Neuro 8 -- Brainstem & Cerebellum 2

Nuclei of cranial nerves

• Heterogenous — different from spinal nerves — as well as in their additional components some cranial nerves have —

  • Special visceral innervation (muscles of branchiomeric origin (pharyngeal arch origin))

  • Special visceral sensation (taste)

  • Special somatic sensation (vestibular & acoustic information)

• Located in the tegmentum of the brainstem (just like reticular formation)

• Responsible for processing new information coming from cranial nerves with a sensory component, or giving rise to an output traveling outside the brainstem

• Grey matter of spinal cord = 4 functional territories, but in the cranial nerves we can find 7 territories — not all throughout the brainstem but just in specific regions depending on the cranial nerve emerging from that point

7 components (functional columns) of cranial nuclei

• General somatic efferent component (GSE) —

  • Innervation of muscles with somatic origin (paraxial mesoderm)

• Special visceral efferent (SVE) (or special branchio-motor component) —

  • Motor innervation of branchiomeric musculature (not smooth muscle, even though visceral)

• General visceral efferent (GVE) —

  • Some motor nuclei of the brainstem can control smooth muscle fibers — parasympathetic preganglionic neurons

• General visceral afferent (GVA) —

  • Sensory information from viscera — from periphery to nervous system

• Special visceral afferent (SVA) —

  • Taste

• General somatic afferent (GSA) —

  • Collects information from somatic components

• Special somatic afferent (SSA) —

  • Relates to the vestibular & cochlear component (hearing & balance)

Oculomotor nerve —

• 3rd cranial nerve — motor neuron — in the mesencephalon

• Emerges through the interpeduncular fossa of the brainstem — thus originating from the midbrain

  • Motor nucleus located in the rostral part of the tegmentum of the midbrain

• Innervates most of the extraocular eye muscles

  • These originate from paraxial mesoderm —thus nerve has a general somatic efferent component

• Also contains parasympathetic fibers to the intrinsic smooth muscles of the eye — thus also hase a gneral visceral efferent component

  • This parasympathetic innervation originates from the Edinger Westphal nucleus — parasympathetic preganglionic nucleus in the tegmentum of the midbrain

    • (The oculomotor nerve originates from 2 nuclei in the midbrain[1]: Oculomotor nucleus. Accessory parasympathetic nucleus (Edinger-Westphal nucleus))

  • Fibres of the GVE innervation along their course will find a ganglion — ciliary ganglion — where they will synapse & then innervate the smooth muscles

• Superior orbital fissure, just like trochlear

Trochlear nerve

• 4th cranial nerve — only nerve originating dorsally — in the mesencephalon

• Only has a general somatic efferent component — innervates one of the extraocular muscles — the superior oblique

• Its motor nucleus is located in the caudal portion of the tegmentum of the midbrain

• The only nerve whose fibers cross after their origin from the brainstem —

  • Cross the superior medullary velum — contralateral innervation

• Exits at the level of the superior orbital fissure

Muscles of oculomotion (extrinsic eye muscles) —

• Oculomotor nerve divides into superior & inferior branches in the anterior part of the cavernous sinus —

  • Superior branch innervates the superior rectus & the levator palpebrae

  • Inferior branch innervates medial rectus, inferior rectus, & inferior oblique

• 7 extrinsic eye muscles — only 2 of which are not innervated by CN3 —

  • Lateral rectus — innervated by abducent nerve (VI)

  • Superior oblique — innervated by trochlear nerve (IV)

Clinical drop — oculomotor nerve palsy

• Condition resulting from damage to oculomotor nerve — most common structural causes —

  • Raised intracranial pressure (compresses the nerve against temporal bone)

  • Posterior communicating artery aneurysm

  • Cavernous sinus infection

  • Trauma

• Other pathological causes —

  • Diabetes, multiple sclerosis, myasthenia gravis, & giant cell arteritis

• Symptoms —

  • Ptosis — drooping upper eyelid, due to paralysis of levator palpebrae superioris & unopposed activity of orbicularis oculi muscle

