Fig. 9.1
Major branches of the trigeminal nerve: ophthalmic, maxillary, and mandibular nerves. The roots of the trigeminal nerve arise from the base of the pons and form the Gasserian ganglion
A smaller motor root lies underneath the main trigeminal ganglion and supplies motor fibers to the mandibular nerve. These motor fibers supply mostly the muscles of mastication, namely, the masseter, temporalis, medial and lateral pterygoids, as well as the anterior belly of the digastric, mylohyoid, tensor tympani, and tensor veli palatini muscles.
The three major divisions of the trigeminal nerve exit the cranium through three distinct foramina – the ophthalmic nerve enters the orbit through the superior orbital fissure, where it divides into its three main branches, the frontal, lacrimal and nasociliary nerves. The maxillary nerve passes through the foramen rotundum to cross over the top of the pterygopalatine fossa and then enters the floor of the orbit through the inferior orbital fissure. After exiting the skull through the foramen rotundum, the maxillary nerve courses anteriorly over the pterygopalatine fossa and enters the floor of the orbit through the infraorbital fissure as the infraorbital nerve; the infraorbital nerve courses in the infraorbital canal, and descending branches include the greater palatine, lesser palatine, and nasopalatine and superior alveolar nerves. The mandibular nerve courses into the infratemporal fossa through the foramen ovale and divides into its two major branches, the lingual nerve and inferior alveolar nerve. The terminal branches of these three divisions, destined for the face, namely, the supraorbital, infraorbital, and mental nerves, exit the skull through the supraorbital, infraorbital, and mental foramina, respectively, and usually lie vertically in-line with each other in the plane of the pupil (neonate, Fig. 9.2a; adolescent, Fig. 9.2b). The greater palatine, lesser palatine, and nasopalatine nerves exit the palate through corresponding foramina of the posterior aspect of the secondary palate and anterior aspect of the primary palate. Although the terminal branches are the nerves that we focus on for many common regional anesthesia blocks, it is important to remember that each of the trigeminal divisions send branches internally to supply the oral and nasal mucosa as well as the mucosa of the paranasal sinuses external branches to supply the lateral aspect of the face.
Fig. 9.2
Neonate (a) and adolescent (b) skull models depicting three distinct foramina, through which branches of the trigeminal nerve exit the cranium. The ophthalmic, maxillary, and mandibular nerves exit through the supraorbital foramen (SO), infraorbital foramen (IO), and mental foramen (M), respectively
9.1.1 Ophthalmic Nerve (V1 Division of the Trigeminal Nerve, Pure Sensory)
The uppermost ophthalmic branch of the trigeminal nerve is a pure sensory nerve and passes through the superior orbital fissure (sphenoidal fissure) into the orbit.
The nerve divides into three main branches – lacrimal, frontal, and nasociliary – just prior to entering the orbit through the superior orbital fissure.
The lacrimal nerve is the smallest of the three divisions. It enters the orbit through the superior orbital fissure and communicates with the zygomaticotemporal branch of the maxillary nerve. It courses through the lateral aspect of the orbit over the lateral rectus muscle. The lacrimal nerve then enters the lacrimal gland and innervates it with parasympathetic secretomotor fibers (from the zygomaticotemporal nerve) and the adjoining conjunctiva with sensory fibers. The nerve eventually pierces the orbital septum to innervate the skin of the lateral aspect of the upper eyelid, joining with filaments from the facial nerve.
The division’s largest branch, the frontal nerve, courses anteriorly through the orbit along the upper surface of the levator palpebrae superioris muscle and bifurcates into its terminal sensory branches, the larger supraorbital nerve laterally and the smaller supratrochlear nerve medially.
The supraorbital nerve courses anteriorly between the levator palpebrae superioris and the periosteum of the orbital roof, continues anteromedially towards the trochlea, exits the orbit through the supraorbital foramen (or notch in some cases), and finally curves superiorly into the forehead before branching into its medial and lateral branches. Anesthesia of the supraorbital nerve blocks the conjunctiva and skin of the upper eyelid as well as the ipsilateral upper forehead as far back as the lambdoidal suture (Figs. 9.3 and 9.4). It also supplies sensory fibers to the mucous membrane of the frontal air sinuses.
Fig. 9.3
Cutaneous innervation by the trigeminal nerve and cervical plexus
Fig. 9.4
Cutaneous innervation by the trigeminal nerve, cervical plexus, and dorsal nerve roots
The supratrochlear nerve traverses the orbit along the same plane as the supraorbital nerve but just medial to it, passes over the trochlea, and enters the front of the face via the frontal notch (or just medial to the supraorbital foramen or notch). It then courses superiorly into the forehead. As it passes through the orbit, the nerve sends a branch to join the infratrochlear branch of the nasociliary nerve. This nerve supplies sensation to the medial part of the upper eyelid, the conjunctiva, and the lower-middle forehead (Fig. 9.4).
The nasociliary nerve is the intermediate branch (in terms of size) of the frontal nerve and passes obliquely through the orbit to reach its medial aspect. Its anterior and posterior ethmoidal branches pass through the ethmoidal foramina to enter the cranium. The nerve then travels on the upper surface of the cribriform plate of the ethmoid bone underneath the dura mater and descends through a slitlike opening in the crista galli to enter the nasal cavity. The nasociliary nerve supplies two internal nasal branches that innervate the lateral wall of the nasal cavity and the mucous membrane of the front part of the nasal septum.
The ethmoidal branches also supply the mucosa lining the ethmoid air cells. The posterior branch leaves the orbit through the posterior ethmoidal foramen and innervates the sphenoidal sinus. The nasociliary nerve terminates in the infratrochlear and anterior ethmoidal branches; the former supplies the conjunctiva and skin of the medial aspect of the upper eyelid, while the latter supplies the anterior and middle ethmoid cells and then sends two internal branches that innervate the mucous membranes of the front part of the nasal septum and lateral nasal wall. The anterior ethmoidal branch eventually exits the face as the external nasal nerve, which innervates the skin of the nasal ala and apex of the nose.
9.1.2 Maxillary Nerve (V2 Division of the Trigeminal Nerve, Pure Sensory)
The maxillary nerve, like the ophthalmic, also contains purely sensory fibers. It exits the skull through the foramen rotundum, courses beneath the skull base anteriorly, and crosses over the upper part of the pterygopalatine (sphenopalatine) fossa (medial to the lateral pterygoid plate on each side) before entering the orbit through the inferior orbital fissure as the infraorbital nerve (Fig. 9.5).
Fig. 9.5
Anatomy of the maxillary and mandibular nerves
Suspended from the nerve as it courses over the pterygopalatine fossa is the parasympathetic pterygopalatine, or more commonly called sphenopalatine ganglion, which lies within the fossa. Several branches of the maxillary nerve appear to arise from the pterygopalatine ganglion; however, the nerve fibers within them simply pass through the sphenopalatine ganglion without synapsing. Although they seem to be branches of the ganglion, they are in reality branches of the maxillary nerve just coursing through the ganglion and very intimately associated with it.Related posts:
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