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Lesser petrosal nerve

The lesser petrosal nerve is a slender parasympathetic nerve arising primarily from the glossopharyngeal nerve (cranial nerve IX), with contributions from the facial nerve (cranial nerve VII) via the nervus intermedius and the vagus nerve (cranial nerve X) via its auricular branch, serving as the preganglionic pathway for secretomotor innervation to the parotid gland.^[1]^[2]^[3] Originating from preganglionic parasympathetic fibers in the inferior salivatory nucleus of the medulla oblongata, these fibers exit the brainstem via the glossopharyngeal nerve and enter the middle ear as the tympanic branch (Jacobson's nerve), where they form part of the tympanic plexus alongside contributions from the facial and vagus nerves.^[1]^[2] From the tympanic plexus, the lesser petrosal nerve emerges, traveling a short distance along the floor of the middle cranial fossa beneath the dura mater, often parallel to the greater petrosal nerve.^[2]^[3] It then descends to the infratemporal fossa, typically passing through the foramen ovale (though variants include the foramen spinosum or sphenopetrosal fissure), to synapse at the otic ganglion located inferomedial to the foramen ovale.^[1]^[3] Postganglionic fibers from the otic ganglion hitchhike along branches of the mandibular division of the trigeminal nerve (cranial nerve V), particularly the auriculotemporal nerve, to reach and innervate the parotid gland, providing parasympathetic control over its salivary secretion.^[1]^[2] Although primarily parasympathetic, the nerve may occasionally incorporate minor sympathetic fibers from the carotid plexus via the mandibular nerve, but it lacks sensory or motor components.^[2] Clinically, the lesser petrosal nerve is significant in middle cranial fossa surgeries, such as those for trigeminal neuralgia or acoustic neuromas, where inadvertent damage can lead to reduced parotid secretion or, in combination with greater petrosal nerve injury, contribute to aberrant reinnervation syndromes such as gustatory sweating (Frey's syndrome) or crocodile tears syndrome (gustatory lacrimation) due to misdirected parasympathetic fibers.^[2] Its anatomical variability underscores the need for precise identification to avoid iatrogenic complications during otologic or neurosurgical procedures.^[2]

Anatomy

Origin

The lesser petrosal nerve arises primarily from the tympanic plexus located within the middle ear cavity. This plexus is formed by the tympanic nerve, also known as Jacobson's nerve, which is a branch of the glossopharyngeal nerve (cranial nerve IX).^[1]^[4] The preganglionic parasympathetic fibers composing the nerve originate from cell bodies in the inferior salivatory nucleus situated in the medulla oblongata. These fibers constitute general visceral efferent (GVE) components responsible for parasympathetic innervation. The pathway begins with axons exiting the inferior salivatory nucleus, traveling through the glossopharyngeal nerve to reach the tympanic nerve, and then integrating into the tympanic plexus where they form the lesser petrosal nerve.^[1]^[5] In addition to its primary glossopharyngeal origin, the lesser petrosal nerve incorporates minor contributions from the facial nerve (cranial nerve VII) via the nervus intermedius and from the vagus nerve (cranial nerve X) through its auricular branch (Arnold's nerve), providing supplementary parasympathetic modulation within the tympanic plexus.^[6] These preganglionic fibers ultimately synapse at the otic ganglion.^[1]

