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Morphology of the Parasympathetic Nervous System

Pars parasympathica

For medical students2 min readUpdated 2026-10-10

The parasympathetic division of the autonomic nervous system is characterized by a craniosacral outflow. Its central structures are located in the brainstem and sacral spinal cord, while its peripheral ganglia lie near or directly within the target organs.

Outflow typeCraniosacral (from the brainstem and sacrum)

Ganglia localizationIntramural (in the walls) or near the target organs

Fiber lengthPreganglionic are long, postganglionic are short

Schematic representationTraditionally highlighted with green lines in anatomical diagrams

General Organization and Outflow Type

The parasympathetic nervous system (pars parasympathica) has a strictly defined anatomical organization classically described as the craniosacral outflow. This means that the cell bodies of the first (preganglionic) neurons are concentrated in two distinct regions of the central nervous system: the brain (cranial division) and the lower segments of the spinal cord (sacral division).

In educational anatomical and physiological diagrams, parasympathetic pathways are traditionally marked with green lines to visually distinguish them from other divisions of the autonomic nervous system.

Cranial Division

The centers of the cranial division are localized in various parts of the brainstem, specifically the midbrain, pons, and medulla oblongata. Fibers from these centers travel within several pairs of cranial nerves, providing innervation to structures of the head, neck, and the thoracic and abdominal organs.

Oculomotor nerve (n. oculomotorius, cranial nerve III): provides parasympathetic innervation to the intrinsic eye muscles. Its activity causes pupillary constriction (miosis) and drives accommodation.
Facial nerve (cranial nerve VII) and Glossopharyngeal nerve (cranial nerve IX): responsible for the secretomotor innervation of the lacrimal gland and major salivary glands (submandibular, sublingual, and parotid).
Vagus nerve (n. vagus, cranial nerve X): serves as the primary conduit for parasympathetic pathways to the visceral organs, innervating the vast majority of structures in the thorax and abdomen.

The target organs of the vagus nerve include the heart, lungs (including the bronchial tree), stomach, liver, pancreas, entire small intestine, and the upper part of the large intestine.

Sacral Division

The centers of the sacral parasympathetic division are located in the spinal cord. Neuronal cell bodies lie in the lateral gray horns at the S2–S4 spinal segments.

Preganglionic fibers emerge from these segments to form the pelvic splanchnic nerves (nn. splanchnici pelvini). These nerves are responsible for innervating the pelvic viscera and the distal parts of the digestive tract.

The target organs for the pelvic splanchnic nerves include:

Descending colon
Rectum
Urinary bladder
Internal and external reproductive organs

Structure of Ganglia and Fibers

A crucial morphological feature of the parasympathetic division is the topography of its autonomic ganglia. Unlike other divisions, parasympathetic ganglia are located as close as possible to the target organs or intramurally—meaning they are embedded directly within the walls of the innervated organs (frequently depicted as small circles directly on organ contours in diagrams).

This ganglion topography accounts for the characteristic difference in nerve fiber lengths:

Preganglionic fibers are very long, as they travel from the brainstem or sacral spinal cord all the way to the target organ.
Postganglionic fibers are extremely short, since impulse transmission occurs directly inside or immediately adjacent to the target organ.

Mnemonic

The "Long Arms" rule: the parasympathetic system extends its long "arms" (preganglionic fibers) all the way from the brain directly to the organ wall, where it transmits the signal over a short distance via an intramural ganglion (short postganglionic fiber).

Frequently asked questions

In which brainstem nucleus do the parasympathetic fibers of the vagus nerve originate?

The parasympathetic fibers of the vagus nerve originate in the dorsal motor nucleus of the vagus nerve (nucleus dorsalis n. vagi).

It is located in the medulla oblongata in the floor of the fourth ventricle. The efferent fibers of this nucleus represent preganglionic parasympathetic fibers heading toward the thoracic and abdominal viscera.

Which specific eye muscles are innervated by the parasympathetic component of the oculomotor nerve?

The parasympathetic component of the oculomotor nerve innervates the intrinsic smooth muscles of the eye.

Sphincter pupillae muscle (m. sphincter pupillae) — innervated via fibers from the paired Edinger-Westphal nuclei, regulating pupillary constriction (miosis).
Ciliary muscle (m. ciliaris) — innervated via fibers from the unpaired nucleus of Perlia, regulating lens curvature (accommodation).

What are the names of the parasympathetic ganglia associated with the facial and glossopharyngeal nerves?

The parasympathetic ganglia associated with the facial and glossopharyngeal nerves include:

In the facial nerve (cranial nerve VII) pathway: pterygopalatine ganglion (ganglion pterygopalatinum) and submandibular ganglion (ganglion submandibulare).
In the glossopharyngeal nerve (cranial nerve IX) pathway: otic ganglion (ganglion oticum).

Where are the central nuclei of the parasympathetic nervous system located?

They are located in the brainstem (midbrain, pons, medulla oblongata) and in the lateral horns of the spinal cord at the S2–S4 segments.

What is the main feature of parasympathetic ganglia?

They lie intramurally (directly in the walls of the innervated organs) or very close to them, resulting in very long preganglionic fibers and short postganglionic fibers.

What areas does the sacral division of the parasympathetic system innervate?

Fibers of the sacral division (pelvic splanchnic nerves) innervate the descending colon, rectum, urinary bladder, and reproductive organs.

What is the function of the parasympathetic component of the oculomotor nerve (cranial nerve III)?

It innervates the intrinsic smooth muscles of the eye, mediating pupillary constriction and accommodation.

Go deeper

Detailed anatomy of vagal nuclei in the medulla oblongata
Histological structure of intramural ganglia in the gastrointestinal tract
Innervation pathways of the salivary and lacrimal glands (facial and glossopharyngeal nerves)
Topography of pelvic plexuses and splanchnic nerves
Mechanisms of accommodation and the pupillary light reflex