The sympathetic nervous system

How does the body respond to stressful situations? Why does the heart beat suddenly and the body sweat in the face of danger?

This response is triggered whenever a threat is perceived, whether it is simply a potentially embarrassing situation or a truly terrifying one(such as an attack by a stranger).

Where does the fight or flight response originate from? This response is triggered by the sympathetic nervous system, responsible for helping humans cope with stress. The sympathetic nervous system(SNS) constitutes one of the parts of the autonomic nervous system and contains a sensory and a motor component. This means that the SNS is responsible for regulating functions such as cardiac activity, respiration, digestion, etc.

In this way, the brain controls other actions such as thinking, speaking, and walking. Meanwhile, the autonomic nervous system controls other functions, thus promoting a balance in the body.

Divisions and functions of the autonomic system

  1. Sympathetic nervous system. It stimulates your fight or flight response, a physiological response to a pernicious event(threat to survival).
  2. Parasympathetic nervous system. It allows to maintain normal functions(such as digesting and keeping the body at rest).

Structure of the sympathetic nervous system

The transmission of signals in the system is accomplished through a network of nerve cells called neuronsThere are two types of neurons:

  • Preganglionic neurons. They have short fibers that originate from the thoracolumbar segments of the spinal cord. These communicate with the ganglia adjacent to the spinal column, synapsing with the longest postganglionic neurons.
    • When preganglionic neurons synapse with ganglia, they release a chemical (neurotransmitter) called acetylcholine. This is how the receptors are activated in postganglionic neurons.
  • Postganglionic neurons. These release norepinephrine, which targets adrenergic receptors in various organs and tissues. Stimulation of these receptors elicits the fight or flight responses

Exceptions

There are two exceptions to the processes mentioned above. These are the postganglionic neurons located in the sweat glands and the chromaffin cells found in the adrenal medulla.

Postganglionic neurons discharge acetylcholine to activate muscarinic receptors, except on the palms of the hands, soles of the feet, and other areas with thick skin. In these areas, norepinephrine acts on adrenergic receptors.

Chromaffin cells found in the adrenal medulla are equivalent to postganglionic neurons. Preganglionic neurons communicate with chromaffin cells, stimulating them to release epinephrine and norepinephrine into the bloodstream.

Hormones behind sympathetic nerve activation

The sympathetic nervous system releases two hormones in response to stress. This results in an “adrenaline rush,” or a sense of urgency during stressful conditions. These hormones are called epinephrine and norepinephrine, which help the body function optimally during such events.

Epinephrine, better known as adrenaline, is a hormone that is secreted by the adrenal glands. It is released in situations of stress, danger or fear. It is in charge of preparing the body for these situations: it increases tension, glucose and sugar, dilates the bronchial tubes and accelerates the heartbeat.

Noreprinephrine, or norepinephrine, is another hormone that has an excitatory effect, maintains wakefulness and consciousness, and is also in charge of the body’s fight-and-flight response. Thus, the flow of blood to the muscles increases to enhance the motor response.

Upon activation of the sympathetic nervous system, norepinephrine is released  preparing the body for the initial stages of stressIf stress is resolved quickly, bodily functions return to normal.

However, if the stressful event persists, your body produces epinephrine to increase these effects and activate various parts of the body to react accordingly.

What happens if the sympathetic nerve is activated?

When faced with a stressful situation, the sympathetic nervous system is activated automatically. Several functions of the body are activated almost simultaneously, such as:

  • On the one hand, the relaxation of the bladder, which allows you to retain urine while you are stressed. However, in worse situations, some people lose bladder control due to paralyzing fear.
  • On the other hand, the airways(bronchioles) in the lungs are widened to allow more air, increasing the supply of oxygen to the blood.
  • In turn, the adrenal glands are stimulated to release norepinephrine and epinephrine, responsible for reactions associated with stress.
  • There is also an increase in heart rate, translated into greater delivery of oxygen and nutrients to the brain and muscles, preparing them for stress.
  • Also, digestive activity is decreased, helping to conserve the body’s energy and can be used to defend against stress.
  • An increase in glucose, released from the liver into the bloodstream to provide more energy to the muscles.
  • Lastly, the pupils are also dilated, which is often seen when you are surprised or feel threatened.

These are some common functions involved in the fight response regulated by the sympathetic nervous system. Due to such bodily reactions, the body is prepared to run, fight or react as needed, depending on the situations.

Finally, when the situation is resolved, the sympathetic functions return to their resting state. This allows the heart rate to normalize, breathing to slow, and body functions to return to a balanced state.

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