Pain Perception - Why Does Your Back Hurt?

4 Theories of Pain Perception

Pain treatments through the millennium started with electrical fish.
Pain treatments through the millennium started with electrical fish.

What makes it so you feel pain? Is there anything other than damage to your tissues that leads you feeling, for example, like a truck ran over you, or experiencing burning pain in your back, hip or legs?

Scientific theories of pain perception go back to the ancients when torpedo fish - which emit an electrical charge as a way of protecting themselves from an attacker - were harnessed for use in early pain management treatments: Headache relief, labor pain and more.

Other theories were fostered through the millennium. Nowadays (in the 20th and 21st centuries), 4 main theories of pain perception have taken root in medical thought. While none have all the answers, each provides some guiding light for development of new pain relief treatments by drug and device companies.

Defining Pain

So what is pain, anyway? I'm sure you have your own definition but in order to talk about it effectively it's probably a good idea to work from a consensus-based understanding. According to the International Association for the Study of Pain (IASP), pain is:

“An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of tissue damage.”

Basically IASP's definition says that pain is unpleasant and may or may not be due to actual tissue damage. This definition also says that not only is the pain physical (felt) but there's an emotional component, as well.

Now that we are on the same page about what pain is, let's look at the 4 main schools of thought about how it comes to be felt in your body.

Specificity Theory

Pain, as well as touch are first received by the body as raw stimulus. The specificity theory says that specialized sense organs in your skin, muscles and joints quickly pick up the stimuli and send it to your brain via "pathways" through the nervous system.

Each type of sensation - pain and touch - have their own type of specialized sense organ and their own unique pathway enroute to the brain. The stimulus is sent to your brain for interpretation.

You might think of a pathway as the road traveled in a relay race - where one of several runners pass a baton until a finish line is crossed by the last team member. The analogy goes like this: One nerve along the path is like one of the runners and the baton represents the nerve signal (the stimulus) that gets moved along. When the last runner crosses the finish line represents the nerve impulse reaching a place in the brain that is capable of interpreting its message. In Specificity Theory, each relay team has its own road up to the brain.

Related:  What is Nerve Pain?

Intensity Theory

The intensity theory discards the idea of distinct pathways of travel for a nerve signal to the brain. Instead, it classifies the stimuli that develops into nerve signals as either low or high threshold, and postulates that the number of impulses in the nerve cells determines how intense that stimulus is.

Once the nerve signals reach the spinal cord, they are encoded according to their degree of intensity. Low level nerve signal activity is interpreted as "innocuous" while higher levels are "understood" by the central nervous system as "noxious."

Pattern Theory of Pain

As the name suggest, the Pattern Theory of Pain is about patterns of nerves that "fire." This theory says that when nerves fire, a sensation results. But what type of sensation - and how intense is it? This is determined by the pattern of the firing, i.e. when, how often and where. When the stimulus is intense, pain is perceived.

The Gate Control Theory of Pain

The Gate Control Theory of Pain is likely the most influential of all modern pain theories. It explains that aspect of pain that doesn't tend to cooperate with the stimulus-reaction model - in other words, many forms of chronic pain, and many pain management practitioners were originally attracted to the Gate Control Theory for this reason. Not only that, but the Gate Control Theory can be structured to explain other types of clinical signs and symptoms as well, according to Davis and Massieh in their 2012 report, "Theories of Pain."

The theory says that a "gate," essentially an area of specialized nerve cells located inside the spinal cord, is either excited or inhibited, depending the type of nerve fiber that relays the signal. A-fibers are large and close this "gate," while C-fibers are small, and open it.

The Gate Control Theory brought together the most accurate data from the Specific and Pattern Theories. Developed in 1965 by Melzack and Wall, it formed the basis for the first dorsal column stimulator, now known as spinal cord stimulation, or neuromodulation, which was introduced in 1967.

Related:  Understand spinal cord stimulation systems.

Sources:

IASP. IASP Taxonomy. IASP website. Accessed March 2015. http://www.iasp-pain.org/Taxonomy?navItemNumber=576

Massieh Moayedi , Karen D. Davis. Theories of Pain: From Specificity to Gate Control. Journal of Neurophysiology Published 1 January 2013 Vol. 109 no. 1, 5-12 DOI: 10.1152/jn.00457.2012http://jn.physiology.org/content/109/1/5

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