Identification of muscle afferents subserving sensation of deep pain in humans

1. Intraneural microstimulation (INMS) and microneurography were used in combination to stimulate and record from muscle nociceptor primary afferent fibers of the common peroneal nerve of healthy volunteers. When pain evoked by INMS was projected to muscle, afferent activity could be evoked by innocuous and noxious pressure applied within the projected painful area. Conduction velocity of single fibers was determined by stimulating the receptive fields (RFs) electrically via needle electrodes inserted into the RF and measuring conduction latency and distance between the RF and recording electrode. 2. Pain projected to muscle during INMS trains 5-10 s in duration at threshold intensity for pain sensation was typically described as cramping and was well localized. Subjects mapped the area of the painful projected field (PF) over the skin using a pointer. 3. Fourteen slowly adaping mechanoreceptors with RF in muscle and with moderate to high receptor threshold were identified within or near the painful PF. Conduction velocities were in the range of Group III (n = 8) and Group IV (n = 6) fibers. Mean RF areas of Group III and Group IV afferents, determined by applying pressure percutaneously, were 2.71 ± 1.14 (SE) cm² and 3.40 ± 1.08 (SE) cm², respectively. Only one Group III afferent unit exhibited spontaneous activity (<1 Hz). 4. One additional high-threshold mechanoreceptor was identified, with its RF located in the extensor tendon at the base of the big toe. This fiber had a conduction velocity of 32 m/s. During INMS, a well-localized sharp pain was projected to the tendon. The RF was contained within the painful PF. 5. RF areas of all units usually overlapped PFs of sensation evoked during INMS and areas of PFs evoked during INMS did not differ from RFs in size. These observations indicate that the recorded afferent was stimulated during INMS. 6. We conclude that muscle pain in humans is subserved by Group III and Group IV nociceptors and that the elementary sensation evoked by their activation is cramplike pain. Unlike cutaneous nociceptors, the sensation evoked by myelinated and small unmyelinated nociceptive muscle afferents is of the same quality. Furthermore, in agreement with classic psychophysical studies it is suggested that the subjective quality of muscle pain and of tendon pain are clearly distinguishable.