Analgesic effect of vibration and cooling on pain induced by intraneural electrical stimulation

Psychophysical experiments were carried out on 16 human subjects to determine how low intensity mechanical and thermal skin stimuli interfere with the sensation of pain. Moderate or intense pain was induced by low frequency (2 Hz) electrical stimulation within cutaneous fascicles of the median nerve at wrist level, and vibration, pressure, cooling or warming were applied for short periods (usually 20-60 sec) within or outside the skin area to which the pain was projected. Vibration within the area of projected pain reduced the sensation of pain more efficiently than vibration outside that area. Moderate pain was sometimes completely inhibited but intense pain was only moderately reduced. Pressure and cooling produced some pain relief whereas mild warming had an ambiguous effect. Since the painful input derived from stimulation of fibres in the nerve trunk, and not from peripheral nociceptors, the pain suppressing effects of vibration and cooling are not explicable in terms of lowered excitability of the nociceptive nerve endings in the skin. Instead, the results indicate that activity in low threshold mechanoreceptive and cold sensitive units suppresses pain at central (probably segmental) levels.