Parasympathetic nervous system
The parasympathetic nervous system (PSNS) is a division of the autonomic nervous system (ANS) that controls the activity of the smooth and cardiac muscles and glands. It works in synergy with the sympathetic nervous system (SNS), which complements the PSNS activity. The parasympathetic nervous system is also called the craniosacral division of the ANS, as its central nervous system components are located within the brain and the sacral portion of the spinal cord.
The functions of the PNS are commonly described as the “rest and digest” response, since it is involved in slowing down the heart rate, relaxing the sphincter muscles in the gastrointestinal and urinary tracts and increasing intestinal and gland activity. The final result is conserving energy and regulating basic bodily functions such as digestion and urination. It is contrasted to the sympathetic nervous system, which is described as the “fight and flight” response that occurs in stressful situations and has mainly opposite functions.
This article will discuss the anatomy and functions of the parasympathetic nervous system.
|Cranial outflow||Oculomotor nerve (CN III) – iris, ciliary muscles
Facial nerve (CN VII) – lacrimal, nasal, palatine, pharyngeal, sublingual, submandibular glands
Glossopharyngeal nerve (CN IX) – parotid gland
|Vagus outflow||Vagus nerve (CN X) – heart, larynx, trachea, bronchi, lungs, liver, gallbladder, stomach, pancreas, kidney, small intestine, proximal large intestine|
|Sacral outflow||Pelvic splanchnic nerves – descending colon, sigmoid colon, rectum, bladder, penis or clitoris|
- Clinical correlation
The parasympathetic nervous system consists of many pathways that connect its craniosacral components with the peripheral tissues. Each parasympathetic pathway consists of two neurons, the presynaptic (preganglionic) and postsynaptic (postganglionic) neurons, which are connected by the axons of the presynaptic neurons.
The presynaptic neurons of the parasympathetic system are located within the medulla oblongata and sacral spinal cord. They give off long axons (presynaptic fibers) that leave the CNS and travel towards the postsynaptic neurons. Once they reach them, the presynaptic fibers synapse with the bodies of the postsynaptic neurons. This synapse uses the acetylcholine as a neurotransmitter, which is why the parasympathetic pathways are referred to as the cholinergic pathways. The presynaptic neurons of the parasympathetic pathways are located within the two major parts of the central nervous system:
- The presynaptic parasympathetic neurons which innervate the structures of the head and neck are found in the medulla oblongata; within the nuclei of the oculomotor (CN III), facial (CN VII), glossopharyngeal (IX) and vagus (X) cranial nerves.
- The presynaptic parasympathetic neurons that innervate the viscera of the thorax, abdomen and pelvis lie within the gray horn of the sacral segments (S2 – S4) of the spinal cord.
The postsynaptic neurons are found within the parasympathetic ganglia, which typically lie near or within the target organs. After receiving the impulse from the presynaptic neuron, the postsynaptic neuron conveys the neural impulse further down its axon (postsynaptic fiber). The postsynaptic fibers are significantly shorter than the presynaptic ones, given that the postsynaptic neuronal bodies lie in the close proximity of their target organs.
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The cranial portion of the parasympathetic nervous system stems from the nuclei of the cranial nerves III, VII, IX and X. The presynaptic fibers of these neurons join these cranial nerves in order to reach the parasympathetic ganglia of the head.
The cranial nerves that carry the cranial parasympathetic outflow are the following:
- Oculomotor nerve (CN III): carries presynaptic parasympathetic fibers from the accessory oculomotor nucleus (Edinger-Westphal). These fibers reach the ciliary ganglion and synapse with the postsynaptic parasympathetic neurons that are found within it. The neurons from the ciliary ganglion project postsynaptic fibers via the short ciliary nerves to innervate the ciliary muscle and sphincter pupillae muscle.
- Facial nerve (CN VII): carries presynaptic parasympathetic fibers from the superior salivatory nucleus and synapses with the postsynaptic neurons found in the pterygopalatine and submandibular ganglia. These ganglia then project postsynaptic fibers via two nerves;
- Glossopharyngeal nerve (CN IX): conveys the presynaptic fibers from the inferior salivatory nucleus which synapse with the postsynaptic neurons found in the otic ganglion. The ganglion then projects postsynaptic fibers via the auriculotemporal nerve to supply parasympathetic innervation to the parotid gland.
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The vagus nerve (CN X) is functionally not included in the cranial parasympathetic outflow since it provides no parasympathetic innervation for the head. Instead, it gives the parasympathetic innervation to the thoracic and abdominal viscera.
The presynaptic parasympathetic fibers of the vagus nerve originate from the dorsal nucleus of vagus nerve and the nucleus ambiguus in the brainstem. The presynaptic fibers travel within the vagus nerve to reach numerous parasympathetic ganglia around and within the organs of the thorax and abdomen. Upon synapsing, the ganglia project short postsynaptic fibers, which then innervate the target organs.
