Understanding Autonomic Nervous System from Neuroscience Perspective

Understanding Autonomic Nervous System from Neuroscience Perspective

It is important to know the Autonomic Nervous System (ANS) to understand the real effect of Yoga for healthy living. There is no other system of the human body where Yoga has a direct influence than the Autonomic Nervous System and hence it is of great relevance to the understanding of the effects of Yoga practice on the body, mind, and emotions.

The autonomic nervous system is a part of the human nervous system that acts largely unconsciously and behaves like a control system for functions such as heart rate, respiration rate, digestion.  It contains nerves that have special functions related to the glands and visceral organs (internal organs with circulatory, endocrine, reproductive and digestive functions). It regulates the body’s subconscious actions that are carried out by these organs. It is so-called because it is autonomic and hence normally not in the conscious control (except through the control on the breath – like the practice of Pranayama, which is a separate topic and will be covered in a different post). The job of the autonomic nervous system is to respond to external threats whenever they arise, and in a normal course, to continuously monitor the internal body parameters and make adjustments to keep these parameters within the optimum range.

The autonomic nervous system has three branches – sympathetic, parasympathetic and enteric nervous system.

1. Sympathetic Nervous System

The sympathetic nervous system (also called “fight or flight” response or “stress response”) is meant to respond during stress and emergencies. It is activated when there is physical arousal (any arousal of the senses – in fact including active and rigorous Yogasana poses) or emotional arousal (any thought that creates fear e.g. receiving a notice from the tax department or just remembering a past traumatic event). When it is activated, the body and mind are prepared for the condition to either defend (fight) or run away (flight) from the perceived threat. Hence, the stress response is an automatic physiological reaction to any situation (real or imagined) that is threatening, overwhelming or requires immediate preparation for survival. In the first level of the stress response, the following happens:

  • breathing rate increases to increase the intake of oxygen
  • bronchioles in the lungs dilate to furnish more oxygen to muscles
  • heart rate increases to pump more blood throughout the body to supply more oxygen and sugar as this is an emergency situation
  • pupils of the eyes dilate
  • various stress hormones are released
  • sugar and fat enter the blood as a fuel to the brain and muscles
  • the blood clotting process is activated to handle any injury to the body
  • muscle tone increases
  • sweating increases
  • the digestive process (like peristalsis – churning of food) is put on the back seat as the blood is to be directed to muscles and brain
  • the beta waves dominate in the left cerebral hemisphere

Following the rapid activation of the first level of the stress response as listed above, if the hypothalamus continues to perceive the danger, the second level of stress response called the “HPA-axis” is initiated to keep the stress response ON. The HPA-axis consists of the hypothalamus, the pituitary gland, and the adrenal glands. The hypothalamus releases corticotropin-releasing hormone (CRH), which travels to the pituitary gland. The pituitary glands in turn release adrenocorticotropic hormone (ACTH). This hormone travels to the adrenal glands triggering them to release cortisol. The body thus stays on high emergency alert. When the threat passes, the parasympathetic nervous system dampens the stress response and cortisol level eventually falls. We will cover hormones and HPA-axis in detail in future posts.

The elevated levels of stress hormones depress healthy immune function. Under normal conditions, the immune system releases cytokines to help the body heal wounds and fight infection. But severe and chronic stress causes inflammation levels to be abnormally high, resulting in vulnerability to many physical problems and disease.

2. Parasympathetic Nervous System

When the sympathetic nervous system is not active, the parasympathetic nervous system (also called “rest and digest” response or “relaxation” response) dominates. The parasympathetic nervous system controls the body processes during ordinary situations and is restorative i.e. it calms your body and mind. It brings the blood pressure and heart rate to normal, stimulates the digestive tract to process food and eliminate wastes. When it is active, the following changes happen:

  • heart rate and blood pressure decrease
  • pupils of the eyes contract
  • digestion and peristalsis (churning of food in the digestive tract) increase
  • insulin production increases (as the sugar in the blood is not a necessity)
  • the mind feels relaxed and calm
  • muscle tone decreases
  • the alphas waves dominate in the right cerebral hemisphere

3. Enteric Nervous System

This is also called “Second Brain”. It consists of sheaths of 100 million neurons embedded in the walls of our digestive tract (alimentary canal) from the esophagus to the anus. It has its own reflexes and senses, and can behave independently of the brain for the process of digestion and excretion. The business of digestion is delegated to this nervous system in our gut. This is precisely the reason why our psychological/emotional state drastically affects our gut and vice versa. This phenomenon is also known as the “Gut-Brain Axis.” It is well known that the stress and emotions originating from the brain can affect gastrointestinal function. Also, a sensation from the gastrointestinal tract can affect emotions and pain. For example, we get butterflies in our stomach when we are anxious, or when we have indigestion we may feel disturbed. We covered the enteric nervous system previously in our post titled “Humans have second brain”.

These three autonomic subsystems are not active simultaneous because their actions are in many cases conflict with one another. For example, activation of the sympathetic nervous system automatically turns off the digestion that is operated by the enteric nervous system. Similarly, when the sympathetic nervous system is active, the parasympathetic response takes a back seat.

The stimulation of the sympathetic nervous system is complete in just a few seconds, whereas the corresponding relaxation through parasympathetic stimulation requires around twenty minutes. The sympathetic nervous system plays a very active role as it is an emergency response – a question of life and death. While the parasympathetic nervous system plays a passive role as its main job is to moderate the stress response created by the sympathetic branch, and to bring the body back to homeostasis once the emergency has passed. Homeostasis is a medical term for a healthy balance of interdependent elements maintained by physiological processes. Hence, they are actually complementary to each other – and both of them are perpetually ON to some extent. It is only the relative proportion of each that changes based on the situation.

The autonomic nervous system normally works in an automatic way – silently adjusting the balance between the sympathetic and parasympathetic nervous system, and through that manipulating various functions of the body so as to keep the operating parameters of the body within the desired range.

It is important to note that not only physical stress but also emotional conditions that are responsible for the sympathetic nervous system activation. If it remains activated for a long duration without the parasympathetic nervous system getting a chance to calm your body and mind, it can cause havoc in your life. And, it may result in various physical, physiological and emotional disorders. This is the main reason for a significant rise in emotional disorders (anxiety and depression) in the modern world. A regular Yogasana and Pranayama practice not only alleviates physical stress by relaxing the muscles and irrigating the internal organs but also reduces the arousal in the mind by shifting from sympathetic to parasympathetic mode, which is restorative by nature.

Also, too much parasympathetic mode can be harmful and is known to cause disorders like depression, sleep difficulties, dizziness. Hence a good mix of active and restorative Yogasana poses will help maintain homeostasis between sympathetic and parasympathetic modes.

In the Yogic science, the sympathetic corresponds to “Pingala” Nadi, and the parasympathetic corresponds to “Ida” Nadi. This aspect will be cover in another post. Also, how breath influences the Autonomic Nervous System and vice versa will be covered in a separate post.

We will cover how certain actions of the autonomic system, which are subconscious in nature, can be brought to the level of consciousness by a serious practice of Yoga. Stay tuned!

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