Form and Function

As a doula I've seen a lot of benefit from the use of Transcutaneous Electrical Nerve Stimulation (TENS) for decreasing the intensity of labor pain. A TENS unit is a small battery-powered system that sends a mild electrical current along lead wires to sticky electrode pads that are placed on the body on either side of the mid-sagittal plane in the area that is experiencing pain. The electrical stimulation runs between the electrodes creating a buzzing feeling in the skin and, if turned up to higher levels, some involuntary muscle contraction. I've long understood that the stimulation works based on the "Gate Control Theory of Pain" by competing with pain reception in the brain, but decided to go digging for the details on how and why this theory explains the benefit of TENS use.

I found these lovely diagrams on Wikipedia and managed to decipher the explanation sufficiently (would have probably been easier to do after neuro A & P but oh well) to satisfy my curiosity, and I am including the explanation as follows.


Part One: No Competing Stimulus
In this diagram, stimulus (represented by the lightening bolt) is carried along a C fiber (which is specialized for pain reception) and arrives at the projection neuron which then passes the message along unimpeded toward the brain, resulting in the experience of the sensation of pain.

Part Two: With Competing Stimulus
Pain stimulus is carried along the C fiber to the projection neuron. At the same time, non-painful stimulus (in this example the buzzing sensation generated by the TENS) is carried by a A β fiber to both the projection neuron and an inhibitory interneuron. Firing of the inhibitory interneuron inhibits firing of the projection neuron, and no stimulus signal is sent to the brain, essentially closing the "gate" by which pain signals were being sent and thereby reducing the overall experience of pain. Presumably not all inhibitory interneurons are being fired by the competing stimulus and some sensation of pain is still experienced.