AUTONOMIC NERVOUS SYSTEM: ANATOMY, FUNCTIONS AND DISORDERS - GEOGRAFÍA - 2025

The autonomic nervous system, neurovegetative nervous system or visceral nervous system is in charge of regulating the functioning of internal organs, such as the stomach, intestine or heart. It consists of a very complex neural network whose objective is to maintain a homeostasis or internal physiological balance.

To begin, it is important to clarify the divisions of the nervous system. This is differentiated into the central nervous system and the peripheral nervous system. The first includes the brain and spinal cord. The second encompasses nerves and ganglia throughout the body.

The autonomic nervous system is divided into the parasympathetic (blue) and the sympathetic (red) systems.

This, in turn, is divided into the somatic nervous system and the autonomic nervous system. The somatic controls voluntary movements and is made up of sensory neurons. While the autonomic regulates involuntary functions and is divided into the sympathetic system and the parasympathetic system. Their functions are described below.

The autonomic nervous system encompasses ophthalmic (pupillary), cardiovascular, thermoregulatory, gastrointestinal, and genitourinary systems.

Regulates the activity of different glands in the body. As well as the muscles of the skin (surrounding the hair follicles), around the blood vessels, in the iris of the eye, the stomach, the intestines, the bladder and the heart.

This system works involuntarily, that is, it escapes our consciousness. However, some patients can be trained to control their own autonomic nervous system reactions. Like heart rate or blood pressure, through relaxation techniques.

The autonomic nervous system participates in two types of situations. Thus, it is activated in stressful situations in which the body must prepare to face them or flee.

On the other hand, it is activated in those moments of rest so that the body can recover from its daily activities, digest food, eliminate waste, etc.

It is important to note that the autonomic nervous system is always in operation, as it works to maintain internal functions at an appropriate level. It is in continuous interaction with the somatic nervous system.

How does the autonomic nervous system work?

The main regions that control the autonomic nervous system are found in the spinal cord, brain stem, and hypothalamus. Although there are also parts of the cerebral cortex that can transmit impulses that modulate autonomic control. For example, the limbic system.

This system is essentially an efferent system, that is, it transmits signals from the central nervous system to peripheral organs. The autonomic nerves are made up of all the fibers that start from the central nervous system, except those that control skeletal muscles.

It also has some afferent fibers (those that carry information from the periphery to the central nervous system). These serve to regulate visceral sensation and respiratory and vasomotor reflexes.

Normally, the autonomic nervous system works through visceral reflexes. Specifically, sensory signals from the viscera and organs reach the autonomic ganglia, the spinal cord, the brain stem, or the hypothalamus.

This produces appropriate reflex responses that are returned to the organs to modulate their activity. The simplest reflexes end in the organ of interest, while the more complex ones are controlled by higher autonomic centers such as the hypothalamus (Ramos, 2001).

Autonomic nervous system anatomy

An autonomous nerve pathway includes two nerve cells. One of them is located at the base of the brain or spinal cord. It is connected by nerve fibers to another neuron located in a group of nerve cells called the autonomic ganglion.

There are two types of neurons depending on which ganglia it belongs to. The preganglionic, which is part of the central nervous system, and the postganglionic, which is found in the autonomic ganglion.

Thus, the nerve fibers of these ganglia connect with the internal organs. Most of the ganglia of the sympathetic nervous system are located outside the spinal cord, on both sides of it. While the ganglia of the parasympathetic division are located near or in the organs with which they connect.

The parts of the central nervous system that integrate and regulate autonomic functions are: the insular and medial prefrontal region of the cerebral cortex, the amygdala, the hypothalamus, the terminal stria…

As well as areas of the brain stem such as the periaqueductal gray matter, the nucleus of the solitary tract, the intermediate reticular zone of the spinal cord and the parabrachial nucleus.

The autonomic nervous system is a complex network made up of roots, plexuses, and nerves. Within the roots are the cervical, thoracic, lumbar and sacral roots.

The plexuses are a set of nerve fibers, both efferent and afferent, in addition to the ganglia. There are several plexuses according to the organs they innervate. These are: cardiac plexus, carotid plexus, pharyngeal plexus, pulmonary plexus, splenic plexus, epigastric plexus, and lumbosacral plexus. While the nerves involved are the cranial nerves.

The autonomic nervous system can be divided into three subsystems, the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system.

The sympathetic and parasympathetic systems often work in opposite ways. It can be said that both divisions complement each other, the sympathetic system functioning as an accelerator and the parasympathetic as a brake.

However, sympathetic and parasympathetic activity do not only involve fighting or resting situations. For example, when we are sitting and getting up, a sharp drop in blood pressure would occur if there were no compensatory increase in arterial sympathetic activity.

In addition, it has been discovered that both systems can participate in sexual arousal and orgasm.

These systems must be considered in an integrated way, working together for the continuous modulation of vital functions, keeping them balanced.

Sympathetic nervous system

This system is mainly activated in contexts that require immediate reactions, such as fight or flight. It originates from the spinal cord, specifically, encompassing the lumbar and thoracic areas.

Some of its functions are to move blood from the intestine and skin to the skeletal muscles and lungs so that they are activated. It also produces the dilation of the pulmonary bronchioles to increase the level of oxygen, and the increase of the heart rate.

The two main neurotransmitters released by this system are acetylcholine and norepinephrine.

Other effects of sympathetic stimulation are:

- Dilation of the pupils.

- Reduction in saliva production.

- Decrease in mucosa production.

- Incrise of cardiac frecuency.

- Relaxation of the bronchial muscle.

- Reduced intestinal motility.

- Greater conversion of glycogen into glucose by the liver.

- Decrease in urine secretion.

- Release of norepinephrine and adrenaline through the adrenal medulla.

