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Transcutaneous (through the skin) Electrical Nerve Stimulation (TENS) is a treatment that uses low voltage electrical currents to relieve pain. The gentle electrical current is passed from the device through electrode pads which are placed near the area of pain, creating a circuit of electrical impulses that travel along nerve fibers.
In order to understand how TENS helps relieve different types of pain, it is important to have a basic understanding of how pain works and how we sense it. Here is an example of how this process works:
Let’s say you prick your finger on something sharp, causing tissue damage. The tissue damage is registered by microscopic pain receptors (nociceptors) in your skin. Each pain receptor forms one end of a nerve cell and the other end is connected by a long nerve fiber (axon) to the spinal cord. When the pain receptor is activated, it sends an electrical signal up the nerve fiber. Nerve fibers are bundled up with many others to form a peripheral nerve, the electrical signal passes up the neuron within the peripheral nerve to reach the spinal cord. Within the spinal cord there is an area called the dorsal horn where the electrical signals are transmitted from one neuron to another across junctions (synapses) by using chemical messengers (neurotransmitters). These signals are then transmitted up the spinal cord, eventually reaching the brain. Inside the brain, the signals travel to the thalamus, a sorting station that sends signals to different parts of the brain. In the case of pain they are sent to the somatosensory cortex (responsible for physical sensations), the frontal cortex (in charge of thinking), and the limbic system (which controls emotions and drives). At the conclusion of all this, you feel a sensation of pain in your finger, feel annoyed or irritated, and think “Ouch! What was that?”, or something similar to that.1, 2
Now with a basic understanding of pain, we can explore how TENS therapy relieves pain. One school of thought looks to the Gate Control Theory of Pain for explanation. The gate control theory of pain was developed by Ronal Melzack and Patrick Wall in 1965. They proposed that there is a ‘gate’ mechanism in the central nervous system that opens and closes to allow pain messages to travel through or prevent them from proceeding. As explained earlier, pain nerves bundle up with other nerve fibers of different and/or identical diameter sizes to create peripheral nerves that connect to the spinal cord, sharing the same ‘gate’ to the central nervous system. Large diameter nerve fibers are responsible for transmitting signals of touch to the brain and smaller diameter nerve fibers transmit pain, Melzack and Wall state that the bigger nerve fibers have the ability to close the pain gate due to their bigger diameters and in doing so block the other smaller nerve fibers signals from reaching the gate..3 The electrical current being passed from a TENS machine stimulates the bigger, sensory nerves.5. As the signals caused by a TENS machine travel to the spinal cord and temporarily block the transmission of pain sensations to the brain, essentially ‘closing the gate’ for the pain signals and so the body does not experience pain4.
Another camp looks at endorphins for explanation because endorphins have been proven to decrease the feeling of pain.7. The word endorphin can be derived into two root words endo- and –orphin which are short forms of the words endogenous and morphine, intended to mean “a morphine like substance originating from within the body” so by definition endorphins are the human body’s naturally occurring painkillers..8 TENS therapy has been proven to stimulate local endorphin production.6, in other words TENS therapy has been proven to relieve the sensation of pain on/near the location where the therapy is being applied