N. VI Abducens nerve – Nervus abducens
Peripheral nervous system
The abducent nerve originates at the border between the pons and the medulla oblongata, the medullopontine sulcus (sulcus bulbopontinus). It travels ventrolaterally on the internal surface of the base of the skull, penetrates the meninges and runs medially within the cavernous sinus. It reaches the orbit through the superior orbital fissure and common tendinous ring and innervates the lateral rectus, which causes the eyeball to abduct.
Dorello canal / canal of abducent nerve is a passage between the apex of the petrous part of the temporal bone and the petrosphenoideal ligament of Gruber. It contains the abducent nerve and inferior petrosal sinus. The abducent nerve is in an extradural position as it travels in this canal.
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• 1 Abducens/abducent nerve
The gustatory nucleus is located in the rostral part of the solitary tract. It receives taste fibres from the facial nerve and sends impulses to the thalamus and cortex.
Nucleus
• 2 Nucleus of abducens nerve
(nucleus nervi abducentis) – a somatomotor nucleus – is located in the pons, inferior to the facial colliculus – innervates the lateral rectus
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Pons
Course
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1. emerges from the medullopontine sulcus between the pons and medulla oblongata 2. leaves the brainstem on the ventral side 3. runs on the base of the skull, penetrates the dura mater then runs through Dorello canal to enter the cavernous sinus 4. enters the orbit via the superior orbital fissure and common tendinous ring
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Middle level of orbit, right side
N. VII Facial nerve – Nervus facialis
The facial nerve develops from the 2nd pharyngeal arch. It innervates all the facial muscles. It emerges from the pons near the cerebellopontine angle as two roots: the facial motor root and the intermediate nerve. The facial motor root carries somatomotor fibres and the intermediate nerve carries visceromotor and special sensory fibres. The two roots run in the internal acoustic meatus and then pass through the facial nerve canal where they unite. The facial nerve leaves the skull through the stylomastoid foramen. It gives off its terminal branches within the substance of the parotid gland. Nuclei
• 1 Motor nucleus of facial nerve (nucleus nervi facialis)
– a somatomotor nucleus for innervation of all facial muscles, platysma, stylohyoid and posterior belly of the digastric 2 Superior salivatory nucleus (nucleus salivatorius superior) – a visceromotor nucleus for parasympathetic innervation of the lacrimal gland, nasal glands, palatine glands, nasopharyngeal glands, submandibular gland, sublingual gland and lingual glands 3 Gustatory nucleus (nucleus gustatorius) – a special sensory nucleus – the rostral part of the nuclei of the solitary tract – receives impulses from the chorda tympani 4 Spinal nucleus of trigeminal nerve 1 (nucleus spinalis nervi trigemini) – somatosensory fibres from the facial nerve 2 synapse in this nucleus
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Peripheral paralysis of the facial nerve may arise from a lesion at any point of the nerve. It presents as dysfunction of both the upper and lower branches, so that one whole half of the face is affected. Apart from paresis of the orbicularis oris it has other manifestation, including:
Xerostomia – dryness of the mouth due to dysfunction of the sublingual and submandibular glands.
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Pons
3 4
Central paralysis of the facial nerve presents as a drop of the contralateral angle of the mouth. However, innervation of both eyelids remains unaltered. The most common cause is a stroke.
Xerophthalmia – conjunctival and corneal dryness caused by dysfunction of the lacrimal gland. It increases the risk of corneal scaring and subsequent blindness and eye infections.
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1 Facial nerve – somatomotor fibres 2 Nervus intermedius 2.1 Visceromotor fibers (for glands) – greater petrosal nerve, chorda tympani 2.3 Special sensory fibers (taste) – chorda tympani, greater petrosal nerve
Unilateral paresis of the abducent nerve manifests as convergent strabism and diplopia (double vision) when looking to the side.
Lagophthalmus – an inability to close the eye caused by dysfunction of the orbicularis oculi.
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Parts
Clinical notes
Hyperacusis – pain when hearing loud sounds due to an impaired stapedial reflex.
Medulla oblongata
Hypogeusis – decreased perception of taste caused by dysfunction of the chorda tympani.
Medulla spinalis
If the facial nerve is transsected (e.g. in parotidectomy), the lesion can be bridged by a graft from either the sural or hypoglossal nerve.