Pain science: Part 2: How Does it Happen?

Pain Science Part 2: How does it happen?

This is finally a continuation of the previous post entitled Pain Science Part 1. As you already know, I am very excited to introduce this and hopefully be able to explain pain in relatively simple terms. The last post talked about what pain is and what pain isn’t. Now it is time to explain why we experience pain and how it all happens. Let’s start peripherally.

Input to the system includes:


  • There is no such as a pain fiber. Tissues contain nociceptive fibers which are not pain fibers. They contain danger receptors- not pain fibers.
  • We don’t conduct pain in the periphery. We experience pain through the brain.
  • The tissues send signals to the spinal cord then up to the brain, the brain decides to call something painful or not based on its overall threat.
  • For example: If you sprain your ankle in the street you experience pain. If a bus is coming the brain will reduce the threat of the sprained ankle and make you move to get out of the way of the bus. Once you are in the clear, the pain comes back because this is the highest threat to the system.
  • Pain does NOT come from the tissue itself!
  • Another example: arthritis. Arthritis does not cause pain. How many patients have you had with, let’s say back pain, and when you have them perform exercises you can hear their knee crunching but they experience no knee pain. Pain is NOT from the periphery.

Peripheral nervous system

  • The PNS sits right below a threshold baseline. The input from the PNS either fires at a rate higher than threshold which is sent to the spinal cord or it fires just below baseline in which no response is sent further on into the system.

The injury:

  • All nerves are myelinated. When an injury occurs these myelinated axons become unmyelinated.
  • At the dorsal root of the spinal cord, ion channels are formed from DNA/mRNA and then travel down axon plasma and insert into specific areas that are unmyelinated.
  • After an injury, these ion channels form at the injury site. For example, if you develop a stress fracture/reaction in your tibia this causes a decrease in myelination-> there will then be an increase in ion channels in the area.

Ion Channels:

  • Channels open and close based upon a whole host of other influences:
    • Temperature, stress, movement, immunity, blood flow
  • They are always in a balance and if there is an increased input from one of the influences it will open the channel and send chemicals up the axon to the spinal cord.
  • For example, stress reaction= ion channels= cold temperatures= increased activity in the spinal cord. This does not = pain. Pain will be determined by the brain
  • If you are more stressed about the injury then the signal to the spinal cord will increase. If you are sick and also have this injury then there will be an even larger increase in signal to the spinal cord.
  • Ion Channels have a half-life of 48 hours. They are replaced naturally by neuroplasticity.
  • The signal is sent via chemical release from the ion channels opening. They fire in both directions. If the ion channels keep opening then chemicals are continually released to the spinal cord and can have long lasting persistent effects that have nothing to do with the current injury.
  • By educating the patients about pain, we can down regulate the system and decrease the activity of the ion channels and therefore decrease the chemicals released.

The Spinal Cord:


  • The chemical released by the ion channels travel to the dorsal horn of the grey matter of the spinal cord.
  • These then are picked up by the spinal interneuron
    • The job of the interneuron is to determine what gets inhibited (stopped there) or what gets relayed up the spinal cord to the brain.
  • There are two fibers that come into the spinal cord
    • A-fibers which regulate everyday light touch activities like the feeling of your pants against your skin
    • C-fibers which are the nociceptive fibers that will alert to danger
  • The interneurons will then take the inputs and determine if the threshold is met and then either send the signal on or stop it there.
  • There is a convergence of information at this point. Not only from the left side and at the L3 level but also from the right side and everything below that level because it receives information from each segment.
    • This is why many people with chronic pain will have a bilateral presentation.

Chronic pain/sensitization

  • Persistent firing by the ion channels cause increased chemicals to be released at the dorsal root of the spinal cord.
  • This causes increased input to the interneuron, increased above thresholds signals relayed up to the brain and therefore consistent awareness down the chain at the site of the tissue.
  • This persistent input can degrade the interneuron and this also can then affect the levels around it because they are receiving even more input due to the lack of one of the levels not regulating the input.
  • These DON’T regenerate!
  • The A-fibers then begin to grow more and now light touch becomes an issue leading to difficulty with discrimination of your left/right sides.
    • Mirror therapy!

