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Libreros-Jiménez, H.M.; Manzo, J.; Rojas-Durán, F.; Aranda-Abreu, G.E.; García-Hernández, L.I.; Coria-Ávila, G.A.; Herrera-Covarrubias, D.; Pérez-Estudillo, C.A.; Toledo-Cárdenas, M.R.; Hernández-Aguilar, M.E. Cranial Nerve VII: Facial Nerve. Encyclopedia. Available online: https://encyclopedia.pub/entry/53396 (accessed on 08 July 2024).
Libreros-Jiménez HM, Manzo J, Rojas-Durán F, Aranda-Abreu GE, García-Hernández LI, Coria-Ávila GA, et al. Cranial Nerve VII: Facial Nerve. Encyclopedia. Available at: https://encyclopedia.pub/entry/53396. Accessed July 08, 2024.
Libreros-Jiménez, Hugo M., Jorge Manzo, Fausto Rojas-Durán, Gonzalo E. Aranda-Abreu, Luis I. García-Hernández, Genaro A. Coria-Ávila, Deissy Herrera-Covarrubias, César A. Pérez-Estudillo, María Rebeca Toledo-Cárdenas, María Elena Hernández-Aguilar. "Cranial Nerve VII: Facial Nerve" Encyclopedia, https://encyclopedia.pub/entry/53396 (accessed July 08, 2024).
Libreros-Jiménez, H.M., Manzo, J., Rojas-Durán, F., Aranda-Abreu, G.E., García-Hernández, L.I., Coria-Ávila, G.A., Herrera-Covarrubias, D., Pérez-Estudillo, C.A., Toledo-Cárdenas, M.R., & Hernández-Aguilar, M.E. (2024, January 03). Cranial Nerve VII: Facial Nerve. In Encyclopedia. https://encyclopedia.pub/entry/53396
Libreros-Jiménez, Hugo M., et al. "Cranial Nerve VII: Facial Nerve." Encyclopedia. Web. 03 January, 2024.
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The twelve cranial nerves play a crucial role in the nervous system, orchestrating a myriad of functions vital for our everyday life. These nerves are each specialized for particular tasks. Cranial nerve VII, the facial nerve, facilitates facial expressions and taste perception.
vision
hearing
tasting
smelling
head
face
brain stem
1. Introduction
The seventh cranial nerve, also known as the facial nerve, is a composite entity encompassing motor, general sensory, special sensory, and visceral components. It originates embryologically from the second pharyngeal arch and emerges ventrolaterally from the lower pons. Significant milestones in the understanding of this nerve include Gabriel Fallopius’ seminal work in 1550 detailing its course through the temporal bone. In 1829, British neurologist Sir Charles Bell made another groundbreaking contribution by distinguishing that, while cranial nerve V handles sensory innervation of the face, cranial nerve VII controls its motor function [1][2][3]. We now know that the facial nerve is responsible for various functions in the head and neck. It innervates the muscles responsible for facial expression, motor fibers of the middle ear, taste receptors in the anterior two-thirds of the tongue, parasympathetic fibers to the salivary glands, and somatic afferent fibers to the external auditory canal and auricle [4][5].
The complex trajectory of the facial nerve begins from its origin in the pons and extends to its terminal distribution in the face. The motor nuclei of this nerve are located beneath the floor of the fourth ventricle. As the nerve extends, its motor fibers encircle the nuclei of the abducens nerve (CN VI). Upon exiting the pons at the cerebellopontine angle, these motor fibers take an anterolateral direction. While traversing the pons, these motor fibers intersect with special visceral sensory fibers originating from the solitary nucleus, responsible for taste sensation. They also intersect with parasympathetic motor fibers that regulate salivary gland function [6]. The nerve enters the temporal bone through the internal auditory meatus. Within the petrous part of the temporal bone, it navigates through the facial canal, which is divided into three segments: labyrinthine, tympanic, and mastoid. Originating from the base of the skull, the nerve exits through the stylomastoid foramen and courses through the parotid gland. The mastoid segment gives rise to the nerve of the stapedius muscle and the chorda tympani. The sensory fibers originating from receptors in the anterior two-thirds of the tongue reach the geniculate ganglion via the chorda tympani. These fibers traverse along the nervus intermedius through the internal auditory meatus and the cerebellopontine cistern, ultimately terminating in the solitary tract nucleus. The parasympathetic fibers of the nervus intermedius originate from the superior salivary nucleus. At the geniculate ganglion, the greater petrosal nerve supplies parasympathetic fibers to the lacrimal gland as well as to the mucosa of the mouth, nose, and pharynx [7][8][9].
