How autonomic nervous system works? – anatomy, function & its control
- What is the autonomic nervous system meaning? What is the difference between SNS and ANS?
- How does autonomic nervous system maintain homeostasis?
- What organs are controlled by the autonomic nervous system? What is difference between sympathetic and parasympathetic?
- Autonomic nervous system and heart rate
- Autonomic nervous system and blood pressure
- Autonomic nervous system and digestion
- Autonomic nervous system and stress/anxiety
- Autonomic nervous system and pain
- When does the autonomic nervous system work?
- Sympathetic nervous system (SNS)
- Parasympathetic nervous system (PSNS)
- Where does the autonomic nervous system originate from? What nerves are in the autonomic nervous system?
- How does the nerve pathway occur in the autonomic nervous system (SNS and PSNS)?
- Autonomic nervous system pathway and structure
- Autonomic nervous system with neurotransmitter
- Autonomic nervous system receptors
- How does the autonomic nervous system work?
- Take home message
- Q 1. Is blinking autonomic or somatic?
- Q 2. Is breathing autonomic or somatic?
- Q 3. Which autonomic nervous system is activated during stress?
- Q 4. What are autonomic nervous system disorders?
Today, we will discuss how autonomic nervous system works in our day to days life. Suppose you are strolling in the street and suddenly you see a big black dog in front of you.
In this circumstance, you will observe some changes in your body like – blown pupils, triggered goosebumps, elevated heart rate, sweating, etc.
Have you ever thought, why do these changes occur instantly in your body?
What does the underlying mechanism behind every thought, action, emotion, and feeling?
In this post, I will provide you depth information about the autonomic nervous system (ANS).
Let’s quickly understand with basic concept.
What is the autonomic nervous system meaning? What is the difference between SNS and ANS?
According to the medical definition of the autonomic nervous system, the autonomic nervous system is part of the nervous system that controls your main involuntary body’s functions like heart rate, respiration, digestion, etc.
Our human nervous system is broadly categorized into the central nervous system (CNS) and peripheral nervous system (PNS).
The autonomic nervous system is a network of interconnected neurons that controls your involuntary internal muscles or organs like the heart, lungs, kidney, liver, stomach, intestine, etc.
It is an autonomic process that requires no conscious control, e.g.,
- You can’t decide to digest your food.
- You can’t control your breathing
- You can’t control your heart rhythm
Conversely, the somatic nervous system is under voluntary control, e.g., skeletal muscles of your limbs. With this system, you can move your hands or legs at your own will.
How does autonomic nervous system maintain homeostasis?
The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems.
Sympathetic nerves usually activate the organs like the heart (rapid heartbeat) and lungs (increase breathing). On the other hand, parasympathetic nerves help calm your body down like slow heart rate, long and deep breathing, etc.
The autonomic nervous system works by one side activating and inhibiting another side to keep things running smoothly.
So, these two divisions work together to maintain homeostasis or prepare your body for appropriate action.
What organs are controlled by the autonomic nervous system? What is difference between sympathetic and parasympathetic?
In our body, the primary function of the autonomic nervous system is to control and regulate those organs that do not have voluntary control.
As I mentioned above, the parasympathetic and sympathetic nervous systems constantly work together to maintain a stable environment for survival. For example –
Autonomic nervous system and heart rate
Your heart is controlled by two nerves of the autonomic nervous system – the sympathetic and parasympathetic nerves.
Sympathetic nerves increase heart rate by releasing adrenaline and noradrenaline neurotransmitter (chemical) that binds on the Beta-1 receptor.
Whereas parasympathetic nerves slow down your heart rate by releasing acetylcholine neurotransmitter that binds to the M2 receptor.
Autonomic nervous system and blood pressure
Under stress conditions, your blood pressure rises by the sympathetic nerve while decreasing when the parasympathetic nervous system gets activated.
Autonomic nervous system and digestion
The parasympathetic nervous system is usually activated if you have a meal. It helps to digest and metabolize your food.
Conversely, if you are in danger or stressful situation, then the digestion process gets stopped by the sympathetic nervous system.
Autonomic nervous system and stress/anxiety
The sympathetic nervous system has a direct role in a stressful situation, either life-threatening or non-life-threatening.
It generally stimulates in danger situation where fight or flight condition is associated.
But it may also release normal stress conditions like being stuck in a traffic jam, having academic stress, financial stress, etc.
Autonomic nervous system and pain
The difference between the sympathetic and parasympathetic nervous system are based on their action –
When does the autonomic nervous system work?
