Introduction

All the functions (processes) of the human body are either voluntary or involuntary. Functions like sight, movement, speech, etc. are some of the few voluntary actions. And many functions vital for the human body like breathing, heart rate, sweating, etc are involuntary. All these conscious or otherwise functions are controlled by the nervous system. The nervous system consists of the central nervous system (CNS) and the peripheral nervous system (PNS). The peripheral nervous system has two further divisions:

1. Somatic Nervous System
2. Autonomic nervous system

The autonomic nervous system is one of the two divisions of the peripheral nervous system. As the name suggests, ANS is largely responsible for the automatic (involuntary) functions of the body. ANS performs to control functions of internal organs such as the heart, stomach, digestive tract, etc. ANS keeps the body functioning smooth and also takes measures in times of emergency. Reflex functions like sneezing, coughing, and vomiting are also included in its responsibilities.

Divisions

ANS is made up of nerves and neuronal cell bodies called ganglia, which connect the central nervous system to the different organs of the body. ANS has further two divisions; the sympathetic nervous system and the parasympathetic nervous system.

Sympathetic Nervous System (SNS)

The main purpose of the sympathetic nervous system is to prepare the body to tackle some type of emergency. It is activated in response to fight or flight situations. The SNS takes steps to not only release adrenaline, increase heart rate, and respiration but also to make changes in the functionality of body organs during times of stress. All the communications between CNS and SNS are carried out by two types of nerves; pre-ganglionic and post-ganglionic.

When we face some kind of danger (like facing a rabid dog), the sympathetic nervous system steps in and alters the body processes to enhance the muscular energy and slows down or halts the non-essential functions. The respiration and heart rates elevate. Blood flow to the skin and digestive tract organs decreases. Many hormones like epinephrine, acetylcholine, and norepinephrine add to the bloodstream. So, the SNS coordinates different body processes as such to increase the chances of survival.

Parasympathetic Nervous System (PSNS)

The parasympathetic nervous system is active when the human body is in a state of rest. Most of the time, when the body is in normal condition, the parasympathetic nervous system controls involuntary functions. The basic principle of PSNS is “rest and digest” i.e. energy consumption is lowered by decreasing processes like muscular activity, heart rate, etc. and processes like digestion, urination, defecation, salivation, and sexual arousal remain normal. The parasympathetic nervous system also works as a counter-part of the sympathetic nervous system, as it also works to return the body processes to the normal (rest) state. The changes made in the functionality of organs by the SNS are countered by PSNS when it is no longer necessary.

Parasympathetic nerves originate from the central nervous system and extend to respective target glands and organs. The nerves involved in the PSNS are the Vagus nerve, cranial nerves, and groups of cell bodies (Ganglia).

Functions of ANS

We have developed until now that the autonomic nervous system is necessary for human survival as it controls the essential involuntary actions of the body. Now we will discuss some of the functions performed by ANS in some detail.

Digestion

The process of digestion is largely controlled by the autonomic nervous system. When we eat or swallow something, the course of the food from the pharynx to the outside of the body is handled by the ANS. The movements necessary for crushing and absorption of food itself are also under the control of the ANS. The autonomic nervous system can diminish or speed up the process of digestion according to the needs of the body. For example, when a fight or flight situation occurs, the ANS halts the digestion processes, but when the body is in a state of rest, the digestion process is resumed. So, the divisions (SNS and PSNS) of the autonomic nervous system regulate digestion according to the situation.

Blood Circulation

The autonomic nervous system smartly controls the flow of blood to different organs of the body, depending upon their specific need and situation. For example, in cases of emergency, like running away from danger, the blood flow to muscles is increased while it is decreased for digestive organs. After the situation is overcome, the autonomic nervous system works to bring the blood flow to normal. So, the ANS can increase or decrease blood supply to specific organs to handle a specific situation.

