The sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS) are the two main divisions of the autonomic nervous system (ANS). Both systems have an enormous effect on our well-being and while the former prepares your body for the “fight or flight” response, the latter prevents it from overworking.
Their effects are both complementary and in opposition to each other, which causes many to incorrectly see them as rivals.
In this article, I will explain both their differences and how they are linked together. Furthermore, to understand what makes the SNS and PSNS so unique and important, we will also take a closer look at the nervous system in general and its many subdivisions.
What is the nervous system?
The nervous system is the major and highly complex controlling, regulatory, and communicating system in your body. It is the center of all mental activity including thought, learning, memory, and feeling and controls everything you do.
What does the nervous system do?
The nervous system’s various activities can be grouped together as three main, overlapping functions:
- sensory input
- integration (of data)
- motor output
Sensory input is all of the information that your body gathers through its many sensory receptors, such as your eyes, ears, nose, tongue, and skin. These sensory organs are constantly sensing the environment and are monitoring any changes (or stimuli) that are occurring both inside and outside your body.
Integration of data
The extracted information (i.e., sensory input) is now transmitted to the brain where it gets processed and interpreted. This, in turn, creates sensations, generates thoughts, or adds to memory and an appropriate response is selected.
This processing of information is carried out by the central nervous system (CNS), which consists of your brain and spinal cord (more on that below).
Based on the sensory input and integration, signals are sent from the brain and spinal cord to muscles and glands to initiate a response. This is called motor output.
In short: Everything you do, think, and feel can be boiled down into three principal functions: sensory input, integration, and motor output.
The nervous system takes in information through the senses, processes it, and triggers reactions, such as contracting your muscles or causing you to feel pain. All of this happens in a matter of seconds.
Ever held a scolding hot plate in your hands, for example? Well, if your nervous system is doing its job, then you will most likely reflexively pull back your hand “thanks to” the pain signals that were sent to your brain via your nerves.
This is why, although the idea of being exempt from any pain sensation might sound like a superpower, it is anything but when it comes to the standpoint of survival. Pain lets us know that something is not right and it helps us to prevent further injuries.
What are the divisions of the nervous system?
The nervous system has two major parts, or subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
The CNS is where information (sensory input) is evaluated (integrated) and includes the brain and spinal cord. The brain is the body’s control center and the spinal cord is the major highway to and from the brain.
The PNS, on the other hand, is a network of nerves linking all parts of the body to the brain and spinal cord. The nerves carry information to and from the body, so the brain can interpret them and take action (motor output).
The PNS can be further broken down into two components: the somatic nervous system and the autonomic nervous system (ANS).
The somatic nervous system controls the body’s voluntary functions and is responsible for muscle movements, whereas the autonomic nervous system controls the body’s involuntary, or visceral, functions such as the beating of the heart and the expansion or contraction of blood vessels or pupils, for example.
Both the SNS and the PSNS are part of the ANS, which means that both systems are entirely involuntary and cannot be switched off or on at will.
What is the sympathetic nervous system?
The SNS originates in the spinal cord and controls your body’s fight or flight response, or how the body reacts to perceived danger.
Its goal is to help you deal with the threat at hand and it focuses entirely on what your body needs to do right now to survive. Because it is one of the key players in the physiology of stress, the SNS often gets a bad rep, but we are, in fact, very lucky to have one as we would not be able to survive without it.
The problem lies in the fact that the body does not distinguish between different types of dangers and this overreaction to non-life-threatening and non-immediate stressors, such as an impending deadline, for example, causes chronic stress.
- With sympathetic nervous responses, the body speeds up, tenses up, and becomes more alert and all functions that are not essential for survival are shut down.
- Dilate pupils
- Decrease saliva production
- Increase the heartbeat
- Relax airways
- Inhibit stomach activity
- Inhibit the gallbladder
- Secrete epinephrine and norepinephrine
- Relax the bladder
- Contract the muscles
- Shut down all the processes that are not critical for survival
What is the parasympathetic nervous system?
The PSNS is located between the spinal cord and the medulla and controls homeostasis. Many view the PSNS as a kind of antidote to the SNS, but although the PSNS counterbalances the SNS by stimulating your body’s “rest and digest” response, it also does much more than that.
You could say that while the SNS focuses on the crisis of the now, the PSNS regulates everything else.
- The PSNS restores your body to a calm state and prevents it from overworking.
- Constrict pupils
- Stimulate saliva
- Slow down the heartbeat
- Constrict airways
- Stimulate stomach activity
- Stimulate the gallbladder
- Contract the bladder
- Relax the muscles
What are the differences between the sympathetic nervous system and the parasympathetic nervous system? And how are they linked together?
While the SNS prepares your body for the fight or flight response, the PSNS prevents it from overworking.
Their effects are both complementary and in opposition to each other, which causes many people to incorrectly see the two systems as rivals when they are, in fact, two sides of a scale. Finding a nice balance in the middle is always the goal, but sometimes you will lean more toward one side or the other, depending on what is happening at that exact moment.
Eventually, however, homeostasis will be achieved again.
Or to draw another analogy: the SNS is the accelerator and the PSNS is the brake. Both are needed to drive safely, but neither should be used for an extended period of time.
Everything you do, think, and feel can be boiled down into three principal functions which are all controlled by your nervous system: sensory input, integration, and motor output.
The nervous system has two major subdivisions: the CNS and the PNS. The PNS can be further broken down into two components: the somatic nervous system and the autonomic nervous system, of which the SNS and the PSNS are the two main divisions.
Both systems have an enormous effect on our well-being and while the former prepares your body for the “fight or flight” response, the latter prevents it from overworking. Because of this, they are often viewed as rivals, when they are actually two sides of a scale.
Would you like to know how to reset your nervous system? Then take a look at this article where I discuss this topic in more detail and share useful tips and tools.
Sound therapy is also a great tool for a reset. Click here to listen to Sonic Yogi tracks.
article by Eline Stone