Cranial Nerve 7 – Facial Nerve

@   SVE、GVE、SVA、GSA

@   Nucleus Related with the Motor Root of the Facial Nerve Þ 含Branchio-motor neurons (SVE)，可分為四個細胞群：

1.     Dorsomedial group Þ 支配Posterior auricular m.、Occipital belly of the occipito-frontalis m.。

2.     Ventromedial group Þ 支配Plastysma m.、Stapedius m.。

3.     Intermediate group Þ 支配Orbicularis oculi m.、Upper mimetic facial m. (Frontalis、Corrugator supercilli、Zygomaticus m.)。

4.     Lateral group Þ 支配Buccinator m.、Buccolabial m.。

5.     Other group Þ 支配Stylohyoid m.、Posterior belly of the digastric m.。

@   Nuclei or Ganglion Related with the Intermediate Nerve

(Intermediate nerve = Sensory root : Parasympathetic roots of the facial nerve，位於Facial motor root與Vestibular nerve之間)

1.     Superior salivary nucleus and Lacrimal nucleus Þ 由位在Facial motor nucleus內側的混合小細胞 (GVE neurons) 所組成。

(1)  Superior salivary nucleus Þ intermediate nerve Þ By pass the geniculate ganglion Þ Chorda tympani branch of the facial nerve : Lingual branch of the mandibular division of the trigeminal nerve Þ Synapse in the submandibular ganglion Þ Secretory and vasomotor (Postganglionic) fibers Þ Submandibular and sublingual glands : Mucous membrane and vessels of the mouth and pharynx

(2)  Lacrimal nucleus Þ intermediate nerve Þ By pass the geniculate ganglion Þ Greater petrosal nerve Þ Synapse in the pterygopalatine ganglion Þ Secretory and vasomotor (Postganglionic) fibers via lacrimal branch of the ophthalamic division of the trigeminal nerve Þ Lacrimal gland : Mucosa and vessels of the nasal cavity

2.     Geniculate ganglion (膝狀神經節) Þ 位於External genu of the facial nerve。

(1)  含有味覺的Primary sensory (SVA) neurons，可傳遞舌前三分之二及軟腭味蕾的Gustatory sense (Taste)。Taste buds Þ Lingual branch of the mandibular division of the trigeminal nerve : Chorda tympani branch of the facial nerve以及Greater petrosal nerve Þ Cell bodies in the geniculate ganglion Þ Intermediate nerve Þ Solitary tract Þ Gustatory nucleus (位於Rostral part of the nucleus of the solitary tract) Þ Central tegmental tract Þ Hypothalamus : Parabrachial nucleus Þ Ventral posteromedial, pars parvicellularis (VPMpc) of the thalamus Þ Primary gustatory cortex (= Area 43)

(2)  含有一般感覺的Primary sensory (GSA) neurons，可傳遞Cutaneous sensory impulses from the external auditory meatus and the region back of the ear，經由Intermediate nerve再進入Dorsomedial part of the spinal trigeminal tract，最後終止於Spinal trigeminal nucleus。

@   Lesions of the Facial Nerve (= Bell’s Palsy or Facial Nerve Paralysis)

1.            Lesion of the nerve as it emerges from the stylomastoid foramen (= Peripheral type facial palsy) Þ 受傷側不能皺額頭、閉眼、露齒、縮唇，眼瞼裂張開，鼻唇皺折(法令紋)平坦。受傷側的Corneal reflex消失但對側的Consensual response保留。

2.            Lesion distal to the geniculate ganglion Þ 除上述(1)症狀外，另外Submandibular gland與Sublingual gland不能分泌唾液，聽覺過敏(Hyperacusis)，受傷側的舌前2/3喪失味覺。

3.            Lesion proximal to the geniculate ganglion Þ 除上述(2)症狀外，另外受傷側不能流淚。當創傷痊癒、神經再生後，原先在Chorda tympani nerve內可支配Submandibular ganglion的Preganglionic parasympathetic fibers，很有機會發生Cross reinnervation而進入Greater petrosal nerve去支配Lacrimal gland。因此，在吃飯時Submandibular and sublingual glands受刺激而分泌唾液，另一方面也會流下眼淚，此現象稱之為Syndrome of “crocodile tears”。

4.            Central facial palsy Þ 涉及到Corticobulbar and corticoreticular fibers的受傷。只造成受傷側的對側下半部臉的顏面表情肌之麻痺，尤其是Perioral region。當要求病患做笑的動作時，他無法笑；但是當他說笑話時，卻能笑。【The affected muscles may not respond to voluntary stimulation but may respond to an emotional stimulus.】

5.            Mimetic facial palsy (皮笑肉不笑) Þ 可能涉及到Limbic system的傷害。【The facial muscles may respond to voluntary stimulation but not to emotional stimulation.】

@   Facial Nerve in the PNS Þ Facial nerve與Vestibulocochlear nerve一起鑽入Internal acoustic meatus後，Cochlear nerve微向內側進入Cochlea的底部，Vestibular nerve則穿進Ampullae of the semicircular canals。而Facial nerve仍在Internal acoustic meatus中繼續向前並微向外側走，當抵達Genicuate ganglion時，大約在Middle ear的上外側方，Facial nerve會明顯向後彎曲，然後再往下行，從Stylomastoid foramen穿出顱骨。

1.     Greater petrosal nerve (岩大神經) Þ Pterygopalatine ganglion Þ Lacrimal gland : Mucous membrane of nose and mouth。此神經亦含支配軟腭味蕾的SVA fibers。

2.     To stapedius muscle (鐙骨肌)

3.     Communicating branch with tympanic plexus

4.     Chorda tympani nerve (鼓索神經) Þ Submandibular ganglion Þ Submandibular and Sublingual glands : 舌前三分之二的味覺。

5.     Communicating branch with vagus nerve

6.     Communicating branch with glossopharyngeal nerve

7.     Posterior auricular nerve (耳後神經) Þ To posterior auricular muscle (耳後肌) and occipital belly of occipito-frontalis muscle (枕額肌的枕肌腹)

8.     To stylohyoid m. (莖突舌肌) and posterior belly of digastric m. (二腹肌的後腹)

9.     Terminal branches

(1)  Temporal branch (顳枝) Þ To reach the scalp and part of the eyelid muscles

(2)  Zygomatic branch (顴枝) Þ To reach the eyelids and nose

(3)  Buccal branch (頰枝) Þ To reach the muscles around the mouth

(4)  Mandibular branch (下頜枝) Þ To reach the chin

(5)  Cervical branch (頸枝) Þ To the platysma muscle