How autonomic nervous system works? - anatomy, function & its control
SUMIT SHARMA
Senior Pharmacist at ESIC, Ministry of labour and employment, Government of India | Pharmacy practice from NIPER | Medical blogger
Autonomic nervous system overview
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.
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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).
CNS includes the brain and?spinal cord , whereas PNS divides into the somatic nervous and autonomic nervous systems.
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.,?
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.
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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
Your blood pressure is also regulated via the sympathetic and parasympathetic nerves. The short-term regulation of blood pressure is controlled by the baroreflex mechanism .?
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
Whenever you feel body pain , it activates your sympathetic nervous system. Due to this, pain induces heart rate, blood pressure , stress, and anxiety.?
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 –
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 –?
The primary function of the PSNS is to conserve energy so that it can use later when required.
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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.?
The information process starts to travel through the thalamus , present on both sides of our brain. Then signal communicates with the cerebral cortex and stimulates the?hypothalamus .?
Hypothalamus is generally present below the thalamus and above the?pituitary gland . Hypothalamus is divided into two regions: -
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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.
Then it innervates the sympathetic nerve from T1 to L3. So, the sympathetic system represents thoracolumbar (thoracic ?and?lumbar ?vertebrae) outflow.
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.
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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.
From the ganglia, long postganglionic fibres are connected to a target organ or internal organ, e.g., cardiac muscles , lungs, liver , kidney, etc.
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.
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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.
The autonomic nervous system releases the two most common neurotransmitters - acetylcholine ?(Ach) and noradrenaline .
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).
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Autonomic nervous system?receptors ?
In the sympathetic nervous system, adrenaline or noradrenaline binds to its specific receptors.
All tissues have alpha 1 and beta 2 receptors except
On the other hand, the parasympathetic nervous system – binds on a specific muscarinic receptor.
Nicotinic receptor present on the ganglia where acetylcholine binds on it.
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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 –?
These all information go to your limbic system
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It produces fear and stress, which stimulate the posterior lateral area of the hypothalamus
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They carry information by polysynaptic descending pathway and stimulate lateral horn of spinal cord present in the thoracolumbar region
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Then it diffuses central sympathetic outflow to various target organs like –
After that, angiotensinogen-I converted to angiotensinogen-II by angiotensin-converting enzyme. Eventually, angiotensinogen II leads to vasoconstriction and causes?high blood pressure .
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 –?
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.?
References?
2. Anatomy , Autonomic Nervous System