PAIN FROM EXCITATION OF IDENTIFIED MUSCLE NOCICEPTORS IN HUMANS

The technique of intraneural microstimulation (INMS) combined with mic roneurography was used to excite and to record impulse activity in ide ntified afferent peroneal nerve fibers from skeletal muscle of human v olunteers. Microelectrode position was minutely adjusted within the im paled nerve fascicle until a reproducible sensation of deep pain proje cted to the limb was obtained during INMS. During INMS trains of 5-10 s in duration and at threshold for sensation, Volunteers perceived a w ell defined area of deep pain projected to muscle. Psychophysical judg ements of the magnitude of pain increased with increasing rates of INM S between 5 and 25 Hz. Also, the area of the painful projected field ( PF) evoked during trains of INMS of various duration but constant inte nsity and rate typically expanded with duration of INMS. The intraneur al microelectrode was alternatively used to record neural activity ori ginating from primary muscle afferents. Eight slowly adapting units wi th moderate to high mechanical threshold were identified by applying p ressure within or adjacent to the painful PF. Conduction velocities ra nged from 0.9 to 6.0 m/s, and fibers were classed as Group III or Grou p 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 conc ept that the primary sensory apparatus that encodes the sensation of c ramping 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 lo cognosia for muscle pain becomes blunted as a function of duration of the stimulus.