The Role of The Sympathetic Nervous System in Persistent Pain

Firstly, what is the sympathetic nervous system?

The sympathetic nervous system (SNS) is a part of the autonomic nervous system (ANS) which is that part of the nervous system which controls most of our automatic bodily functions. In other words those that take place without us having to ‘think’ about them like pumping of the heart and breathing.

Most of our organs are under control of the ANS as well as other physiological functions such as the release of hormones and chemicals needed to keep the body operating smoothly. The ANS is also an important regulator of stress in the body and can affect how we think and feel at an emotional level (Hallman, 2013).

The ANS can be divided into the two parts, the sympathetic and parasympathetic nervous systems (SNS & PNS). Each part has a different effect on our bodily processes (Hallman, 2013).

The PNS generally functions to calm, relax and slow down as therefore will decrease heart rate and blood pressure for example.

The function of the SNS by comparison is to prepare the body for physical or mental challenge – the so called “Fight or Flight” response − through a variety of physiological changes such as increased heart and breathing rates, increased blood pressure and blood flow to the muscles of the body (Hallman, 2013).

This is essentially a ‘wind-up’ of the nervous system, getting the body ready for action. For example if a dog were to start chasing you or you are getting ready to present at a large conference. The chemicals which support the SNS are adrenaline and noradrenaline which are released by the adrenal glands in response to signals from the brain.

This response is useful when it is temporary. Problems begin when the SNS response is persistent and does not get a chance to ‘wind down’ such as during sustained periods of busyness, rushing around, long hours at work which results in higher levels of adrenaline circulating through the body for longer periods of time. Other possible contributing factors can be anxiety, low mood, poor sleep, stress and worry.

What are the effects on pain?

The nervous system, specifically the sympathetic nervous system, has been shown to be very responsive to social influences and stress. This explains why if you have a stressful week at work, or a loved one is unwell, you may find pain you may be experiencing becomes worse than normal. In some cases you may also realise that the start of your painful experiences began at a time when there were many stressful events going on in your life suggesting the pain may be more a product of the nervous system as opposed to a physical injury.

Higher than normal levels of circulating adrenaline can over time increase our sensitivity to painful stimuli and sometimes be responsible to generating pain all on its own. In this ‘wound-up’ state painful sensations can become more painful and sensations that are not normally painful can become very sore.

For example, brushing your hand off a towel to dry your hands may result in pain. This then causes you to become more worried, which in turn winds the nervous system up more, and you enter a vicious cycle: nervous system winds up resulting in more pain which may result in more worry, stress & poor sleep causing more wind-up and then further pain.

When you experience pain anywhere in the body, the sensory centres of the brain then begin to adapt and change resulting in what some experts have called a ‘pain memory’. The more persistent your pain is and the longer you have it, the larger the area affected in the brain. If this process, which has been referred to as central sensitisation, has been occurring for more than 3 months the pain may be referred to as persistent or chronic.

Well what does this mean?

1. That our lifestyle, our social situation and our work can all influence the nervous system and, therefore, our pain.
2. It is possible that you can experience pain caused by a dysfunctional nervous system but have no structural injury.
3. Educating people as to the role of the nervous system and how it can trigger pain is a useful treatment technique. A high quality randomised controlled study has shown that neurophysiology education can alter pain cognitions (i.e. how we think about pain) and physical performance (Moseley, 2004)
4. Physiotherapy techniques such as medical acupuncture as well as the neurostructural integration technique (NST) which help wind down the sympathetic nervous system can be very helpful in reducing pain and sensitivity. These can be used in conjunction with pain management education and in some cases supplementary medication.

Take home message?

1. The nervous system is a complicated system which can have a real effect on pain
2. The longer you have had pain for, the larger the impact of the nervous system on your pain
3. Personal and social stresses affect the nervous system, and therefore, your pain levels
4. Education and coping strategies and certain physiotherapy techniques are affective methods of managing persistent pain

• Hallman, D. 2013. Autonomic nervous system regulation in chronic neck-shoulder pain: Relations to physical activity and perceived stress. Acta Universitatis Upsaliensis. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine 851. 68 pp. Uppsala. ISBN 978-91-554-8561-0.
• Moseley, G.L. (2004) Evidence for a direct relationship between cognitive and physical change during and education intervention in people with chronic low back pain. European Journal of Pain, 39-45.
• Pomeranz B. Scientific basis of acupuncture. In: Stux G, Pomeranz B, eds. Acupuncture Textbook and Atlas. Heidelberg: Springer-Verlag; 1987: 1-18.

Written by Rachel Neary.