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Understanding Pain, A Ten Week Ecourse: Part 4

By Jonathan Blood Smyth 

Week 4 - The Structure of the Pain System

This is Jonathan Blood Smyth, Founder of The Physiotherapy Site and author of Secrets of Pacing, and welcome to the fourth instalment of my Ten Week Ecourse, I hope you find it interesting and stimulating. It is certainly long.

Part Four covers the anatomy of the pain producing system, the way that pain is perceived by the tissues and conveyed up through the central nervous system to the brain.

You are welcome to publish this course in your newsletters of on your web site, on condition you do not alter it and that you keep the resource box at the bottom of each instalment.

Last time we talked about the unsatisfactory division of the mind and the body when we are thinking about pain. The organism is a unified whole, with the mind and the body cooperating to bring in the necessary information and make appropriate decisions.

The next stage is to understand the underlying anatomy of the pain producing system. The way the nervous system works has many good lessons for us as we try and understand how people are affected by their pain and what we might do about it.

I hope you don’t mind if the 10 week ecourse becomes much longer. As I write I realise how much there is to go over, and I’d rather cover the stuff I feel would be useful than skate over a lot of important topics. Stick with me if you agree…

The Pain Producing System

The pain system is very complex and wonderfully designed to give us important information about what is happening to us and our body tissues.

The pain receiving system is also known as the nociceptive system, from the latin “nocere”, to injure.

Pain is considered to have three dimensions:

  1. Sensory-discriminative. This refers to the ability to analyse the intensity, position, type and behaviour of the pain.
  2. Cognitive-evaluative. This includes knowledge, attitudes and beliefs, anticipation and attention giving.
  3. Motivational-affective. This refers to emotional responses to pain, such as anxiety, fear and depression.

Today we are starting to cover part of the first dimension, the sensory aspects of pain, how we feel it.

The Anatomy of Pain Perception

The peripheral nociceptors. These structures convert the energy of the incoming feelings into the electrochemical energy used by the nervous system. Peripheral means outside the central nervous system and nociceptors are the specialised pain nerve receptors which are sensitive to changes in their environment.

Pain receptors are found in skin, bone, muscle, joint capsules (fibrous bags surrounding joints), internal organs and blood vessels.

There appear to be no pain receptors in joint cartilage (the shiny, hard, joint covering), lung, brain or spinal cord.

Pain is mainly transmitted by two types of nerve:

  1. The small, thinly insulated nerves (known as A-delta nerves), which conduct messages at a speed of 5 to 30 metres per second. These give us sharp, stabbing type pains which we can localise easily to a particular place.
  2. The small, un-insulated nerves (known as C fibres), which conduct more slowly at 0.5 to 2 metres per second. These give us pains which are harder to pin down in area and can be dull and persistent. An example is a long term ache.

Skin Pain Receptors

  1. Mechanical pain receptors respond to strong mechanical input like blows or pressure. They have fast nerves transmitting their information to the central nervous system. These receptors are very numerous in the skin.
  2. Polymodal (”many types”) receptors also respond to strong mechanical input as above, but also to heat, chemicals and even cooling. These receptors send their messages along the slower C nerves, and are the commonest of this kind of pain receptors in skin.
  3. Insensitive receptors. These receptors may not respond to mechanical stimulus, or only to very high levels of force. They can become more sensitive in the presence of inflammation from injury or other reasons.

The Dorsal Root Ganglion (DRG)

There is no easy name for this structure! When the messages pass from the skin receptors up the nerve they reach the DRG where the nerve cells are, then travel on further to reach the spinal cord on their journey towards the brain.

Primary Afferents

The nerves which conduct sensations towards the central nervous system are called afferents (”bringing towards”). They make their way towards the spinal cord, joining together to form a “spinal nerve”.

Each spinal nerve deals with a particular patch of skin, known as a dermatome, although there is some overlapping with the nearby nerves.

The afferent nerves head into the spinal canal until they meet the spinal cord, where they join the next connections.

The Dorsal Horn of the Spinal Cord

In cross-section, the spinal cord varies in shape but elliptical is easy to visualise. Much of the structure is made of “white matter”, the upgoing and downgoing nerves which are conducting messages in each direction.

The whiteness is from myelin, the insulating material surrounding many kinds of nerve which allows fast, accurate transmission.

Apart from the white matter, there is a broad “H” shape of darker material, the grey matter. This is areas of nerve cell bodies which accept incoming messages and start to process them.

The uprights of the “H” are referred to as the dorsal (back) and ventral (belly) horns. The dorsal horn is towards the back of a person and is responsible for sensory, incoming information.

The dorsal horn is structured in layers, called Rexed Laminae in honour of the eminent swedish anatomist Bror Rexed who described the structure. This is important as each lamina has a distinct functional part to play. We’ll only think about laminae 1 and 2 to keep it simple for me and you.

Lamina 1 has a high number of nerves which look after incoming pain messages. Lamina 2 is called the substantia gelatinosa (”jelly-like essence!”) and is important as this area is the site of a multiple-connection system.

In lamina 2 there are remarkable structures called glomeruli which make multiple connections with incoming and other structures, allowing many influences to affect incoming messages. This means that pain can be affected by lots of other nerve impulses before it gets up to the head to be felt.

Where Incoming Nerve Fibres Connect in the Dorsal Horn

The incoming nerves make a web of connections with the nerves in the dorsal horn. The constantly changing pattern of inputs determines how the dorsal horn nerves fire and send signals on up towards the brain.

There is a degree of predictability in where the incoming nerves finish up in the various layers of the dorsal horn.

The larger A-delta nerves end up in lamina 1 and lamina 5. The smaller, uninsulated nerves end in the superficial areas of the dorsal horn, with the skin nerves ending in lamina 2 and internal organ nerves ending in laminae 1,2 and some in 3.

All this is what happens in normal situations. In some pathological pain conditions the architecture of the nerve connections in the spinal cord can change, leading to long term pain problems.

Check out the fifth instalment of the Pain Ecourse.

Understanding Pain, A Ten+ Week Ecourse by Jonathan Blood Smyth.
The Physiotherapy Site is your resource for orthopaedics, joint replacement, physiotherapy and pain.
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