Parasympathetic fibers, postganglionic

Overview

Postganglionic parasympathetic fibers are a crucial component of the parasympathetic nervous system, which is part of the autonomic nervous system. These fibers originate from parasympathetic ganglia and extend to various target organs, where they exert their effects by releasing the neurotransmitter acetylcholine.

Anatomy

Postganglionic parasympathetic fibers are relatively short compared to their preganglionic fibers counterparts. This is because parasympathetic ganglia are typically located near or within the walls of their target organs. The main parasympathetic ganglia include the ciliary ganglion, pterygopalatine ganglion, submandibular ganglion, and otic ganglion.

Ciliary Ganglion

The ciliary ganglion is located in the orbit and is associated with the oculomotor nerve (CN III). Postganglionic fibers from this ganglion innervate the sphincter pupillae and ciliary muscle, controlling pupil constriction and lens accommodation.

Pterygopalatine Ganglion

The pterygopalatine ganglion is located in the pterygopalatine fossa and is associated with the facial nerve (CN VII). Postganglionic fibers from this ganglion innervate the lacrimal glands and nasal mucosa, facilitating tear production and nasal secretion.

Submandibular Ganglion

The submandibular ganglion is located near the submandibular gland and is also associated with the facial nerve (CN VII). Postganglionic fibers from this ganglion innervate the submandibular and sublingual glands, promoting salivation.

Otic Ganglion

The otic ganglion is located just below the foramen ovale and is associated with the glossopharyngeal nerve (CN IX). Postganglionic fibers from this ganglion innervate the parotid gland, stimulating salivary secretion.

Function

The primary function of postganglionic parasympathetic fibers is to mediate the "rest and digest" activities of the body. They achieve this by releasing acetylcholine, which binds to muscarinic receptors on target tissues, leading to various physiological responses.

Cardiovascular System

In the cardiovascular system, postganglionic parasympathetic fibers decrease heart rate and reduce the force of cardiac contractions. This is primarily mediated through the vagus nerve (CN X), which innervates the heart.

Respiratory System

In the respiratory system, these fibers cause bronchoconstriction and increase bronchial secretions, aiding in the humidification and warming of inhaled air.

Digestive System

In the digestive system, postganglionic parasympathetic fibers enhance peristalsis and increase the secretion of digestive enzymes and gastric acid, facilitating digestion and nutrient absorption.

Urinary System

In the urinary system, these fibers promote bladder contraction and relaxation of the internal urethral sphincter, aiding in urination.

Clinical Significance

Dysfunction of postganglionic parasympathetic fibers can lead to various clinical conditions. For example, damage to the ciliary ganglion can result in Adie syndrome, characterized by a dilated pupil and loss of accommodation. Similarly, dysfunction in the submandibular or otic ganglia can lead to dry mouth due to reduced salivary secretion.

Conclusion

Postganglionic parasympathetic fibers play a vital role in maintaining homeostasis by regulating the body's "rest and digest" functions. Understanding their anatomy and function is essential for diagnosing and treating disorders of the autonomic nervous system.