The Autonomic Nervous System

The central nervous system(CNS) is a collection of nerves and neurons that transmit signals throughout the body influencing the senses, movement, and organ regulation & function. The nervous system initially branches into the central nervous system (brain, spinal cord, & nerves) and the peripheral nervous system (neurons, ganglia, cranial and spinal nerves).
The peripheral nervous system (PNS) is made of up of the somatic nervous system (SNS) and the autonomic nervous system (ANS). Very simply stated the somatic nervous system controls voluntary actions of the body and the autonomic nervous system involuntary. The ANS is off the branch of the PNS which carries messages to and from the CNS and is not under voluntary or conscious control.

The ANS branches again into the sympathetic and parasympathetic nervous system. Sympathetic nervous system relates to our response to fear (or anticipation of something fearful) arousing the body to expend energy and reach maximal performance (fight, flight, freeze). The parasympathetic nervous system relates to our conservation and maintenance of energy and recovery (rest & digest). The enteric nervous system is sometimes mentioned as a third branch of ANS or under the PNS. ENS relates to the gastrointestinal tract, is often referred to as our second brain because it has its own reflexes independent of the CNS.

The sympathetic nervous system physiologically prepares us for activity – in defense of an attack or run from it –fight, flight, and freeze. The response to a stress/fear triggers the adrenal gland to release of catecholamine (including adrenaline, noradrenaline, & cortisol), which results in an increase of heart rate, blood pressure, and breath rate. The release of cortisol then causes the liver releases glucose for muscle energy. The eyes dilate providing more light improving vision. While there is an increase in many things other functions of the body decrease to conserve energy–digestion and elimination slows or stops, sleep is compromised by the increase of energy. Thoracolumbar nerves send and receive these signals.

• ⇧ Metabolic rate (Mobilization of energy -adrenaline, etc)
• ⇧ cardiovascular tone (helps to pump the energy throughout the body)
• ⇧ respiration rate (delivers oxygen)
• ⇩ digestion ⇩ saliva production
• ⇩ growth } ‘turn off long-term projects’
• ⇩ reproduction
• Enhancement of immune system
• Sharpening of cognition – ⇧ beta waves
• Pupils dilate

The parasympathetic nervous system physiologically restores calm– preparing us for recovery and growth through the relaxation response of rest and digest/feed and breed. Cranial sacral nerves send and receive these signals.

• Breathing calms
• ⇩ heart rate ⇩ blood pressure
• ⇩ sweat production
• ⇧ saliva stimulates digestive secretions
• Muscles repair
• Ideal elimination
• Stimulation of the vagus nerve (nerve X)
• Pupils contract- ⇧ tears
• ⇩ muscle tone
• ⇧ insulin
• ⇧ alpha state

For many humans in the western world – the necessity of fight, flight or freeze to escape immediate threat of physical harm (grizzly bear/mountain lion) on a near daily basis isn’t as great as it was during other ages. I do find it necessary to point out that people do live under immediate threat (and anticipatory fear) of domestic and/or gang violence, and that of hate crimes. As humans have evolved, tension and stresses related our job, finances, traffic, social encounters and relationships can commonly and chronically trigger the sympathetic nervous system. And this is causes the domino effect of the above mentioned response reactions.

Our system strives for homeostasis and does so pretty well most of the time – regulating body temperature, oxygen, glucose, sodium levels, etc, etc-; however, when stressors become chronic and we live more under the rule of the sympathetic nervous system our body’s ability to restore to optimal balance can be skewed. This is thought to promote the conditions for many disorders, conditions, and diseases— for example: hypertension, heart disease, type 2 diabetes, insulin resistance, osteoporosis, infertility, loss of libido, anxiety, and depression.

Good news…The stimulation of the vagus nerve occurs when the parasympathetic nervous system is dominant promoting the rest and digest/relaxation response. Much of what is practiced in breath centric asana and restorative yoga promotes this stimulation of the vagus nerve and helps the body restore balance. Another promising note is that due to the sympathetic nervous system’s reactions being ‘bound together’ –”if one particular body function can be turned from sympathetic dominance to parasympathetic dominance it than can have the effect of turning off all sympathetic functions.” This is why breath centric yoga practices, including conscious breath work, has such a noticeable effect.