For the purpose of systematisation, we will follow the pattern of Policeni and Smoker [3]. According to these authors, the path of the lower cranial nerves is divided into the brain stem, cisternal, cranial base (to which we will add the parasympathetic pathway leaving the main trunk of the GPN at the cranial base) and cervical segments, which, in this case, is practically reduced to the suprahyoid compartment [4].
Origin in the brain stem
The GPN and vagus nerve are mixed nerves that contain motor, branchial, sensory and autonomic fibres [16]. Both nerves have a common origin in the upper medulla oblongata and share three nuclei: the motor, the parasympathetic and the special sensory nuclei. Moreover, they convey general sensory information into the spinal trigeminal tract [1,2,3, 8, 11, 15,16,17]. Briefly, the motor nucleus involves the upper end of the nucleus ambiguous and innervates the stylopharyngeus muscle. The lower salivary nucleus sends efferent fibres to the parotid gland. General cutaneous and visceral sensory information travels through the GPN, joins the spinal trigeminal tract and ends in the spinal trigeminal nucleus. Finally, the special sensory nucleus is the solitary tract or gustatory nucleus. It receives taste sensation fibres through the solitary tract. The nucleus of the solitary tract also receives afferent impulses from the carotid sinus through the GPN [1,2,3, 7, 8, 10, 13, 15,16,17]. All nuclei are located behind the inferior olivary nucleus (Fig. 1).
Brain stem exit and cisternal segment
The GPN emerges in the cerebellomedullary cistern from the medulla oblongata immediately below the bulbopontine sulcus at the level of the retro-olive groove, between the inferior olivary nucleus and the inferior cerebellar peduncle [7, 8, 10] (Fig. 1). At this level, it is immediately above the vagus and spinal nerves (Fig. 2). Within the cistern, the three nerves rest on the posterior margin of the jugular tubercle of the occipital bone (Figs. 2 and 3), crossing laterally close to the anterior and inferior margin of the flocculus and the choroid plexus protruding from the fourth ventricle (Fig. 3). When it arrives at the skull base, the GPN enters an exclusive cranial exit near the top of the jugular foramen [10, 13, 14] (Fig. 2). The cranial nerves are visible in the cistern with MRI [4, 15, 18, 19] but not with CT, so the cerebellar flocculus, the choroid plexus and the jugular tubercle become critical anatomical references.
The jugular foramen and parasympathetic segment
The cochlear aqueduct, opening just above the GPN entrance in the jugular foramen [14, 15, 19,20,21], is the first anatomical reference (Figs. 3 and 4). In the jugular foramen, the GPN runs through the anteromedial portion or pars nervosa (petrosal fossula), separated from the posterolateral portion or pars vascularis (exit for the vagus and spinal nerves and the internal jugular vein) by the jugular spine (Fig. 2) and a fibrous septum (petro-occipital ligament), which is sometimes ossified [3, 6, 8, 13, 14]. At the entrance to the foramen, the GPN, vagus and spinal nerves are initially arranged in an obliquely posteroanterior-lateromedial distribution, between the jugular bulb (posterolateral) and the inferior petrosal sinus (anteromedial) [21] (Fig. 2). Once at the exit of the jugular foramen, the GPN is generally in front of the inferior petrosal sinus before draining into the internal jugular vein [6, 20, 21] (Figs. 2 and 5). Still within the foramen, the GPN shows two focal expansions or nodes [11] (Fig. 1), which are normally not visible in sectional high-resolution clinical images at 1.5T MRI [18] but have been observed with 3T MRI [21]. The superior node conveys general sensitive information and is located next to the opening of the cochlear aqueduct [21]. The lower node (Andersch node) handles the visceral sensory, gustatory and carotid [6, 16] innervations, and is located approximately 3 mm below [21].