  • “Down & out” position of eye at rest — due to paralysis of superior, inferior, & middle rectus, and inferior oblique (therefore the unapposed activity of lateral rectus & superior oblique) — leading patient to be unable to elevate, depress, or adduct the eye

  • Dilated pupil — due to unopposed action of dilator pupillae muscle

Clinical drop — examination of the trochlear nerve

• Examined in conjuction with oculomotor & abducens nerves by testing eye movements —

  • Patient is asked to follow a point (commonly the tip of a pen) with their eyes without moving their head

  • Target is moved in an “H-shape,” and the patient is asked to repot any blurriness of vision or diplopia (double vision)

Clinical drop — Palsy of the trochlear nerve

• Causes —

  • Microvascular damage from diabetes mellitus or hypertensive disease

  • Other causes — congenital malformation, thrombophlebitis of the cavernous sinus, and raised intracranial pressure

• Symptoms —

  • Commonly presents with vertical diplopia — exacerbated when looking downwards & inwards (such as when reading or walking down the stairs)

  • They can also develop a head tilt away from the affected side

Trigeminal nerve

• 5th cranial nerve — has both a motor & sensory nucleus — located in the metencephalon (pons)

• Motor nucleus — located in pons, and has a special visceral efferent (SVE) component — innervates the muscles of mastication & others of branchiomeric origin

  • Only branch of trigeminus with a motor component — mandibular branch — masticatory muscles (medial pterygoid, lateral pterygoid, masseter & temporalis), but also innervates the anterior belly of digastric, mylohyoid, tensor veli palatini, and tensor tympani

• Large general sensation territory at level of head & of face — general somatic afferent (GSA) component, thus presnece of a sensory nucleus in the pons belonging to trigeminus

• Sensations (from trigeminus) carried into the brain through 3 nuclei —

  • Pontine/principal (found in the pons)

  • Mesencephalic nucleus (in the midbrain)

  • Bulbospinal or spinal nuclei

• Sensory ganglion of the trigeminus = ganglion of Gasser

  • not in brainstem — root goes to this ganglion then breaks off into the 3 branches

• Exits at the level of the foramen rotundum, ovale, and superior orbital fissure

Abducens nerve

• 6th cranial nerve, innervating lateral rectus muscle (extraocular) — located in metencephalon (pons)

• Only has a somatic general efferent component (GSE)

• Originates from the pons, with its motor nucleus very clos to the midline & very superficial — so much so that it forms a protrusion of the floor of the 4th ventricle

  • Moreover, close to it — motor nucleus of facial nerve, whose fibers run over the abducens nucleus as they exit the brainstem —

  • Loop formed by fibers of facial nerve around nucleus of abducens nerve — Genu of the facial nerve

    • Also protrudes at level of floor of 4th ventricle

Facial nerve

• 7th cranial nerve — originates from the pons (metencephalon) — in particular its motor nucleus can be found in the pons

• Exits from the brainstem at level of supra-olivary groove — in the bulbopontine groove

• Made by a larger bundle of fibers, forming the facial nerve proper, and a smaller bundle of fibers, forming an intermediate component of the facial nerve

• Has many different afferent & efferent components —

  • Special visceral efferent (SVE) (branchiomeric) — innervates muscles of facial expression

    • Linked to a motor nucleus in the tegmentum of pons

  • General visceral efferent (GVE) — specifically a parasympathetic preganglionic component — comtrongs sublingual, submandibular, lacrimal, nose, palate, & pharynx glands

    • Preganglionic parasympathetic neurons located in superior salivatory nucleus — sends fibers out of branstem to reach submandibular, sublingual, and sphenopalatine ganglia

  • Special visceral afferent (SVA) — formed by fibers directed to collect information from the tongue, generating taste — information is relayed to most rostral pat of nucleus of solitary tract — located in medulla oblongata

