What is pain?
The science of pain is a truly fascinating and developing field. Everyday we are learning more and more about how our bodies and brains detect, compute, and respond to pain. Pain is a unique sensation that has special receptors and special pathways for processing. These special pain receptors, called nociceptors, fire when a pain stimulus like a sharp knife or hot surface come in contact with our skin. These nociceptors carry this ‘pain signal’ to the brain; however, the brain can modify this signal at multiple locations and thus can modulate the perception of pain. This means that our brains have the ability to lower the amount of pain we perceive!
One of the first points of pain processing occurs in the spinal cord. Here, there are special pain processing cells called Wide Dynamic Range (WDR) cells. WDR cells receive input from both nociceptors (pain nerve fibers), as well as cutaneous afferents (non-pain nerve fibers). Cutaneous afferent nerve fibers carry other forms of sensory information, like touch, pressure, and temperature to our brains. It is here, at the WDR cells, that TENS exhibits its modulatory pain effect thru a phenomena known as the “Gate Control Theory”.
Gate Control Theory of Pain
When nociceptor signals reach the WDR cell, they excite it. This causes the WDR cell to fire and relay a message to the brain that pain is occurring. However, when cutaneous afferent signals reach the WDR cell at the same time, they compete with the nociceptors signals and tend to decrease WDR cell activity. This decrease in activity represents a reduction in the pain signal that is sent to the brain. This gate control theory mechanism explains why shaking or rubbing your toe after stubbing it feels so good! The shaking and rubbing are forms of mechanical sensory input that are relayed to the WDR cell on cutaneous afferent fibers. They compete with the nociceptors signal at the WDR cell, and subsequently reduce the pain signal to the brain.
Now, wouldn’t it be great if you could shake and rub your aching knee or shoulder all day to compete with the nociceptor signals at the WDR cell and decrease your pain? Unfortunately, sustaining these behaviors is impractical. However, those same cutaneous afferents nerve fibers can also be stimulated electrically via transcutaneous electrical nerve stimulation. TENS works by electrically stimulating cutaneous afferent nerve fibers, which will compete with the nociceptor signals at the WDR cell, ultimately decreasing the amount of pain that is eventually perceived by the brain. If you have a TENS unit, follow the instructions below to begin your pain relief.
Electrode size and placement: Try to use the largest electrode pads possible for the treatment area. Bracket the joint or area of pain by placing electrodes on either side of the painful area.
Frequency: Frequency is a measure of how often the electrical pulse is delivered. Most TENS units offer a range between 50-120 pulses per second. The higher the frequency the better, as more pulses per second means more competition at the WDR cell.
Phase Duration: Phase duration is a measure of how long the electrical pulse lasts. Most TENS units offer a phase duration between 50 – 150 microseconds. Generally a shorter phase duration around 100 microseconds is recommended.
Amplitude: Once the frequency and phase duration of the TENS unit is set, you can adjust the intensity, or amplitude, of the TENS unit. The sensation of electrically stimulating your cutaneous afferents is a little unusual, so slowly raise the amplitude until you begin to feel a little tickle or buzzing sensation. This signifies the TENS unit activating the cutaneous afferents. For best pain management, now raise the amplitude as high as possible without a muscular contraction. At higher amplitudes, the electrical signal has the potential to stimulate both cutaneous afferents, as well as muscle motor neurons. For pain management with TENS, you do not want to stimulate your muscles, only the cutaneous afferents.
And that’s it! You can leave the TENS unit on for as long as you like. At a minimum, keep the TENS unit on for at least 20 minutes, as it takes a while for the TENS unit to exert its effect. If you have any questions with setting up your TENS unit, please leave me a comment below.