Facial nerve anatomy

Surgical anatomy plastic surgery point of view

1. Facial Nerve Anatomy and management of bells palsy Moderarator : Dr Upendra Sharma Presenters: Dr Meghna Dr Shamendra

2. Nuclei of origin Roots: Sensory root (nervous intermedius ) Motor root Course – Intracranial/ Intrapetrous part,Extracranial part. Branches – of Communication of Distribution

3. Nuclei of Origin : Motor Nucleus : Site: situated in reticular formation in caudal part of pons below and in front of abducent nucleus. At the cranial end of the abducent nucleus,the fibres bend abruptly downwards and forwards forming an internal genu (First genu ) And finally emerge at the lower border of the pons through the motor root.

4. Superior salivatory nucleus : situated dorsolateral to caudal part of motor nucleus Gives origin to pre ganglionic secreto motor fibres (parasympathetic) which emerge through the sensory root Sensory Root =misnomer (conveys secretomotor fibres also) Upper part of the nucleus of tractus solitarius : acts as a afferent column Recieves taste sensation from ant 2/3 of the tongue via corda tympani nerve from soft palate through greater petrosal nerve.

5. Course : Divided by stylomastoid foramen into intracranial – intrapetrous part and extracranial part. Intracranial- intrapetrous part : From the lower border of the pons , both roots of facial nerve along with vestibulococlear nerve, labyrinthine artery enter the internal acoustic meatus . Within the Internal acoustic meatus , motor root lodges in a groove on the anterio superior surface of vestibulo coclear nerve and sensory root intervenes between them.

6. At the bottom of the internal acoustic meatus , both roots combine to form trunk and enter the bony facial canal. In the bony facial canal, nerve passes above the vestibule of the inner ear , and reaches the medial wall of epitympanic part of tympanic cavity. In epitympanic part of tympanic cavity, facial nerve bends backwards forming a 2 nd genu (external genu ). At the genu , facial nerve presents an asymetrical swelling known as genicular ganglion(contains cell bodies of pseudo unipolar sensory neurons).

7. From the genu,facial nerve passses into another bony canal above the promontary and fenestra vestibuli and reaches the medial wall of the aditus of mastoid antrum.Here nerve is related to bulging of the lateral semicircular canal. finally nerve passes vertically downwards along the posterior wall of tympanic cavity and leaves the temporal bone through stylomastoid foramen.

8. Extracranial part : After exit from stylomastoid foramen, it changes its direction and passes forwards superficial to the styloid process of the temporal bone and pierces the posterio medial surface of the parotid gland. With in the gland,nerve runs forwards for about 1 cm superficial to retromandibular vein and ext carotid artery and then divides into temporo facial and cervico facial trunks. Temporo facial trunk turns abruptly upwards and subdivides into Temporal Zygomatic branches .

9. The Cervico facial branch passes downward and forward and subdivides into Buccal , marginal mandibular Cervical branches. The five terminal branches radiate like goose feet through the anterior border of the gland and supply the facial muscles. Such branching pattern is known as the pes anerinus .

10. Points of special importance : The stylomastoid foramen lies about 2 cm deep to the middle of the anterior border of the mastoid process.In the new born, stylomastoid foramen lies at the surface of the mastoid bone ,since the mastoid process grows about the 2 nd year after birth.Therefore , incisions behind the auricle are taken carefully in a child in order to avoid injury to facial nerve.

11. Branches: 1) In the facial canal : a) Nerve to stapedius : Hyperacusis b) Chorda tympani nerve : Taste sensations, Preganglionic secretomotor fibres to submandibular and sublingual gland. 2) Below the stylomastoid foramen : a)Posterior auricular nerve : passes behind the auricle. Supplies intrinsic muscles on the cranial surface of the auricle , auricularis posterior and occipital belly of occipitofrontalis .

12. b ) Nerve to post belly of digastric c) Nerve to the stylohyoid muscle 3) In the face : 5 terminal branches: a) Temporal branch :consists of 3 or 4 branches. They pass upwards and forwards in front of the auricle and crosses over the zygomatic arch 3 to 5 cm from lateral orbital margin. After that,they run deep to temporo parietal fascia(sup temporal fascia). Supplies , Auricularis anterior Auricularis superior Upper part of orbicularis occuli Frontalis Corrugator supercilli