  • General somatic afferent component (GSA) (quite small) — small cutaneous territory referring to general sensation from skin of the ear, with this information relayed to the bulbospinal trigeminal nucleus

• Geniculate ganglion — sensory ganglion containing fibers for the tongue (SVA)

Clinical relevance — corneal reflex

Involuntary blinking of eyelids — stimulated by tactile, thermal, or painful stimulation of the cornea

• In the corneal reflex, the ophthalmic nerve acts as the afferent limb, detecting the stimuli, while the facial nerve is the efferent limb, causing contraction of the orbicularis oculi muscle

• If the corneal reflex is absent, it is a sign of damage to the trigeminal/ophthalmic nerve, or the facial nerve

Damage to the facial nerve

• Due to the wide range of functions of the facial nerve, damage to it can produce a variety of symptoms, depending on the site of the lesion —

• Intracranial lesions —

  • Ocur during intracranial course (proximal to stylomastoid foramen) — paralyses or severely weakens muscles of facial expression, with other symptoms depending on location of lesion —

    • Chorda tympani — reduced salivation & loss of taste on ipsilateral (same side) 2/3 of tongue

    • Nerve to stapedius — ipsilateral hyperacusis (hypersensitivity to sound)

    • Greater petrosal nerve — ipsilateral reduced lacrimal fluid production

  • Most common cause — infection related to external or middle ear

  • If no definitive cause can be found — labeled Bell’s palsy

• Extracranial lesions —

  • Extracranial course of facial nerve (distal to stylomastoid foramen) — only motor function is affected, resulting in paralysis or severe weakness of facial expression muscles

  • Various causes —

    • Parotid gland pathology — tumour, parotitis, surgery

    • Infection of the nerve (ex. herpes_

    • Compression during forceps delivery (neonatal mastoid process not fully developed)

    • Idiopathic — if no definitive cause — Bell’s palsy

Vestibulocochlear nerve

• 8th cranial nerve — some sensory nuclei located in medulla and some in caudal portion of the pons (myencephalon & metencephalon)

• Special somatic afferent nerve — carries hearing & balance information coming from cochlear & vestibular parts of the middle ear

• 2 cochlear nuclei — superior & inferior (/dorsal & ventral)

• 4 vestibular nuclei — superior, inferior, medial, & lateral

• Nuclei are found in the tegmentum of the pons & of the medulla

Vestibular neuritis

Inflammation of the vestibular branch of the vestibulocochlear nerve —

• Aeitology(causes) of the condition not fully understood, but some cases are though to be due to reactivation of the herpes simplex virus

• Presents with symptoms of vestibular nerve damage —

  • Vertigo (false sensation of oneself or surrounding spinning)

  • Nystagmus (repetitive, involuntary to-&-fro oscillation of the eyes

  • Loss of equilibrium (especially in low light)

  • Nausea & vomiting

• Usually self-resolving — treatment is symptomatic, usually in form of anti-emetics or vestibular suppressants

Labryinthitis

Inflammation of the membranous labyrinth, resulting in damage to vestibular & cochlear branches of vestibulocochlear nerve

• Symptoms = similar to vestibular neuritis — also include indicators of cochlear nerve damage — sensorineural hearing loss & tinnitus (false ringing or buzzing sound)

Glossopharyngeal nerve

9th cranial nerve, located in the medulla, with 5 components —

• SVE (branchiomotor) component — originates from nucleus ambiguous, innervates stylopharyngeus muscle

• GVE component — formed by parasympathetic preganglionic fibers directed to the parotid gland —

  • Originates from inferior salivatory nucleus, with its fibers passing through the otic ganglion

• SVA component — originates from nucleus of solitary tract in medulla, collects taste information

• GVA component — collects information from viscera such as carotid body & sinus, pharynx, & middle ear — originates from nucleus of solitary tract

• GSA — collects general sensation from the ear, relaying it to the bulbospinal trigeminal nucleus

  • (small territory around ear lobe shared by facial & vagus nerve)

