Parasympathetic: Definition, Anatomy, Function, Differences and Treatment

It is part of the autonomic nervous system, which controls involuntary acts and functions.

The nervous system’s division controls automatic processes, such as digestion, respiration, and heart rate.

It works with the sympathetic nervous system and conserves the body’s energy by returning bodily functions to homeostasis, particularly after the sympathetic nervous system activates the fight or flight response.

Definition of the parasympathetic nervous system

The parasympathetic nervous system, or PSNS, is part of the nervous system. The nervous system sends signals to and from different body parts through the nerves.

The parasympathetic nervous system is responsible for all bodily activities when a body is at rest.

For this reason, the PSNS is known as the “rest and digest” part of the nervous system. These actions may include digesting food, excreting waste, crying, salivating, or sexually arousing.

The counterpart of the parasympathetic nervous system is the sympathetic nervous system (SNS), which is responsible for the “fight or flight” activities that occur when a body decides to be in motion.


The function of the parasympathetic nervous system

The sympathetic and parasympathetic nervous systems are collectively known as the autonomic nervous system. This division of the nervous system regulates bodily actions that occur without conscious thought.

The parasympathetic nervous system works with the sympathetic nervous system to maintain homeostasis in the body.

For example, the SNS increases the heart rate and blood pressure during a fight or flight response. The PSNS then works to lower the heart rate and lower blood pressure.

It also starts bodily processes that were temporarily suspended when the SNS was activated, such as digestion.

During periods of rest, the body can devote energy to processes that are not directly related to fight or flight.

Parasympathetic nerves begin in the brain’s medulla and in the spinal column’s middle area, which contains the spinal cord.

One of the nerves in the spinal cord is the vagus nerve, which is a part of the body that helps control the heart, lungs, and organs of digestion.

The brain and spinal cord are essential structures in the nervous system; Together, they form the central nervous system (CNS). The nerves of the PSNS that originate in the brain are called cranial nerves.

Ganglia, or groups of nerve cell bodies, are extensions of the PSNS nerves and are found near or on organs in the body to send signals to their target areas.

The sympathetic and parasympathetic nervous systems regulate many parts of the body, such as:

  • The muscles of the heart.
  • Blood vessels.
  • The muscles of the digestive system.
  • The salivary glands and the adrenal glands (which produce epinephrine, also called adrenaline, during fight or flight).

People dilate during a fight or flight period to allow us to see more clearly and make faster decisions.

During the break and the summary, the PSNS makes people constrict. PSNS also causes increased digestion of food, increased production of saliva and mucosa, and increased secretion of urine from the kidneys.


The parasympathetic division of the ANS originates (bilaterally) in the brainstem and sacral segments of the spinal cord.

Synapses of preganglionic neurons in discrete autonomic ganglia, except in the case of the vagus nerve.

The preganglionic axons at the vagus nerve synapse in the terminal ganglia located within the plexuses within the visceral organ are innervated. A microscope is required to view the terminal nodes.

Sacral parasympathetic preganglionic neurons are located in the intermediate gray matter of the spinal cord segments (S1,2,3 in the carnivore).

Bilaterally, the preganglionic axons traverse the lumbosacral plexus and the pelvic nerve to form synapses on the pelvic ganglia within the pelvic plexus.

The posterior ganglionic axons innervate the pelvic viscera, including the descending colon.

Vagal parasympathetic preganglionic neurons arise from the parasympathetic nucleus of the vagus nerve (cardiac preganglionic reside in the nucleus ambiguous).

In the chest, the preganglionic axons leave the vagus nerves to innervate the terminal ganglia of the heart and lungs.

Bilaterally, the preganglionic axons enter the abdomen through ventral and ventral vagal nerve trunks.

The axons reach the terminal ganglia of the abdominal viscera, including the transverse colon, running into the nerve plexuses in the celiac and cranial mesenteric vessels.

The parasympathetic and sympathetic nervous systems are the two subsystems that make up the autonomic nervous system.

These systems control the reflexive and involuntary actions of your body, such as:

  • Pupil dilation.
  • Heart rate
  • The digestion.
  • The production of saliva.
  • The size of the blood vessels.
  • Sweating and other automatic functions of your body’s organs.

Differences between the parasympathetic and sympathetic nervous system

The sympathetic and parasympathetic nervous systems work together so that the body can respond appropriately to everyday scenarios.

The sympathetic nervous system is dedicated to fast-responding stress signals, such as responding to life and death situations.

The parasympathetic nervous system treats less immediate situations, such as digestion and tearing the eyes.

Here are a couple of tips to help you remember the difference between the sympathetic nervous system and the parasympathetic nervous system:

  • Parasympathetic: controls “rest and digestion” functions when everyday situations are calm.
  • Sympathetic: controls the “fight or flight” response during more stressful situations.

The parasympathetic nervous system is found in the brain and sacrum, specifically in the S2, S3, and S4 vertebrae, at the end of the spine.

Problems with the parasympathetic nervous system are severe and can cause the following symptoms:

  • Digestive difficulty
  • Inability to breathe correctly.
  • Increase or decrease in blood pressure.
  • Heart problems.

Treatment of nerve-related conditions

Spinal conditions, such as foraminal stenosis, spinal stenosis, bulging or herniated discs, and spinal arthritis can affect the nervous system.

Once a spinal condition is diagnosed, a doctor will generally recommend conservative treatments, such as physical therapy, chiropractic care, or anti-inflammatory medications to reduce stress on nerve tissue.

Treatment options to provide pain relief is surgery that becomes an option.