A Review of Facial Nerve Anatomy Essay Sample
An intimate cognition of facial nervus anatomy is critical to avoid its accidental hurt during face lift. parotidectomy. maxillofacial break decrease. and about any surgery of the caput and cervix. Injury to the frontlet and fringy inframaxillary subdivisions of the facial nervus in peculiar can take to obvious clinical de?cits. and countries where these nervousnesss are peculiarly susceptible to hurt hold been designated danger zones by old writers. Appraisal of facial nervus map is non limited to its extratemporal anatomy. nevertheless. as many clinical de?cits originate within its intratemporal and intracranial constituents. Similarly. the facial nervus can non be considered an entirely motor nervus given its parts to savor. otic esthesis. sympathetic input to the in-between meningeal arteria. and parasympathetic excitation to the lacrimal. submandibular. and sublingual secretory organs.
The configuration of de?cits ensuing from facial nervus hurt is correlated with its complex anatomy to assist set up the degree of hurt. predict recovery. and steer surgical direction. KEYWORDS: Extratemporal. intratemporal. facial nervus. frontal nervus. fringy inframaxillary nervus he anatomy of the facial nervus is among the most complex of the cranial nervousnesss. In his initial description of the cranial nervousnesss. Galen described the facial nervus as portion of a distinguishable facial-vestibulocochlear nervus composite. 1. 2 Although the anatomy of the other cranial nervousnesss was accurately described shortly after Galen’s initial descriptions. it was non until the early 1800s that Charles Bell distinguished the motor and centripetal constituents of the facial nervus.
Facial nervus anatomy is categorized in footings of its relationship to the braincase or temporal bone ( intracranial. intratemporal. and extratemporal ) or its four distinguishable constituents ( branchial motor. splanchnic motor. general sensory. and particular sensory ) . The plastic sawbones bene?ts from a basic cognition of the intracranial and intratemporal constituents of the facial nervus to assist place facial nervus pathology and distinguish extratemporal from facial nervus lesions at other anatomic locations. Similarly. a cognition of the four distinguishable constituents of the facial nervus reminds the sawbones that the facial nervus is composed non entirely of voluntary motor ?bers but besides of parasympathetics to the lacrimal. submandibular. and sublingual secretory organs ; centripetal excitation to portion of the external ear ; and parts to savor at the anterior two tierces of the lingua.
INTRACRANIAL ANATOMY OF THE FACIAL NERVE Voluntary control of the branchial subdivision of the facial nervus is initiated intracranially by supranuclear inputs originating from the intellectual cerebral mantle projecting to the facial karyon. These cortical inputs are arranged with forehead representation most rostral and palpebras. midface. and lips consecutive caudal to this. 5 The pyramidal system is composed of corticobulbar piece of lands that project voluntary. ipsilateral cortical inputs via the knee of the internal capsule to the 7th cranial nervus karyon of the pontine tegmentum. Cell organic structures of the upper facial motor nervousnesss giving rise to the frontal subdivision receive bilateral cortical inputs. and nerve cells to the balance of the facial karyon receive contralateral cortical excitation. Spontaneous facial motions are centrally transmitted via the extrapyramidal system. which involves diffuse axonal connexions between multiple parts including the basal ganglia. amygdaloid nucleus. hypothalamus. and motor cerebral mantle. The extrapyramidal system regulates resting facial tone and stabilizes the voluntary motor response ; hypothalamic inputs modulate the emotional response.
The facial karyon contain the cell organic structures of facial nervus lower motor nerve cells. These cell organic structures receive supranuclear inputs via synapse formation with axons going through both the pyramidal and extrapyramidal systems. The con?uence of these postsynaptic lower motor nerve cells round the abducents nucleus and organize the facial colliculus at the ?oor of the 4th ventricle ( Fig. 1 ) . The branchial motor subdivision of the facial nervus exits the brain-stem at the cerebellopontine angle. where it is joined by the less robust nerve intermedius. These nervousnesss resemble the nervus roots of the spinal cord in that they are barren of epineurium but covered in pia mater and bathed in cerebrospinal ?uid. The branchial motor nerve–nervus intermedius complex travels about 15. 8 millimeter from the cerebellopontine angle before it begins its class within the temporal bone. 6 The parasympathetic constituent of the facial nervus is composed of splanchnic motor ?bers whose arising cell organic structures are scattered within the pontine tegmentum and jointly known as the superior salivatory karyon.