Course

The lesser petrosal nerve emerges from the tympanic plexus within the middle ear and ascends through a small opening in the petrous portion of the temporal bone, known as the hiatus for the lesser petrosal nerve (also called the canaliculus for the lesser petrosal nerve or hiatus canalis nervi petrosi minoris), to enter the middle cranial fossa.^[2]^[7] In the middle cranial fossa, the nerve courses anteriorly along the floor of the fossa, typically running parallel and just lateral to the greater petrosal nerve, beneath the dura mater in a shallow groove formed by the temporal and sphenoid bones.^[2]^[7] From there, it descends to exit the skull base, most commonly (in approximately 70% of cases) through the canaliculus innominatus—a small foramen located posterior to the foramen ovale and foramen spinosum in the greater wing of the sphenoid bone—although it may alternatively pass directly through the foramen ovale, the foramen spinosum, the sphenopetrosal fissure, or the petrosal foramen.^[2]^[8] This exit leads the nerve into the infratemporal fossa. The intracranial segment of the nerve measures approximately 15 mm on average (ranging from 11.5 to 19.3 mm).^[8]^[2] Upon entering the infratemporal fossa, the lesser petrosal nerve proceeds a short distance to reach the otic ganglion, where it synapses with postganglionic parasympathetic neurons.^[2] The postganglionic fibers then hitchhike along the auriculotemporal nerve—a branch of the mandibular division of the trigeminal nerve (CN V3)—to ultimately innervate the parotid gland.^[2]^[8] Variations in the exit point from the middle cranial fossa can influence the precise trajectory in this region, with the nerve occasionally diverging earlier from its parallel path to the greater petrosal nerve at an average angle of about 12 degrees.^[7]

Relations

In the middle cranial fossa, the lesser petrosal nerve lies anterolateral and parallel to the groove for the greater petrosal nerve on the floor of the petrous temporal bone, positioned laterally and inferiorly to the impression for the trigeminal ganglion.^[9]^[7] It runs adjacent to the trigeminal ganglion and in close proximity to the horizontal segment of the internal carotid artery, often between the petrous branch of the middle meningeal artery medially and the superior tympanic artery laterally.^[10]^[11] Near the foramen ovale, the lesser petrosal nerve is proximate to the mandibular division of the trigeminal nerve (CN V3) and the accessory meningeal artery, with which it may share a common canal in anatomical variants.^[1]^[11] In the infratemporal fossa, the nerve is closely associated with the otic ganglion, to which it provides preganglionic parasympathetic fibers; the ganglion itself is positioned medial to the mandibular nerve and lateral to the tensor veli palatini muscle.^[1]^[11] Potential sites of compression for the lesser petrosal nerve include impingement by the middle meningeal artery along its course in the middle cranial fossa or injury during temporal bone fractures affecting the petrous apex.^[11]^[1]

Function

Parasympathetic innervation

The lesser petrosal nerve conveys preganglionic general visceral efferent (GVE) fibers originating from the inferior salivatory nucleus of the glossopharyngeal nerve (CN IX), which travel through the tympanic plexus before entering the nerve proper.^[1] These fibers reach the otic ganglion, located inferior to the foramen ovale in the infratemporal fossa, where they synapse with postganglionic neurons.^[12] The synapse at the otic ganglion reflects the limited divergence typical of parasympathetic ganglia.^[13] Postganglionic parasympathetic fibers exit the otic ganglion and join the auriculotemporal branch of the mandibular division of the trigeminal nerve (CN V3), traveling to the parotid gland to form the intraparotid plexus around the facial nerve branches within the gland.^[12] This plexus distributes secretomotor fibers exclusively to the parotid salivary gland, with no sensory or somatic motor components in the lesser petrosal nerve itself.^[3] The primary target is the serous acinar cells of the parotid gland, where postganglionic fibers release acetylcholine that binds to muscarinic cholinergic receptors (primarily M3 subtype), stimulating watery serous secretion essential for initial digestion.^[14]

