This is a summary of section 2 of “Explain Pain” by David Butler and Lorimer Moseley.

Alarm Signals

Our body’s alarm system alerts us to danger or potential danger. This alarm system is composed of sensors throughout the body, the eyes, nose, and ears. It is these sensors that are our first line of defense against harm. If one sensor fails the others take over.

Most of these sensors are located in the brain and respond to various stimuli. Some to mechanical movement, some to temperature change; the sensors in the brain particularly respond to chemical activity.

What is important to know with sensors is that they have a very short life expectancy of a few days. This cycling means our body’s sensitivity is constantly changing. It is with these life cycles that there is hope for those with chronic pain.

Moreover, the rate at which sensors are made is normally stable but can change very quickly in regards to a particular stimulus. So if we take for example one with persistent pain, the rate at which pain sensitivity occurs can be changed.

Nociception

We lack pain receptors in our bodies. Instead, the various tissues have special neurons that respond to different stimuli. These receptors are called nociceptors, which translates into “danger receptors.” Nociception is occurring all the time, but only sometimes will it end in pain.

Nociception is neither necessary nor sufficient for pain.

The sensors correspond to particular neurons. In order for these neurons to become excited and send signals to the brain, an action potential must occur. An action potential is a spike in which nerves relay messages. These spikes require a certain amount of stimulation to go over a certain threshold. Think of it the same as if someone were doing many things to make you mad. Eventually, that person will cross the line and may cause you to get very angry. Action potentials act very much the same way.

These nociceptors can become very active very quickly depending on resting stimulation. If at rest these sensors are stimulated high enough, then small changes could cause the action potential threshold to more quickly be reached. This happens in acute injuries for example. Suppose you scrape your leg. The skin sensors along the scrape have increased sensitivity. This change may lead to even a slight touch to the injured area creating a nociceptive response.

Notice that above I have been neglecting to say pain. This is because the brain and spinal cord have to analyze the incoming nociceptive information prior to pain being felt. This processing is why not all nociceptive responses are painful.

The Pathway

When nociceptive fibers activate, a signal is sent to the spinal cord. Here at the cord, chemicals are released to activate surrounding neurons. The various neurons are built to respond to particular chemicals and not to others.  This is called the Lock and Key Principle.

From the spinal cord, neuronal messages are relayed to the brain. In the brain, all relevant stimuli are processed and sorted out to determine the best course of action. There is not one part of the brain, but several, that deal with pain. These areas are called “ignition nodes.”

The response that can occur from the brain can affect multiple body systems in order to get us out of trouble. Here are the different effects that can occur with an injury.

• Sympathetic nervous system – Increase heart rate and vigilance, mobilize energy stores, sweat.
• Motor system – Run away, fight, protect damaged area.
• Endocrine system – Mobilize energy stores, reduce gut and reproductive activity.
• Pain production system – Motivate to escape and seek help, attract attention.
• Immune system – Occurs later post-injury, but cleans up injured area, increases sensitivity, produces fever, makes one sleepy.
• Parasympathetic system – Occurs later post-injury, but promotes healing.