Sympathetic nervous system (SNS)
If you are in any stressful situation like –
- Physical stress
- Financial stress
- Academic stress
- Visit a haunted house
- Watch scary movies
In these conditions, your sympathetic nervous system is get activated.
It is easy to remember as
S = stress and sympathetic
SNS is generally concerned with the “fight or flight” response.
It triggers during stressful or emergency situations. If someone is a threat to you. So, you have two options either you fight or flight.
It means that if your opponent is weaker than you. Then you can fight, but if your opponent is stronger than you. It’s better to defend yourself and run away quickly. This is the concept of fight or flight response.
Parasympathetic nervous system (PSNS)
The parasympathetic nervous system is stimulated whenever you are in a pleasant mood or feeling well, especially when your body is at rest.
It is a concern with the “rest or digest” response.
It is associated with our day-to-day life. Some activities are involved –
- Reading books
- Having meal
- Watching romantic movies
- Sexual arousal
- Secretion from glands like lacrimation, nasal secretion, watery secretion, and saliva secretion.
The primary function of the PSNS is to conserve energy so that it can use later when required.
Where does the autonomic nervous system originate from? What nerves are in the autonomic nervous system?
Autonomic nervous system physiology –
Whatever we think, listen, watch, feel, learn, memorize, and emotions, this information is kept in a limbic system.
The limbic system is a collection of structures like the thalamus, hypothalamus, amygdala, and hippocampus.
Hypothalamus is generally present below the thalamus and above the pituitary gland. Hypothalamus is divided into two regions: –
- Anteromedial area – perceives information related to the parasympathetic nervous system
- Posterior lateral area – perceives information that associated with the sympathetic nervous system
In a sympathetic nervous system, the poly-neuronal or polysynaptic descending pathway (reflex arc) carries information from the posterior lateral area of the hypothalamus. It stimulates the lateral horn of the spinal cord.
On the other hand, the parasympathetic system represents craniosacral outflow (CN3, CN7, CN9, CN10, S2, S3, S4).
CN3 – Oculomotor nerve (coordinate eye move movement)
CN7 – Facial nerve (controls muscles of facial expression)
CN9 – Glossopharyngeal nerve (provides sensation to the parotid gland, oropharynx, and tongue)
CN10 – Vagus nerve (control blood pressure and heart functioning)
Here ‘S’ represents sacral as the spinal cord is divided into 5 parts – cervical (C1 – C8), thoracic (T1-T12), lumbar (L1-L5), sacral (S1-S5), and a coccyx.
How does the nerve pathway occur in the autonomic nervous system (SNS and PSNS)?
Autonomic nervous system pathway and structure
There are two nerve fibers that affect the autonomic nervous system – preganglionic nerve fibers and postganglionic nerve fibres.
If we talk about the structural difference between sympathetic and parasympathetic nerve fibre that is the axon.
SNS – short preganglionic nerve fibre and long postganglionic nerve fibre.
PSNS – long preganglionic nerve fibre and short postganglionic nerve fibre.
Sympathetic preganglionic nerve fibre is myelinated and white rami communicantes, while sympathetic postganglionic nerve fibre is unmyelinated and gray rami communicantes.
As stated earlier – In SNS (sympathetic nervous system), neurons arise from the thoracic – lumbar region, and their nerve fibers end up with ganglia. These ganglia present on the side of the vertebral column and make a chain.
Some of the nerve fibers travel up and down before synapsing. Some pass through without synapsing.
Let’s understand through an example.
Suppose your spinal cord is like a highway, and your car is traveling on the highway up and down. Likewise, the nerve fibres transmit information or messages from your brain to all body parts.
On the other hand, the preganglionic parasympathetic nerve arises from the craniosacral region.
Parasympathetic preganglionic fibers are located near or within target organs. So, postganglionic nerves fibers are relatively short.
Autonomic nervous system with neurotransmitter
The neuronal junction also called a synapse, releases chemicals or neurotransmitters to help in the communication, which influences the target organs.
All presynaptic neuron or preganglionic fibers which comes out from the central nervous system and release acetylcholine (Ach). They all are known as cholinergic.
All neuromuscular junctions are cholinergic.
Postganglionic sympathetic nerve fibers release chemicals – Noradrenaline (norepinephrine) or adrenaline (epinephrine).
While postganglionic parasympathetic nerve fibers release chemical acetylcholine (Ach).
Autonomic nervous system receptors
In the sympathetic nervous system, adrenaline or noradrenaline binds to its specific receptors.
