Ion channels, receptors and action potentials
Last month we began to explore the role of nerves in the pain experience. We talked about 'nociception' – danger messages that a nerve or nerves will send to the brain. You may recall that these electrical impulses themselves are not a 'pain message' and that the pain experience is a brain based construct using all relevant information for that time, place and experience.
Let's look at nerves in more detail. Nerves from the tissues (muscles, skin etc) to the brain will only send quantitative data and not qualitative. The nerves do this via receptors or ion channels which are positioned at the end of the nerve and 'open and shut' in response to stimuli. They are made of proteins and respond to 3 differing stimuli:
- Temperature changes, such as hot and cold
- Mechanical changes or stresses, such as stretching and vibration
- Chemical changes in the tissues, such as inflammatory by-products, histamine, adrenalin, stress hormones like cortisol.
Opening ion channels allow for enough positive charges to flow into the nerve, thereby 'exciting it'. At a critical level of 'excitement', the nerve will 'fire off' an electrical impulse to the brain via the spinal cord. This is called an action potential. It is this message (and the intensity of it) that the brain uses to decide whether a pain experience will result from it. Can you see that the ion channels and therefore the messages to the brain will revolve around the stimuli of temperature, chemical or mechanical?
Another amazing fact about receptors is that they are in a constant state of change or remodelling (like the rest of our body eg skin cells). Receptors close, move and open every few days and will do so in response to these 3 stimuli. When a nerve is under greater levels of stimulation, more receptors will form and open. We can call this an 'over-excited' or 'over-sensitive' nerve. Here are some common examples we see all of the time:
- poor postures of the neck and upper back, increasing the pulling stresses in shoulder and upper back nerves
- anxiety and stress can increase the number of open chemical ion channels in all nerves
- inflammatory products such as histamine in a newly sprained ankle will increase the opening of the chemical ion channels.
An 'over-sensitive' nerve will send increased intensities of danger messages and your brain will need to sit up and take more notice of this. The brain is more than likely to say "hey, this is enough of a danger message to decide I am in pain."
And there is always a more hopeful and positive side to these facts! Receptors and ion channels can close by decreasing the levels of these 3 stimuli. And less ion channels opening will mean less danger messages to the brain. Sound great?
How do we do this? By reducing the chemical and mechanical loads on the nerves. There are many many ways, but here are a few:
- unload the nerve mechanically such as through movement, changes in posture, better muscle strength and endurance to get you through your days, better work place ergonomics
- improve the health of your nerve tissues such as through nerve gliding exercises to improve the blood flow to the nerves and their protective sheaths
- de-stressing and reducing anxieties, such as through education, psychology, relaxation techniques, drugs such as anti-depressants.
So as you continue to work on your good postures, build 'core' and trunk strength and slide your nerves in all those fancy ways, remember that you are not only having a positive impact on your muscles and joints, but also your nerves. You are helping to 'dampen them', making them less sensitive and reducing the levels of nociceptive danger messages to your brain.