How does the body respond to stressful situations? Why does the heart suddenly beat and the body sweats before the danger? This response originates whenever a threat is perceived, whether it is simply a potentially embarrassing situation or a truly terrifying one (like the attack of a stranger).
And from where does the fight or flight response originate? This is caused by the sympathetic nervous system, responsible for helping the human being to 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 certain functions such as cardiac activity, breathing, digestion, sweating patterns, and so on. In this way, the brain can control other actions such as thinking, speaking and walking, while the autonomic nervous system controls other functions, thus promoting a balance in the body.
Divisions and functions of the autonomic system
- Sympathetic nervous system. Stimulate your fight or flight response, which is a physiological reaction that occurs in response to a perceived pernicious event (attack or threat to survival).
- Parasympathetic nervous system. It allows maintaining 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 achieved through a network of nerve cells called neurons. There are two types of neurons:
- Preganglionic neurons. These have short fibers that originate from the thoracolumbar segments of the spinal cord, which communicate with the ganglia adjacent to the spinal column and synapse with the longer postganglionic neurons.
- When the preganglionic neurons synapse with the ganglia, they release a chemical (neurotransmitter) called acetylcholine, which activates the receptors in the postganglionic neurons.
Postganglionic neurons These, in turn, release a hormone called norepinephrine, which targets adrenergic receptors in various organs and tissues. The stimulation of these receptors results in the characteristic fight or flight responses
- There are two exceptions to the processes mentioned above, which are the postganglionic neurons that are found in the sweat glands and the chromaffin cells that are found in the adrenal medulla. Postganglionic neurons discharge acetylcholine to activate muscarinic receptors, except in the palms of the hands, soles of the feet and other areas with thick skin. In these areas, norepinephrine acts on adrenergic receptors.
The chromaffin cells found in the adrenal medulla are equivalent to postganglionic neurons. Preganglionic neurons communicate with chromaffin cells and stimulate them to release epinephrine and norepinephrine directly into the bloodstream.
Hormones behind sympathetic nerve activation
The sympathetic nervous system releases two hormones in the body in response to stress, resulting in an “adrenaline rush”, or a sense of urgency that occurs during stressful conditions. These hormones are called epinephrine and norepinephrine, which help your body to function optimally during such events.
After the activation of your system, norepinephrine is released to prepare the body for the initial stages of stress. If 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 one faces a dangerous or stressful situation, the sympathetic nervous system is automatically activated without conscious control. Several body functions are activated almost simultaneously, such as:
- Relaxation of the bladder, which allows you to hold your urine while you are stressed. However, in worsening situations, some people involuntarily lose bladder control due to a paralyzing fear that allows their body to release it.
- Expansion of the airways (bronchioles) in the lungs to allow more air, which increases the supply of oxygen to the blood and the rest of the body.
- Stimulation of the adrenal glands to release norepinephrine and epinephrine, which are responsible for the cascade of reactions associated with stress.
- An increase in heart rate, which results in increased delivery of oxygen and nutrients to the brain and muscles to prepare them for stress.
- Decreased digestive activity, which helps conserve your 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.
- Dilation of the pupils, which is often seen when you are surprised or feel threatened.
These are just some of the common functions involved in the fight or flight response regulated by the sympathetic nervous system. Due to such bodily reactions, the body is ready to run, fight, lift weights or react as needed, depending on the threat situations.
When the situation is resolved, the sympathetic functions return to their resting state, which allows the heart rate to return to normal, the breathing to slow down and the functions of the body to return to a balanced state.