The tympanic nerve or Jacobson nerve leaves the GPN from the lower node [10,11,12,13,14]. Fibres of the GPN course through this nerve and have a long cranial and parapharyngeal path, jumping from cranial nerve VII (the facial nerve) to cranial nerve V (the trigeminal nerve), and targeting the parotid gland finally. The first anatomical reference for the Jacobson nerve is the inlet of the lower tympanic canaliculus in the jugular spine, which enters just when leaving the GPN [3, 6] and through which it reaches the medial wall of the tympanic cavity at the level of the cochlear promontory [7, 14, 22] (Figs. 2, 4 and 6). Inside the tympanic cavity, the nerve forms a submucosal plexus that conveys sensitive information from the middle ear mucosa, antrum, mastoid air cells and Eustachian tube [3, 8, 10, 13,14,15]. At the level of the tendon of the tensor tympani muscle, it gives off a small nerve branch (deep great petrosal nerve) that will bind the lesser superficial petrosal nerve coming from the geniculate ganglion of the facial nerve [6, 11, 12, 23]. The course of the resultant nerve (the lesser petrosal nerve) can be followed by identifying the geniculate ganglion and the ducts in front, until the nerve enters the middle cranial fossa through the accessory hiatus, lateral to the exit of the greater petrosal nerve, which exits through the fallopian hiatus [10,11,12] (Fig. 6).
Once in the middle cranial fossa, it crosses forward and medially to leave the skull variably through different exits: the foramen spinosum, foramen ovale, innominate canaliculus (located between these two foramen) or the sphenopetrosal junction [11, 23] (Figs. 6 and 7). Immediately below the skull, the GPN fibres synapse in the otic ganglion, medial to the mandibular nerve and just under the foramen ovale [10,11,12,13] (Fig. 7). The GPN postganglionic fibres leave the otic ganglion through the auriculotemporal branch of the trigeminal nerve. Now, the reference is the line that connects the mandibular nerve with the middle meningeal artery. The auriculotemporal nerve embraces the artery and then goes back through the parapharyngeal space medial to the lateral pterygoid muscle first and the neck of the mandibular condyle later, to reach the deep lobe of the parotid gland, providing it with parasympathetic innervation [8, 10, 11, 14, 22] (Fig. 7).
Cervical segment
Immediately after leaving the jugular foramen, the GPN is located medial to the internal jugular vein and behind the internal carotid artery (Figs. 5 and 8). Now it is located in the retro-styloid or carotid space [4, 24]. When proceeding down the neck, the GPN initially has the same vascular relationships. The styloid process, the most lateral reference at this level, is also a useful mark [10, 12]. At the level of the C1 transverse process, the nerve goes around the carotid artery laterally and descends behind the styloid process and the stylopharyngeus muscle [14] (Fig. 8). When it crosses between the internal carotid artery and the internal jugular vein, the GPN gives off the carotid sinus and carotid body nerve (Hering’s nerve), which descends along the anterior wall of the internal carotid artery [11] (Fig. 9).
Once the GPN is anterior to the vascular structures, it leaves the close relationship with the internal carotid artery approximately at the level of the soft palate [24] and runs behind the styloid muscles (stylohyoid, styloglossus, stylopharyngeus) (Fig. 10). These muscles form a tripod that will be the reference until the nerve enters the oropharynx, especially the medial (stylopharyngeus) and the anterior (styloglossus) muscles. The styloid muscles are surrounded by a fibrous fascia (styloid diaphragm); muscles and fascia form the styloid pyramid [14] (Fig. 9). The GPN surrounds the external side of the stylopharyngeus muscle to reach its anterior surface inside the pyramid [11]. In this course, the GPN provides motor innervation to the muscle [14, 25, 26]. Once within the pyramid, the most common position of the GPN is in a triangle, in which the posterior margin is the external carotid artery, the lower one is the facial artery and the anterior one is the styloglossus muscle [27] (Fig. 9). However, this reference appears not to be suitable for the common axial slices, in which the long axis of the styloid pyramid area might be an easier anatomical key point (Fig. 10).
At approximately the point where the stylopharyngeus muscle merges with the constrictor muscles, the GPN enters the pharynx between the upper and middle constrictors [14]. At this point, the stylopharyngeus and hyoglossus muscles separate the hypoglossal nerve (lateral) from the GPN (medial). This circumstance occurs immediately above the level of the intermediate tendon of the digastric muscle [14] (Fig. 10).
Once in the pharynx, the GPN splits into pharyngeal branches, which contribute to the pharyngeal plexus of the vagus, and the lingual branch [4, 6, 10, 14, 25]. The references of the lingual branch are the lower edge of the palatine tonsil (palatoglossus and styloglossus muscles) and the hyoglossus muscle. The lingual branch reaches the tongue medial to the hyoglossus muscle to innervate the posterior third of the tongue [6, 11] (Fig. 10).
All anatomic references are summarised in Fig. 11.
