Session 8: Oculomotor, Trigeminal and Facial Nerves Flashcards
Give a quick recap of the cranial nerves and foramina (general)
The components of the 12 cranial nerves pass in and out of the cranial cavity through the foramina in the base of the skull
[*] The foramina are located in the anterior, middle and posterior cranial fossae.
The 12 cranial nerves all contribute to the innervation of the skin, viscera and muscles of the head and neck.
[*] Cranial nerves are bundles of sensory fibres or motor fibres that innervate muscles or glands, carry impulses from sensory receptors or have a combination of motor and sensory fibres.
[*] They are called cranial nerves because they emerge through foramina or fissures in the cranium, and are covered by tubular sheaths derived from the cranial meninges.
Nerves derived from the cervical plexus innervates a portion of the skin of the region.
Describe the Oculomotor Nerve. What types of fibres does it carry?
The oculomotor nerve (III) is of significance clinically owing to anatomical structures it innervates. It carries
- Somatic (motor) fibres to all extra-ocular muscles apart from the LR and SO.
- Autonomic (parasympathetic) fibres to the pupil of the eye
- Autonomic fibres to the eyelid
Competent assessment of oculomotor nerve function requires a secure understanding of anatomical connections of this nerve. This is because the oculomotor nerve does not control its effector organs on its own. Instead, it shares them with other cranial nerves. It is one of the most heavily tested nerves in neurological examination.
[*] It is a mixed nerve: it exerts its function through either the somatic division of the nervous system or through its autonomics.
[*] Regarding its actions, the oculomotor nerve contributes to the control of some eye movements (apart from LR6 and SO4), determining size of the pupil of each eye and also the extent of opening of the eye.
[*] Regarding its role in the control of pupillary size, firstly, it is necessary that the **optic nerve is intact as its afferents drive the oculomotor nerve to constrict the pupil via its parasympathetic fibres. **
Differentiate between the “anatomical oculomotor nerve proper” and the “oculomotor nerve that is examined clinically”
[*] The above 2 variants are not the same nerve.
[*] The anatomical oculomotor nerve proper (in red) is a pure somatic efferent nerve. It supplies:
- All extra-ocular muscles except lateral rectus and superior oblique
- Somatic muscle that opens the upper eyelid (levator palpebrae superioris)
[*] The oculomotor nerve originates from the anterior aspect of the midbrain. It moves anteriorly, passing below the posterior cerebral artery, and above the superior cerebellar artery. The nerve pierces the dura mater and enters the lateral aspect of the cavernous sinus. Within the cavernous sinus, it receives sympathetic branches from the internal carotid plexus. These fibres do not combine with the oculomotor nerve – they merely travel within its sheath.
[*] The nerve leaves the cranial cavity via the superior orbital fissure. At this point, it divides into superior and inferior branches in the orbit.
Describe the superior and inferior branches of the oculomotor nerve + the separate parasympathetic fibres
Superior branch: motor innervation to the superior rectus and levator palpabrae superioris. Sympathetic fibres run with the superior branch to innervate the superior tarsal muscle. The superior (dorsal) branch splits into 2 branches.
Inferior branch: motor innervation to the inferior rectus, medial rectus and inferior oblique. Parasympathetic fibres to the ciliary ganglion, which ultimately innervates the sphincter pupillae and ciliary muscles. The inferior (ventral) branch splits into 3 branches.
[*] Parasympathetic functions:
- Sphincter pupillae: constricts the pupil, reducing the amount of light entering the eye
- Ciliary muscles: contracts, causes the lens to become more spherical and thus more adapted to short range vision
- The parasympathetic fibres arise from the Edinger-Westphal Nucleus travel in the inferior branch of the oculomotor nerve. Within the orbit, they branch off and synapse in the ciliary ganglion. The fibres are carried from the ganglion to the eye via the short ciliary nerves.
Why are parasympathetic fibres not considered strictly speaking part of the oculomotor nerve?
Although parasympathetic fibres to the eye travel in the oculomotor nerve, strictly speaking, they arise from a separate brain nucleus and are not part of the oculomotor nerve. Instead they “hitch-hike” or “joy-ride” or “take advantage” of the oculomotor nerve as a means to reach their targets. Inevitably, damage to the oculomotor nerve is likely to catch them too (but not necessarily all the time)
What does complete palsy of the “anatomical oculomotor nerve proper” lead to?
Complete palsy of the “anatomical oculomotor nerve proper” leads to denervation of most muscles that move the eye and denervation of the main muscles that keeps the upper eyelid open => the eye on the affected side assumes a down and out position due to the unopposed actions of lateral rectus and superior oblique muscles, drooping of the upper eyelid (as a result of paralysis of levator palpebrae superioris). This results in an oculomotor nerve palsy **without pupillary involvement. **
What is a dilated pupil due to?