13. b) Zygomatic branch : consists of 5 to 8 branches with significant functional overlap. Runs along the zygomatic arch and then deep to parotid messeteric fascia in the same plane as that of parotid duct. Supplies lower part of orbicularis occuli . c) Buccal branch : Consists of superficial and deep parts. Superficial branches : supply procerus Deep branches : subdivide into upper and lower sets. upper buccal : passes forwards above the parotid duct and supply Zygomaticus major Zygomaticus minor Levator anguli oris Levator labi superioris Levator labi superioris alaeque nasi Muscles of the nose – dialator naris , Dialator septi

14. Lower buccal : passes below the parotid duct Supplies Buccinator Orbicularis oris . d) Marginal mandibular branch : consists of 1 to 3 branches .appears first in the neck(2 cm below the ramus ) ,then curves upwards and forwards across the lower border of the mandible at the anterio inferior angle of the masseter and appears in the face after crossing superficial to facial artery and vein. Supplies Risorius Depressor angulli oris Depressor labi inferioris Mentalis

15. e) Cervical branch : consists of single branch. comes out of apex of parotid gland and appears in anterior triangle of neck and Supplies Platysma by entering the muscle at its cranial and middle 1/3rds.point of entry is 2-3 cm caudal to platysmal muscle branch of facial artery.

16. Facial muscles responsible for facial expression: Frowning : Corrugator supercilli forms vertical wrinkles on forehead.Procerus forms transverse wrinkles across the root of the nose. Surprise,horror , fright : Frontalis by elevation of eyebrows and transverse wrinkles of the forehead. Anger : Dialator nares , Depressor septi.They dialate the anterior nasal apertures and depressing the mobile part of the septum.

17. Sadness : Levator anguli oris , Levator labi superioris , zygomaticus minor by accentuating the naso labial furrow. Depressor anguli oris by drawing the angle of the mouth downwards and laterally. Laughing : zygomaticus major which draws angle of the mouth upwards and laterally. Irony : Depressor labi inferioris by drawing the angle of mouth inferiorly and some what laterallly . .

18. Grinning : Risorius by retracting the angle of mouth laterally

19. Disdain : Mentalis , by puckering the chin and protruding the lower lip.

20. Salient features about extra cranial facial nerve : The five divisions of facial nerve are to some extent artificial . There is no distinct seperation between zygomatic and buccal branches As the branches exit from the parotid gland , there may be 8 to 15 branches making up to five divisions. Distal to parotid , there is significant arborization and interconnection of these branches. There is significant functional overlap and frequent duplication of branches providing same function .

21. Each of the divisions consist of multiple branches ,except for cervical division which consists of single branch. There are fewer crossover communications between the frontal branches and marginal mandibular branches. Hence,injuries to these nerves are less likely to recover function in their respective muscles

22. The branches of the facial nerve can be seen where they exit from the anterior margin of the parotid. They have been separated from the surrounding tissue in preparation for stimulation and functional identification

23. Facial musculature: There are 18 paired muscles of the face, including the orbicularis oris . Facial muscles are arranged in layers about the mouth . The most superficial are the depressor anguli oris , zygomaticus minor, and the orbicularis oris . The deepest layer consists of the buccinator , mentalis,and the depressor anguli oris .

24. SMAS : Five layers of the face: Skin; Subcutaneous fat Superficial musculoaponeurotic system (SMAS)–muscle layer; Transparent fascia Branches of the facial nerve. The muscles of facial expression are part of the SMAS layer. In the temporal region, this layer is not muscular but is fascial in quality and is represented by the superficial temporal fascia (or temporoparietal ) fascia.

25. The fourth layer = layer of areolar tissue. In the temporal area, this layer is known as the innominate or subgaleal fascia. In the cheek-parotid– masseteric fascia; and Neck-superficial cervical fascia. Once under the SMAS, the facial nerve branches can be seen through this fourth layer. Though not totally accurate, galea–frontalis–temporoparietal fascia–SMAS– orbicularis oculi–platysma as a single layer, Similiarly , subgaleal fascia–innominate fascia parotid/masseteric fascia–superficial cervical fascia as a single layer.

26. The anatomic layers of the face. Although the quality of the layers differs in various areas of the face, the arrangement of layers is identical. The facial nerve (cranial nerve [CN] VII) branches innervate their respective muscles via their deep surfaces.

27. Facial nerve branches travel deep to the SMAS layer, but at some point these branches turn superficially to innervate the overlying muscles . Ex : Any dissection in the sub-SMAS plane in the cheek necessitates a change of surgical planes at the zygomaticus major muscle to avoid transection of the branch to this muscle. The dissection plane changes from sub-SMAS to subcutaneous by passing over the superficial surface of the zygomaticus major ,thereby preserving its innervation .