The parasympathetic nervous system is a division of the autonomic nervous system. Unlike the consciously controlled somatic nervous system, the parasympathetic division is involuntary and acts together with the sympathetic nervous system to maintain bodily homeostasis. The actions of the parasympathetic nervous system are associated with the ‘rest and digest’ response, by which physiological functions return to their normal baseline levels after being temporarily increased or decreased. In this article, we shall look at the anatomy of the parasympathetic innervation of the head and neck region - its structure, anatomical course, and clinical implications.
The parasympathetic fibres originate in the central nervous system. They are situated within four nuclei, located within the brainstem. Each nucleus is associated with a specific cranial nerve – the oculomotor, facial, glossopharyngeal, and vagus - which carry the parasympathetic fibres out of the brain. After leaving the brain, the parasympathetic fibres from each nucleus synapse in a peripheral ganglion, a collection of neurone cell bodies located outside of the CNS. These ganglia are typically located near to the target viscera of the head and neck, and from here post-ganglionic parasympathetic fibres to the organs.
There are four parasympathetic ganglia located within the head: the ciliary, otic, pterygopalatine, and submandibular. These receive fibres from the oculomotor, facial, and glossopharyngeal nerves. The vagus nerve only innervates structures in the thorax and abdomen. We shall now examine these ganglia in more detail.
Note: Most ganglia are associated with some sensory and sympathetic nerves, but these do not synapse in the ganglia; they merely travel through it.
The ciliary ganglion is located within the bony orbit, situated anteriorly to the superior orbital fissure and between the lateral rectus muscle and the optic nerve. It is supplied by fibres from the Edinger-Westphal nucleus, associated with the oculomotor nerve. The parasympathetic fibres leave the ganglion via the short ciliary nerves, continuing into the orbit to innervate structures of the eye. The post-ganglionic fibres from the ciliary ganglion provide secretomotor innervation to the sphincter pupillae, which contracts the pupil, and the ciliary muscles, which accommodate for near vision.
Two sets of nerve fibres also pass through the ciliary ganglion without synapsing. These are sympathetic nerves from the internal carotid plexus, which innervate the dilator pupillae muscle, and sensory fibres from the nasociliary nerve, a branch of the ophthalmic division of the trigeminal nerve, which innervate the cornea, ciliary body, and iris.
The pterygopalatine ganglion (also known as sphenopalatine) is the largest of the four parasympathetic ganglia. It is located within the pterygopalatine fossa, a space located inferiorly to the base of the skull and posteriorly to the maxilla. The pterygopalatine ganglion is supplied by fibres from the superior salivatory nucleus, associated with the facial nerve. These fibres travel within the greater petrosal nerve and nerve of the pterygoid canal to reach the ganglion. The parasympathetic fibres leave the ganglion by hitch-hiking on branches of the maxillary nerve, derived from the trigeminal nerve. The post-ganglionic fibres from the pterygopalatine ganglion provide secretomotor innervation to the lacrimal gland, mucous glands of the posterosuperior nasal cavity, nasopharynx, and palate.
The four parasympathetic ganglia of the head and neck are connected to the heart, lungs, eyes, nose, and throat, illustrating the importance of the parasympathetic nervous system in regulating these vital organs and systems. Parasympathetic stimulation is essential in maintaining the physiological balance and functioning of the body. Any interference in the parasympathetic innervation can lead to a variety of clinical disorders. The most common conditions are related to impaired vision, such as ptosis (drooping eyelid), chronic dry eye syndrome, and optic neuropathy. Other disorders can affect the lacrimal gland, the mucous glands of the nasal cavity and throat, and the salivary glands.
Parasympathetic dysfunction can have a significant impact on quality of life, as it can lead to a variety of uncomfortable and debilitating conditions. It is important for healthcare professionals to understand the anatomy and clinical correlations of the parasympathetic nervous system, in order to develop effective treatments for patients affected by disorders associated with parasympathetic dysfunction.
The parasympathetic fibres from each nuclei synapse in a peripheral ganglion (a collection of neurone cell bodies outside the Central Nervous System). These specialized ganglia are located near to the target viscera and provide post-ganglionic parasympathetic innervation to the organs in the head and neck. There are four parasympathetic ganglia located within the head - the ciliary, otic, pterygopalatine and submandibular - which receive fibres from the oculomotor, facial and glossopharyngeal nerves (the vagus nerve only innervates structures in the thorax and abdomen).
The ciliary ganglion is located within the bony orbit, anteriorly to the superior orbital fissure and between the lateral rectus muscle and the optic nerve. It is close to the optic nerve, superiorly to the annulus of Zinn. Pre-ganglionic fibres are supplied by fibres from the Edinger-Westphal nucleus (associated with the oculomotor nerve). These fibres travel with the motor root of the oculomotor nerve, to reach the ciliary ganglion. Post-ganglionic fibres leave the ganglion via the short ciliary nerves and continue into the orbit to innervate structures of the eye.
Target Organs: The post-ganglionic fibres from the ciliary ganglion innervate the sphincter pupillae (contracts the pupil) and the ciliary muscles (accommodates for near vision).
The pterygopalatine ganglion (also known as sphenopalatine) is the largest of the four parasympathetic ganglia. It is located within the pterygopalatine fossa - a space located inferiorly to the base of the skull, and posteriorly to the maxilla. Pre-ganglionic fibres are supplied by fibres from the superior salivatory nucleus (associated with the facial nerve), which travel within the greater petrosal nerve and the nerve of the pterygoid canal to reach the ganglion. Post-ganglionic fibres then leave the ganglion by hitch-hiking on branches of the maxillary nerve (derived from the trigeminal nerve).
