Afferent nerve fibers and acupuncture.

Acupuncture has been used for analgesia, for treating visceral function disorders and for improving motor functions. It is well established that stimulation of the skin and muscles, either electrically or with noxious or non-noxious stimuli, induces a variety of somato-motor and autonomic responses. This strongly suggests that acupuncture acts by exciting cutaneous and/or muscular afferent nerve fibers. A question of considerable scientific and practical interest is what kinds of somatic afferent fibers are stimulated by acupuncture and are involved in its effects. There are several types of afferent fiber: thick myelinated Aα and Aβ (group I and II), thin myelinated Aδ (group III) and thinner unmyelinated C (group IV) fibers. In recent studies we have tried to establish which ones of these types of somatic afferent fiber are stimulated by acupuncture. In this article we first review the experimental evidence showing that the effects of acupuncture are mediated by the activation of afferent nerve fibers innervating the skin and muscles. Secondly, we discuss what types of afferent nerve fiber are activated by electrical acupuncture, and what types are involved in its effects on somato-motor functions and on visceral functions. Finally, we present some new findings based on recordings from single afferent nerve fibers excited by manual acupuncture.