These karyons are in?uenced by nonvoluntary hypothalamic inputs. Cell bodies interceding the general centripetal map of the facial nervus reside in the general centripetal trigeminal karyon of the rostral myelin and receive sensory nerve inputs from projections of the geniculate ganglion within the temporal bone. The gustatory karyon within the pontine tegmentum besides receives particular centripetal inputs from the geniculate ganglion. These urges. nevertheless. were ab initio generated by gustatory sensation receptors in the anterior two tierces of the lingua. Ascending centripetal inputs from the trigeminal and gustative karyons are in?uenced by the thalamic karyon prior to their response within the centripetal cerebral mantle. Patients with supranuclear lesions affecting the motor cerebral mantle or internal capsule present clinically with loss of volitional control of the lower facial muscular structure but relentless facial tone and self-generated facial motions.
Voluntary control of the forehead muscular structure is retained because the upper halves of the facial karyon. which are populated by frontal nervus subdivision cell organic structures. receive bilateral cortical excitation and so non all input is lost after a one-sided supranuclear lesion. Voluntary lip. nose. and cheek motions. nevertheless. are lost. It should besides be noted that facial musculus disfunction caused by cardinal hurt is often accompanied by motor disfunction of the lingua and manus given the propinquity of these cortical control centres within the motor cerebral mantle and internal capsule. Re?ex arcs affecting the facial karyon. such as the corneal wink ( trigeminalfacial ) . are preserved following supranuclear lesions.
INTRATEMPORAL FACIAL NERVE
The intratemporal anatomy of the facial nervus has been extensively studied to minimise morbidity in skull base surgery while maximising exposure. In add-on. its intraneural topography has been investigated in corpses and carnal theoretical accounts. 7–9 Whereas the topography in certain carnal theoretical accounts. such as the cat. is shown to be consistent. the topography of the intratemporal facial nervus in the human is extremely variable and spacial relationships to other intratemporal constructions such as the carotid arteria and sigmoid fistula are besides variable. 10–13 The ramification form of the intratemporal facial nervus. nevertheless. is moderately consistent. The branchial motor and nervus intermedius constituents of the facial nervus are slackly associated as they enter the internal auditory meatus of the temporal bone. Both the seventh cranial nerve and acoustic nervousnesss enter the temporal bone at the same time with the facial nervus located superior to the acoustic nervus. The facial nervus. along with the acoustic and vestibular nervousnesss. travel 8 to 10 millimeters within the internal audile canal before merely the facial nervus enters the fallopian canal. The fallopian canal consists of labyrinthine. tympanic. and mastoid sections.
The labyrinthine section is the narrowest section and extends 3 to 5 millimeter from the border of the internal audile canal. The geniculate ganglion resides within the distal portion of the labyrinthine section of the facial nervus and gives rise to the ?rst subdivision of the facial nerve—the greater petrosal nerve—which carries splanchnic motor parasympathetic ?bers to the lachrymal secretory organ ( Fig. 2 ) . The external petrosal nervus is a 2nd. threadlike subdivision that is on occasion present and provides sympathetic excitation to the in-between meningeal arteria. The lesser petrosal nervus is the 3rd subdivision widening from the geniculate ganglion. This subdivision typically carries parasympathetic ?bers associated with the glossopharyngeal nervus ( 9th cranial nervus ) to the parotid secretory organ. Salivary ?ow from the parotid secretory organ may non. nevertheless. be interrupted by lesions to the glossopharyngeal nervus.
In fact. parasympathetic ?bers going along the nervus intermedius of the facial nervus can short-circuit the glossopharyngeal subdivision to the parotid and supply an alternate beginning of parasympathetic excitation to keep salivary ?ow. Compaction of the facial nervus within the labyrinthine section is peculiarly common given the canal’s narrow dimensions. The facial nervus occupies up to 83 % of the labyrinthine canal cross-sectional country compared with merely 64 % of the more distal mastoid country. 14 The junction of the labyrinthine and tympanic constituents of the fallopian canal is formed by an acute angle. and shearing of the facial nervus normally occurs as the nervus traverses this knee. 8 The tympanic or horizontal section extends 8 to 11 millimeters through the temporal bone. The midtympanic canal represents a 2nd part of fallopian canal narrowing and is a less common point of nervus compaction compared with the narrow labyrinthine section. 15 The tympanic section connects with the mastoid section at a 2nd knee.
The voluntary motor constituent of the facial nervus exits the cerebellopontine angle with the nervus intermedius before come ining the porous acusticus. The facial nervus traverses the labyrinthine section before come ining the geniculate ganglion. The greater petrosal. external petrosal. and lesser petrosal nervousnesss are given off at this degree. The temporal or horizontal section forms the 2nd constituent of the intratemporal facial nervus and is located merely distal to a crisp knee formed at the distal geniculate ganglion. A 2nd knee separates the temporal and mastoid sections of the intratemporal facial nervus. The general centripetal subdivision of the facial nervus is given off at this degree and often travels with the general centripetal subdivision of the pneumogastric nervus ( Arnold’s nervus ) and gives esthesis to the external ear. The nervus to stapedius is a motor nervus that helps dampen loud sounds. The chorda kettle is the last subdivision of the intratemporal facial nervus and is the terminal subdivision of the nervus intermedius.
