General characteristics and functions
The parasympathetic division of the autonomic nervous system is responsible for the innervation of internal organs during rest. Its action is aimed at restoring and maintaining homeostasis. Unlike structures that mobilize the body, parasympathetic impulses trigger resource-conserving processes:
- Decreased heart rate (HR).
- Enhanced motor function of the digestive tract (peristalsis).
- Activation of glandular function (increased secretion).
Structurally, this division is organized according to the classical principle and is subdivided into the central division and the peripheral division.
Structural features of the peripheral division
A key anatomical feature of the parasympathetic system lies in the ratio of the lengths of its nerve pathways. Preganglionic fibers here are very long because they extend from centers in the nervous system practically to the target organ itself.
In turn, postganglionic fibers are extremely short. This is due to the fact that autonomic ganglia are located either in the immediate vicinity of the organs (extramurally) or directly inside their walls (intramurally). It is in these peripheral structures that the nerve impulse is relayed from the preganglionic fiber to the postganglionic one.
Central division: localization of nuclei
The centers of the parasympathetic nervous system do not form a single cluster, but are divided into two major foci:
- In the brainstem — forms the cranial (cephalic) division.
- In the sacral spinal cord — is responsible for the innervation of pelvic organs.
Cranial (cephalic) division
Parasympathetic nuclei located in the brainstem are anatomically and functionally connected to specific cranial nerves. Preganglionic fibers emerge from the brain as part of the following nerves:
- III pair — oculomotor nerve (nervus oculomotorius).
- VII pair — facial nerve (nervus facialis).
- IX pair — glossopharyngeal nerve (nervus glossopharyngeus).
- X pair — vagus nerve (nervus vagus).
The ganglia of the head are peripheral extra- or intramural structures that provide innervation to the corresponding structures of the craniofacial region.
Briefly: answers to topical questions
What are the names of the parasympathetic cranial nerve nuclei located in the brainstem?
The brainstem houses the following parasympathetic (autonomic) nuclei of the cranial nerves:
- Accessory nucleus (nucleus oculomotorius accessorius / Edinger-Westphal nucleus) — nucleus of the oculomotor nerve (III pair).
- Superior salivatory nucleus (nucleus salivatorius superior) — nucleus of the facial nerve (VII pair), part of which forms the lacrimal nucleus (nucleus lacrimalis).
- Inferior salivatory nucleus (nucleus salivatorius inferior) — nucleus of the glossopharyngeal nerve (IX pair).
- Posterior (dorsal) nucleus of the vagus nerve (nucleus dorsalis nervi vagi) — nucleus of the vagus nerve (X pair).
In which segments of the spinal cord are the nuclei of the sacral parasympathetic division located?
The nuclei of the sacral parasympathetic division lie in the gray matter of the spinal cord at the level of segments S2–S4 (SII–SIV). Some sources also indicate their location in the intermediate zone of the sacral and coccygeal segments up to CoIII. These centers are called the sacral parasympathetic nuclei (nuclei parasympathici sacrales).
Which parasympathetic ganglia belong to the cranial division (ganglia of the head)?
The cranial division of the parasympathetic nervous system includes the following autonomic ganglia of the head:
- Ciliary ganglion (ganglion ciliare) — connected to the oculomotor nerve (III pair).
- Pterygopalatine ganglion (ganglion pterygopalatinum) — connected to the intermediate (facial) nerve (VII pair).
- Submandibular ganglion (ganglion submandibulare) — connected to the facial nerve (VII pair).
- Otic ganglion (ganglion oticum) — connected to the glossopharyngeal nerve (IX pair).
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 lower parts of the gastrointestinal tract. Specific innervated organs include:
- Large intestine (areas below the distal third of the transverse colon, including the descending colon).
- Rectum.
- Urinary bladder (specifically, the detrusor muscle and the internal urethral sphincter).
- Genitals (provides penile erection).
As part of which nerves do preganglionic fibers run from the sacral nuclei to the pelvic organs?
Preganglionic fibers from the sacral parasympathetic nuclei travel to the pelvic organs as part of the pelvic splanchnic nerves (nn. splanchnici pelvici / nn. splanchnici pelvini). In some sources, they are also referred to as pelvic nerves (nn. erigentes). These nerves carry fibers to the inferior hypogastric (pelvic) plexus and autonomic ganglia located in the muscle layer of the walls of the innervated organs.
The numbers of the cranial nerves 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).
Class and exam questions
Where are the centers of the parasympathetic nervous system located?
The central division is located in the brainstem and the sacral spinal cord.
What is the difference between preganglionic and postganglionic fibers in this division?
A characteristic feature of the parasympathetic system is long preganglionic fibers (extending all the way to the organs) and very short postganglionic fibers.
Where are the parasympathetic ganglia located?
They are extra- or intramural structures, meaning they lie near the innervated organs or directly within their walls.
Which cranial nerves carry the preganglionic fibers?
They travel as part of the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves.
What else to study on this topic
- Topography of parasympathetic nuclei in the brainstem
- Path of fibers within the vagus nerve (n. vagus)
- Localization and connections of parasympathetic ganglia of the head
- Structure of intramural ganglia in the walls of internal organs
- Sacral spinal cord as the center of parasympathetic innervation
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