Autonomic Nervous System (ANS)

As you all know, all the processes that happen in the human body are controlled by the nervous system. A person has control over only a limited body functions such as movements, speech, sight, thinking, etc. Most of the functions of the body that are not under our conscious control are controlled by the autonomic nervous system.

The autonomic nervous system is a division of peripheral nervous system that is not under voluntary control. It is often regarded as a self-regulating system. It controls the functions of internal body organs such as stomach, heart, lungs, urinary bladder, etc.

In this article, we will talk about the structure,
divisions, functions, and diseases of the autonomic nervous system. We will
also talk about some drugs that act on the autonomic nervous system.

Structure

The autonomic nervous system is a division of the peripheral nervous system. It means that it is the system by which centrally located brain and spinal cord control the organs located at the periphery.

Thus, it consists of the nerves (bundles of axons) that originate from or lead to the brain and spinal cord (central nervous system). It also consists of collections of the neuronal cell bodies located outside the central nervous system. These collections of cell bodies are called ganglia.

Divisions

The autonomic nervous system is further divided into
three divisions:

Sympathetic Nervous System This division of autonomic nervous system is responsible for controlling fight or flight response. It controls the involuntary responses of the body when a person is in some serious situation. The sympathetic responses prepare the body to deal with some fight or flight conditions.

Parasympathetic Nervous System: This division controls the body functions in a calm state. The involuntary responses of the body under calm conditions are controlled by the parasympathetic nervous system. It is responsible for regulating body functions under normal conditions.

Enteric Nervous System: It is the third division of the autonomic nervous system. It is confined to the control of the gut. It contains branches from both sympathetic and parasympathetic nervous systems and is responsible for regulating the functions of gastrointestinal tract.

Functions

Most of the functions of the body essential for
maintaining life are under the control of the autonomic nervous system. We will
take a look at these functions one by one.

Blood pressure

The autonomic nervous system controls the blood pressure of a person and keeps it within the limits. The blood pressure is controlled by regulating the tone of blood vessels, the excretion of fluids and heart rate. The autonomic nervous system senses any change in the blood pressure and tends to return it to the normal within a few seconds.

Heart rate

Heart rate is also controlled by the ANS. It not only controls the heart rate but also controls the force of contraction, the automaticity of the cardiac cells and their refractory period (the period after which they are ready to undergo another contraction).

The control of heart rate is dependent on other factors such as blood pressure, venous return to the heart, exercise, temperature, etc. In response to any of the above-mentioned factors, the heart rate is regulated accordingly by the autonomic nervous system.

Blood flow

The autonomic nervous system not only regulates the
blood pressure and heart rate but also controls the blood flow to a particular
organ.  Depending on the needs of the
organ, it can increase or decrease its blood supply.

Digestion

The process of digestion is also controlled by the ANS. Once you swallow a meal and it reaches the pharynx, the rest of the course of the bolus you swallowed is decided by the autonomic nervous system.

The movement of the bolus from the esophagus to the large intestine and outside the body, all is under the control of the autonomic nervous system. It also controls the release of enzymes and gut movements involved in crushing and digesting the food particles.

Breathing

The process of breathing is controlled by the ANS. It
not only controls the rate of breathing but also controls the diameter of the
airways and the secretions present in them. It is also involved in the removal
of any foreign particle that enters the respiratory tract.

Urination

The autonomic nervous system controls the process by urination by controlling the tone of the bladder muscles. It also regulates the tone of urinary sphincters and smooth muscles of urethra. When the bladder is full, it is sensed by the autonomic nervous system and signals are sent for the excretion of the urine from the bladder.

Pupillary response

The response of the pupils to light is under the
control of this system.  It also controls
the accommodation of vision to near or far objects. The closing of the eyelid
on exposure to a foreign particle is also under the control of autonomic
nervous system.

Sexual responses

The sexual responses are also under the control of the ANS. The processes such as erection and ejaculation are controlled by it. The vaginal secretions, the firmness of breasts and other sexual responses in females are also under the control of ANS.

