Human Anatomy - Kotsan I. Ya. 2009

Autonomic (Vegetative) Nervous System
Parasympathetic part of the autonomic nervous system

The Parasympathetic division of the Autonomic Nervous system, much like the sympathetic division, comprises both central and peripheral components.

The central component includes the parasympathetic nuclei of the III, VII, IX, and X cranial nerve pairs located within the Brainstem (the cranial outflow), as well as the sacral parasympathetic nuclei situated in the Gray matter of Spinal Cord segments S2–S4 between the anterior and posterior horns (the sacral outflow).

The peripheral component of the parasympathetic nervous system encompasses the parasympathetic fibers of the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) Cranial Nerves along with their branches; the ciliary, pterygopalatine, submandibular, sublingual, and otic ganglia (the cranial part of the peripheral component); and the pelvic ganglia, pelvic splanchnic nerves, and their branches (the sacral part of the peripheral component).

Let us examine each component of the parasympathetic nervous system in greater detail.

The parasympathetic component of the Oculomotor nerve is represented by the accessory (parasympathetic) Nucleus (nucleus accessorius), also known as Edinger-Westphal nucleus, the ciliary ganglion, and the processes of the Cells located within this nucleus and ganglion. The axons of the cells originating in the accessory oculomotor nucleus, which lies in the tegmentum of the Midbrain, form preganglionic fibers running within the III cranial nerve pair. Within the orbital cavity, these fibers diverge from the inferior branch of the oculomotor nerve as the oculomotor ROOT (radix oculomotoria), enter the ciliary ganglion, and terminate on its cells.

The ciliary ganglion (ganglion ciliare) is a flat Structure, approximately 1.5–2 mm in length, embedded in the adipose tissue of the posterior Orbit lateral to the Optic nerve. This ganglion is formed by a cluster of second-row Neurons of the parasympathetic autonomic nervous system. Preganglionic parasympathetic fibers synapse with the Cells of the ciliary ganglion. Postganglionic nerve fibers travel via the short ciliary nerves (nn. ciliares breves) to supply the ciliary Muscle and the sphincter pupillae. Passing through the ganglion are transit fibers, which convey general somatic sensory information from the First Division of the Trigeminal nerve, and sympathetic postganglionic fibers entering via the sympathetic root from the internal carotid plexus.

The parasympathetic component of the Facial Nerve consists of the superior salivatory nucleus, along with the pterygopalatine and submandibular autonomic ganglia. The axons of cells from the superior salivatory nucleus, located in the tegmentum of the Pons, run within the nervus intermedius and subsequently the facial nerve proper through the facial canal. In the region of the geniculum of the facial nerve, a portion of the parasympathetic fibers diverges to form the greater petrosal nerve (n. petrosus major), exiting the facial canal through the hiatus for the greater petrosal nerve. The greater petrosal nerve courses along the corresponding groove on the petrous part of the Temporal bone, traverses the fibrocartilage filling the foramen lacerum, and enters the pterygopallatine canal. Here, the greater petrosal nerve joins the sympathetic deep petrosal nerve to form the nerve of the pterygoid canal (Vidian nerve). The nerve of the pterygoid canal emerges into the pterygopalatine fossa, where its preganglionic parasympathetic fibers terminate on the cells of the pterygopalatine ganglion.

The pterygopalatine ganglion (ganglion pterygopalatinum), measuring 4–5 mm and irregular in shape, is situated in the pterygopalatine fossa, inferior and medial to the maxillary nerve. Cell processes from this ganglion—the postganglionic fibers—join the maxillary nerve and course along its branches. From the zygomatic nerve, parasympathetic fibers transition to the lacrimal nerve via the communicating branch with the zygomatic nerve (r. communicans cum n. zygomaticus) to innervate the lacrimal gland. Additionally, nerve fibers from the pterygopalatine ganglion extend via its branches (the nasopalatine, greater palatine, lesser palatine, posterior superior nasal, lateral posterior nasal, and pharyngeal nerves) to innervate the mucous Glands of the Nasal cavity, palate, and Pharynx. A fraction of the preganglionic parasympathetic fibers that do not join the greater petrosal nerve branch off from the facial nerve as its second major branch, the chorda tympani. Upon joining the lingual nerve, these parasympathetic fibers travel along with it and ultimately reach the submandibular and sublingual ganglia via ganglionic branches.

