What is a “Pain Pathway”? An Introduction

Orthopaedic & Spine Center

Jenny L. F. Andrus, MD

As an Interventional Pain Management Physician, I see patients who suffer from chronic pain as a part of life.  In understanding pain, it important to know that pain not only has many emotional, psychological, and sensory components, but that it also is a physiological activity as well.  There is a complex process that happens in your body between stepping on a tack and having that pain register in your brain so that you quickly react and pull the tack out of your foot.  In this article, I’m going to discuss pain pathways, what they are and how they communicate different types of pain and why that is important to understand in the treatment of chronic pain.

Imagine, if you will, the nerves in your body as a superhighway.  Your brain is the powerful police officer in charge of managing traffic through the busy intersection of your spinal cord and all the nerve root roads that emerge from it and head to distant parts of your body.  The body is filled with sensory receptors, called nociceptors, that can detect unpleasant stimuli (pain) and transform these into electrical signals, which are sent to the central nervous system and brain.  They can be found in the skin, joints, muscles, internal organs, etc. and can be activated by heat, cold, chemical or mechanical stimuli.

Nociceptors are also two types of primary afferent fibres that carry noxious sensory information, type ADelta and C.  There is another type called ABeta that carries stimuli considered to be pleasant or not noxious.  Below is a description of each type of fibre:

ABeta fibres have a large diameter and can carry a lot of information rapidly.  They are also highly myelinated (myelin is the covering over nerves, made up of fats and proteins, that allows them to transmit electrical impulses well), respond to light touch and transmit any pleasant or non-noxious stimuli.

ADelta fibres have a smaller diameter and are less myelinated, so they don’t carry as much information as quickly as ABeta fibres.  However, they carry our initial, reflexive response to acute, sharp pain and respond to thermal and mechanical stimulus.

C fibres are the smallest in diameter and aren’t myelinated, so they have the slowest conduction time of all the primary afferent fibres.  C fibres are responsible for burning, slow pain and are polymodal, meaning that they are activated by chemical, thermal and mechanical sources.

It is important for me to understand nociception and how each of these fibres function as a pain pathway because it often can help me to find an effective treatment option for your pain.  If you have heard of the procedure called Radio Frequency Ablation, I use heat to burn the myelin sheath off nerves so that they do not transmit pain signals well.  Hence, the patient is provided pain relief.  Without this in-depth understanding how nerves function polymodally in the body and how they transmit information to the brain, including pain, researchers would not have been able to develop this effective treatment.

In this article, I discussed a small portion of the “pain pathway”.  In my next article, I’ll talk about the role neurotransmitters and ascending tracts in the spinal cord play in chronic pain.