The fibers stemming from the dorsal nucleus of the vagus nerve primarily innervate the structures of the lungs and gastrointestinal tract up to the splenic flexure of the large intestine, while the fibers from the nucleus ambiguus supply the smooth musculature of the heart, pharynx, larynx and soft palate.
|Thorax||Pharyngeal, superior laryngeal, recurrent laryngeal, superior and inferior cardiac, anterior and posterior bronchial and esophageal branches|
|Abdomen||Gastric, celiac and hepatic branches|
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The presynaptic parasympathetic neurons of the sacral part of the PSNS lie within the gray matter of the S2-S4 segments of the spinal cord. These fibers leave the spinal cord through the anterior roots of the sacral spinal nerves S2-S4 and the pelvic splanchnic nerves that arise from their anterior branches.
These fibers synapse with the parasympathetic ganglia around the descending and sigmoid parts of the colon, the rectum, and the viscera of the pelvic cavity. These ganglia then project the postsynaptic fibers which innervate the mentioned organs.
The autonomic nervous system (ANS) controls and regulates the functions of the visceral organs, smooth and cardiac muscles and glands without any conscious effort of an individual, and is thus referred to as involuntary. The ANS plays a crucial role in the maintenance of homeostasis through the complementary functions of its two anatomically and functionally distinct divisions; the parasympathetic and sympathetic nervous systems. Both systems provide some degree of nervous input to a given tissue at all times. Each of them either increases or decreases the activity of the innervated structure, complementing each other’s functions.
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Parasympathetic fibers are sent to various viscera to ensure different involuntary functions. In general, it is evident that the cranial outflow provides parasympathetic innervation to the head, and the sacral outflow provides the parasympathetic innervations of the pelvic viscera. However, an estimated 75% of all parasympathetic outflow in the whole body comes from the vagus nerve, that supplies the thoracic and abdominal viscera.
The parasympathetic innervation carried through the oculomotor nerve results in two events in the eye:
- The contraction of the sphincter pupillae muscle, which leads to the constriction of the pupil (myosis).
- The contraction of the ciliary muscle that results in loosening the zonular fibers of the lens, allowing the lens to shrink and increase its convexity. This process leads to increasing the refractive power of the lens in order to maintain a clear image and focus on a near object, and is a part of the accommodation of the eye.
In addition, the parasympathetic stimulation of the pterygopalatine ganglion via the facial nerve causes increased secretion of the lacrimal gland, resulting in increased tear production.
The parasympathetic innervation carried through the facial and glossopharyngeal nerves stimulates the secretion of the submandibular, sublingual and parotid glands. The final result is increased salivation, which promotes digestion.
The parasympathetic outflow through the vagus nerve has a more direct effect on digestion, increasing the release of digestive juices and enzymes from the stomach, pancreas and gallbladder as well as increasing the peristalsis of the gastrointestinal tract. The PNS also causes contraction of the rectum and relaxation of the internal anal sphincter to enable defecation.
The vagus nerve innervates the abdominal viscera by forming the two plexuses along the entire length of the intestine;
- Submucosal nervous plexus (of Meissner) found in the submucosa of the digestive tube and contains only parasympathetic input from the vagus nerve (CN X).
- Myenteric nervous plexus (of Auerbach) located in the muscularis externa of the digestive tube. It contains the parasympathetic fibers from the vagus nerve, as well as the sympathetic fibers from the thoracic splanchnic nerves.
The parasympathetic outflow through the vagus nerve plays an important role in heart rate. The vagus nerve acts on atrioventricular (AV) node, slowing its conduction and thus slowing the heart rate.
Parasympathetic stimulation also relaxes the smooth musculature of the peripheral blood vessels, which results in the vasodilation of peripheral vasculature. In contrast to this, acting on the smooth muscles of the coronary vessels, the parasympathetic nerves cause their vasoconstriction in response to reduced demand for oxygen. Acting on the lungs, the PSNS contracts the smooth muscles of the tracheobronchial tree, causing bronchoconstriction and promotion of mucus secretion in the bronchi.
The sacral parasympathetic outflow acts on the pelvic viscera. It causes the relaxation of the internal sphincter of urinary bladder and simultaneous contraction of the detrusor muscle of the urinary bladder wall. This increases the intraluminal pressure within the bladder, thereby promoting urination. In addition, the PNS causes stimulation of erectile tissues of the external genitalia, enabling an erection of the penis/clitoris to occur.
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Diabetic cardiac autonomic neuropathy is a serious and common complication of diabetes mellitus that is often under-diagnosed but can lead to severe morbidity and mortality, due to the associated cardiovascular burden. In the early periods of this condition, there is degeneration of the sympathetic control of the heart, which is followed in later stages by the degeneration of the parasympathetic stimulation of the heart. Also, its effects on the parasympathetic nervous system cause several cardiovascular disturbances, including resting tachycardia, exercise intolerance and postural hypotension.
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