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The Nervous System


The nervous system has three primary functions. They are:

1. Sensing: Through our sensory, or afferent, nerves, the nervous system gathers information about our body and our environment.

2. Integration: This information goes to the brain for processing.


3. Response: The brain sends out the appropriate motor response through the efferent (motor) nerves based on this information.

The brain coordinates our movement by responding to signals sent by the afferent (sensory) nerves to the sensorimotor cortex of the brain. A motor response is sent out through the efferent (motor) nerves.

In a very basic example, suppose you want to pick up a relatively heavy reference book. As you touch the book, you are unconsciously evaluating its density and dimensions and getting a sense of its weight. The brain processes this information and adjusts to the appropriate amount of muscular tension needed to lever the book. As you lift the book, your brain ideally continues to take in information about it and adjust muscular tension accordingly so you can lift the book while levering appropriately, and without taxing your lower back, collapsing your wrist, etc.

In short, the nervous system is the information gathering and response system. 

Fight or Flight vs. Parasympathetic Nervous System State

The autonomic nervous system can either be in the the sympathetic nervous system state or the parasympathetic nervous system state. The sympathetic nervous system state is also called fight or flight or, in popular language, the stressed nervous system state. This is optimal for responding to situations of immediate physical danger, or for investigating whether or not this danger exists.


The parasympathetic state is sometimes referred to as the “rest and digest” nervous system state. This state is optimal for times when we don’t need    

to respond to immediate physical danger and also allows for optimal functioning of other systems of our body. In fight or flight, numerous physiological changes occur that prepare us for fighting, fleeing, playing dead to avoid a predator, or other survival functions.


Here are some examples of changes that occur:

1. The Achilles tendon on the back of the ankle shortens. This protects vital blood flow, or from disabling damage to the ankle that could keep us from moving. As a side effect, this also collapses the bony arch structure of the foot by shifting the position of the heel bone (to which the Achilles is attached). This can contribute to foot, knee, and hip issues.

2. Joints compress. This is helpful because the bone-to-bone contact provides more surfaces of friction in the body to create fast and powerful movement that support running, fighting, moving heavy objects, etc. This also compresses nerves, which impedes the sending of nerve signals, which is helpful so that we can keep fighting even if we are wounded. If joint compression is sustained over a period of time, this eventually leads to deterioration of the bones and tissues in the joint.

3. The jaw dislocates. This compresses the trigeminal nerve, a major nerve in the body that collects a substantial percentage of sensory information that the brain receives, including information about taste, touch, and sound. This nerve also enervates the face. An example of how this can be helpful is that if we are punched in the face, we can continue fighting without distraction.


4. Blood vessels dilate in the arms and legs and constrict in the organs. This sends more energy (transmitted by blood) to the muscles for maximal force output, again for fighting or fleeing.

5. Digestion shuts down (as a result of a change or decrease in activity of the valve system, a series of diaphragms or pumps that circulate blood throughout the body and create peristalsis, which is a constant wave or pumping motion in the body that allows digestion to occur). Digestion requires a lot of the body’s energy, so this change conserves energy to respond to the immediate crisis at hand. If this continues for a long period of time, this also means that we stop absorbing food normally and may experience digestive issues.

MRI showing minimal space in the knee joint, a result of the activation of fight or flight motor reflexes

6. Adrenalin and cortisol are released. Excessive amounts of these hormones dehydrates muscle and connective tissue, which changes movement and also negatively affects our first-line-of-defense immune response that occurs in a type of connective tissue called fascia.

7. The neck and shoulder muscles tighten.

When we have spent a lot of time in the fight or flight nervous system state, and particularly when we have activated it inappropriately (not in response to immediate physical danger), it is not uncommon for these changes to become the dominant habits in the body.

Peristalsis ceases in fight or flight, halting the digestive process

7 Stages of Fight or Flight

The structural and physiological changes in the sympathetic nervous system state can be grouped into 7 different stages. The earlier stages are active stages, while the later stages are conservation of energy stages.

The Nervous System and Health

The changes mentioned above are just a few examples of the changes that occur during fight or flight.


These examples are not meant to be comprehensive, but rather to illustrate a few points:

1. Numerous physiological changes occur in the fight or flight state that alter the way the body functions.

2. The physiological changes that occur during fight or flight are changes that aid a response to immediate physical danger.

3. These same physiological changes will affect motor efficiency and body function if sustained over long periods of time and if activated not in response to physical danger (which is common).

Information Delivery in Fight or Flight

Sympathetic nervous system changes can result in a decrease of sensory information through compressed or demyelinated nerves. 


Thus the brain is not receiving sufficient information about the environment and is incapable of responding appropriately. This can make it more challenging to leave the fight or flight nervous system state, as well as cause some difficulties with movement, health, memory, information processing, and general functioning in life.

Selection of Nervous System State

In order to appropriately select a nervous system state, the brain needs to know through the receipt of sufficient and accurate sensory information whether or not there is physical danger in the immediate environment. If there is or there may be, then the brain activates the fight or flight nervous system state. If the brain receives signals that the body is safe from physical harm, then it is able to select the parasympathetic state. If the brain is not receiving sufficient sensory information, then it will assume there is a problem and activate the fight or flight system. In this way, the physiological and structural changes associated with this nervous system state may persist over time and begin to cause a lack of ease in the body.

Fight or Flight TNS
Chronic Health Issues
Sensory Info TNS
Info Delivery
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