[*] A dilated pupil results from palsy of autonomic parasympathetic fibres that associate themselves very closely to the oculomotor nerve
- Denervation of the sphincter pupillae muscle of the eye
- Unopposed actions of dilator pupillae muscle supplied by the intact sympathetics
What happens in progressive compressive oculomotor nerve damage?
In progressive compressive oculomotor nerve damage, compression of the parasympathetic fibres before any disruption of the motor fibres can occur since the parasympathetic fibres run on the outside of the nerve. In such a situation, therefore, a patient could present with mydriasis (i.e. a “blown” pupil) with or without lid ptosis before the “down and out” position of the eye is seen.
In the fullness of the compression, ptosis and “down and out” position of the eye will be seen.
Describe the anatomical course of the division of the Oculomotor Nerve Supplying Pupil of the Eye
Division of Oculomotor Nerve Supplying Pupil of the Eye (seen in blue line)
[*] It is the parasympathetic portion of the oculomotor nerve. It forms the outer layer of the oculomotor nerve.
[*] Origin: Edinger-Westphal Nucleus (Midbrain)
[*] Anatomical Landmarks Along Course of Nerve
- Cavernous Sinus
- Uncus (a part of the brain)
- Tentorial Notch
- Ciliary Ganglion (where it ends)
[*] Site of Exit from Cranium: Superior Orbital Fissure
[*] Targets
- Ciliary ganglion
- Short ciliary nerve (post ganglionic fibres that supply the pupil)
How would you determine where the Oculomotor Nerve lesion was?
Determining Sites of Oculomotor Nerve Lesions Based on Presenting Signs
[*] Complete palsy of the oculomotor nerve anywhere proximal to the ciliary ganglion will lead to the following signs
- Displacement of the eye to a down and out position
- Drooping of the upper eyelid
- A dilated pupil
- Loss of accommodation reflex (normally the pupils constrict while fixating on an object being moved from far away to near the eyes)
[*] Complete palsy of the oculomotor nerve anywhere distal to the ciliary ganglion will lead to the following signs
- Displacement of the eye to a down and out position
- Drooping of the upper eyelid
- Pupils of the two eyes will be equal; this is known as pupil-sparing third nerve palsy and e.g. could be caused by diabetes
[*] Damage only to the nerve fibres from Edinger-Westphal Nucleus, but sparing the “anatomical oculomotor nerve proper”
- Unilateral dilation of one pupil
- Loss of accommodation reflex
- Normal oculomotion
- Normal palpebral fissure
What are the causes of an oculomotor nerve lesion?
3 main causes of an oculomotor nerve lesion:
[*] Increasing intracranial pressure – this compresses the nerve against the temporal bone.
[*] Aneurysm of the posterior cerebral artery
[*] Cavernous sinus infection or trauma.
Other pathological causes include diabetes, multiple sclerosis, myasthenia gravis and giant cell arteritis
What fibres do the Trigeminal Nerve carry? Any autonomic fibres?
The trigeminal nerve (V) is the great sensory nerve of the head and neck and is the largest cranial nerve. It is probably the most widely distributed sensory nerve in the head region. It is called the trigeminal nerve owing to 3 separate nerve roots of the brainstem in which it arises.
The first 2 (ophthalmic and maxillary) are entirely sensory but the mandibular nerve also contains motor fibres which innervate the muscles of mastication so the trigeminal nerve is a mixed nerve.
The trigeminal nerve is also a transit for some autonomic nerves from other sources (sympathetic fibres from the superior cervical ganglion) but it does not give rise to autonomic fibres. The autonomic nerves are responsible for sweating and vasomotor function.
[*] Clinical examination is based on testing sensory function (NB distribution of varicella zoster rash - ophthalmic division).
Remember that the skin at the back of the head is supplied by cervical spinal nerve roots and not the trigeminal nerve
Describe the origins of the Trigeminal Nerve
[*] The trigeminal nerve originates from 3 sensory nuclei (mesencephalic, principal sensory, spinal nuclei of trigeminal nerve) and one motor nucleus (motor nucleus of the trigeminal nerve) extending from the midbrain to the medulla.
- At the level of the pons, the sensory nuclei merge to form a sensory root. The motor nucleus continues to form a motor root. These roots are analogous to the dorsal and ventral roots of the spinal cord.
- In middle cranial fossa, the sensory root expands into the trigeminal ganglion. The trigeminal ganglion is located lateral to the cavernous sinus, in a depression of the temporal bone known as the trigeminal cave.
- The peripheral aspect of the trigeminal ganglion gives rise to 3 divisions.
- The motor root passes inferiorly to the sensory root, along the floor of the trigeminal cave. Its fibres are only distributed to the mandibular division.
