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Gate Control Theory - The Delusion of Pain

I want you to remember the last time you played hard and sprained an ankle or pulled a muscle. Now, I want you to think about how you sprayed a pain relief spray or applied an ointment over that area and felt a soothing warmth and cold spread over. Slowly your pain dissipates a little and you relax. By then, the pain killer medication you took has taken effect and you fall into a relaxed sleep. Sound familiar? 

I am sure all of us have been subjected to various degrees of pain in our lives. Every mother has her own way of dealing with her child’s pain. From mind control with lollipops and ice creams, to fear tactics to make babies forget the pain from vaccinations, everything seems fair game. But we don’t really, fully understand the actual mechanism of pain. 

Yet, pain is one of the most common yet complicated phenomena that human beings are subjected to and the alleviation of pain is one of the principle pillars of medicine. Imagine a world without in which you had to live with pain for the rest of your life. A life like that surely belongs in one of the seven circles of Dante’s hell. 

In todays article, we’ll try to understand one of the theories of pain and its implications. In 1965, Ronald Mezlack and Patrick Wall proposed the gate control theory of pain. But to understand this theory fully, we must first understand the electrical wiring that governs our body :  the nervous system. 

In 1924 Erlanger, Gasser and Bishop classified the peripheral nerves. These nerves which are responsible for carrying messages from the various parts of the body to the spinal cord are divided into three classes based on diameter of the nerve fibre and their myelination: A, B, C. 

A fibers are the thickest fibers.

They are myelinated and have the fastest conduction velocity of the three types. They are further divided into four classes based on their diameter:

  • A alpha: these carry impulses from the Golgi tendon and muscle spindles and help with proprioception
  • A beta: these carry impulses from cutaneous mechanoceptors - basically pressure
  • A delta: carry impulses from free nerve endings which transmit pain and temperature
  • A gamma: motor neurons which control the intrinsic tone of the muscle

B fibers are also myelinated by they are thinner than A fibers. They carry autonomic information.

C fibers are the thinnest and they are unmyelinated which means they have the slowest conduction velocity. They carry deep pain from the visceral structures and the muscles. Typically the pain carried by the C fibers is slower, longer lasting and throbbing in nature. 

So now that we know the various types of fibers, let us see how they play a role in changing our perception of pain. The A delta fibers and the C fibers both carry pain to the spinal cord and they synapse on the same projection neuron that carries the impulses to the pain centres in the brain. 

Remember that A delta fibers are larger and faster than C fibers and as a result when both of these fibers fire together, the A delta fibers will not allow the impulses of the C fibers to be transmitted, which effectively limits the deep throbbing visceral pain. 

So when you spray that pain relief spray onto your skin, the cool burning sensation that you feel is picked up by the A delta fibres and carried to the spinal cord. These fibers inhibit the already firing C fibres which are responsible for the pain of the muscle pull or sprain and give you an illusion of pain relief. This gating the pain already being carried by the C fibers, by the A delta fibers is known as the Gate Control Theory of pain. 

The gate control theory does not stop with just pain however. Other mechanisms can also be used to trick your body into feeling pain relief. Vigorously rubbing an area that has recently injured, activates the A beta fibers which carry mechanical stimuli like pressure will also overshadow the deep slow pain carried by the C fibers.

Sympathetic overdrive states like fear and anxiety may also do the trick -  this is how scared babies don’t feel shots. This also explains how soldiers and athletes are able to bear inhuman levels of pain in war zones or on the fields. It is said that higher centres of pain can also be manipulated by endogenous opioids like enkephalins to reduce pains. 

So next time you injure yourself or see someone else injured, don’t worry so much. You might not be able to cure their pain, but you can definitely fool them into not feeling it!

Author: Narendran Sairam (Facebook)

Sources and citations

ABPP, William W. Deardorff PhD. “The Gate Control Theory of Chronic Pain.” Spine-Health, 3 Nov. 2003, (www.spine-health.com/conditions/chronic-pain/gate-control-theory-chronic-pain.
Moayedi, Massieh, and Karen Davis. “Theories of Pain: from Specificity to Gate Control.” (http://www.physiology.org, www.physiology.org/doi/10.1152/jn.00457.2012.