Wider cross-sectional country than the other sections. and the facial nervus gives off three subdivisions within this part. The nervus to the stapedius is the ?rst subdivision and innervates the stirrups musculus of the interior ear. Because the cell organic structures of this motor nervus are non located in the facial karyon. patients with inborn ? facial paralysiss such as Mobius syndrome retain excitation to the stirrups when the other facial mimetics are paralyzed. 8 The centripetal subdivision of the facial nervus is typically the 2nd subdivision. Ramsay Hunt ?rst noted this general sensory nervus in 1907 when patients showing with facial palsy related to herpes shingles besides demonstrated a vesicular eruption limited to parts of the external ear. 16 Ten cadaverous temporal bone dissections revealed a little subdivision off the perpendicular constituent of the intratemporal facial nervus that arced laterally and inferiorly to provide the buttocks and inferior external auditory canal. Tumor encroachment upon this centripetal nervus. which is thought to consist 10 to 15 % of the nerve cells within the intratemporal facial nervus. 17 consequences in hypoesthesia of the external ear canal and is known as Hitselberger’s mark. after the doctor who described it.
The general centripetal subdivision of the facial nervus travels with Arnold’s nervus. a centripetal subdivision of the pneumogastric nervus that exits the jugular hiatuss and so joins the class of the facial nervus merely distal to the nervus to the stapedius subdivision. 8 The chorda kettle is the terminal extension of the nervus intermedius. It branches off the facial nervus in the distal tierce of the mastoid section and runs between the bonelets of the in-between ear before go outing the tympanic pit through the temporal bone at the petrotympanic ?ssure. It joins the linguistic subdivision of the trigeminal nervus to supply parasympathetic excitation to the submandibular and sublingual secretory organs. Particular centripetal sensory nerves from the anterior two tierces of the lingua besides travel with the chorda kettle. and on juncture the centripetal subdivision of the facial nervus travels with the chorda kettle alternatively of posteriorly to the chief facial nervus bole. Advocates of this technique note that harm to a little subdivision of the facial nervus during the initial geographic expedition is far less lay waste toing than an accidental hurt to the full motor bole. However. these peripheral subdivisions are more dif?cult to place because of their smaller size and a deficiency of consistent landmarks.
The arborization of the extratemporal facial nervus typically begins within the substance of the parotid secretory organ and finally gives rise to the cervical. fringy mandibular. buccal. zygomatic. and frontal ( or temporal ) nervus subdivisions. Davis et al dissected 350 cadaverous facial halves and were the ?rst to categorise the ramification form of the facial nervus into six distinguishable forms. 20 The facial nervus bole typically gave rise to superior and inferior divisions. The fringy mandibular and cervical subdivisions of the facial nervus were entirely derived from the inferior division. whereas the buccal subdivision ever received some part from the inferior division and either no or a variable part from the superior division ( Fig. 3 ) . The frontal subdivision systematically represented a terminal subdivision of the superior division of the facial nervus bole. Baker and Conley reviewed the extratemporal facial nervus anatomy in 2000 parotidectomy instances. 21 Their ?ndings suggested that the facial nervus ramifying form was more variable than that noted in Davis’ cadaverous surveies. including the presence of a facial nervus bole trifurcation with a direct buccal subdivision in a few cases.
EXTRATEMPORAL FACIAL NERVE
The extratemporal constituent of the facial nervus starts when the facial nervus exits the stylomastoid hiatuss. In the grownup. it is protected laterally by the mastoid tip. tympanic ring. and inframaxillary ramus. whereas in kids younger than 2 old ages it is comparatively super?cial. Postauricular scratchs in this younger population must be carefully planned because the bole of the facial nervus is a hypodermic construction at this degree. After go outing the stylomastoid hiatuss. the facial nervus gives off motor subdivisions to the posterior abdomen of digastric. stylohyoid. and the superior auricular. posterior otic. and occipitalis musculuss. The facial nervus so travels along a class front tooth to the posterior abdomen of the digastric and sidelong to the external carotid arteria and styloid procedure before spliting into its chief motor subdivisions at the posterior border of the parotid secretory organ. The facial nervus bole is normally identi?ed about 1 cm deep and merely inferior and median to the tragal arrow. The parotid and super?cial musculoaponeurotic system ( SMAS ) can so be carefully divided to expose the facial nervus for facial nervus Reconstruction.
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A REVIEW OF FACIAL NERVE ANATOMY/MYCKATYN. MACKINNON
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