The nervous system that is primarily responsible for resting and digesting is the parasympathetic nervous system. The parasympathetic nervous system controls the “rest and digest” functions of the body and is dominant when the body is relaxed. Let’s explore this in more detail:
The Autonomic Nervous System
The nervous system can be divided into two main parts – the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS connects the CNS to the rest of the body and consists of all the nerves and ganglia outside of the brain and spinal cord.
The PNS can be further divided into:
- Somatic nervous system – controls voluntary muscle movements
- Autonomic nervous system (ANS) – controls involuntary bodily functions
The autonomic nervous system is the part of the PNS that regulates involuntary bodily processes such as heart rate, respiration, digestion, etc. It has two main divisions:
- Sympathetic nervous system
- Parasympathetic nervous system
Parasympathetic vs Sympathetic Nervous System
The sympathetic and parasympathetic nervous systems often have opposing effects on bodily functions:
| Parasympathetic Nervous System | Sympathetic Nervous System |
|---|---|
| Associated with “rest and digest” functions | Associated with “fight or flight” response |
| Slows heart rate | Increases heart rate |
| Constricts pupils | Dilates pupils |
| Increases digestion | Decreases digestion |
| Relaxes muscles | Contracts muscles |
In general, the parasympathetic system promotes homeostasis and energy conservation, allowing the body to rest and recover. The sympathetic system activates the body’s “fight or flight” response in stressful situations.
Functions of the Parasympathetic Nervous System
When the body is relaxed and not under stress, the parasympathetic nervous system takes over to promote rest and digestion by:
- Slowing the heart rate and lowering blood pressure
- Constricting bronchioles and pupils
- Stimulating salivary gland secretion
- Activating digestion by stimulating gut motility and digestive juice secretion
- Promoting urine and feces elimination
- Relaxing muscles and reducing muscle tone
The main neurotransmitter used by the parasympathetic nervous system is acetylcholine. The parasympathetic preganglionic neurons originate in the brainstem and sacral spinal cord. The nerve fibers extend to terminal ganglia near or within the target organs. Postganglionic neurons then complete the pathway to the target tissues.
Key Parasympathetic Nervous System Structures
- Cranial nerves – Parasympathetic fibers from the brainstem travel in cranial nerves III, VII, IX, and X to control eye muscles, salivary glands, and digestive organs.
- Vagus nerve – This is the longest cranial nerve and innervates the heart, lungs, and most of the digestive tract.
- Sacral spinal nerves – Nerves originating in the sacral region of the spinal cord control urination and defecation.
- Terminal ganglia – Located near or within target organs like the heart and gut.
The Parasympathetic System at Rest
When the body is at rest and not under stress, parasympathetic nerve impulses dominate. This allows metabolic resources to be diverted away from muscles toward internal organs to promote functions like digestion. The parasympathetic stimulation of digestion includes:
- Increased motility and secretion in the gastrointestinal tract
- Increased blood flow to the gut
- Relaxation of the pyloric sphincter to allow stomach emptying
- Stimulation of pancreatic secretions
- Increased production of digestive enzymes and bile
In addition, the parasympathetic system slows the heart rate through the vagus nerve to conserve energy. It also constricts the bronchioles to slow respiration rate. The pupils constrict to protect the retina from excessive light when the body is relaxed.
Conclusion
In summary, the parasympathetic nervous system controls the “rest and digest” functions of the body when it is relaxed and not under stress. It promotes energy conservation by slowing the heart rate, constricting pupils, relaxing muscles, and stimulating digestion and elimination. Key parasympathetic structures include the cranial nerves, vagus nerve, sacral spinal nerves, and terminal ganglia that innervate the internal organs. The parasympathetic system allows the body to direct energy and resources toward rest, recovery, and internal homeostasis when the body is at rest.