What is the Autonomic Nervous System?

The autonomic nervous system (ANS) has already been introduced; let’s summarize what we know about it so far. Part of the peripheral nervous system, the ANS consists of the sympathetic nervous system (SNS), which consists of outflow from the thoracic and upper lumbar spine, and the parasympathetic nervous system (PNS), including outflow from the cranial nerves emanating from the upper spine and also from the mid-lumbar to the sacral areas at the buttock region. Several neurochemicals help transmit autonomic instructions. These include epinephrine (adrenaline), norepinephrine (noradrenalin), dopamine, and acetylcholine. This chapter will focus on how abnormalities in the regulation of the ANS cause many of the symptoms and signs observed in fibromyalgia. Our body has numerous receptors or surveillance sensors that detect heat, cold, and inflammation. These ANS sensors perform a function known as autoregulation. As an example of how the ANS normally works, why don’t we pass out when we suddenly jump out of bed? Because the ANS instantly constricts our blood vessels peripherally and dilates them centrally. In other words, as blood is pooled to the heart and the brain, the ANS adjusts our blood pressure and regulates our pulse, or heart rate, so that we don’t collapse. On the local level, these sensors dilate or constrict flow from blood vessels. They can secondarily contract and relax muscles, open and close lung airways, or cause us to sweat. For instance, ANS sensors can tone muscles, regulate urine, and regulate bowel movements, as well as dilate or constrict our pupils. The SNS arm of the ANS is our “fight or flight” system, releasing epinephrine and norepinephrine as well as a neurochemical called dopamine. Whereas the SNS often acts as an acute stress response, the PNS arm tends to protect and conserve body processes and resources. The SNS and PNS sometimes work at cross purposes, but frequently they work together to permit actions such as normal sexual functioning and urination. How do the workings of the ANS relate to fibromyalgia? The SNS is underactive in fibromyalgia in the sense that an increased ratio of excitatory to inhibitory responses from central sensitization results in lower blood flow rates, leaky capillaries, at relatively low baseline blood pressure.