The Brain:

  • There is no pain area in the brain!
  • Some common areas light up but the pain experience is a combination of multiple inputs from multiple areas of the brain.
  • On brain imaging from fMRI, chronic pain patients have a brain that is very alert and hyperaware. There is too much going on! If our goal is herniated disc -> pelvic tilt -> and we make them think that they have to maintain this all we are doing is increasing input to the system that is already overloaded
    • On a side note: chronic pain and abdominal exercises. No, they usually aren’t “weak” but actually they are already co-contracting too much. If we focus on this even more then all we are doing is scaring the patient into thinking that they are too weak and this will increase the system even more! We need to down regulate!
  • Typical Pain Nueromatrix taken from A. Louw (these are the areas that light up the most)
    • Premotor cortex/motor cortex
      • Movement
    • Cingulate cortex
      • Concentration and “fog”
    • Prefrontal cortex
      • Memory
    • Amygdala
      • Fear and addiction
    • Sensory cortex
      • Right and left discrimination, joint position
    • Hypothalamus/thalamus
      • Stress responses, chemical output and motivation
    • Cerebellum
      • Movement and fine motor control (ie. Those patients who can’t even perform those simple lift leg up exercise)
    • Hippocampus
      • Fear conditioning
    • Spinal cord
      • Peripheral inputs
    • As you can see from the above list, increased input from the spinal cord influences all of these areas. Think about your patients who are too tired to get out of bed, who are too fearful to move, lack motivation, have a difficult time even with simple tasks, etc
    • If a patient has difficulty with simple tasks because the brain is too active and that area has such a low threshold. The communication between the brain and the body is impaired. This can lead to those imbalances and poor movement that we see.
      • Not necessary weak but rather inability for brain to focus on that one specific movement

The output:

  • The brain puts every other system on hold except for the most important ones. It decides this based upon threats. If one threat is larger than another then this system will take precedence.
  • The brain releases a whole host of chemicals in response to the perception of a threat/pain
    • Adrenaline
      • Stress chemical (see below)
    • Cortisol
      • Shunts blood and can make tissues sore/tired/sensitive/fatigued
      • Impacts sleep, memory, concentration
      • Increases nerve sensitization (ion channels), persistent inflammation (makes the body think that an area is still injured when not thereby increasing sensitivity to an area), and general malaise.

In summary:

Once you understand that the brain controls everything. Questions begin to answer themselves. For instance, general “patella femoral” knee pain. We can all agree that the research shows an impaired onset of the activation of the VMO versus the other quad muscles. But why? Well, if you believe that the brain truly runs the system, then you have to believe that there is a reason for it. Not because there is pain there. Pain is not there! Pain is in the brain! So if you have someone that collapses in with running and there medial knee is painful with a reduced VMO activation (we can’t measure this in clinic). Then look further. If there is a dynamic valgus then the patella no longer needs to track medially. It make sense for the patella to stay in the groove and this would be best accomplished by activating the vastus lateralis first. The problem would be at the hips and proper control over the musculature in that area. This does NOT include just SLR 4 way and call it a day but much more.

If everything is regulated by the brain then so is manual therapy! Yes, we like to think that we are doing this awesome adjustment. But really all we are doing to sending a neurological feedback into the system which hopes to “reset” the system. At least give us a way in. Watch the Medbridge Chad Cook cervical lecture and he points to some great research showing the chemical effects that manual therapy has and the short time effects if it.  I will also go into how to explain it to patients (creating handouts soon) and also what this means in respect to different cases and “painful” experiences.


I really like to use pain science to better understand what we do as therapists. But it should never be used alone. There is always something we can do to help the patient. Use reset systems (like Mackenzie, mulligans, any manual therapy), use proper words and also improve movements. Give the patient hope and a purpose to getting better.

~ TJ Slowik

Thank you to Adriaan Louw for one of the best continuing courses I have taken (through medbridge).

Thank you to Adriaan Louw for one of the best continuing courses I have taken (through medbridge).


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