Peripheral nerve stimulation suppression of C-fiber-evoked flexion reflex in rats. Part 2: Parameters of low-rate train stimulation of skin and muscle afferent nerves.

Transcutaneous electrical nerve stimulation (TENS) is an acknowledged clinical technique to alleviate chronic pain. Its effectiveness is still limited, however, and the stimulation parameters used are based on subjective reports from patients. In the present study, the systematic investigation of conditioning stimulation that was previously reported by the author has been extended to include short trains of stimuli at a low repetition rate (0.1 to 5 Hz) delivered to dissected skin and muscle nerves in the lightly anesthetized rat. The size of a C-fiber-evoked flexion reflex was utilized as a measure of transmission from nociceptive afferent nerve fibers in the spinal cord and was tested repeatedly after 30 minutes of conditioning TENS-like stimulation to adjacent nerves. At these low rates, stimulation of a muscle nerve was usually more effective in suppressing transmission from C-fibers to second-order neurons than was stimulation of a skin nerve. Furthermore, a stimulation strength recruiting both Group I-II and III muscle afferent fibers was more effective in depressing the C-fiber-evoked activity at all frequencies studied than was that activating Group I-II fibers only. A pulse-train repetition rate of around 1 Hz was most effective. These findings should be taken into account when carrying out clinical TENS treatment.