In humans, this mixed nerve comprises approximately 7000 fibers, the majority of which are myelinated and range in diameter from 7 to 10 μm. The superior terminal motor branches of the facial nerve traverse the parotid plexus and advance anteriorly through the zygomatic arch towards the frontal, orbicular, and corrugator muscles. Additional branches extend horizontally towards the zygomatic, orbicularis oculi, and adjacent muscles enveloping the buccal region, including the buccinator and masseter muscles. Furthermore, an inferior cervical branch innervates the platysma muscle, which lies superficially and contributes to skin tautness in the neck. This muscle may become visibly prominent during facial gestures. According to traditional anatomical teachings, there are five recognized terminal branches of the facial nerve: the temporal, zygomatic, buccal, marginal mandibular, and cervical branches.
The motor function that supplies the muscles responsible for facial expressions as well as the stapedius muscle in the ear originates from the facial nucleus within the pons. This motor component then courses alongside cranial nerve VI [10]. Predominantly, the study of the seventh cranial nerve focuses on its somatic motor component, which is responsible for innervating muscles present in the face and, to a lesser extent, the neck. In total, this nerve innervates 28 muscles, each with various roles in controlling the movement of the eyelids, eyebrows, lips, mouth, cheeks, and chin [11].
2. Pathophysiological Conditions
The facial nerve is implicated in various diseases, the most common of which is Bell’s palsy. This condition is characterized by facial paralysis, and its etiology often remains idiopathic. Several theories suggest that vascular spasms of the arteries in the facial canal, which supply blood to the nerve, may contribute to this condition. Alternatively, the inflammation and swelling of the nerve within the bony canal may also play a role in the development of Bell’s palsy. Facial paralysis has substantial impacts on both functionality and appearance, resulting in profound psychological and aesthetic challenges. Numerous approaches for relief have been explored, including primary repair and the utilization of nerve grafts or conduits. Tissue engineering has emerged as a pivotal field in the development of synthetic materials that can mimic nerve properties, thus minimizing any further patient harm. Moreover, ongoing research is increasingly examining the potential benefits of incorporating neurotrophic factors or stem cells within or around the repair site to further enhance neuronal recovery [12].
Additionally, other conditions can affect the motor functionality of the seventh cranial nerve. These include tumors infiltrating the temporal bone, temporal bone fractures, Ramsay–Hunt syndrome (characterized by herpes zoster affecting the geniculate ganglion and causing severe facial paralysis along with vesicular rashes in the external auditory canal), acoustic neuromas, basilar artery dilatations due to aneurysms, leprosy, and infectious mononucleosis. The latter has the potential to manifest as sudden-onset, single, or multiple cranial paralyses, with bilateral facial paralysis being the most common combination. Furthermore, the motor function of the seventh cranial nerve can also be impacted by myasthenia gravis.
3. Comparative Features
In other mammals, such as dogs, the facial nerve shares characteristics with its human counterpart. Originating from the rostral and ventral medulla, it controls the muscles responsible for facial expression, taste sensation in the rostral two-thirds of the tongue, and skin sensitivity on the inner surface of the auricle. Typical reactions observed in dogs associated with proper facial nerve function include symmetrical facial expression; normal movements of lips, ears, and eyelids; and the presence of the palpebral reflex. Ears respond to stimulation by moving, and there is an immediate negative taste response. On the sensory level, a behavioral reaction occurs, and the ears contract. Altered mental states or deficits in the seventh cranial nerve also indicate a central vestibular disorder; however, certain polyneuropathies may affect this nerve as well [13].
Rats, serving as a model for rodents, possess a substantial number of facial muscles, amounting to 20 in total, but a debate persists regarding the specific similarities between facial muscles in monotremes and in other mammals [14]. Mammalian facial muscles represent a subgroup of the hyoid muscles, the muscles innervated by the seventh cranial nerve. In strepsirrhines, the facial muscles resemble those in non-primate mammals, such as tree shrews. Notably, there are some differences; strepsirrhines possess a muscle known as the depressor supercilii, which is generally not differentiated in tree shrews. Conversely, strepsirrhines lack two muscles commonly differentiated in these mammals: the zygomatico-orbicularis and the sphincter colli superficialis. In contrast, macaques often lack certain muscles, such as the risorius, anterior auricular, and temporoparietal muscles, which are present in hominids, such as humans. However, they possess muscles that are typically not differentiated in certain hominid groups, for instance, the cervical platysma in orangutans, panins, and humans and the posterior auricular in orangutans.
