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Parasympathetic Nervous System

Pars parasympathica systematis nervosi autonomici

For medical students2 min readUpdated 2026-10-10

Parasympathetic nervous system (PSNS) is a vital division of the autonomic nervous system, primarily responsible for restoring and maintaining bodily homeostasis. It regulates internal organ function to conserve energy: decreasing heart rate and stimulating digestion through enhanced peristalsis and secretion.

Central componentLocated in the brainstem and the sacral spinal cord
Fiber lengthPreganglionic fibers are long; postganglionic fibers are short
Ganglia topographyLocated close to target organs or directly within their walls
Main functionResource restoration and maintenance of homeostasis (decreased heart rate)

General Characteristics and Functions

The parasympathetic division of the autonomic nervous system is responsible for innervating visceral structures during rest. Its action aims at restoring and maintaining homeostasis. Unlike sympathetic structures that mobilize the body, parasympathetic impulses trigger energy-conserving processes:

Structurally, this division is organized into a central division and a peripheral division.

Structural Features of the Peripheral Division

A key anatomical feature of the parasympathetic system is the relative length of its nerve fibers. Preganglionic fibers are very long because they extend from central nervous system centers almost all the way to the target organ.

In turn, postganglionic fibers are extremely short. This is because autonomic ganglia are located either in close proximity to the organs (extramural) or directly inside their walls (intramural). It is within these peripheral structures that the nerve impulse is relayed from the preganglionic fiber to the postganglionic neuron.

Central Division: Nuclear Localization

The centers of the parasympathetic nervous system do not form a single continuous cluster, but are divided into two major regions:

  1. In the brainstem — forming the cranial division.
  2. In the sacral spinal cord — responsible for pelvic organ innervation.

Cranial Division

Parasympathetic nuclei located in the brainstem are anatomically and functionally associated with specific cranial nerves. Preganglionic fibers leave the brain within the following nerves:

Cranial ganglia are peripheral extra- or intramural structures providing innervation to corresponding craniofacial structures.

Mnemonic

The cranial nerve numbers carrying parasympathetic fibers are easy to remember in sequence: 3, 7, 9, 10 (III — n. oculomotorius, VII — n. facialis, IX — n. glossopharyngeus, X — n. vagus).

Frequently asked questions

What are the names of the parasympathetic cranial nerve nuclei located in the brainstem?

The brainstem contains the following parasympathetic (autonomic) cranial nerve nuclei:

  • Accessory oculomotor nucleus (nucleus oculomotorius accessorius / Edinger-Westphal nucleus) — nucleus of the oculomotor nerve (CN III).
  • Superior salivatory nucleus (nucleus salivatorius superior) — nucleus of the facial nerve (CN VII), part of which forms the lacrimal nucleus (nucleus lacrimalis).
  • Inferior salivatory nucleus (nucleus salivatorius inferior) — nucleus of the glossopharyngeal nerve (CN IX).
  • Posterior (dorsal) motor nucleus of vagus nerve (nucleus dorsalis nervi vagi) — nucleus of the vagus nerve (CN X).
Which spinal cord segments contain the nuclei of the sacral parasympathetic division?

The nuclei of the sacral parasympathetic division lie in the gray matter of the spinal cord at the level of segments S2–S4 ($S_{II}$–$S_{IV}$). Some sources also note their location in the intermediate zone of the sacral and coccygeal segments up to $Co_{III}$. These centers are called the sacral parasympathetic nuclei (nuclei parasympathici sacrales).

Which parasympathetic ganglia belong to the cranial division (head ganglia)?

The cranial division of the parasympathetic nervous system includes the following head autonomic ganglia:

  • Ciliary ganglion (ganglion ciliare) — associated with the oculomotor nerve (CN III).
  • Pterygopalatine ganglion (ganglion pterygopalatinum) — associated with the intermediate (facial) nerve (CN VII).
  • Submandibular ganglion (ganglion submandibulare) — associated with the facial nerve (CN VII).
  • Otic ganglion (ganglion oticum) — associated with the glossopharyngeal nerve (CN IX).

In these ganglia, preganglionic fibers form synapses with postganglionic neurons.

Which specific organs are innervated by the sacral parasympathetic division?

The sacral parasympathetic division innervates the pelvic organs, external genitalia, and the lower gastrointestinal tract. Specific organs include:

  • Large intestine (segments distal to the left third of the transverse colon, including the descending colon).
  • Rectum.
  • Urinary bladder (specifically the detrusor muscle and internal urethral sphincter).
  • Genitalia (mediates penile erection).
Through which nerves do preganglionic fibers travel from the sacral nuclei to the pelvic organs?

Preganglionic fibers from the sacral parasympathetic nuclei travel to the pelvic organs via the pelvic splanchnic nerves (nn. splanchnici pelvici). In some literature, they are also referred to as pelvic nerves (nn. erigentes). These nerves deliver fibers to the inferior hypogastric (pelvic) plexus and autonomic ganglia located in the muscular walls of the target organs.

Where are the centers of the parasympathetic nervous system located?

The central division is located in the brainstem and the sacral spinal cord.

How do preganglionic and postganglionic fibers differ in this division?

A characteristic feature of the parasympathetic system is long preganglionic fibers (reaching the target organs) and very short postganglionic fibers.

Where are parasympathetic ganglia located?

They are extra- or intramural structures, meaning they lie close to the innervated organs or directly within their walls.

Which cranial nerves carry parasympathetic preganglionic fibers?

They travel within the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves.

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