The submandibular ganglion (ganglion submandibulare) is irregularly shaped, measures about 3–3.5 mm, and lies on the submandibular gland inferior to the lingual nerve. Preganglionic fibers originating from the superior salivatory nucleus (via the chorda tympani of the facial nerve) reach this ganglion via the lingual nerve. The submandibular ganglion houses The Cell bodies of parasympathetic neurons whose processes form postganglionic fibers directed to the submandibular and sublingual glands for secretomotor innervation.

The sublingual ganglion (ganglion sublinguale) represents a cluster of neuronal cell bodies along the course of the glandular Branches of the lingual nerve supplying the sublingual gland, where preganglionic parasympathetic fibers from the superior salivatory nucleus synapse with postganglionic neurons. Occasionally, the sublingual ganglion appears as a distinct, small nodal structure on the external surface of the sublingual salivary gland.

The parasympathetic component of the Glossopharyngeal nerve is formed by the inferior salivatory nucleus, the otic ganglion, and the cell processes associated with them. Axons originating in the inferior salivatory nucleus, located within the Medulla Oblongata, exit the cranial cavity as part of the glossopharyngeal nerve through the jugular foramen. At the level of the lower margin of the jugular foramen, preganglionic parasympathetic fibers branch off as the tympanic nerve (n. tympanicus), which initially enters the tympanic cavity to form a plexus. These fibers then emerge from the tympanic cavity through the hiatus for the lesser petrosal nerve as the lesser petrosal nerve (n. petrosus minor). Following the corresponding groove, the lesser petrosal nerve exits the cranial cavity via the sphenopetrosal fissure and reaches the otic ganglion.

The otic ganglion (ganglion oticum), measuring approximately 3 mm, lies medial to the mandibular nerve just inferior to the foramen ovale. It receives preganglionic fibers originating from the inferior salivatory nucleus that travel within the parasympathetic component of the glossopharyngeal nerve and its branches—the tympanic and lesser petrosal nerves. Postganglionic secretomotor fibers from the otic ganglion proceed to the parotid gland via the auriculotemporal nerve (a branch of the mandibular nerve).

The parasympathetic component of the Vagus nerve comprises the posterior (parasympathetic) vagal nucleus, numerous ganglia embedded within visceral Autonomic plexuses, and the cell processes associated with this nucleus and ganglia. Axons of the cells in the posterior vagal nucleus, situated in the medulla oblongata, travel within the branches of the vagus nerve to reach the parasympathetic ganglia of peri-organ and intra-organ autonomic plexuses. Such ganglia are integral to the cardiac, esophageal, pulmonary, gastric, intestinal, and other autonomic plexuses. These peri-organ and intra-organ parasympathetic ganglia contain the cell bodies of the second-order neurons of the efferent pathway; the processes of these neurons form bundles of postganglionic fibers that innervate the smooth muscle and glands of the visceral Organs of the neck, Thorax, and abdomen.

The sacral division of the parasympathetic nervous system is represented by the sacral parasympathetic nuclei located in the lateral intermediate substance of spinal cord segments S2–S4, the pelvic parasympathetic ganglia, and the associated cell processes. The axons of the sacral parasympathetic neurons emerge from the spinal cord within the anterior roots, join the anterior rami of the sacral Spinal Nerves, and after exiting through the pelvic sacral foramina, diverge to form the pelvic splanchnic nerves (nn. splanchnici pelvini). These nerves extend to the inferior hypogastric plexus and, via its branches, reach the external and internal genitalia, the Urinary Organs situated within the lesser pelvis, and the segments of the Large Intestine distal to the left colic flexure. Within or adjacent to the walls of these organs lie organ-specific plexuses (rectal, prostatic, uterovaginal, vesical) containing parasympathetic pelvic ganglia (ganglia pelvica), upon whose cells the preganglionic fibers of the pelvic splanchnic nerves terminate. The cell processes of the pelvic ganglia act as postganglionic parasympathetic fibers, projecting to the target organs to innervate their smooth musculature and glands.