Physiological role

The lesser petrosal nerve plays a key role in the parasympathetic control of salivary secretion by conveying preganglionic fibers that stimulate the parotid gland, promoting the release of serous saliva in response to autonomic activation. This activation originates from the inferior salivatory nucleus, which receives inputs from higher centers triggered by sensory stimuli such as the sight, smell, and taste of food, initiating salivation as part of the anticipatory cephalic phase of digestion.^[15]^[2]^[16] The nerve's parasympathetic efferents facilitate the production of watery, enzyme-rich saliva from the parotid gland, which aids in oral lubrication, initial food breakdown through amylase activity, and protection of the oral mucosa. This serous secretion contrasts with the more viscous, protein-laden output from other salivary glands and integrates with sympathetic innervation—delivered via the auriculotemporal nerve—to modulate overall saliva composition and volume for balanced digestive preparation.^[17]^[18]^[10] In autonomic reflexes, the lesser petrosal nerve contributes to homeostasis by enhancing salivary flow during reflexive responses to oropharyngeal stimuli, supporting digestive readiness without direct involvement in gastric acid secretion. Compared to the greater petrosal nerve, which innervates the lacrimal gland and mucosal glands of the nasal and palatine regions (producing mixed secretions), the lesser petrosal nerve specifically targets the parotid gland to drive predominantly serous salivation essential for enzymatic digestion.^[16]^[10]^[3]

Development

Embryological origin

The lesser petrosal nerve is a component of the glossopharyngeal nerve (CN IX), which derives from the third pharyngeal arch.^[4] The parasympathetic fibers originate in the inferior salivatory nucleus and travel via the tympanic branch (Jacobson's nerve) to form the nerve, providing secretomotor innervation to the parotid gland.^[4] At week 7 of gestation, the lesser petrosal nerve runs straight and parallel to the facial nerve along the otic capsule.^[19] During the fetal period, its course becomes more winding due to the expansion of the tympanic cavity epithelium.^[20] The hiatus for the lesser petrosal nerve develops as part of the endochondral ossification of the petrous temporal bone in the otic capsule, which begins around week 6 and progresses through the fetal period.^[21]

Anatomical variations

The lesser petrosal nerve exhibits variations in its course, particularly in exiting the skull base. It typically passes through the foramen ovale, but alternative routes include the sphenopetrosal fissure or foramen spinosum.^[11] The nerve receives preganglionic parasympathetic fibers primarily from the tympanic branch of the glossopharyngeal nerve (CN IX), with contributions from the nervus intermedius of the facial nerve (CN VII) and the auricular branch of the vagus nerve (CN X). Sympathetic fibers from the meningeal branch of the mandibular division of the trigeminal nerve (V3) join in approximately 33% of cases.^[11] In the middle cranial fossa, the nerve typically emerges via the canaliculus innominatus and runs in a groove on the petrous bone floor, diverging medially from the greater petrosal nerve by an average angle of 11.6°. It is exposed without bony covering in 75% of cases.^[11]

Clinical significance

Surgical considerations

The lesser petrosal nerve is particularly vulnerable during middle cranial fossa approaches, such as those employed for trigeminal neuralgia or access to the petrous apex, where it courses along the floor of the fossa just beneath the dura mater, often parallel and lateral to the greater superficial petrosal nerve. Surgeons identify and preserve it by recognizing its position within the petrous groove or relative to the tensor tympani muscle and facial hiatus, typically after dural elevation, to avoid iatrogenic injury that could disrupt parasympathetic innervation.^[22]^[23] In temporal bone surgery, including mastoidectomy and translabyrinthine approaches for acoustic neuroma resection, the nerve risks damage during bone removal near the petrous ridge; dissection proceeds from lateral to medial and posterior to anterior to spare it, with intraoperative nerve monitoring recommended to detect proximity and guide preservation.^[24]^[25] During parotidectomy, the postganglionic parasympathetic fibers derived from the lesser petrosal nerve via the otic ganglion travel along the auriculotemporal nerve and are susceptible to transection, potentially leading to aberrant reinnervation if not meticulously preserved.^[26] Preoperative imaging with high-resolution computed tomography (CT) visualizes the lesser petrosal nerve canal in up to 75% of cases on angled axial sections, aiding surgical planning by delineating its course through the temporal bone; magnetic resonance imaging (MRI) complements this for soft-tissue relations, while endoscopic techniques facilitate its identification in the infratemporal fossa during transsphenoidal or extended approaches.^[27] Historically, the lesser petrosal nerve's anatomical landmarks for surgical orientation were detailed in the 20th edition of Gray's Anatomy (1918), emphasizing its extradural path as a reference in temporal and cranial base procedures; contemporary practices build on this with advanced neuromonitoring and navigation systems.^[28]