Target Organs: The post-ganglionic fibres from the pterygopalatine ganglion provide secretomotor innervation to lacrimal gland, mucous glands of posterosuperior nasal cavity, nasopharynx, and the palate.
The submandibular ganglion is located inferiorly to the lingual nerve, from which it is suspended. Pre-ganglionic fibres are supplied by fibres from the superior salivatory nucleus (associated with the facial nerve). These fibres are carried within a branch of the facial nerve, the chorda tympani. This nerve hitchhikes along the lingual branch of the mandibular nerve to reach the ganglion. Post-ganglionic fibres leave the ganglion and travel directly to the submandibular and sublingual glands.
Target Organs: Secretomotor innervation to the submandibular and sublingual salivary glands.
The otic ganglion is located inferiorly to the foramen ovale, within the infratemporal fossa. Pre-ganglionic fibres are supplied by fibres from the inferior salivatory nucleus (associated with the glossopharyngeal nerve). These fibres travel within a branch of the glossopharyngeal nerve, the lesser petrosal nerve, to reach the otic ganglion. Post-ganglionic fibres then leave the ganglion by hitch-hiking on the auriculotemporal nerve (associated with the mandibular nerve) and innervate the parotid gland.
Target Organs: Secretomotor innervation to the parotid salivary gland.
The four parasympathetic ganglia located within the head - the ciliary, otic, pterygopalatine and submandibular - provide vital secretomotor innervation to the organs of the head and neck. Pre-ganglionic fibres are supplied to the ganglia from each nerves’ associated nuclei. The post-ganglionic fibres then hitchhike to their respective target organs via branches of other cranial nerves, or travel directly to the terminal organ. Each of the ganglia provide secretomotor innervation to different organs, which will ultimately result in a variety of salivary, lacrimal or ciliary responses.
Post-ganglionic fibres are the set of nerve fibres which transmit signals from the brain to the target organs. They are composed of Parasympathetic fibres, which hitchhike along the auriculotemporal nerve (a branch of the mandibular division of the trigeminal nerve) to provide secretomotor innervation to the parotid gland, as well as sympathetic fibres from the superior cervical chain which pass through the otic ganglion and travel with the middle meningeal artery to innervate the parotid gland.
The post-ganglionic fibres from the otic ganglion provide innervation to the parotid gland while sympathetic fibres from the superior cervical chain pass through the otic ganglion and travel to innervate the parotid gland.
The ciliary ganglion is responsible for providing innervation to the sphincter pupillae muscle, which acts to constrict the pupil. If the ciliary ganglion is damaged, there is a loss of innervation to the sphincter pupillae, resulting in a permanently dilated pupil that does not constrict in the presence of light, which is known as Adie's pupil - named after the British neurologist William John Adie. It is generally thought to result from inflammation caused by a viral or bacterial infection.
Post-ganglionic nerve fibres are a vital component of the body’s nervous system, responsible for carrying impulses from the brain to target organs throughout the body. These fibres are composed of Parasympathetic and Sympathetic fibres, which are carried by various nerves. The post-ganglionic parasympathetic fibres travel via the auriculotemporal nerve to provide secretomotor innervation to the parotid gland, while the sympathetic fibres travel through the otic ganglion and along the middle meningeal artery to innervate the parotid gland. In addition, the ciliary ganglion provides innervation to the sphincter pupillae muscle, which is necessary for normal pupil constriction in response to light. A damage to the ciliary ganglion will result in the paralysis of this muscle, resulting in what is known as Adie’s pupil - a permanently dilated pupil that does not constrict in the presence of light.
In terms of specifics, there are five main patterns of post-ganglionic nerve fibre distribution. The first is in relation to the Edinger-Westphal nucleus, which is part of the oculomotor nerve and travels along with the motor root of this nerve to the ciliary ganglion in order to innervate the sphincter pupillae and ciliary muscles. The second pattern is in relation to the superior salivatory nucleus, which is part of the facial nerve and travels along with the greater petrosal nerve and the nerve of pterygoid canal to the pterygopalatine ganglion, providing innervation to the lacrimal gland, nasopharynx, palate, and nasal cavity. The third pattern is in relation to the nervous fibres travelling within the chorda tympani, a branch of the facial nerve, which reach the submandibular ganglion and travel directly to the target organs. The fourth pattern is in relation to the inferior salivatory nucleus, which is part of the glossopharyngeal nerve and travels within the lesser petrosal nerve to the otic ganglion, providing innervation to the parotid gland. Finally, the fifth pattern is in relation to the dorsal vagal motor nucleus, which is part of the vagus nerve and travels within the vagus nerve itself, reaching many ganglia located within the target organs in order to innervate the smooth muscle of the trachea, bronchi and gastro-intestinal tract.
Overall, post-ganglionic nerve fibres are essential in the functioning of the body’s nervous system. They provide essential innervation to target organs by travelling through various pathways, with some fibres even travelling directly to the target organs. Damage to these fibres can have a plethora of effects, such as a permanently dilated pupil that does not constrict in the presence of light. It is therefore important to have knowledge of the various pathways that these fibres take in order to properly diagnose and treat any damages or issues caused by them.
