Basic science studies show that high and low frequency TENS produce their effects by activation of opioid receptors in the central nervous system. Specifically, high frequency TENS activates delta-opioid receptors both in the spinal cord and supraspinally (in the medulla) while low frequency TENS activates mu-opioid receptors both in the spinal cord and supraspinally. Further high frequency TENS reduces excitation of central neurons that transmit nociceptive information, reduces release of excitatory neurotransmitters (glutamate) and increases the release of inhibitory neurotransmitters (GABA) in the spinal cord, and activates muscarinic receptors centrally to produce analgesia. Low frequency TENS also releases serotonin and activates serotonin receptors in the spinal cord, releases GABA, and activates muscarinic receptors to reduce excitability of nociceptive neurons in the spinal cord.
In palliative care and pain medicine, TENS units are used in an attempt to alleviate neuropathic pain (pain due to nerve damage). Some patients benefit from this approach, while others may not, depending on individual differences , and pain threshold. Further use is documented in the attached references: in obstetric care, particularly in labour; knee pain; lithotripsy or bladder-stone removal; limb pain.
Electrical stimulation for pain control was used in ancient Greece, 63 A.D. It was reported by Scribonius Largus that pain was relieved by standing on an electrical fish at the seashore. In the 16th through the 18th century various electrostatic devices were used for headache and other pains. Benjamin Franklin was a proponent of this method for pain relief. In the 1900s a device called the electreat, along with numerous other devices were used for pain control and cancer cures. Only the electreat survived into the twentieth century, but was not portable, and had limited control of the stimulus.
The first modern, patient-wearable TENS was patented in the U.S.A. on June 18, 1974 (). It was initially used for testing the tolerance of chronic pain patients to electrical stimulation before implantation of electrodes in the spinal cord dorsal column. The electrodes were attached to an implanted receiver, which received its power from an antenna worn on the surface of the skin. Although intended only for testing tolerance to electrical stimulation, many of the patients got so much relief from the TENS itself that they never returned for the implant.
A number of companies manufacturing TENS appeared after the commercial success of the Medtronic device became known. The neurological division of Medtronic, founded by Don Maurer, Ed Schuck and Dr. Charles Ray, developed a number of applications for implanted electrical stimulation devices for treatment of epilepsy, Parkinson's disease, and other disorders of the nervous system. Maurer founded Empi, Inc. in 1977, and in the late 1980s purchased the TENS product line from Medtronic. Today many people confuse Tens with Ems. Ems and Tens devices look similar and both use long electric lead wires and electrodes. Tens is for blocking pain, where Ems is for stimulating the muscle.
TENS Electrodes should never be placed:
TENS should also be used with caution in people with epilepsy or pregnant women (do not use over area of the uterus as the effects of electrical stimulation over the developing fetus are not known). TENS should not be used by people with an artificial cardiac pacemaker due to risk of interference and failure of their implanted device. Possible failure of these warnings can result in a cardiac arrhythmia.