Heart rate

Regulating heart rate, depending on the situation, is also the responsibility of the autonomic nervous system. This function is performed by controlling the cardiovascular activity including contractions of cardiac muscles, automaticity, and refractory period of cardiac cells. A simple example of controlling heart rate is when the body undergoes a change in posture (like from sitting to standing). During exercise, heart rate is controlled to deliver a necessary amount of oxygen and other nutrients to the muscles. The autonomic nervous system decreases heart rate when a person is resting. This feat of the autonomic nervous system is also vital for the normal functionality of the body.

Blood Pressure

Keeping the blood pressure within the limits is also the task of the autonomic nervous system. This function is handled by the careful control of blood vessels, hormones, and heart rate. If the limits of blood pressure are crossed for some reason, the autonomic nervous system works to normalize the situation immediately. This process is generally very fast, as the situation is handled within a matter of seconds.

Urination and Defecation

The processes of urination and defecation are under the supervision of the autonomic nervous system. These processes can be halted by the ANS in fight or flee situations. This task is accomplished by controlling the muscles of the urinary bladder, rectum, and urethra. Appropriate measures are taken from the autonomic nervous system when there is a need for the removal of wastes from the body.

Pupillary Response

The autonomic nervous system controls the constriction and dilation of pupils. When we are exposed to bright light, the ANS constricts the pupils to minimize light exposure. The ANS also controls the response of pupils in emergencies. The sympathetic nervous system dilates the pupils so that the person can see easily and decide faster. And conversely, the parasympathetic nervous system constricts the pupils, thus limiting light exposure, so that the person can rest.

Respiratory rate

The process of breathing is also controlled by the autonomic nervous system. The ANS performs this function by controlling the blood supply, the diameter of air passages, and the amount of mucus. The respiratory rate is increased in times of need or emergency and is decreased when the body returns to a normal resting state. Involuntary responses (sneezing, coughing, etc.) of the body, when an unwanted particle is discovered in the respiratory tract, are also the responsibility of the autonomic nervous system.

Sexual Response

Sexual response is also controlled by the autonomic nervous system. The functions like sexual arousal, ejaculation, firmness of breasts, and vaginal secretions of the human body, are all under the control of the ANS. The activation of the sympathetic nervous system causes ejaculation.

Body Temperature

Regulation of normal body temperature is the task of the autonomic nervous system. The ANS accomplishes this task by using a number of methods. For example, when the temperature of the body falls, the ANS reduces blood flow to the skin to conserve heat. And if the body temperature is exceeding the optimal temperate, the ANS can reduce heat by using methods like sweating. So, the autonomic nervous system keeps the body temperature optimal.

Metabolism

Controlling body metabolism is also one of the functions of the autonomic nervous system. It is done by stimulating hormones like insulin and glucagon. The ANS is directly involved in the metabolism of essential body nutrients like lipids. Thus, the autonomic nervous system is necessary for the energy balance of the body.

Production and Secretion of body fluids

The ANS plays a key role in the production and secretion of essential body fluids. These body fluids include hormones and secretions of glands including sweat glands, respiratory glands, salivary glands, etc. The activation of the SNS usually decreases the secretion of hormones, but it is not always the case. The SNS works in coordination with the PSNS to control the secretion of the hormones.

Disorders of the ANS

The autonomic nervous system is vital for the normal functioning and survival of the human body, as we already discussed above. Any disorder of the ANS can cause serious complications. Some of the disorders of the ANS are the following:

Autonomic Paralysis

Autonomic paralysis is a severe body disorder generally caused by injury to the spinal cord. It severely affects the autonomic nervous system. If not treated on time, it can cause serious damage to ANS, ultimately leading to breathing failure, disruption in heart rate and blood flow control, and strokes. Its symptoms vary from individual to individual, but some of the common symptoms include high blood pressure, excessive sweating, nausea, slow pulse, severe headache, and a flushed face.

