Parasympathetic Nervous System (PSNS)

There are many processes that are taking place in your body every time. When you are calmly sitting on your resting chair, you are breathing, your heart is pumping, your blood pressure is being controlled, and the cellular metabolic processes are also regulated. You are not aware of any of these processes, but parasympathetic system is there to take care of all these processes for you.

The parasympathetic nervous system is a division of the autonomic nervous system that controls the internal body organs in resting state. It is active all the time and promotes life by regulating the vital body functions, although the person is unconscious about all these processes.

In this article, we will talk about the organization
of the parasympathetic nervous system, its neurotransmitters and receptors, and
the process by which it controls the internal body organs.

In the end, we will talk about some disorders that
change the activity of the parasympathetic nervous system. We will also discuss
some drugs acting on the PSNS.

Anatomy and Organization:

The parasympathetic nervous system is also a subdivision of the peripheral nervous system. Like other subdivisions of the PNS, it also consists of nerve fibers leading to or originating from the brain and spinal cord. It also has neuronal cell bodies located in ganglia.

Nerva fibers

The parasympathetic system also has two types of nerve
fibers:

Pre-ganglionic nerve fibers: These nerve fibers originate from the central nervous system and terminate at the ganglia of the parasympathetic nervous system. Contrary to the sympathetic nervous system, the pre-ganglionic fibers in PSNS are long.

Post-ganglionic nerve fibers: They originate from the ganglia of the parasympathetic nervous system and terminate at the target organs. The post-ganglionic fibers are short in case of the parasympathetic system because the ganglia are present near the target organs. 

The parasympathetic system has craniosacral outflow.
The pre-ganglionic nerve fibers originate from the nuclei in the brain and the
sacral segments of the spinal cord and terminate in the ganglia.

Ganglia

Ganglia are the collections of neuronal cell bodies located outside the CNS. As stated earlier, the parasympathetic system has longer preganglionic fibers because the ganglia are present away from the CNS. They are present near the effector organs.

Examples of the parasympathetic ganglia include
ciliary ganglia, submandibular ganglia, etc. Ciliary ganglion controls the
muscles of the eye and is present just behind the eyeball. The submandibular
ganglia control the secretions of salivary glands and are present in the close
proximity to these glands.

Neurotransmitters

These are the chemicals released by the axons at the nerve terminals. They bind to the specific receptors present on the target tissue and initiate chemical responses.

The main neurotransmitter present in the parasympathetic system is acetylcholine. Acetylcholine is released by both preganglionic and postganglionic nerve fibers. Thus, it acts on both the neurons present in the ganglia as well as tissues present in the target organ.

Acetylcholine performs its function by binding to the
specific receptors called cholinergic receptors.

Receptors

The parasympathetic system also contains two types of receptors. The neurotransmitter for both these receptors is the same, i.e. acetylcholine.

Ganglionic receptors: These are the nicotinic receptors present on the neuronal cell bodies in the ganglia of the parasympathetic system.

Target receptors: These are the muscarinic receptors activated by acetylcholine. Nicotine can’t activate these receptors.  Depending on their response to activation, they are further divided into three types:

M1 receptors: These are the inhibitory receptors. They are present in heart especially on SA node and AV node. Activation of M1 receptors decrease heart rate. It does not have any effect on the force of contraction.

M2 receptors: These receptors are stimulatory in nature. They are present in the CNS. We have included them in the discussion of the parasympathetic nervous system because they are also activated by acetylcholine. The drugs that modify the activity of PSNS can also act on these receptors.

M3 receptors: These receptors are stimulatory in nature. They are present in smooth muscles of blood vessels, bronchi, gastrointestinal tract, bladder, eyes, etc. The activation of these receptors can cause relaxation or contraction of smooth muscles, depending on their location. 

Effects

The parasympathetic nervous system is active all the time. It controls many important functions of the body. It is the most important component of the nervous system involved in regulating body functions in resting state. Below we will give an account of different processes in the body that are under the control of parasympathetic nervous system.

Blood Pressure:

The parasympathetic system is important in regulating the blood pressure under resting conditions. It prevents any abnormal increase in blood pressure. If the blood pressure increases due to any reason, it is sensed by the baroreceptor system.

