Parasympathetic nervous system functions, neurons, divisions

The parasympathetic nervous system (SNP) is one of the two divisions of the autonomic nervous system, which is a division in turn of the peripheral nervous system, part of the nervous system of the human body. This system regulates the functions of many organs in our body..

The parasympathetic nervous system is said to be responsible for "resting and digesting" and "feeding and breeding," behaviors that occur when the body is resting. Some of the responses of this system are digestion, salivation, and defecation..

This system conserves energy, slowing the heart rate, increasing bowel activity, and relaxing the sphincter muscles in the gastrointestinal tract. The opposite of the sympathetic nervous system, which is involved in generating the fight or flight response.

Article index

• 1 A part of the autonomic nervous system
• 2 Functions of the parasympathetic nervous system
  • 2.1 What does the parasympathetic system do?
  • 2.2 Examples of parasympathetic responses
• 3 Reactions caused by the parasympathetic nervous system in the body systems
  • 3.1 In the visual system
  • 3.2 In the cardiac system
  • 3.3 In the digestive system
  • 3.4 In the respiratory system
  • 3.5 In the genital system
  • 3.6 In the excretory system
• 4 Types of neurons of the SNP
  • 4.1 Preganglionic neurons
  • 4.2 Postganglionic neurons
• 5 Divisions of the parasympathetic nervous system
  • 5.1 Cranial division
  • 5.2 Sacral division
• 6 References

A part of the autonomic nervous system

The nervous system of humans consists of a group of specialized cells, neurons, which are distributed throughout the body and communicate with each other, controlling the functions of all organs..

The nervous system is considered subdivided into two:

• The central nervous system: represents the command center of the entire nervous system, as it is the one who receives information and sends precise orders to the rest of the body. It is made up of the brain and spinal cord.
• The peripheral nervous system: it is represented by all the neurons and nervous structures that are not part of the central nervous system, but that connect all the organs with the latter.

The peripheral nervous system is made up of two components:

• The somatic nervous system: it is in charge of the control of our conscious functions.
• The autonomic nervous system: regulates the activity of our visceral organs, which are unconscious functions.

The autonomous subdivision of the peripheral nervous system is composed of two more divisions, which are responsible for the innervation and control of the internal organs of our body:

• The sympathetic nervous system.
• The parasympathetic nervous system.

The parasympathetic division is formed by the vagus nerve and the lumbar spinal nerves, which innervate the heart, intestines, etc..

Functions of the parasympathetic nervous system

The internal functioning of our body occurs thanks to the balance between the activities of the sympathetic and parasympathetic systems of the autonomic nervous system..

Many texts state that the main function of the sympathetic nervous system is to act during the "fight or flight" responses that we have in the face of stressful or frightening situations, and that the parasympathetic nervous system is responsible for restoring the normal conditions prior to the situation of stress.

However, it is important to clarify that both divisions of the autonomic nervous system are always in operation, inhibited or activated depending on the time of day, the physiological condition or the needs of our body.

What this means is that, although they participate in the aforementioned responses, the two systems fulfill very complex and important permanent functions for us..

What does the parasympathetic system do?

The parasympathetic division of the autonomic nervous system has a predominant activity in the conditions of "rest and digestion", conserving energy to be used later and regulating different bodily functions, such as digestion and urination..

Like the sympathetic nervous system, this division consists of a complex network of neurons whose main function is to maintain the integrity and functionality of the organs of our body..

This division, as well as the sympathetic division, exerts its functions through pairs of neurons, one preganglionic and the other postganglionic, which we will talk about later..

This system is involved in the control of many of the organs of our body:

• In the male reproductive system
• In the cardiovascular system
• In the respiratory system
• In the digestive system (salivary glands, stomach, intestines and associated glands)
• In the eyes
• In the excretory system

Examples of parasympathetic responses

Examples of responses involving the parasympathetic nervous system are:

• Defecation- The parasympathetic nervous system contracts the sphincters and moves digested food through the digestive tract to continue defecation.
• Digestion: stimulates the production of saliva and the movement of the stomach and intestines.
• Urination / urination: contracts the bladder for urine output.
• Tearing: stimulates the production of tears to keep the eyes lubricated.
• Salivation: stimulates the production of saliva, which contains enzymes for digestion.

Reactions caused by the parasympathetic nervous system in the body systems

The actions of the parasympathetic nervous system on its different target organs can be separated according to the systems of our body, since it performs different functions at the level of the eyes, the heart, the digestive tract, the lungs, the genitalia and the excretory organs..

In the visual system

The parasympathetic nervous system causes miosis, that is, the constriction of the pupil of the eyes. On the contrary, the sympathetic division causes mydriasis in the eyes, which is the dilation of the pupil.

This effect on the visual system is achieved through the stimulation, by acetylcholine released by postganglionic fibers, of cholinergic receptors in the sphincter muscle of the iris, which causes the constriction of the pupil.

In the cardiac system

The activity of the parasympathetic nervous system on this system causes a decrease in heart rate, contrary to what produces the sympathetic division, which accelerates it.

