Lang-Lasting Analgesia fallawing TENS and Acupuncture: Spinal Mechanisms beyand Gate Contral

Stimulation of sensory nerve fibers can alleviate acute and chronic pain. The application of thermal or tolerably painful stimuli (e.g., via acupuncture needles) has a long-standing tradition in pain therapy. Shortly after the formulation of the gate control theory (Melzack and Wall 1965), a new form of afferent stimulation was successfully introduced into clinical practice: the excitation of thick, myelinated Aß fibers, either by transcutaneous electrical nerve stimulation (TENS), by dorsal column stimulation, or by mechanical excitation of sensory nerve endings during vibratory stimuli (Wall and Sweet 1967). Two basically distinct forms of TENS have proven clinically useful. First, stimulation at a high frequency (e.g., at 100 Hz) and low intensity evokes paresthesia but no painful sensations. This form of stimulation recruits only Aa/ß fibers and typically produces an analgesia or hypoalgesia throughout the duration of stimulation. Analgesia that lasted up to 2 hours after termination of stimulation was, however, achieved in less than 20% of patients (Johnson et al. 1991). Thus, this form of paresthetic TENS is used in a patient -controlled administration für several hours every day. In animal experiments the antinociceptive effect can be blocked by the GABAA -receptor antagonist bicuculline (Duggan and Foong 1985). This form of lowintensity, high-frequency, paresthetic TENS is best explained by a spinal segmental mechanism involving an inhibitory, probably GABAergic interneuron as described in the gate control theory (see Fig. lA; Melzack and I Wall 1965). I The second form of TENS requires stimulation at an intensity that pro-