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),
facial (CN VII),
glossopharyngeal (CN IX),
and vagus (CN X) nerves.
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.
From this nucleus, preganglionic fibers emerge and travel to the ciliary ganglion.
Then postganglionic fibers in the short ciliary nerves
innervate the following structures:
- Ciliary muscle. The parasympathetic effect is the contraction of this 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.
CN VII – Facial nerve
Its autonomic nucleus is the superior salivatory nucleus, which is located in the pons.
From this nucleus, preganglionic fibers travel within the greater petrosal nerve
to the 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.
After the chorda tympani joins the lingual nerve,
the preganglionic fibers proceed to the submandibular ganglion
and the 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.
Preganglionic fibers as a part of the lesser petrosal nerve
reach the otic ganglion.
Postganglionic fibers from the ganglion travel to the parotid gland within the 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.
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.
After synapsing in the corresponding visceral and intramural ganglia, the postganglionic fibers innervate the following structures:
- Internal urethral sphincter
and the 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.
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