Associated disorders

Damage to the lesser petrosal nerve, which carries parasympathetic fibers essential for parotid gland secretion, results in reduced salivary flow from the parotid, contributing to xerostomia or dry mouth.^[29] This autonomic dysfunction can occur following trauma or surgical intervention near the nerve's course through the middle cranial fossa. In cases of aberrant regeneration after injury, such as during parotidectomy, parasympathetic fibers from the lesser petrosal nerve pathway may cross-innervate sympathetic fibers of the auriculotemporal nerve, leading to Frey's syndrome characterized by gustatory sweating, facial flushing, warmth, and sometimes neuralgia or pruritus in the preauricular and temporal regions.^[26] Symptoms of Frey's syndrome typically manifest 6 to 18 months post-injury and are provoked by gustatory stimuli, particularly acidic or spicy foods, with an incidence ranging from 4% to 96% after parotid surgery depending on diagnostic methods.^[26] Rare isolated lesions of the lesser petrosal nerve can arise from petrous temporal bone fractures due to high-energy trauma, disrupting its canal and causing ipsilateral parotid hypofunction or autonomic disturbances.^[29] Similarly, compressive tumors such as schwannomas arising from related petrosal nerve segments or meningiomas at the skull base can impinge on the nerve, leading to progressive xerostomia or sensory deficits.^[30] These lesions are infrequent, with most reported cases involving adjacent structures like the tympanic branch (Jacobson's nerve).^[31] Diagnosis of lesser petrosal nerve involvement relies on clinical assessment combined with targeted testing. An analog to Schirmer's test for lacrimal function, the salivary flow test measures stimulated parotid secretion via cannulation of Stensen's duct, comparing ipsilateral and contralateral output to quantify hypofunction.^[32] For structural lesions, magnetic resonance imaging (MRI) is the modality of choice to visualize nerve compression by tumors like meningiomas or post-traumatic changes in the petrous bone, often showing enhancement or displacement along the nerve's extradural course.^[29] Computed tomography (CT) complements MRI by delineating bony fractures or canal disruptions.^[33] Current understanding of lesser petrosal nerve disorders is limited by the scarcity of isolated case studies, as most pathologies affect it in conjunction with other cranial nerves, complicating attribution of symptoms.^[29] This gap underscores the need for more focused research on selective lesions to better delineate clinical presentations and long-term outcomes.

References

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Jul 24, 2023 · Otic ganglion is one of the four parasympathetic ganglia within the head region. An anatomist named Arnold first described it in 1828.
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The human lesser petrosal nerve (LPN) is a branch of the glossopharyngeal nerve (IX), with contributions from the facial (VII) and vagus (X) nerves.Missing: StatPearls | Show results with:StatPearls
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lesser petrosal n. synapses in the otic ganglion and postganglionic axons distribute to the parotid gland by joining the auriculotemporal n. lingual n.
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The lesser petrosal nerve crosses the middle fossa floor anterior to the greater petrosal nerve, exiting through the canaliculus innominatus, foramen spinosum, ...
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The lesser petrosal nerve averages a 15 mm (range: 11.5-19.3 mm) course along the skull base.
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The parasympathetic innervation of the parotid gland is from the lesser petrosal branch of the glossopharyngeal nerve. The preganglionic fibers synapse in ...
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Parasympathetic fibers leave the plexus as the lesser petrosal nerve, re-enter the middle cranial fossa to exit the skull via the foramen ovale along with ...
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... lesser petrosal nerve. Motor fibers of the GN originate in the nucleus ambiguous of the medulla, and innervate stylopharyngeal muscle [32,47]. VN, a part of ...
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Aug 9, 2025 · Recently, our group described changes in the courses of the greater and lesser petrosal nerves in human embryos and fetuses (Jin et al., 2017).
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