The autonomic nervous system has three branches – the sympathetic, parasympathetic and enteric nervous systems. Working automatically, the ANS silently adjusts the balance between the sympathetic and parasympathetic nervous systems. The enteric nervous system, whilst an independent system, works side-by-side with the parasympathetic nervous system.
The job of the autonomic nervous system is to respond to external threats when they arise, to continuously monitor internal body parameters making adjustments to keep them within optimum range. These functions include heart rate (circulatory system), breathing rate (respiratory system), peristalsis (digestive system), neurotransmitters (nervous system), hormone release (endocrine system) and muscle tone (skeleton-muscular system).
The sympathetic nervous system (ANS) – your “fight or flight” response – is designed to respond during stress and emergencies. It is activated when you are physically or emotionally aroused. When activated the body and mind are prepared to either defend or run away from a perceived threat in an automatic physiological reaction to situation (real or imagined) which is threatening or overwhelming.
If perception of danger continues following the activation of the first level of the stress response, the second level of stress response called the “HPA-axis” – consisting of the hypothalamus, pituitary gland and adrenal glands – is initiated and your body remains on high alert. The hypothalamus releases corticotropin-releasing hormone which travels to the pituitary gland where adrenocorticotropic hormone is released, travelling to the adrenal glands triggering the release of cortisol. When the threat passes, the parasympathetic nervous system dampens the stress response and cortisol levels fall.
When the sympathetic nervous system is inactive, the parasympathetic nervous system (PNS) – “rest and digest” response – dominates. The PNS controls bodily processes during ordinary situations. It is restorative, calms body and mind, brings blood pressure and heart rate to normal, and stimulates your digestive tract to process food and eliminate waste.
The enteric nervous system is also called the “Second Brain”. It consists of 100 million neurons embedded in the walls of your digestive tract and has its own reflexes and senses. This is where the business of digestion occurs, which is why your emotional state drastically affects your gut – for example, you get butterflies in your stomach when you feel anxious. Conversely, a sensation from the gastrointestinal tract can also affect your emotions – for example, when you have indigestion you may feel disturbed.