The baroreceptor reflex stimulates the parasympathetic system. The PSNS causes relaxation of blood vessels, decreasing total peripheral resistance. It also decreases heart rate. As a result, the blood pressure comes back to the normal level.

Heart Rate:

Under resting conditions, heart rate is under the control of parasympathetic nervous system. It prevents any abnormal increase in heart rate. A balance between sympathetic and parasympathetic stimulation of the cardiac cells keep the heart rate within normal limits.

The parasympathetic system may be artificially
stimulated to decrease heart rate in conditions like surgery etc. This can be
done by giving cholinergic drugs or by massaging the neck of the person. The
massage of neck stimulates the baroreceptors that increase parasympathetic
stimulation of the heart, thus decreasing the heart rate.

Respiration

The activation of M3 receptors present in bronchi causes contraction of smooth muscles. As a result, the diameter of the airways decreases, causing bronchiolar constriction.

The parasympathetic stimulation also increases the
secretions of glands present in the respiratory tract. The resultant increase
in mucous further blocks the air passages. The resultant effect of
parasympathetic stimulation is to inhibit the process of breathing. 

Digestion

The parasympathetic activity increases the contraction of smooth muscles present in the wall of the gut. Thus, it promotes peristalsis and the process of digestion.

The parasympathetic system also promotes the
secretions of different glands associated with the alimentary tract. 

The smooth muscles present in sphincters are inhibited
by the parasympathetic system. It causes opening of the sphincters.

The overall effect of parasympathetic system is to
promote the process of digestion.

Urination:

The process of urination is also stimulated by the parasympathetic system. The parasympathetic stimulation contracts the bladder muscles and relaxes the smooth muscles present in sphincters. The combined effect helps in voiding the bladder.

Secretions

The secretions of the sweat glands as well as the
glands associated with the digestive tract, respiratory tract, etc. are under
the control of parasympathetic system. It promotes the secretions of all these
glands.

Sexual Response:

The activation of the parasympathetic system causes
erection.

Pupillary Response:

The muscarinic receptors are present in the ciliary
muscles as well as the circular muscles of the eye. The activation of
muscarinic receptors by parasympathetic activity causes smooth muscle
contraction. The contraction of circular muscles constricts the pupil while
contraction of ciliary muscles cause accommodation for near vision.

Disorders affecting the activity of Parasympathetic system

The activity of the parasympathetic system is either
increased or decreased in the following disorders:

• Hypertension
• Heart failure
• Organophosphate poisoning
• Orthostatic hypotension
• Erectile dysfunction
• Neurogenic bladder

Drugs

The drugs acting on the parasympathetic system are
classified into two broad categories:

Cholinomimetic Drugs: These drugs act by binding to the muscarinic receptors and activating them.  This category also includes the drugs that increase the levels of acetylcholine in nerve synapses. They are used to treat conditions like Parkinson’s disease, myasthenia gravis, atropine toxicity, etc.

Anti-cholinergic drugs: These drugs either act by blocking the muscarinic receptors or by decreasing the available amount of acetylcholine. They are used in organophosphate poisoning, asthma, heart failure, etc.

Conclusion/Summary

The parasympathetic system is the division of the autonomic
nervous system that regulates important body functions at rest. It is
responsible for the maintenance of vital functions under calm situations.

It consists of long preganglionic fibers and short
postganglionic fibers. The ganglia of the parasympathetic system are present
near the target organ.

Like the sympathetic system, the ganglia of
parasympathetic system also have nicotinic receptors. However, the receptors in
the target tissue are different. These include:

• M1 receptors (in heart)
• M2 receptors (in CNS)
• M3 receptors (in smooth muscles and
  glands)

All the receptors in the parasympathetic system are
activated by acetylcholine, the main neurotransmitter present in the
parasympathetic system.

All the vital functions of the body are regulated
through parasympathetic system in the resting state. These include:

• Blood pressure
• Heart rate
• Breathing
• Digestion
• Secretions
• Urination
• Sexual responses
• Pupillary response

The activity of the parasympathetic system may be
increased or decreased in different disorders. The abnormalities of the system
itself can also give rise to a number of disorders.

The drugs acting on the parasympathetic system act by:

• Stimulating muscarinic receptors
• Inhibiting receptors
• Increasing the amount of available
  acetylcholine
• Decreasing the amount of
  acetylcholine

References

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