- α- 1 (Alpha-1) receptor
- α – 2 (Alpha-2) receptor
- β- 1 (Beta-1) receptor
- β -2 (Beta-2) receptor
All tissues have alpha 1 and beta 2 receptors except
- Beta-1 receptor present in heart, kidney (juxtaglomerular cells), and adipocytes (fat cells)
- Alpha-2 is present in the presynaptic junction, platelets, and β-cells of the pancreas.
On the other hand, the parasympathetic nervous system – binds on a specific muscarinic receptor.
- M1 receptor – gastric, salivary gland, and cerebral cortex
- M2 receptor – heart
- M3 receptor – located throughout the body, e.g., smooth muscles, glands, iris, etc.
Nicotinic receptor present on the ganglia where acetylcholine binds on it.
How does the autonomic nervous system work?
Imagine you are in a good mood, delighted, and strolling in the street. Suddenly, you see there is a tiger.
When you were in a pleasant mood, your parasympathetic nervous system was activated! As soon as you saw a tiger, your nervous system switched to the sympathetic nervous system.
In this condition, your all-stressful information like –
- tiger roar sound information from the temporal lobe
- tiger visual information from the occipital lobe
- thinking from the frontal lobe
These all information go to your limbic system
It produces fear and stress, which stimulate the posterior lateral area of the hypothalamus
They carry information by polysynaptic descending pathway and stimulate lateral horn of spinal cord present in the thoracolumbar region
Then it diffuses central sympathetic outflow to various target organs like –
- Hair stands up(goosebumps) – arrector pili muscle contracts by stimulating an alpha-1 receptor.
- Eye– Mydriasis (pupil will dilate and focus for far vision) by stimulating an alpha-1 receptor
- GIT(Gastrointestinal tract) – stop digestion, motility, peristalsis, and blood diverted to skeletal muscles to utilize energy.
- Respiratory system– stimulate respiratory center, which increases respiratory rate.
- Metabolic changes– stimulate glycogenolysis (breakdown glycogen to glucose) because your body needs glucose in this circumstance.
- Urinary bladder– stop urination by stimulating the alpha-1 receptor and contract trigone urethral sphincter.
- Kidney– Juxtaglomerular (JG) apparatus releases renin, then stimulates angiotensin and converts to angiotensinogen I.
After that, angiotensinogen-I converted to angiotensinogen-II by angiotensin-converting enzyme. Eventually, angiotensinogen II leads to vasoconstriction and causes high blood pressure.
- Cardiovascular system
As far as the concerned cardiovascular system, your heart is generally well decorated by Beta-1 adrenergic receptors. In a stressful situation, it stimulates the Beta-1 receptor that affects the various parameters of the heart –
- Positive chronotropic action (SA node) – increase heart rate (HR)
- Positive dromotropic action (AV node) – increase conduction velocity
- Positive ionotropic action (strongly contracts ventricles) – increase stroke volume (SV)
CO = SV x HR
Increase systolic blood pressure = ↑ SV x ↑ HR
In contrast, all the actions will be reversed in the parasympathetic nervous system like – constricting pupil, stimulating saliva flow, constricting bronchi, slowing heart rate, stimulating digestion and peristalsis, urination, defecation, etc.
Take home message
We know that human behavior is complex.
In our daily life activities, the autonomic nervous system regulates unconscious actions to maintain homeostasis like a heartbeat, digestion, breathing, blood pressure, and sexual arousal.
Therefore, it is an essential system of our body.
It is all about the autonomic nervous system. If you found this post informative, please share it on social media.
Ans. The blinking of the eyes is typically influenced by the autonomic nervous system. So, it has involuntary control, but it may also carry out voluntary control (somatic nervous system).
Q 2. Is breathing autonomic or somatic?
Ans. Breathing is an autonomic process, and it is controlled by the respiratory center. Due to this, we can breathe even in an unconscious condition and during sleep. We can also control our breathing rate ourselves but only for few times.
Q 3. Which autonomic nervous system is activated during stress?
Ans. The sympathetic nervous is usually activated during stressful conditions such as physical stress, psychological stress, financial stress, and academic stress. It triggers the “fight or flight” response and pumps the adrenaline hormone for action.
Q 4. What are autonomic nervous system disorders?
Ans. When sympathetic or parasympathetic nerves get damaged, your autonomic nervous system can be dysfunctional. A person with this condition is said to have autonomic neuropathy or dysautonomia. For example, Parkinson’s disease, multiple system atrophy, familial dysautonomia, etc.
2. Anatomy, Autonomic Nervous System