Parasympathetic nervous system

The neurons in this system appear to start in the cranial nerves. Specifically, in the oculomotor nerve, the facial nerve, the glossopharyngeal nerve and the vagus nerve. It also has nerves that start from the sacral region of the spinal cord.

One of its functions is to dilate the blood vessels, causing a constriction of the pupil and the ciliary muscle. This results in better near vision. It also stimulates the salivary glands, as well as rest and digestion.

In summary, when the parasympathetic nervous system is active, some of the functions are:

- Increased production of the nasal mucosa.

- Decreased strength and heart rate.

- Contraction of the bronchi.

- Increased intestinal motility, secreting more gastric juices.

- Development of digestion.

- Increased urine secretion.

Enteric nervous system

The enteric nervous system is sometimes included within the autonomic nervous system. Although some authors consider it an independent system.

This system is a set of nerve cells that innervate the viscera and internal organs. These cells are organized in numerous ganglia located in the walls of the esophagus, stomach, intestines, pancreas, gallbladder, etc.

Neurotransmitters

Two types of neurotransmitters or chemical messengers predominate to send signals in the autonomic nervous system:

Acetylcholine

Generally, this substance has parasympathetic effects, that is, inhibitory. Although sometimes it has sympathetic effects, for example when it stimulates sweating or makes the hair stand on end. The nerve cells that release acetylcholine are called cholinergic neurons.

Noradrenaline

It usually has stimulating effects. The neurons that secrete them are called adrenergic cells.

Features

The main functions of the autonomic nervous system are:

- Control of the heart rate and the force of contraction of the heart.

- Dilation and contraction of blood vessels.

- Dilation and contraction of the smooth muscle of various organs. Smooth muscle is found in blood vessels in the reproductive and excretory systems, and other structures, such as the iris of the eye.

- Regulation of respiratory rate.

- Control of digestion and intestinal motility.

- Reflex actions such as coughing, sneezing, swallowing or vomiting.

- Visual accommodation and pupil size. This allows us to focus the eye on the desired stimuli and adapt the light input to it.

- Increased activity of the endocrine and exocrine glands. Exocrine secretions refer to the sweat, tears, or enzymes of the pancreas.

- Participates in the thermoregulation or control of body temperature. Through the autonomic nervous system, an adequate and constant temperature is maintained. One way to control it is by sweating.

- Control of waste disposal (urination and defecation)

- Participate in sexual arousal.

- Regulates metabolism. In this way, it manages the consumption of carbohydrates (glucose), influencing our body weight.

- Maintains adequate levels of water and electrolytes, such as calcium or sodium.

Disorders

Autonomic nervous system disorders can involve any part of the body or vital function. These disorders can also result from other conditions that damage autonomic nerves, such as diabetes. Although they can also appear on their own.

The activity of this system can be disturbed by toxins, pain, emotions or trauma involving the hypothalamus or the limbic system. These can be progressive or reversible.

The set of symptoms that cause disorders of this system is known as dysautonomia. Some of the symptoms are:

- Dizziness and low blood pressure. There can also be episodes of rhythmic palpitations at rest and for no apparent reason.

- Small nerve fiber neuropathy.

- Dry eyes and mouth, and lack of sweating. Although excessive sweating can also occur.

- Slow emptying of the stomach that is manifested by the person feeling very full, even eating a small amount of food, even the person may feel nauseous. This is known as gastroparesis.

- Urinary incontinence due to an overactive bladder. Although the opposite process can occur, that is, retention of urine due to lack of bladder activity.

- Constipation or decreased bowel movements. Although diarrhea can also occur, especially at night.

- Difficulty starting and maintaining an erection in men (erectile dysfunction).

- Another symptom may be that the pupils do not adapt to changes in light.

The disorders most associated with dysfunctions of the autonomic nervous system are:

- Diabetes mellitus: characterized by persistently high levels of glucose in the blood. Some of the symptoms that involve the autonomic system are: alterations in sweating, muscle weakness and blurred vision. In addition to problems in intestinal motility with pictures of nocturnal diarrhea or sexual impotence.

- Chronic alcoholism: in this case there are also alterations in intestinal transit, orthostatic hypotension (inability of the body to control blood pressure quickly) and impotence.

- Parkinson's disease: it is a degenerative motor disease in which there is a reduction in salivation, an increase in sweating, orthostatic hypotension and urinary retention.

- Multiple sclerosis: presents aforementioned alterations, in addition to deficits in the body's thermoregulation.

- Shy Drager syndrome: or multisystemic atrophy, which stands out for a progressive deterioration of the autonomic nervous system. It occurs in older people and is rare.

- Riley Dey syndrome: it is a hereditary disorder that affects the functioning of the nerves, it is associated with a congenital insensitivity to pain. These patients have orthostatic hypotension, decreased lacrimation, constipation or diarrhea, insensitivity to changes in temperature.

- In addition, autonomic dysfunction is associated with neuropathies such as Guillain-Barré syndrome, Lyme disease, HIV, or leprosy.

References

1. Autonomic nervous system. (sf). Retrieved on February 28, 2017, from Wikipedia: en.wikipedia.org.
2. Chawla, J. (June 28, 2016). Autonomic Nervous System Anatomy. Obtained from Medscape: emedicine.medscape.com.
3. Chudler, EH (nd). Autonomic Nervous System. Retrieved on February 28, 2017, from the University of Washington: faculty.washington.edu.
4. Low, P. (sf). Overview of the Autonomic Nervous System. Retrieved on February 28, 2017, from Msdmanuals: msdmanuals.com.
5. Ramos, M., Rovira, C., Umfuhrer, L. & Urbina, E. (2001) Autonomous Nervous System. Postgraduate Journal of the Chair VIa Medicina 101 (1-7)