Multiple System Atrophy

Multiple system atrophy is a degenerative neurological disorder, affecting the autonomic nervous system. This disorder causes loss of motor control, high blood pressure, and bladder dysfunction. The major symptoms of MSA include:

• Stiff muscles
• Slow movement
• Poor coordination and body balance

Orthostatic Hypotension

Orthostatic hypotension is a disorder that can affect the autonomic nervous system. When the body changes posture (standing after sitting a while), the blood pressure can fall so abruptly to cause fainting. Some of the orthostatic hypotension symptoms include:

• Dizziness while standing up
• Weakness
• Fainting
• Blurry sight

Other than these disorders, there are several other conditions that can affect the normal functionality of the autonomic nervous system including:

• Alcohol or drugs abuse
• Diabetes
• Cancer
• Aging
• Trauma

Drugs Affecting the ANS

Several drugs can affect the performance of the ANS. The purpose of such drugs may be the treatment of some neurological disorder or they may simply be used to aid the functionality of the ANS. These drugs are divided into two categories following categories:

Sympathomimetic Drugs

These drugs affect the sympathetic nervous system by activating the adrenergic receptors either directly or indirectly. These drugs can stimulate sympathetic responses in the body. These drugs include amphetamine, cathine, cocaine, cathinone, etc.

Sympatholytic Drugs

These drugs interfere with or inhibit sympathetic activities. They generally work as antagonists to the adrenergic receptors. These drugs thus aid parasympathetic activities. Examples of such drugs are Indoramin, Guanethidine, Doxazosin, etc.

Read more about Nerve Impulses

Summary

The autonomic nervous system is a part of the peripheral nervous system that controls the functions of the body that are not under conscious control but are pivotal for sustaining life. These involuntary functions include breathing, blood rate, digestion, urination, etc. In addition, there are some reflex functions that also come under the control of the autonomic nervous system such as sneezing, coughing, vomiting, etc. It is further categorized into two divisions:

1. Sympathetic Nervous System: Responsible for body functionality during the ‘fight or flee’ situations. Eg. Facing a dog, having a nightmare, etc.
2. Parasympathetic Nervous System: Responsible for body functionality during the ‘rest and digest’ condition. It also performs to counter the changes made by the sympathetic nervous system. Eg. Sleeping, resting after doing some exercise, etc.

The body functions under the control of the autonomic nervous system are:

• Digestion
• Blood Circulation
• Heart rate
• Blood Pressure
• Urination and Defecation
• Pupillary Response
• Respiratory rate
• Sexual Response
• Body Temperature
• Metabolism
• Production and secretion of body fluids

The disorders of the autonomic nervous system can cause serious complications including autonomic paralysis, multiple system atrophy, orthostatic hypotension, etc.

Factors like alcohol or drug abuse, diabetes, cancer, ageing, and trauma can affect the normal functionality of the autonomic nervous system.

There are several drugs that can also affect the performance of the autonomic nervous system. These drugs fall into these two categories:

1. Sympathomimetic drugs
2. Sympatholytic drugs

Frequently Asked Questions

What is the autonomic nervous system?

It is a part of the peripheral nervous system that automatically controls the involuntary functions of the body such as heart rate, breathing, respiration, blood pressure, digestion, urination and defecation reflexes, sexual functions, etc.

What are 2 parts of the autonomic nervous system?

The two parts of the autonomic nervous system include the sympathetic nervous system and the parasympathetic nervous system. The sympathetic system is responsible for fight or flight response while the parasympathetic system is responsible for controlling autonomic functions at rest. 

What controls the autonomic nervous system?

The autonomic nervous system is controlled by the hypothalamus and the brainstem. The brainstem has many centres such as the respiratory centre, vomiting centre, etc that control most of the autonomic functions of the human body. 

What diseases cause autonomic dysfunction?

Certain metabolic diseases can cause autonomic dysfunction such as diabetes, hypothyroidism, porphyrias, amyloidoses, etc.

References

1. Schmidt, A; Thews, G (1989). “Autonomic Nervous System”. In Janig, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370.
2. Allostatic load notebook: Parasympathetic Function Archived2012-08-19 at the Wayback Machine – 1999, MacArthur research network, UCSF
3. Langley, J.N. (1921). The Autonomic Nervous System Part 1. Cambridge: W. Heffer.
4. Jänig, Wilfrid (2008). Integrative action of the autonomic nervous system : neurobiology of homeostasis (Digitally printed version. ed.). Cambridge: Cambridge University Press. p. 13. ISBN 978052106754-6.