Definition and Overview
The parasympathetic nervous system is one of the two major divisions that make up the autonomic nervous system. It is activated when the body is in a resting state and performs the functions of slowing the heart rate, promoting digestion, and storing energy. Because of this function, it is often called the ‘rest and digest nerve.’
The autonomic nervous system maintains balance in the body's internal environment through the antagonistic action of the sympathetic and parasympathetic nervous systems. While the sympathetic nervous system is responsible for states of tension and activity, the parasympathetic nervous system is responsible for relaxation and recovery.
The core pathway of the parasympathetic nervous system is the vagus nerve. The vagus nerve starts from the brain and is distributed to major organs in the chest and abdomen, including the heart, lungs, and gastrointestinal tract, and transmits approximately 75% of all parasympathetic nerve signals.
anatomy
The nerves of the parasympathetic nervous system originate from two places: the brainstem and the sacral spinal cord. This is called craniosacral outflow.
The parasympathetic nerves that emerge from the brain stem travel along the four cranial nerves.
- Oculomotor nerve (third cranial nerve): Responsible for pupil constriction and control of the lens.
- Facial nerve (7th cranial nerve): Stimulates the secretion of lacrimal and salivary glands.
- Glossopharyngeal nerve (9th cranial nerve): Controls salivary secretion from the parotid gland.
- Vagus nerve (10th cranial nerve): Controls the chest and abdominal organs, including the heart, lungs, and gastrointestinal tract.
The sacral parasympathetic nerves start from sacral vertebrae 2 to 4 and control the pelvic organs such as the bladder, rectum, and genitals.
A structural feature of the parasympathetic nervous system is that the ganglia (relay stations) are located close to the target organ or within the organ wall. Therefore, the nerve in front of the ganglion (preganglionic nerve) is long, and the nerve behind the ganglion (postganglionic nerve) is short. This is the opposite structure to the sympathetic nervous system.
Both preganglionic and postganglionic nerves use acetylcholine as a neurotransmitter. Acetylcholine in the postganglionic nerve exerts its effects by acting on muscarinic receptors in target organs.
function
The parasympathetic nervous system works to conserve and store energy when the body is in a stable state. The function of resting and digestion is key.
The functions of each major organ are as follows.
- Heart: Lowers the heart rate and slows atrioventricular conduction. The ability to maintain a resting heart rate of 60 to 80 beats per minute is thanks to the continuous inhibitory action of the vagus nerve.
- Lungs: Constricts the bronchi and promotes bronchial mucus secretion.
- Digestive system: Helps digest food and absorb nutrients by promoting gastric acid secretion, digestive enzyme secretion, and intestinal movement. There is research showing that gastric acid secretion increases by about 50% when vagus nerve is stimulated.
- Eyes: Constricts (miosis) the pupil and thickens the lens to focus on nearby objects.
- Secretory glands: Promote the secretion of saliva, tears, digestive juices, etc.
- Bladder: Induces urination by contracting the bladder wall (detrusor muscle) and relaxing the urethral sphincter.
Vagal tone is a representative indicator of parasympathetic nerve function. The higher the vagal tone, the greater the heart rate variability (HRV), which suggests better stress resilience and cardiovascular health.
Symptoms of parasympathetic nerve abnormalities
When parasympathetic nerve function declines, the sympathetic nerve becomes relatively dominant and various symptoms appear. There are reports that approximately 70% of all patients with autonomic dysfunction are accompanied by decreased parasympathetic nerve function.
Typical symptoms are as follows:
- Cardiovascular system: Increased resting heart rate (tachycardia), decreased heart rate variability
- Digestive system: indigestion, decreased gastrointestinal motility, constipation, abdominal distension
- Secretion function: dry mouth, dry eyes
- Urinary system: Difficulty urinating, feeling of residual urine
- Systemic symptoms: chronic fatigue, difficulty sleeping, difficulty relaxing
Diabetes is a major cause of parasympathetic nerve damage. In approximately 60% of patients with diabetic autonomic neuropathy, decreased cardiovascular parasympathetic function appears as an early finding. Parasympathetic nerve function gradually decreases with aging, and the high frequency (HF) component of HRV is known to decrease by about 15% every 10 years.
Inspection method
The test method to evaluate parasympathetic nerve function is as follows.
- Heart rate variability (HRV) analysis: High frequency (HF, 0.15~0.40Hz) component and RMSSD are key indicators reflecting parasympathetic nerve activity. RMSSD of less than 20ms suggests parasympathetic function decline.
- Deep breathing test: Measures heart rate changes (E:I ratio) during 6 deep breaths per minute. An increase in heart rate during inspiration and a decrease during expiration are normal responses, and a decrease in this change indicates a decrease in parasympathetic function.
- Valsalva maneuver: Observe the recovery pattern of heart rate and blood pressure after holding your breath and exerting force. A Valsalva ratio of 1.21 or higher is normal.
- tilt-table test: Comprehensively evaluates autonomic reflex function by observing changes in heart rate and blood pressure when standing.
- Pupil response test: Measures the speed and size of pupil contraction in response to light stimulation to check for abnormalities in the parasympathetic nerve pathway.
life management
Daily management rules to maintain and improve parasympathetic function are as follows.
- Regular aerobic exercise: 3 to 5 times a week, walking, swimming, or cycling for more than 30 minutes increases vagal nerve tone and improves parasympathetic nerve function. There is a study showing that the HF component of HRV significantly increased after a 12-week aerobic exercise program.
- Slow abdominal breathing: Slow, deep breathing of 6 times per minute directly stimulates the vagus nerve and increases parasympathetic nerve activity.
- Sufficient sleep: Regular sleep of 7 to 8 hours a day is essential for autonomic recovery. Lack of sleep causes decreased parasympathetic nerve function and overactivation of the sympathetic nerve.
- Stress management: Meditation, yoga, and progressive muscle relaxation help activate the parasympathetic nervous system.
- Washing your face with cold water: Contacting your face with cold water triggers the diving reflex, activating the vagus nerve and reducing heart rate.
- Avoid excessive caffeine and alcohol: Excessive consumption of caffeine stimulates the sympathetic nervous system and inhibits the parasympathetic nervous system function.
If symptoms persist or interfere with daily life, it is recommended to consult an autonomic neurologist.