Local Effector Functions of Primary Afferent Nerve Fibres
More than a century ago it was discovered that under certain experimental conditions primary afferent nerve fibers can behave as if they were axons of autonomic neurons. Stricker54 was the first to observe that stimulation of the peripheral ends of cut dorsal roots induced hyperaemia in the corresponding skin area. This increase in blood flow results from antidromic conduction of nerve impulses in afferent nerve fibers1 and, in the rat, is associated with an increase in vascular permeability.24 These reactions are embodied in the term “neurogenic inflammation” which denotes inflammatory responses that depend on the afferent innervation of the tissue.5,24 We now know that neurogenic inflammation is due to the release of vasoactive peptide transmitters from the peripheral endings of fine, mostly unmyelinated, afferent nerve fibers.9,16,46 It has also been turned out that in this way afferent nerve fibers control, besides vascular functions, a variety of local effector systems including the immune system, the respiratory tract and the digestive system (Figure 1). Appreciation of these roles of peptidergic afferent neurons has lit up new aspects in the neural control of tissue functions16,18 and has even stirred reconsideration of the principal organization of the autonomic nervous system.52 The present article highlights some of the basic mechanisms of the local effector control by afferent nerve fibers, discusses the relationship of this function to the sensory and afferent activities of the neurons, and addresses some of the questions which have remained open in these respects.
KeywordsAfferent Nerve Fiber Neurogenic Inflammation Plasma Extravasation Sensory Nerve Ending Gastric Mucosal Blood Flow
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