In the digestive system

In this system, the parasympathetic nervous system stimulates the smooth muscles of the intestines, promoting peristalsis (intestinal movements that favor the transit of ingested material). In addition, it also promotes the secretion of digestive enzymes by the pancreas, which is inhibited by the activity of the sympathetic division..

Another of the reactions of the digestive system against the activity of the parasympathetic involves the relaxation of the sphincters. It also participates in the contraction of the muscles associated with the gallbladder, which promotes the release of bile..

In the respiratory system

In the lungs, the parasympathetic division causes bronchoconstriction and increased bronchial secretions, while the sympathetic one causes bronchodilation.

In the genital system

In the male genital system, stimulation mediated by the parasympathetic nervous system causes erection of the penis, in addition to the excitation of the vas deferens, seminal vesicles and prostate (stimulates prostatic secretions).

In the female genital system, stimulation mediated by the parasympathetic nervous system also causes an erection of the clitoris, stimulation of vaginal secretions, rhythmic vaginal contractions during orgasm, and somatic contractions of the pelvic muscles that accompany orgasm..

In the excretory system

Regarding the reactions of the excretory system against the activity of the parasympathetic nervous system, we can say that the latter facilitates urination (urine), thanks to the stimulation of the contraction of the detrusor muscle and the relaxation of the internal sphincter of the urethra (urination it is a very complex process, controlled at the autonomic and somatic level).

Types of neurons in the SNP

Neural pairs are involved in the parasympathetic nervous system. Each pair is made up of a preganglionic neuron and a postganglionic neuron, which communicate via synapses in the parasympathetic ganglia..

Preganglionic neurons form parasympathetic preganglionic fibers, which are fibers derived from the central nervous system and dedicated to parasympathetic control.

Preganglionic neurons

The neurons of the parasympathetic division are derived from some nuclei of the brainstem (in the brain) and the sacral segments S2 and S4 of the spinal cord, in contrast to the neurons of the sympathetic division, which are derived only from the spinal cord in the thoracolumbar segments.

- The cranial nerves that participate in the parasympathetic nervous system are the oculomotor nerve (cranial nerve III), the facial nerve (cranial nerve VII), the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X).

- Fibers deriving from the spinal cord form the splanchnic pelvic nerves, which innervate the viscera of the pelvic cavity.

The parasympathetic preganglionic fibers formed by these neurons are extremely long, especially those that innervate visceral organs, since the ganglia where they communicate with the paired neuron (postganglionic) are very close to the innermost organs..

In addition, the preganglionic neurons of the parasympathetic, as well as those of the sympathetic, are cholinergic, that is, when they synapse with postganglionic neurons they secrete the neurotransmitter acetylcholine..

Postganglionic neurons

The ganglia where the synapse occurs between the axon terminals of the preganglionic fibers and the cell bodies of the postganglionic fibers are at or very close to the target organs, so that the parasympathetic postganglionic fibers have axons much shorter than those of the sympathetic postganglionic neurons..

These neurons are cholinergic, like preganglionic neurons, since they also secrete acetylcholine to control the functions of their target organs (another difference from those of the sympathetic division, which release norepinephrine and adrenaline).

This neurotransmitter secreted by pre- and postganglionic fibers stimulates muscarinic receptors on the surface of target organ cells that respond to nerve stimulation..

These receptors are integral membrane proteins that activate signaling cascades within the cell and, consequently, within the organ..

Divisions of the parasympathetic nervous system

The parasympathetic nervous system can be divided into two: the cranial parasympathetic and the sacral parasympathetic, according to the nerve fibers that are involved in the transmission of electrical impulses from the central nervous system..

Cranial division

This division mainly innervates the organs of the head and the preganglionic nerve fibers derive from the cranial nerves known as "cranial nerves".

The cranial nerves that participate in the parasympathetic system are those mentioned above, responsible for all the neuronal information related to the upper part of the body:

• III pair or oculomotor nerve, which acts in the eyes (pupil constriction)
• VII pair or facial nerve, which regulates the lacrimal and salivary glands, as well as the mucous membranes of the nasal cavity
• IX pair or glossopharyngeal nerve, which acts on the parotid gland
• X pair or vagus nerve, which acts on the viscera of the abdomen and thorax (it represents 75% of the parasympathetic fibers of our body)

Sacral division

This division has to do with the visceral function of the pelvic region, more specifically with the regulatory activity of the digestive tract and sexual functions..

It is formed by the sympathetic preganglionic nerve fibers that derive from the sacral segments of the spinal cord, which act both in the activation of the production of digestive secretions and in the erection of the male genital member..

References

1. Fox, S. I. (2002). Human physiology. McGraw-Hill.
2. Furness, J. B. (2006). The organization of the autonomic nervous system: peripheral connections. Autonomic Neuroscience: Basic and Clinical, 130 (1), 1-5.
3. Ganong, W. F. (1995). Review of medical physiology. McGraw-Hill.
4. Purves D, Augustine GJ, Fitzpatrick D, et al., Editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Autonomic Regulation of Sexual Function. Taken from ncbi.nlm.nih.gov
5. Tindle J, Tadi P. Neuroanatomy, Parasympathetic Nervous System. [Updated 2020 Jan 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Taken from ncbi.nlm.nih.gov