Parasympathetic Nervous System

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Parasympathetic Division of the Autonomic Nervous System

• Preganglionic neurons have their cell bodies in the nuclei of four cranial nerves in the brain stem (III, VII, IX, and X) and in lateral gray horns of the second through fourth sacral segments.

• Also known as craniosacral division.

• Axons of preganglionic neurons emerge as part of a cranial nerve or as part of the anterior root of a spinal nerve.

• There are two types of preganglionic neuron divisions: cranial parasympathetic outflow (vagus n; CN X makes up 80% of this) and sacral parasympathetic outflow (anterior roots of S2-S4; pelvic splanchnic nerves).

• The preganglionic and postganglionic axons synapse in terminal ganglia, which are located in effector tissues.

• Most of these ganglia are actually located in the visceral effector tissue, so parasympathetic preganglionic neurons have much longer axons.

• Preganglionic neurons synapse with only 4-5 postganglionic neurons, all of which supply a single visceral effector.

• Parasympathetic responses can be localized to a single effector.

• All preganglionic neurons are cholinergic.

• All postganglionic neurons are cholinergic.

Parasympathetic System: Brainstem and sacral outflow of spinal cord.


1. Cranial outflow: Visceral structures of the head are supplied by the cranial nerves (oculomotor, facial, glossopharyngeal and vagus nerves) relaying through the ciliary, sphenopalatine, submaxillary and otic ganglion.

2. Sacral outflow (pelvic nerve - S2, 3, 4).

3. Length of fibres: Long preganglionic fibres relay on short postganglionic neurons located adjacent or near the visceral structures.

4. Association with parasympathetic extracranial ganglia (ciliary, pterygopalatine, submandular and otic ganglia).

Stimulation of parasympathetic nervous system:

1. Heart: Bradycardia (slowing of the heart).

2. Lungs (bronchi): Bronchoconstriction.

3. Pupil: Constriction.

4. GIT: Increased peristalsis, sphincters relaxed.

5. Blood vessels: Dilatation of visceral blood vessels.

6. Sweat glands and hair follicles: Regulated by sympathetic system.

7. Responses 1-6 are due to the release of acetylcholine from postganglionic fibres. The results just the opposite (antagonistic) to the effects of sympathetic stimulation.


1. Cranial nerves, eye, lacrimal gland, submandibular gland, sublingual gland and parotid gland. Vagus innervates widely (heart, larynx, trachea, bronchi, lung, small bowel, abdominal vessels, liver, gallbladder, bile ducts, pancreas, kidney and ascending colon.

2. Sacral outflow (S1 - 4). Descending colon, sigmoid and rectum, bladder, sex organs and external genitalia.

Parasympathetic System or Craniosacral System:

1. Cranial parasympathetic outflow: via cranial nerves (3, 6, 9, 10, and 11). Relay in otic, ciliary, submandibular and sphenopalatine ganglia).

2. Branches of Vagus nerve. Distributed to:

a. Cardiac branches.

b. Esophageal branches.

c. Cardiac plexus.

d. Pulmonary plexus.

e. Anterior and posterior gastric nerves.

3. Sacral outflow (S2, 3, 4) - supplies rectum, bladder, uterus, erectile tissue of penis and clitoris, distal colon.

4. Visceral efferent pathways: consist of three sets of neurons:

a. Upper motor neuron (grey matter of brain).

b. Preganglionic neuron (lower motor neuron i.e. sited in grey matter of brainstem or spinal cord).

c. Postganglionic neuron (sited in autonomic ganglion).

Further Topics on Autonomic Nervous System:

1. Introduction to the Autonomic Nervous System
2. Control of the Autonomic Nervous System
3. Divisions the Autonomic Nervous System
4. Notes on Sympathetic Nervous System
5. Notes on Paraympathetic Nervous System
6. Neurons of the Autonomic Nervous System
7. Neurotransmitters and Receptors of the Autonomic Nervous System
8. FAQ on Autonomic System

Additional Reading:

Basic Neurology

1. Peripheral Nervous System
2. Central Nervous System
3. The Ventricular System
4. The Spinal Cord
5. The Brain Stem
6. The Cerebellum
7. Visual Pathways
8. Diencephalon
9. Basal Ganglia
10. Cerebral Cortex
11. Sleep Disorders
12. Autonomic Nervous System
13. Cranial Nerves and Parasympathetic Ganglia
14. Cells of the Nervous System
15. Cerebrospinal fluid
16. Additional short notes on Cerebrum
17. Functions and Diseases of Cerebrum
18. Subcortical Grey Matter
19. Notes on The Spinal Cord
20. Regulation of Heart Rate by Autonomic Nervous System
21. Action Potentials, Axon Conduction, and Neuromuscular Junction
22. Types of Seizures
23. What is a Cough Reflex?
24. Notes on Congenital Prosopagnosia
25. Findings in Parkinson's Disease
26. Types of Heat Strokes
27. Types of Strokes
28. What is Benign Intracranial Hypertension?
29. What is Cauda Equina Syndrome?
30. Cranial Nerve Locations in Brain Stem
31. What is a Cluster Headache?
32. What is a Subarachnoid Hemorrhage?
33. What is a Tension Headache?

Neurology Videos

1. Video of Neurology Examination in a Clinical Setting

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Related Topics

1. Nervous System Disorders
2. Histology of Nervous Tissue
3. Cranial Nerve Reflexes
4. Motor System Examination

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