The Parasympathetic Nervous System: Rest and Digest
The autonomic nervous system is responsible for the involuntary functions of the body, such as breathing, heart rate, and digestion. It is divided into two branches: the sympathetic nervous system and the parasympathetic nervous system. This post will focus on the parasympathetic nervous system.
Introduction
The parasympathetic nervous system (PNS) is one of the main parts of the autonomic nervous system that controls the "rest and digest" functions of the body. Its actions usually oppose those of the sympathetic nervous system and it restores the body to a state of calmness.
Origin
The PNS has pre- and post-ganglionic fibers. It is referred to as craniosacral because of the location of its preganglionic fibers.
Preganglionic Fibers
These fibers are usually long, myelinated, slow-conducting B-fibers. They originate from two main areas:
- Cranial outflow: Arises from nuclei of some cranial nerves located in the brainstem, namely:
- Oculomotor nerve (CN III)
- Facial nerve (CN VII)
- Glossopharyngeal nerve (CN IX)
- Vagus nerve (CN X)
- Sacral outflow: It consists of the 2nd to 4th (S2-S4) sacral segments of the spinal cord.
Postganglionic Fibers
These fibers are relatively short, unmyelinated C fibers. They include:
- CN III: Ciliary ganglion to pupillary constrictor and ciliary muscle.
- CN VII:
- Sphenopalatine/pterygopalatine ganglion to lacrimal and nasal gland from the superior salivatory nucleus.
- Submandibular ganglion which supplies submandibular and sublingual glands.
- CN IX: Otic ganglion which supplies the parotid gland. It originates from the inferior salivatory nucleus.
- CN X: Thoracic and visceral organs up to half of the colon through the nucleus ambiguus and dorsal motor vagal nucleus.
- S2-S4: Supplies pelvic viscera.
Neurotransmitters Used
- Preganglionic: Acetylcholine
- Postganglionic: Acetylcholine
Receptors Used
- Nicotinic receptors: Ligand-gated ion channels found in the autonomic ganglion of the synapse of pre- and post-synaptic neurons.
- Muscarinic receptors: Use G-protein found in effector organs stimulated by postganglionic cholinergic neurons.
Effects on Organs or Functions
| Organ or Function | Response |
|---|---|
| Eye – Pupils – Ciliary muscles |
Contraction Contraction |
| Glands – Lacrimal glands – Salivary glands |
Secretion Profuse, watery solution |
| Stomach and Intestine – Secretion – Mobility and tone – Sphincters |
Stimulation Increased Relaxed |
| Heart – Heart rate – Conduction velocity – Contractility |
Decreases Decreases Decreases |
| Urinary bladder – Detrusor muscle – Sphincter |
Contraction Relaxation |
| Male sex organ | Erection |
| Gall bladder | Contraction |
Clinical Correlates
- Parasympathetic drugs: Drugs that produce effects similar to parasympathetic stimulation. Acetylcholine produced in the body acts for a short time, whereas intravenous injection acts over a longer time. They either act on muscarinic receptors or prolong the effect of acetylcholine (e.g., Methcholine and neostigmine).
- Parasympathetic blocks: These prevent the action of parasympathetic neurotransmitters by blocking muscarinic receptors (e.g., Atropine).
