Sensory and Facial Motor Anatomy
Although the importance of CNs II, III, IV, and VI is obvious, CN V and CN VII also have important impacts on normal ophthalmic function and are frequently involved in neuro-ophthalmic disorders. For example, proper functioning of CN V is essential for preventing corneal damage. In addition, complete loss of corneal sensation may be accompanied by abnormal corneal epithelial growth (neurotrophic keratitis associated with loss of neural secreted growth factors).
Trigeminal Nerve (CN V)
The sensory nerve terminates within the trigeminal nucleus. The nuclear complex of CN V extends from the midbrain to the cervical spinal cord and includes a main sensory nucleus, a mesencephalic nucleus, and a spinal nucleus (Fig 1-36). The main sensory nucleus is located within the pons lateral to the motor nucleus (the most rostral portion of the trigeminal complex except for the mesencephalic nucleus) and receives light-touch information from the skin of the face and the mucous membranes. The mesencephalic nucleus serves proprioception and deep sensation from the facial muscles, including those of mastication and the EOMs. In addition, a spinal nucleus extends caudally to the level of the C4 vertebra, receiving pain and temperature information. The various sensory nuclei of CN V project to the contralateral thalamus and from there to the postcentral gyrus. The motor nucleus of CN V lies in the pons, medial to the sensory nucleus. The motor nucleus sends signals to the muscles of mastication (temporalis, pterygoid, and masseter), the tensor tympani (which dampens sound by tensing the tympanic membrane within the middle ear as a reflex response to loud noises), the tensor veli palatini (which orients the uvula), the mylohyoid muscle, and the anterior belly of the digastric muscle (both of which are strap muscles in the neck).
Figure 1-36 Diagram of the central pathways and peripheral innervation of CN V. (Used with permission from Kline LB. Neuro-
Ophthalmology Review Manual. 6th ed. Thorofare, NJ: Slack; 2008:174.)
The fascicles of CN V enter the brainstem ventrally in the pons and extra-axially traverse the subarachnoid space to penetrate the dura just over the petrous pyramid. Within the subarachnoid space, the trigeminal root often comes in contact with the superior cerebellar artery. This proximity may be a cause of trigeminal neuralgia (atypical facial pain, discussed in Chapter 12) and is the anatomical basis for microvascular decompression. The 3 divisions of CN V synapse in the trigeminal (gasserian) ganglion, located in an extradural space at the floor of the middle cranial fossa (Meckel cave) (Fig 1-37).
Figure 1-37 Lateral view of the orbit, showing its sensory nerves. (Illustration b y Dave Peace.)
The ophthalmic division (V1) is the most anterior branch exiting the trigeminal ganglion. It runs forward within the lateral wall of the cavernous sinus just below CN IV. As it approaches the superior orbital fissure extradurally, it divides into 3 major branches: lacrimal, frontal, and nasociliary. In addition, small branches innervate the dura of the anterior middle cranial fossa, including the cavernous sinus, the parasellar region, the tentorium, and the dura of the petrous apex. These branches also innervate the floor of the anterior cranial fossa, including the falx and the major blood vessels at the skull base.
The lacrimal and frontal nerves enter the orbital apex outside the annulus of Zinn. At its terminus, the frontal nerve divides into supraorbital and supratrochlear branches, which innervate the forehead, frontal sinus, and upper eyelid (including the conjunctiva). The lacrimal nerve also runs anteriorly in the superior lateral orbit just above the lateral rectus to innervate the lacrimal gland and some skin just superotemporal to the orbit. The nasociliary branch is the only branch entering the intraconal space through the annulus of Zinn. The nasociliary branch runs through the ciliary ganglion and anteriorly to innervate the globe through the short and long posterior ciliary nerves. Prior to reaching the globe, branches from the nasociliary division pass through the anterior and posterior ethmoidal foramina to innervate part of the ethmoidal sinuses, the lateral wall of the nose, and the skin of the nose to the nasal tip. This co-innervation of the globe and the nasal skin is the reason behind the development of the Hutchinson sign in patients with zoster ophthalmicus.
The maxillary division (V2) runs forward at the inferior lateral base of the cavernous sinus to enter the foramen rotundum, located just below the superior orbital fissure. Just before entering the canal, V2 gives off the middle meningeal nerve, which supplies the dura of the lateral middle cranial fossa. On the anterior end of the foramen rotundum, V2 enters the pterygomaxillary area. Two large pterygopalatine nerves supply sensation to the nasopharynx, hard and soft palate, and portions of the nasal cavity. Posterior alveolar nerves supply sensation to the upper gums and molars. The zygomatic nerve enters the orbit through the inferior orbital fissure and divides into the zygomaticofacial and the
zygomaticotemporal nerves, which supply sensation to the lateral face (see Fig 1-37). The maxillary nerve continues anteriorly within a canal between the orbit above and the maxillary sinus below to exit through the infraorbital foramen (as the infraorbital nerve) just below the inferior orbital rim. It subsequently divides into palpebral, nasal, and labial branches. The sensation of the cheek as well as the lower eyelid and upper teeth and gums is provided by this division.
The mandibular division (V3) enters through the foramen ovale, lateral to the foramen lacerum and medial to the foramen spinosum (carrying the MMA). V3 innervates the skin of the jaw and carries the motor division of the trigeminal nerve to the muscles of mastication and neck. Motor paralysis results in contralateral deviation of the jaw when it is closed (weakness of the temporalis) and ipsilateral deviation when protruded (because of weakness in the lateral pterygoid).
Facial Nerve (CN VII)
The facial nerve (CN VII) is responsible for the movement of the facial muscles. Voluntary facial movements originate along with other motor activity in the precentral gyrus. White matter tracts pass through the internal capsule and cerebral peduncles along with the other corticobulbar fibers. The motor neurons destined for the upper face receive information from both sides (bilateral innervation), whereas the lower facial musculature receives information only from the contralateral cortex (Fig 1- 38). The CN VII nucleus receives additional information from basal ganglia extrapyramidal connections, which are largely responsible for involuntary blinking. Abnormal blinking, as present in basal ganglia disorders such as Parkinson disease, is probably mediated through alterations in inhibition of the supranuclear control of the blink reflex.
