Sympathetic vs Parasympathetic
Sympathetic vs parasympathetic, what’s the difference and how do I remember which is which?
Sympathetic is the nervous system responsible for your “fight or flight” responses in times of emergencies. It controls the body’s responses to stress, injuries, or perceived threats.
Parasympathetic is the nervous system responsible for your “rest and digest” responses in times of non-emergencies. It controls the body’s responses at rest and helps your body regulate its normal daily organ functions.
One way to remember how to distinguish between the two is to associate the beginning letters of the words with their corresponding attributes.
The sympathetic nervous system kicks in during times of stress, and the parasympathetic nervous system kicks in during times of peace.
Of course, there is much more to both nervous systems than that so let’s look a little bit closer at both.
Sympathetic vs Parasympathetic Background Information
To better understand sympathetic vs parasympathetic nervous systems we first have to understand nervous systems in general. Let’s take a closer look at the different types of nervous systems and what makes sympathetic and parasympathetic unique by comparison.
While most animals have a nervous system, for the purpose of this article we will only be discussing the human nervous system. This is because nervous systems can vary greatly depending on the type of animal.
Humans have a nervous system which is responsible for detecting environmental changes, controlling the body, and transmitting signals to and from different parts of the body.
The nervous system is made up of the brain, the spinal cord, sensory organs, and nerves (that transmit impulses). The nervous system consists of two main parts: the central nervous system and the peripheral nervous system.
The central nervous system is where information is evaluated, and decision making takes place. It consists of the brain and spinal cord.
The peripheral nervous system is where signal transmission (communication) takes place. It consists of the sensory organs and nerves. This is the nervous system that relates to the sympathetic and parasympathetic nervous systems.
The peripheral nervous system can be further broken down into two components: the somatic nervous system and the autonomic nervous system.
The Peripheral Nervous System
The somatic nervous system controls the body’s voluntary functions and is responsible for muscle movements. The system’s primary role is to carry motor and sensor information from the central nervous system to the sensory organs, skin, and muscles.
The somatic nervous system is also responsible for processing sensory information from external stimuli (such as sight, hearing, and touch).
For example, say that you see a jagged rock on the ground. Your visual system perceives the rock and relays this information to your brain (the central nervous system). Your brain then sends signals to your muscles, and they respond by moving your body away from the rock. This is your somatic nervous system at work.
The key point of the somatic nervous system is that it controls voluntary functions and you have control over your movements. If for some crazy reason you wanted to, you could choose to step on the jagged rock.
The autonomic nervous system controls the body’s involuntary functions and is responsible for regulating your body’s internal organs such as your heart, stomach, kidneys, and intestines. The system’s primary role is to stimulate or inhibit our body’s processes to protect us from perceived danger.
For example, you are walking along, and a super scary clown jumps out at you. Your autonomic nervous system kicks in involuntarily and your heart rate increases, your pupils will dilate, and your blood pressure increases. Your body does this so that you can be more aware and react to danger faster. Basically, it enables you with the ability to decide and follow through on either running away from or fighting said evil clown.
The key point of the autonomic nervous system is that you have no control over your body’s responses. You cannot choose to stop dilating your eyes or not have your heart rate increase. Later on, you may be able to control lowering your heart rate after the fact, but during the actual moment, your body reacts involuntarily.
Autonomic Nervous System in Depth
The autonomic nervous system is the one that pertains to sympathetic vs parasympathetic. The autonomic nervous system has three divisions: the enteric nervous system, the sympathetic nervous system, and the parasympathetic nervous system.
A key point to keep in mind with all three systems is that not only are all three involuntary, but all three systems are always at work. They regulate your body functions, at all times and cannot be switched off.
The enteric nervous system consists of neurons (nerve cells) that control your gastrointestinal tract. This system is sometimes referred to as a “second brain,” because it can work independently. While it typically communicates with the central nervous system through the sympathetic or parasympathetic nervous systems, it can function completely on its own.
The sympathetic nervous system is responsible for preparing the body’s reactions to stress, injury, or perceived threats. This is commonly referred to as your “fight or flight” response. It is an involuntary reaction and happens in times of emergencies.