- The ophthalmic nerve and maxillary nerve travel lateral to the cavernous sinus, exiting the cranium via the superior orbital fissure and foramen rotundum respectively. The mandibular nerve exits via the foramen ovale entering the infra-temporal fossa.
Describe the course of the Ophthalmic Nerve. What does it supply?
[*] The Ophthalmic Nerve exits skull through the Superior Orbital Fissure.
- Arises from the trigeminal, semilunar or Gasserion ganglion
- Sensory only – it supplies the cornea, upper conjunctiva, nasal cavity (anterosuperior), frontal sinus, ethmoid sinus, external nose including bridge of nose, upper eyelids, eyebrow forehead and scalp (up to vertex), lacrimal gland, ciliary body, iris, mucous membranes of the nasal cavities, meninges
- Tested via the Corneal Reflex
- Three branches
- Frontal (gives off supratrochlear and supraorbital branches)
- Nasociliary (gives off sensory root of ciliary ganglion, posterior ethmoidal, long ciliary, infratrochlear, anterior ethmoidal (which gives off terminal branches – external nasal nerves))
- Lacrimal: lacrimal gland, conjunctiva, post ganglionic parasympathetic fibres to lacrimal gland
Describe the course of the Maxillary Nerve. What does it supply?
[*] Maxillary Nerve exits skull through the Foramen Rotundum
- Sensory only – supplies the nasal cavity (posteroinferior), lateral external nose (side of the nose), maxillary sinus, superior hard palate, inferior eyelid, upper lip, cheek, uvula, nasopharynx, upper teeth + gums
- It begins at the middle of the trigeminal ganglion and then leaves the skull through the foramen rotundum, crosses the pterygopalatine fossa, inclines lateralward on the back of the maxilla and enters the orbit through the inferior orbital fissure. It traverses the infraorbital groove and canal in the floor of the orbit and appears upon the face at the infraorbital foramen. There it is called the infraorbital nerve, a terminal branch.
14 terminal branches – can be divided into 4 groups, depending upon where they branch off
- In the cranium: middle meningeal nerve
- From the pterygopalatine fossa: infraorbital, zygomatic (divides into zygomaticotemporal and zygomaticofacial), nasal/nasopalatine, superior alveolar (posterior, middle and anterior), palatine nerves (greater, lesser, nasopalatine), pharyngeal nerve
- In the infraorbital canal: anterior superior alveolar nerve, infraorbital nerve
- On the face: inferior palpebral nerve, superior labial nerve, lateral nasal nerve
Describe the parasympathetic fibres of the trigeminal nerve
- Lacrimal gland: post-ganglionic fibres from the pterygopalatine ganglion (derived from the facial nerve), travel with the zygomatic branch of V2 and then join the lacrimal branch of V1. The fibres supply parasympathetic innervation to the lacrimal gland.
- Nasal glands: parasympathetic fibres are also carried to the mucous glands of the nasal mucosa: Post-ganglionic fibres travel with the nasopalatine and greater palatine nerves (branches of V2)
- Innervation of lacrimal, nasal and palate glands
Describe the course of the Mandibular Nerve
[*] The Mandibular nerve exits the skull through the Foramen Ovale. It is a mixed sensory and motor nerve.
- Sensory fibres arise from the trigeminal, semilunar or Gasserion ganglion
- Motor fibres arise from the fifth nerve motor nucleus (in the pons)
- Sensory: supplies the mucous membranes - gum, buccal mucosa, floor of mouth, lower teeth, lower jaw, lower lip, chin, external ear (front of auricle), anterior two thirds of tongue (general sensation, not taste – special taste is supplied by the chorda tympani, a branch of the facial nerve!)
- Motor: (muscles of mastication) masseter, temporalis, medial and lateral pterygoids, mylohyoid, anterior belly of the digastric, tensor tympani, tensor vele palatine
- Mandibular nerve gives rise to 4 terminal branches in the infra-temporal fossa: buccal nerve, inferior alveolar nerve, auriculotemporal nerve and lingual nerve.
What autonomic ganglia is the Mandibular Nerve associated with?
Associated with 2 parasympathetic ganglia – submandibular ganglion (=> submandibular salivary gland and sublingual salivary gland) and otic ganglion (=> parotid salivary gland)
- Post-ganglionic fibres from the submandibular ganglion (derived from the facial nerve), travel with the lingual nerve to innervate the submandibular and sublingual glands.
- Post-ganglionic fibres from the otic ganglion (derived from the glossopharyngeal nerve, CN IX), travel with the auriculotemporal branch to innervate the parotid gland.
What fibres does CN VII carry?
