Pain from excitation of identified muscle nociceptors in humans

The technique of intraneural microstimulation (INMS) combined with microneurography was used to excite and to record impulse activity in identified afferent peroneal nerve fibers from skeletal muscle of human volunteers. Microelectrode position was minutely adjusted within the impaled nerve fascicle until a reproducible sensation of deep pain projected to the limb was obtained during INMS. During INMS trains of 5–10 s in duration and at threshold for sensation, volunteers perceived a well defined area of deep pain projected to muscle. Psychophysical judgements of the magnitude of pain increased with increasing rates of INMS between 5 and 25 Hz. Also, the area of the painful projected field (PF) evoked during trains of INMS of various duration but constant intensity and rate typically expanded with duration of INMS. The intraneural microelectrode was alternatively used to record neural activity originating from primary muscle afferents. Eight slowly adapting units with moderate to high mechanical threshold were identified by applying pressure within or adjacent to the painful PF. Conduction velocities ranged from 0.9 to 6.0 m/s, and fibers were classed as Group III or Group IV. Capsaicin (0.01%) injected into the RF of two slowly conducting muscle afferents (one Group III and one Group IV) produced spontaneous discharge of each fiber and caused intense cramping pain, suggesting that the units recorded were nociceptive. Our results endorse the concept that the primary sensory apparatus that encodes the sensation of cramping muscle pain in humans is served by mechanical nociceptors with slowly conducting nerve fibers. Results also reveal that muscle pain can be precisely localized, although the human cortical function of locognosia for muscle pain becomes blunted as a function of duration of the stimulus.