Secretions

Secretions of different glands of the body such as
sweat glands, salivary glands, glands in the respiratory system are all
controlled by the autonomic nervous system. It also controls the secretions of
glands present in gastro-intestinal tract such as pancreas, gall bladder, etc.

Body temperature

Body temperature is also controlled by the ANS. It controls the loss of heat from body by regulating the blood flow to the skin. The excretion of sweat also plays an important role in regulating the body temperature which is under the control of the autonomic nervous system. 

Metabolism

By controlling the release of hormones like insulin
and glucagon, the autonomic nervous system also exerts its effects on the
cellular metabolism of the body. It also has direct role in the metabolism of
lipids.

Disorders

As evident from the above-stated functions, any
disorder of the autonomic nervous system can have some serious impacts on the
overall functioning of the body. Below are given some of the important
disorders of the autonomic nervous system:

• Autonomic Paralysis: It is the most severe condition that causes the paralysis of the entire autonomic nervous system. The body fails to control the essential processes like breathing, heart rate, blood pressure. The symptoms of autonomic paralysis include high blood pressure, fast pounding heart, increased sweating, flushing face, etc
• Baroreceptor failure: In this disorder, the baroreceptor mechanism fails to sense the changes in blood pressure. As a result, the autonomic nervous system fails to control blood pressure. The local mechanisms may still control the blood pressure, but the most important mechanism involved in controlling the blood pressure is lost.
• Orthostatic hypotension: It is a condition in which the blood pressure immediately falls as a person stands up from a sitting position. The patient may become unconscious because of severe decline in blood pressure. Normally, when a person stands up, the autonomic nervous system causes the blood vessels to contract which results in increased venous return to the heart and blood pressure id controlled. In this disorder, the control of autonomic nervous system is lost. As a result, the blood is pooled in the leg veins, and when a person changes posture from sitting to standing position, the blood pressure immediately falls.

Other
conditions that can influence the normal functioning of the autonomic nervous
system include:

• Alcohol abuse
• Drug abuse
• Cancer
• Diabetes
• Spinal cord injuries
• Peripheral neuropathy

Drugs acting on the ANS

Drugs acting on the autonomic nervous system may be
used to treat disorders of the ANS. They may be used to aid the ANS in
regulating the body state in other conditions such as hypertension, cardiac
failure, hyperlipidemias, etc. The drugs that act on the autonomic nervous
system are classified into two major categories:

Cholinergic drugs: They act on the parasympathetic division of the autonomic nervous system. They may inhibit the parasympathetic responses or increase them. These include drugs such as pyridostigmine, atropine, organophosphates, etc.

Adrenergic Drugs: They act on the sympathetic division of the autonomic nervous system. They may inhibit or increase the sympathetic responses of the body. These include drugs such as tamsulosin, albuterol, propranolol, esmolol, etc.

Conclusion

The autonomic nervous system is the division of the
peripheral system that controls the vital functions of the body.

It is further divided into sympathetic and
parasympathetic nervous systems. It consists of the nerves connected to the
central nervous system and the cell bodies of neurons in the form of ganglia.

It controls the vital body functions such as:

• blood pressure
• heart rate
• breathing rate
• urination and defecation
• digestion
• pupillary response
• sexual responses
• body temperature
• metabolism

The disorders of the autonomic nervous system can
cause the body lose control of vital body functions that can lead to some
serious complications.

The drugs acting on the autonomic nervous system
mainly act by altering the responses of its two subdivisions. Depending on
their site of action, they may increase or decrease the responses of the
sympathetic or parasympathetic nervous system.

References

1. Langley, J.N. (1921). The Autonomic Nervous System Part 1. Cambridge: W. Heffer.
2.  Allostatic load notebook: Parasympathetic Function Archived 2012-08-19 at the Wayback Machine - 1999, MacArthur research network, UCSF
3. Furness, John (9 October 2007). "Enteric nervous system". Scholarpedia. doi:10.4249/scholarpedia.4064. Archived from the original on 8 October 2017. Retrieved 8 October 2017.
4. Willis, William D. (2004). "The Autonomic Nervous System and its central control". In Berne, Robert M. (ed.). Physiology (5. ed.). St. Louis, Mo.: Mosby.