Lampreys and hagfish are the only representatives of the most ancient branch of vertebrates. The facial nerve in lampreys comprises both motor and sensory fibers and is divided into four primary branches: buccal, hyomandibular, thyroid, and pharyngeal. The buccal branch, located anteriorly, innervates the neuromasts, which are sensory organs situated in the epithelial pit. These neuromasts extend from the front of the eyes to the tip of the snout. It has been documented that this branch fuses with the ophthalmic branch of the trigeminal nerve along its distal course [15]. The efferent nuclei responsible for facial (VII) and octolateral (VIIIeff) functions in adults display significant variability in terms of their spatial arrangement and target destinations. Nonetheless, it has been observed that, in most species, branchiomotor neurons VII (VIIbm) and (VIIIeff) predominantly originate in the r4 region. Consequently, divergent locations found in adult specimens are primarily attributed to variations in the extent of caudal and lateral migration. In lampreys, the VIIbm nucleus can be traced from Mauthner’s cell in caudal r4 to the vicinity just behind the accessory Mauthner cell in caudal r5 across all developmental stages, from early larvae to post-morphic adults. In adult lampreys, the VIIIeff neurons are situated ipsilaterally near both Mauthner’s cell and the VIIbm neurons, suggesting that they derive from the r4 region [16].
References
- Ottaiano, A.C.; Gomez, G.D.; Freddi, A.L. The facial nerve: Anatomy and pathology. Semin. Ultrasound CT MR 2023, 44, 71–80.
- Eisenmenger, L.B.; Wiggins, R.H. Cranial Nerve VII: Facial. In Neuroimaging: Anatomy Meets Function; Agarwal, N., Port, J., Eds.; Springer: Cham, Switzerland, 2018; pp. 197–201.
- Brackmann, D.E.; Fetterman, B.L. Cranial Nerve VII. In Textbook of Clinical Neurology, 3rd ed.; Goetz, C.G., Ed.; Elsevier: Amsterdam, The Netherlands, 2007; pp. 185–198.
- Kircher, M.; Thomas, A.; Leonetti, J. 6 Cranial Nerve VII: Facial Nerve Disorders. In Cost-Effective Evaluation and Management of Cranial Neuropathy, 1st ed.; Babu, S.S., Jackson, N.M., Stuttgart, G., Eds.; Thieme: New York, NY, USA, 2020; pp. 46–48.
- Seilesh, C.; Neal, M. Cost-Effective Evaluation and Management of Cranial Neuropathy, 1st ed.; Thieme: New York, NY, USA, 2020.
- Perry, J.R.; Hasso, A.N. Magnetic resonance imaging of cranial nerve VII. Top. Magn. Reason. Imaging 1996, 8, 155–163.
- Romano, N.; Federici, M.; Castaldi, A. Imaging of cranial nerves: A pictorial overview. Insights Imaging 2019, 10, 33.
- Tsutsumi, S.; Ono, H.; Yasumoto, Y. Visualization of the olfactory nerve using constructive interference in steady state magnetic resonance imaging. Surg. Radiol. Anat. 2017, 39, 315–321.
- Thomas, P.N.; Smirniotopoulos, J.G.; Castillo, M.; Soonmee, C. Imaging of the Brain: Expert Radiology Series, 1st ed.; Elsevier: Amsterdam, The Netherlands, 2012.
- Sanders, R.D. The trigeminal (V) and facial (VII). Cranial Nerves: Head and face sensation and movement. Psychiatry 2010, 7, 13–16.
- Lu, G.N.; Byrne, P.J. Facial nerve and muscle anatomy. In Management of Post-Facial Paralysis Synkinesis; Azizzadeh, B., Nduka, C., Eds.; Elsevier: Amsterdam, The Netherlands, 2022; pp. 1–11.
- Bengur, F.B.; Stoy, C.; Binko, M.; Nerone, W.; Fedor, C.; Solari, M.; Marra, K. Facial nerve repair: Bioengineering approaches in preclinical models. Tissue Eng. Part B Rev. 2022, 28, 364–378.
- Lorenz, M.D.; Coates, J.R.; Kent, M. Handbook of Veterinary Neurology, 5th ed.; Elsevier: Amsterdam, The Netherlands, 2010.
- Diogo, R.; Wood, B.A.; Aziz, M.A.; Burrows, A. On the origin, homologies and evolution of primate facial muscles, with a particular focus on hominoids and a suggested unifying nomenclature for the facial muscles of the mammalia. J. Anat. 2009, 215, 300–319.
- Pombal, M.A.; Megías, M. Development and functional organization of the cranial nerves in lampreys. Anat. Rec. 2019, 302, 512–539.
- Gilland, E.; Baker, R. Evolutionary patterns of cranial nerve efferent nuclei in vertebrates. Brain Behav. Evol. 2005, 66, 234–254.
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