The Autonomic Nervous System

Your body’s ability to deal with stress is regulated by the autonomic nervous system (ANS). This system monitors the environmental signals, interprets them, and organizes appropriate automatic behavioural responses.  It is composed of a specialized group of neurons that regulate cardiac muscle (the heart), smooth muscles (walls of the visceral organs and blood vessels) and glands.

The autonomic nervous system has two components that balance each other – Protection – the sympathetic nervous system(SNS) and Growth – the parasympathetic nervous system (PNS). What is important to know is that both systems CANNOT operate optimally at the same time. We unavoidably restrict our growth behaviours when we shift into protective mode (stressed).

Protection – the HPA axis and the Immune system

The sympathetic nervous system (SNS) turns UP your nervous system. It helps us handle what we perceive to be emergencies or threatening situations (can include emotional upset as well as physical stress) and is in charge of the flight-or-fight response. The SNS has two systems to protect the body: the Hypothalamus – Pituitary – Adrenals axis (HPA Axis) which responds to perceived external threats, and the Immune system which protects us from threats originating underneath the skin (like attack by virus or bacteria).

1. HPA axis

• When the body perceives that there are no external threats, the HPA axis is inactive and growth flourishes.
• When brain’s hypothalamus perceives an environmental threat, it engages the HPA axis by sending a chemical signal (CRE) to the pituitary gland.
• The pituitary gland (master gland) is responsible for organizing the body to deal with the impending threat.
• The pituitary gland sends a chemical signal (via ACTH) to the adrenals informing them to coordinate the body’s “fight or flight” response via stress hormones.
• HPA axis interferes with both the immune system (protection) and growth systems:
• HPA axis also interferes with our ability to think clearly
  • The processing of information in the forebrain (the center of executive reasoning and logic) is significantly slower than the reflex activity controlled by the hind brain
  • Adrenal stress hormones constrict the blood vessels in the forebrain reducing its ability to function
  • Stress hormones also repress activity in the brain’s prefrontal cortex (the center of conscious, volitional action)
  • In an emergency, the vascular flow and hormones serve to activate the hindbrain (source of reflexes)
  • Stress hormones inhibit neuronal growth, leading to depression. In chronically depressed people the hippocampus and prefrontal cortex are physically shrunken.
    • When the brain’s stress machinery goes into overdrive, it leads to depression.

2. Immune system

• When the immune system is mobilized, it can consume much of the body’s energy supply.
• Adrenal hormones secreted by the HPA axis actively repress the action of the immune system

Growth – the Vagus Nerve

The parasympathetic nervous system turns DOWN the nervous system and helps us to be calm. It is most active when the body is at rest and not threatened in any way. This division is sometimes called the ‘resting-and-digesting’ system since it is chiefly concerned with promoting normal digestion, with elimination of feces and urine, and with conserving body energy. It promotes relaxation, rest, sleep, and drowsiness by slowing our heart rate, slowing our breathing, constricts the pupils of our eyes, increases the production of saliva in our mouth, and allows us to digest our food and grow.

The vagus nerve is the key instrument of the parasympathetic system. Beginning in the medulla oblongata, the nerve travels to all of the organs of the body sending signals to and from the brain. The two previous posts provide lots of information about the vagus nerve and how to activate the relaxation response. Post 1 – The Vagus Nerve, Post 2 – Activating the Vagus Nerve.

Importance of sleep

A recent survey found that more people are sleeping less than six hours a night, and are having difficulties sleeping (because they are unable to turn off their HPA axis). Chronic sleep loss can contribute to health problems such as weight gain, high blood pressure, and a decrease in the immune system’s power, reports the Harvard Women’s Health Watch.[i]

Some important tips for getting a good night’s sleep and allowing the PNS time to heal and relax are:

1. Keep a regular sleep schedule – set a regular time to go to sleep, and to wake up. After dinner, when you are relaxing on the sofa, try not to fall asleep and then wake up late in the night, get up and do something mildly stimulating. On the weekend, catch up on your sleep with naps.
2. Naturally regulate your light/dark cycles – increase light exposure during the day by taking walks outside in the light, use a light therapy box in the winter to offset SAD symptoms. In the evening, turn off the television and electronic devices in your bedroom and close the blinds so outside lights don’t disturb you.
3. Create a relaxing bedtime routine – keep noise down, keep the temperature cool (adjust your thermostat to automatically drop several degrees during the night), and sleep in a comfortable bed.
4. Eat right and get regular exercise – stay away from really heavy meals late at night, avoid alcohol before bed, cut down on caffeine and avoid drinking too much liquid before bed. Having fruit several hours after dinner or before bed puts the system into ‘detox and cleanse mode’ which turns on the pancreas and liver and may keep you awake. If you are hungry before bed, try a light snack of a small piece of turkey or chicken breast, or avocado, or some soaked nuts with some plain yogurt.
5. Get anxiety and stress in check – try the activation of vagus nerve techniques in the previous post to turn on the PNS.

Today, we live in a stressed-out world and an increasing body of research suggests that our hyper-vigilant lifestyle is severely impacting the health of our bodies. Daily stressors and emotional upsets are constantly activating the HPA axis causing emotional and physical disharmony that cause major illness such as cardio vascular issues, depression, digestive issues, glucose/insulin resistance. Further, these stressors are not released from the body (as they would be in a fight or flight situation) and can build up to become chronic fears and concerns.

A dynamic balance needs to exist between the two divisions of the autonomic nervous system, so that they can continuously make fine adjustments. As a society we need to find new ways to release our fears and stressors and add relaxation time and techniques to our daily life.

Footnotes