- The facial nerve (VII) is a complex nerve as it contains motor (somatic efferents, visceral efferents), general sensory afferents (that later join trigeminal afferents) and special sensory afferents and parasympathetic fibres. It exits the skull through the Stylomastoid Foramen.
- When one refers to CN VII, the nerve in question is clear and unambiguous
- When referring to the facial nerve, the nerve in question is unclear, therefore very ambiguous (the motor branch of CN VII is actually the facial nerve).
- It is important thus, to be clear on the implications of using CN VII vs Facial Nerve terminology
What are the roots of CN VII “proper”?
[*] It has 2 separate roots of fibres with separate origins in the brainstem
- A large motor root arises from the facial motor nucleus in the pons
- A smaller sensory root in the pons constituting nucleus solitarious that receives taste inputs from the sensory fibres of the facial nerve with primary sensory neurones in the geniculate ganglion
What are the roots of CN VII?
[*] It has 3 separate roots of fibres with separate origins in the brainstem
- A large motor root arises from the facial motor nucleus in the pons
- A smaller sensory root known as nervus intermedius
It is found in the pons and constitutes part of nucleus solitarious
It receives taste inputs from the sensory fibres of the facial nerve
Facial nerve primary sensory neurones constitute the geniculate ganglion
- Autonomic fibres arising from the superior salivatory nucleus. To purists, this is not a root of the facial nerve but a “hitchhiker” or “joyrider”
Describe the Parasympathetic secretomotor fibres of CN VII
[*] Parasympathetic secretomotor fibres are widely distributed to the glands of the nose, palate and to the submandibular and sublingual salivary glands (the last 2 are supplied via the chorda tympani)
- Apart from the chorda tympani, the greater petrosal nerve carries the parasympathetic fibres of the facial nerve. The greater petrosal nerve arises just distal to the geniculate ganglion, within the facial canal. Making an antero-medial turn from the geniculate ganglion, the nerve exits the temporal bone into the middle cranial fossa. From here it travels through the foramen lacerum to the pterygopalatine fossa. Within this fossa, the greater petrosal nerve forms the pterygopalatine ganglion. Branches from this ganglion then go on to provide parasympathetic innervation to the mucous glands of the oral cavity, nose and pharynx and lacrimal gland.
Describe the Intracranial Course of CN VII
The nerve arises in the pons (an area of the brainstem). It begins as two roots; a large motor root and a small sensory root (the part of the facial nerve that arises from the sensory root is sometimes known as the intermediate nerve).
The two roots travel through the internal acoustic meatus and enter into the facial canal. Within the facial canal, three important events occur
[*] Firstly the two roots fuse to form the facial nerve
[*] Next the nerve forms the geniculate ganglion (a ganglion is a collection of nerve cell bodies).
[*] Lastly, the nerve gives rise to the greater petrosal nerve (parasympathetic fibres to glands), the nerve to stapedius (motor fibres to stapedius muscle), and the chorda tympani (special sensory fibres to anterior 2/3rd tongue).
The facial nerve then exits the facial canal (and the cranium) via the stylomastoid foramen, which is located just posterior to the styloid process of the temporal bone.
Describe the Course of CN VII once it is has left the cranium? What are its terminal branches?
After exiting the skull, the facial nerve turns superiorly to run just anterior to the outer ear. It gives off the posterior auricular nerve, which provides motor innervation to some of the intrinsic and extrinsic muscles of the outer ear. It also supplies the occipital part of the occipitofrontalis muscle.
Immediately distal to this, motor fibres are sent to the posterior belly of the digastric muscle (innervates a suprahyoid muscle of the neck – responsible for raising the hyoid bone) and nerve to the stylohyoid muscle (also responsible for raising the hyoid bone)
The main trunk of the nerve, now termed the motor root of the facial nerve, continues anteriorly and inferiorly into the parotid gland (NB: the facial nerve does not contribute towards the innervation of the parotid gland).
Within the parotid gland, the nerve terminates into 5 Branches (superior to inferior): Temporal, Zygomatic, Buccal, Marginal Mandibular, Cervical (Ten Zulus Buggered My Cat) which are responsible for innervating the muscles of facial expression
What are the possible causes of a Facial Nerve Palsy?
Facial Nerve Palsy: smoothing out of forehead, eyebrow droop and drooping corner of mouth.
[*] Non-Traumatic Causes
- Inflammation of the facial nerve (within the facial canal)
- Infection (parotitis)
- Viral (Herpes)
- Parotitis (S. Aureus)
- Compression
- Parotid Tumour
[*] Traumatic causes
- Temporal Bone fractures
- Stabbing
- Gunshots
- Childbirth – forceps delivery of babies
- Incision of the facial nerve
- Tympanectomy
- Surgical procedures of the infratemporal fossa
[*] Iatrogenic
- Surgery (Parotidectomy)
