19th February 2024

Parasympathetic nervous system

Main structure 

The preganglionic neurons of the parasympathetic nervous system are concentrated within several cranial nerve nuclei within the medulla oblongata and the sacral spinal cord. In contrast to the sympathetic nervous system, the preganglionic neurons have long courses and will travel close to their effector organ before they synapse with their postganglionic neuron. 

The cranial parasympathetic nervous system originates from CN III, VII, IX, and X. CN III supplies the parasympathetic fibres destined for the ciliary ganglion. Fibres from the Edinger-Westphal nucleus will travel towards the ciliary ganglion before it synapses and supplies the ciliary muscle and pupils with parasympathetic fibres via the short ciliary nerves. 

CN VII carries parasympathetic fibres from the superior salivary nucleus and synapses with the pterygopalatine and submandibular ganglion. It will then supply the lacrimal gland and nasal mucosa via the greater petrosal nerve, while the submandibular nerve will supply the submandibular gland. 

CN IX carries fibres from the inferior salivary nucleus with its synapse within the otic ganglion. The auricular nerve will then carry the parasympathetic fibres towards the parotid gland. 

CN X supplies most of the parasympathetic innervation within the body, with the abdominal and thoracic cavities receiving its parasympathetic innervation from the vagus nerve. The origin of the fibres is from the nucleus ambiguous within the medulla. The presynaptic fibres travel down into the thorax and abdomen to supply the lungs, heart and abdominal organs via various parasympathetic ganglions close to the aforementioned organs. The abdominal organs are supplied up to the level of the splenic flexure of the large intestine. 

The sacral portion of the parasympathetic nervous system arises from the S2-S4 levels. These provide the parasympathetic innervation to the descending colon, sigmoid colon, rectum and pelvic organs.

Image Source: Wikimedia Commons (2014). Blausen 0703 Parasympathetic Innervation, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/f/f0/Blausen_0703_Parasympathetic_Innervation.png (Accessed: 9 January 2024).