The parasympathetic nervous system is responsible for preparing the body’s reactions at rest and maintaining homeostasis. Homeostasis is when a body seeks to maintain a condition of equilibrium regardless of outside influences. A common example is body temperature: regardless of the weather the human body always strives to maintain an internal temperature of 98.6 degrees Fahrenheit.
The parasympathetic nervous system is commonly referred to as your “rest and digest” response. It is also an involuntary reaction and happens in non-emergency situations.
Sympathetic vs Parasympathetic: How They Work
Let’s look at the sympathetic vs parasympathetic systems and how they work within the autonomic nervous system. Now, this gets a little complicated. So, if you just need to know the basics, you can skim through the technical details in this part. But if you need to know all the nitty-gritty details of how they work, buckle in, here we go!
The Sympathetic Nervous System
The sympathetic nervous system is activated by perceived stressors, and in turn, it activates the adrenal medulla (the middle part of your adrenal glands).
The adrenal medulla is activated by way of a preganglionic neuron. Preganglionic neurons are neurons that originate in the spinal cord or brainstem.
The preganglionic neurons have nerve fibers (axons) that extend to ganglion. Ganglion are structures that contain the ganglia (nerve cell clusters). The nerve fibers are called preganglionic fibers, and they originate in the spinal cord or brainstem as well.
So preganglionic neurons and fibers both originate in the spinal cord or brainstem, but preganglionic fibers extend and leave the spinal cord or brainstem as part of either a spinal or cranial nerve. Preganglionic neurons keep their cell body (soma) in the spinal cord or brainstem.
Preganglionic neurons release a neurotransmitter (a chemical messenger) called acetylcholine. The preganglionic neurons synapse with the postganglionic neurons by way of preganglionic fibers, and they use the neurotransmitter acetylcholine during this synapse. Postganglionic neurons are neurons that originate outside of the spinal cord or brainstem.
Preganglionic fibers are considered cholinergic since they utilize the neurotransmitter acetylcholine. Any fiber that utilizes acetylcholine as a transmitter is considered cholinergic so this would include preganglionic fibers and postganglionic fibers.
The acetylcholine neurotransmitter also causes the adrenal medulla to release hormones into your bloodstream (such as epinephrine and norepinephrine).
The postganglionic neurons then use a secondary neurotransmitter (norepinephrine) to target specific glands and muscles in order to activate them so that they produce the desired response.
Since the postganglionic neurons use norepinephrine (to target specific glands and muscles) and the adrenal medulla releases norepinephrine (when activated), this keeps the sympathetic nervous system continuously stimulated in times of danger.
Once the perceived threat is gone the parasympathetic nervous system takes back over in order to counteract the effects of the sympathetic nervous system’s responses.
The Parasympathetic Nervous System
The parasympathetic nervous system is always at work when the body is at rest. It originates in the spinal cord and the medulla oblongata (the medulla of the brain, not the adrenal medulla).
The system stimulates the cranial nerves (specifically nerves 3, 7, 9, and 10). These nerves also have a preganglionic neuron made up of cholerginic fibers (known as preganglionic fibers).
These preganglionic fibers project towards ganglia (nerve cell clusters) to reach their targeted organ. The ganglia are housed in ganglion. In the ganglion, the preganglionic fibers synapse with the postganglionic neuron using the neurotransmitter acetylcholine.
The postganglionic neuron then uses the neurotransmitter acetylcholine again in order to create a synapse with the targeted organ. This, in turn, stimulates the desired response in the targeted organ.
So, while both systems initially use acetylcholine as their neurotransmitter, the sympathetic nervous system uses a secondary neurotransmitter called norepinephrine as well. Whereas the parasympathetic nervous system uses only acetylcholine as a neurotransmitter.
The parasympathetic nervous system is used to replenish the energy and glucose that were depleted by the sympathetic nervous system. It allows the internal organs to rest and recover, but the system is slow in restoring the body back to homeostasis.
This is because the hormones that were released instantly into the bloodstream by the sympathetic nervous system take a long time to be used up by the body or be counteracted by the parasympathetic nervous system.
Sympathetic vs Parasympathetic Differences
It can be a little bit difficult to keep track of which effects the two nervous systems have on the human body.
A key point to keep in mind is that they have opposite effects.
Whichever effect one system has the other system basically does the complete opposite.
