The autonomic nervous system is responsible for controlling many of the unconscious processes in the body, including heart rate, digestion, respiration, and sexual arousal. It has two main divisions – the sympathetic nervous system and the parasympathetic nervous system – that work together to maintain homeostasis. The parasympathetic nervous system specifically promotes rest and digest activities when the body is relaxed, allowing the body to conserve energy and focus on internal tasks like digestion.
Overview of the Autonomic Nervous System
The autonomic nervous system is part of the peripheral nervous system and functions largely below the level of consciousness. It controls many automatic functions such as heart rate, breathing, digestion, and glandular secretion. This system is divided into two divisions:
- Sympathetic nervous system
- Parasympathetic nervous system
These two divisions generally have opposite effects on target organs within the body, allowing precise control of involuntary activities. The sympathetic division typically stimulates organs and mobilizes the body for action, while the parasympathetic division generally inhibits organ function to promote rest and digestion.
The Sympathetic Nervous System
The sympathetic nervous system controls the fight-or-flight response. When faced with stress, danger, excitement, exercise, or low temperature, the sympathetic system is activated. This division originates from the thoracic and lumbar regions of the spinal cord.
Activation of the sympathetic nerves:
- Dilates the pupils
- Increases heart rate and blood pressure
- Diverts blood to skeletal muscles
- Slows or halts digestion
- Inhibits salivation and lacrimation
These widespread effects prepare the body to deal with perceived threats and expend energy in emergency situations. Once the threat has passed, the parasympathetic system brings operations back to normal levels.
The Parasympathetic Nervous System
The parasympathetic nervous system controls rest and digest activities. It conserves energy and restores homeostasis after sympathetic activation. This division originates from cranial nerves III, VII, IX, and X and the sacral region of the spinal cord.
Activation of the parasympathetic nerves:
- Constricts the pupils
- Slows heart rate and breathing
- Stimulates digestion and glandular secretion
- Relaxes sphincter muscles
These responses encourage rest, relaxation, and regeneration of the body’s systems. The vagus nerve (cranial nerve X) plays a major role in mediating the widespread effects of the parasympathetic nervous system.
Parasympathetic Nervous System and Rest & Digest
The parasympathetic nervous system is responsible for stimulating rest and digest activities that occur when the body is relaxed. This division dominates when the body is not under stress. It conserves energy for internal tasks like digestion, while slowing high-energy processes like heart rate.
Promoting Digestion
One of the main roles of the parasympathetic nervous system is controlling digestion. The vagus nerve is critical for mediating the digestive effects of this system. Parasympathetic stimulation:
- Increases secretion from digestive glands
- Increases gut motility to move food through the GI tract
- Relaxes the pyloric sphincter to empty stomach contents into the small intestine
- Relaxes the anal sphincter for defecation
These actions allow the body to efficiently digest food, absorb nutrients, and eliminate waste when the body is relaxed and not expending much energy on other pursuits.
Slowing Heart Rate
The parasympathetic nervous system works antagonistically to the sympathetic system to slow heart rate during rest. It releases acetylcholine, which binds to muscarinic receptors on cardiac pacemaker cells to reduce their spontaneous depolarization rate. This dominant vagal tone on the heart keeps heart rate low during rest.
Heart rate can decrease by up to 30% due to parasympathetic stimulation. This reserves cardiac output for digestion and other restorative functions rather than preparing for fight or flight.
Other Effects
Other effects of the parasympathetic nervous system that promote rest and regeneration include:
- Constricting the pupils to reduce incoming light
- Increasing salivary gland secretion
- Slowing respiration rate
- Relaxing bronchiolar smooth muscle in the lungs
- Stimulating secretion from endocrine glands
These actions coordinate body systems to work efficiently, restore resources, and build up energy stores during periods of rest and recovery.
Comparison to the Sympathetic Nervous System
The parasympathetic and sympathetic nervous systems have contrasting effects that help regulate homeostasis. The balance between the two divisions depends on the body’s immediate needs and activity levels.
| System | Parasympathetic | Sympathetic |
|---|---|---|
| Origin | Cranial nerves III, VII, IX, X; sacral spinal nerves | Thoracic and lumbar spinal nerves |
| Neurotransmitter | Acetylcholine | Norepinephrine, epinephrine |
| Cardiac effects | Decreases heart rate | Increases heart rate |
| Digestive effects | Increases digestion | Inhibits digestion |
| Pupil effects | Constricts pupils | Dilates pupils |
| Bronchiole effects | Relaxes (opens airways) | Constricts (closes airways) |
| Blood flow effects | Promotes blood flow to GI tract and skin | Redirects blood flow to skeletal muscle, heart, brain |
This yin-and-yang balance between parasympathetic and sympathetic allows precise regulation of target organs and bodily systems to meet changing needs and environments.
Disorders of the Parasympathetic Nervous System
Dysfunction of the parasympathetic nervous system can lead to issues like:
- Constipation: Due to reduced GI motility and sphincter dysfunction
- Dry mouth: Due to reduced salivation
- Erectile dysfunction: Due to inability to relax cavernosal smooth muscle
- Tachycardia: Due to reduced vagal tone leading to increased heart rate
- Urinary retention: Due to inability to relax bladder sphincter
- Hypertension: Due to unchecked sympathetic activity
These issues can result from nerve damage, medications, electrolyte abnormalities, and neurodegenerative disorders. Treatment aims to address the underlying cause and manage symptoms.
Treatments
Treatments for parasympathetic dysfunction include:
- Medications that boost parasympathetic activity like anticholinergics, nitrates, and botulinum toxin
- Cholinergic agents to increase acetylcholine levels
- Biofeedback training to help activate the vagus nerve
- Physical therapy and relaxation techniques
- Surgery in some cases of nerve damage
Key Points
- The parasympathetic nervous system controls rest and digest functions when the body is relaxed.
- It originates from cranial nerves III, VII, IX, X and sacral spinal nerves.
- Acetylcholine is the main parasympathetic neurotransmitter.
- The vagus nerve mediates many parasympathetic effects like slowing heart rate and stimulating digestion.
- Parasympathetic and sympathetic divisions have opposite effects that balance homeostasis.
- Disorders like constipation and dry mouth can occur from dysfunction.
Conclusion
The parasympathetic nervous system plays an essential role in regulating rest and digestion when the body is relaxed and not under threat. It works in opposition to the sympathetic system to control unconscious body functions via cranial nerves and sacral spinal nerves. Dysfunction of the parasympathetic system can lead to issues like constipation, erectile dysfunction, and hypertension due to its vital role in controlling organ function and homeostasis. Understanding this vital system provides insight into how the nervous system integrates external and internal needs for optimal health and wellbeing.