonable to consider the receptive characteristics of the finer afferent fibres since these will determine the kind of stimuli able to affect lamina I neurones most directly. In the cat, the common laboratory animal used in neurophysiological experiments, and especially when the lumbo-sacral spinal cord is considered, most of the fine afferent fibres come from the skin, with a much smaller proportion from the subcutaneous and deep tissues and teh pelvic viscera. The Aδ cutaneous afferent units in the cat include a) type D hair follicle afferents; b) mechano-nociceptors, including those effectively excited by needle-like punctiform stimuli and c) some slowly-conducting Aδ axons with receptors responding to intense pressure and to high temperatures. The C afferent units include a) sensitive C-mechanoreceptors, excited by brushing hairs; b) C-nociceptors (some primarily responding to noxious temperatures, others also excited by firm pressure); c) sensitive thermoreceptors, either cold units or warm units.