The sympathetic nervous system stops all resources that are being used for non-essential functions such as eating, reproducing, and having bowel movements.
The parasympathetic nervous system does the opposite and regulates your body back to non-essential functions so that you can then be capable of sustaining yourself and reproducing.
Location of Origin
Both nervous system’s location of origin is in the spinal cord, but they have different regions of origin.
The sympathetic nervous system originates in the thoracic and lumbar regions of the spine. The thoracic is in the middle of the back, and the lumbar is the lower back.
The parasympathetic nervous system originates in the bottom region of the spinal cord (the sacral region) and the medulla oblongata (that connects the spinal cord to the brain stem).
Both nervous systems contain neuron pathways that connect your neurons (nerve cells) to one another.
The sympathetic nervous system has short neuron pathways and a faster system. This is because they must react quickly in times of stress and danger.
The parasympathetic nervous system has longer neuron pathways and is a much slower system. This is because they do not need to react quickly, and it can take a long time to counterbalance the effects of the sympathetic nervous system.
Heart Rate and Muscle Reaction
The sympathetic nervous system increases your heart rate and contracts your muscles. This is so you can respond quickly to stress and perceived dangers.
The parasympathetic nervous system decreases your heart rate and relaxes your muscles. This is so that you can reach homeostasis, and your body can concentrate on sustainability and reproduction.
Stomach Movement and Saliva Production
The sympathetic nervous system decreases your stomach movement and your saliva production. This is because they are considered non-essential during moments of danger.
This is also why when you are very stressed it can be hard to digest your food properly because your body is releasing a lot of cortisol (the stress hormone). This is one of the things you should tell yourself for why you should not eat potato chips when you are stressed. Your body is literally telling you that those chips are unimportant in that moment!
The parasympathetic nervous system increases your stomach movement and your saliva production. This is because when you are not in danger, your body wants you to focus on sustaining it and increasing your chances of survival.
Basically, when you are calm and happy, you can tell yourself that your body is encouraging you to eat those potato chips (but not really)!
Urinary and Sphincter Responses
The sympathetic nervous system decreases your urinary output and contracts your sphincter. This is because it is considered non-essential, and it also gives new meaning to the phrase “butt-clenching fear.” Your body is actually clenching in fear when you are scared!
The parasympathetic nervous system increases your urinary output and relaxes your sphincter. This is because when you are not in immediate danger, you need to release your bowels to survive. This is also why when you are under a lot of stress it can be hard to go to the bathroom. Constipation. It’s not you; it’s your body!
Sympathetic vs Parasympathetic Dangers
The parasympathetic nervous system does not pose any dangers to your health because it is simply carrying out your body’s normal functions. The same cannot be said for the sympathetic nervous system.
Even though the system is meant to help your body in times of need, there are actually times when the sympathetic nervous system can be harmful.
Your body is not meant to be in a constant state of “fight or flight” so being in this state long term can lead to chronic stress illnesses. Some examples would be high blood pressure, constipation, anxiety, and immune system suppression.
With our modernized society also comes staying up all night and constant sources of stress. We are becoming a nation plagued by insomnia and round the clock stress. It’s important to try and reduce your stress in order to help your body maintain homeostasis.
Believe it or not, you can actually trigger your parasympathetic nervous system.
This can be done by deep breathing, yoga, or a relaxing massage. Basically, anything that helps your body relax and de-stress. Deep breathing especially has been shown to reduce heart rate and lower levels of cortisone in the body. So, get your inner yogi on help your parasympathetic nervous system do its job! Here’s a video if you want to give it a try.
Final Thoughts on Sympathetic vs Parasympathetic
Because they sound so similar, it can be confusing to remember which is which. One of the best ways to remember their differences is to look at the beginning letters of the words.
The sympathetic nervous system responds to stress and is your “fight or flight” response. While the parasympathetic nervous system responds to peace and is your “rest and digest” response.
Both nervous systems are part of the autonomic nervous system which is in turn part of the peripheral nervous system.
The sympathetic nervous system shuts down all non-essential body functions and can sometimes lead to longer than optimal cortisol production. This can, in turn, lead to chronic stress illnesses.
The parasympathetic nervous system maintains essential body functions and can also be triggered to help reduce the levels of cortisol your body produces.
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