Parasympathetic nervous system

Parasympathetic nervous systemAnatomy and function of the parasympathetic nervous system. Parasympathetic ganglia
~ 7 min

In this note, we are going to continue studying the anatomy and function of the autonomic nervous system.

Before watching this video, we suggest reviewing the topic “Autonomic Nervous System. Vegetative Reflex Arc”. This will provide you with a solid understanding of the key principles of ANS anatomy and its functions.

Let’s move on to a detailed examination of the parasympathetic division.

In general, the parasympathetic part performs functions aimed at restoration of energy and relaxation of the body.

The central part of the parasympathetic nervous system is located in the autonomic nuclei of the cranial nerves, namely the oculomotor (CN III),

Oculomotor nerve
Oculomotor nerve

facial (CN VII),

Facial nerve
Facial nerve

glossopharyngeal (CN IX),

Glossopharyngeal nerve
Glossopharyngeal nerve

and vagus (CN X) nerves.

Vagus nerve
Vagus nerve

Additionally, it is found in the lateral regions of the sacral segments of the spinal cord (from S2 to S4). These structures somewhat resemble lateral horns, but they are not their complete equivalents.

Before reaching the target organ, the motor component of the parasympathetic arc first synapses in a terminal or intramural autonomic ganglion.

In the parasympathetic nervous system, there are two types of such ganglia:

  • Visceral ganglia, which are located near the internal organs
  • Intramural ganglia, which are located within the walls of the internal organs

Due to the location of these ganglia, the preganglionic fibers in the parasympathetic reflex arc are long, while the postganglionic fibers are short. In both preganglionic and postganglionic fibers, the neurotransmitter is acetylcholine.

Let’s start with the cranial nerves that contain parasympathetic fibers.

CN III – Oculomotor nerve

Its autonomic nucleus is the Edinger-Westphal nucleus, which is located in the midbrain.

Edinger-Westphal nucleus
Edinger-Westphal nucleus

From this nucleus, preganglionic fibers emerge and travel to the ciliary ganglion.

Ciliary ganglion
Ciliary ganglion

Then postganglionic fibers in the short ciliary nerves

Short ciliary nerves
Short ciliary nerves

innervate the following structures:

  • Ciliary muscle. The parasympathetic effect is the contraction of this muscle.
Ciliary muscle
Ciliary muscle

It rounds the lens, facilitating accommodation for better vision of nearby objects.

  • Sphincter pupillae muscle. The parasympathetic effect is the contraction of this muscle, resulting in pupil constriction.
Sphincter pupillae muscle
Sphincter pupillae muscle

CN VII – Facial nerve

Its autonomic nucleus is the superior salivatory nucleus, which is located in the pons.

Salivatory nuclei (superior and inferior)
Salivatory nuclei (superior and inferior)

From this nucleus, preganglionic fibers travel within the greater petrosal nerve

Greater petrosal nerve
Greater petrosal nerve

to the pterygopalatine ganglion.

Pterygopalatine ganglion
Pterygopalatine ganglion

Postganglionic fibers innervate the lacrimal gland, as well as the palatine glands and the nasal glands, increasing their secretion.

The portion of fibers that do not follow the greater petrosal nerve travel within the chorda tympani.

Chorda tympani
Chorda tympani

After the chorda tympani joins the lingual nerve,

Lingual nerve
Lingual nerve

the preganglionic fibers proceed to the submandibular ganglion

Submandibular ganglion
Submandibular ganglion

and the sublingual ganglion.

Sublingual ganglion
Sublingual ganglion

Fibers from these ganglia innervate the corresponding salivary glands, increasing their secretion.

CN IX – Glossopharyngeal nerve

Its autonomic nucleus is the inferior salivatory nucleus, which is located within the lower part of the pons.

Salivatory nuclei (superior and inferior)
Salivatory nuclei (superior and inferior)

Preganglionic fibers as a part of the lesser petrosal nerve

Lesser petrosal nerve
Lesser petrosal nerve

reach the otic ganglion.

Otic ganglion
Otic ganglion

Postganglionic fibers from the ganglion travel to the parotid gland within the auriculotemporal nerve.

Auriculotemporal nerve
Auriculotemporal nerve

Parasympathetic stimulation increases the secretion of saliva by this gland.

CN X – Vagus nerve

Its autonomic nucleus is the dorsal nucleus of the vagus nerve, located in the medulla oblongata.

Dorsal nucleus of the vagus nerve
Dorsal nucleus of the vagus nerve

Axons from the cells of this nucleus travel to the ganglia of various visceral and intramural plexuses. These include the cardiac, pulmonary, esophageal, gastric, celiac, hepatic, and other plexuses. After synapsing in these ganglia, parasympathetic fibers innervate the smooth muscles of the organs in the neck, thoracic, and abdominal cavities. Eventually, this causes the following effects:

  • Lowering of the heart rate and cardiac contractility, which leads to a reducing of blood pressure
  • Bronchoconstriction (narrowing of the bronchial lumen)
  • Activation of digestion (enhanced peristalsis, absorption, and secretion) in the digestive organs, as well as glycogenogenesis
  • Increased urine production and contractility of the ureters
  • Increased insulin production

Let’s move on to the sacral part of the parasympathetic system.

Axons from the cells of the lateral gray matter of the sacral segments (from S2 to S4) form the so-called pelvic splanchnic nerves.

Pelvic splanchnic nerves
Pelvic splanchnic nerves

After synapsing in the corresponding visceral and intramural ganglia, the postganglionic fibers innervate the following structures:

  • Internal urethral sphincter
Internal urethral sphincter
Internal urethral sphincter

and the detrusor muscle.

Detrusor muscle
Detrusor muscle

With parasympathetic stimulation, the contractility of the detrusor increases, while the contractility of the sphincter decreases.

  • Parts of the digestive tract, including the rectum, sigmoid colon, descending colon, and part of the transverse colon.
  • Female and male reproductive organs. Parasympathetic stimulation causes erection of the erectile structures of the penis and clitoris.
Golosary

Parasympathetic nervous system

parasympathetic part
pars parasympathica
oculomotor nerve
nervus oculomotorius
facial nerve
nervus facialis
glossopharyngeal nerve
nervus glossopharyngeus
vagus nerve
nervus vagus
visceral ganglia
ganglia visceralia
intramural ganglia
ganglia intramuralia
ciliary ganglion
ganglion ciliare
ciliary muscle
musculus ciliaris
sphincter pupillae muscle
musculus sphincter pupillae
superior salivatory nucleus
nucleus salivatorius superior
pterygopalatine ganglion
ganglion pterygopalatinum
inferior salivatory nucleus
nucleus salivatorius inferior
otic ganglion
ganglion oticum
dorsal nucleus of vagus nerve
nucleus dorsalis nervi vagi
pelvic splanchnic nerves
nn. splanchnici pelvici
internal urethral sphincter
sphincter internus urethrae
detrusor muscle
detrusor vesicae
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