How does TENS work?

TENS has been used for 50 years to provide pain relief, however the reason it works remains something of a mystery.  The most common explanation is based on the pain regulation theory proposed by Drs. Ronald Melzack and Patrick Wall in 1965 called the “pain gate theory.”  This theory states that stimulation of nerves carrying non-painful sensations (called A-beta nerve fibers) closes a neurological “gate” in the spinal cord that reduces transmission from pain nerves (called A-delta and C nerve fibers) to the brain.  By preventing pain signals from reaching the brain, the perception of pain decreases.  An illustration of the Melzack and Wall theory is shown below.

Let’s say you have inflammation in the knee causing pain.  This signal will be carried from your knee to the spinal cord by C and A-delta nerve fibers.  Once in the spinal cord, the signal is passed to a projection neuron that sends it on to your brain (sensory cortex) which results in you feeling the pain.  However, if you simultaneously stimulate the A-beta nerve fibers in the vicinity of the pain with a TENS device, then that non-painful signal will be carried to the spinal cord where it activates an interneuron that inhibits the projection neuron from sending the pain signal from the C and A-delta nerve fibers to your brain, leading you to feel less pain.  If you think of the projection neuron as a “pain gate” then the stimulation of the A-beta nerve fibers “closes the gate.”  In reality the biological mechanisms are considerably more complicated than depicted here, nevertheless the pain gate theory remains a useful tool for understanding how TENS works.