2 sensory ganglia — superior & inferior (or petrosal ganglia), and it exits at the jugular foramen

Gag reflex

• Glosssopharyngeal nerve supplies sensory innervation to the oropharynx, thus carrying the afferent information for gag reflex

• When a foreign object toughes the back of the mouth, this stimulates CN9, beginning the reflex

• The efferent nerve in this process is the vagus nerve — CN10 — however as it is triggered by CN9, an absent gag reflex signifies damage to the glossopharyngeal nerve

Vagus nerve

10th cranial nerve, originating from the medulla at the level of the retro-olivary groove — very large territory of innervation with 5 components —

• SVE (branchial motor) component —

  • Innervates muscles of larynx & pharynx, with nucleus ambiguous giving rise to these fibers

• GVE —

  • All viscera in thorax & abdomen are innervated through parasympathetic preganglionic fibers —

  • Originate from dorsal motor nucleus of vagus nerve in medulla, and then are linked to many ganglia & ENS (enteric NS)

• GVA —

  • Collects informatin from all the viscera in which it passes — originate from nucleus of solitary tract

    • (same nucleus that collects taste info in its most rostral part)

• GSA —

  • Collects general sensation from the ear (with facial & glossopharyngeal brings information from most of territory of the head)

    • Related to bulbospinal trigeminal nucleus

• SVA —

  • Collects taste information from the tongue, originating from nucleus of solitary tract

Taste information from the tongue

• In anterior 2/3 of tongue — collected from intermediate component of facial nerve

• Slightly more posteriorly — collected by glossopharyngeal nerve

• At level of taste buds at root of tongue, epiglottis, and initial part of larynx & pharynx — vagus nerve

  • Here we find 2 sensory ganglia — superior & inferior or nodose ganglia

    • Superior contains somatosensory neurons

    • Inferior contains viscerosensory neurons

Innervation of muscles of pharynx

• Most pharyngeal muscles are innervated by pharyngeal branches of the vagus nerve —

  • Superior, middle, & inferior pharyngeal constrictor muscles

  • Palatopharyngeus muscle

  • Salpingopharyngeus muscle

  • Muscles of the larynx

  • Palatoglossus & majority of muscles of soft palate

• Since vagus nerve innervates muscles responsible for movement of soft palate, in case of problems in the vagus nerve, we have —

  • Ipsilateral paralysis of the soft palate, muscles of pharynx & larynx

    • This can cause dysphonia, dyspnea, dysarthria, & dysphagia

    • Could have a loss of cough reflex

Accessory nerve

• 11th cranial nerve, with a spinal & cranial part —

  • Many of its fibers originate from C1-C5 spine

  • Cranial part — fibers originating from medulla oblongata

• Fibers originating from spinal cord enter skull via foramen magnum, joining to components originating by medulla oblongata

  • SVE — fibers originating from spinal cord motoneurons are responsible for innervation of — trapezius & sternocleidomastoid muscles

• Fibers originating from cranial component, once exiting jugular foramen, abandon XI nerve to join the vagus nerve (accessory)

Hypoglossal nerve

12th cranial nerve — located in medulla

• GSE — innervates intrinsic muscles of the tongue, which have a somatic origin — characterized by branches of motor nucleus located in tegmentum of medulla

Clinical relevance — examination & palsy of accessory nerve

Examination —

• Patient is asked to rotate their head & shrug their shoulders — both normally & against resistance

• Simply observing the patient may also reveal signs of muscle wasting in sternocleidomastoid & trapezius in cases of long-standing nerve damage

Palsy —

• Most common cause of accessory nerve damage is latrogenic —

  • Prodcedures such as cervical node excision biopsy or central line insertion can cause trauma to the nerve

Clinical features —

• Muscle watering & partial paralysis of sternocleidomastoid — resulting in inability to rotate head or weakness in shrugging shoulders. Damage to the muscles can also result in an asymmetrical neckline

General rule of cranial nerves & components —

The nerves that emerge close to the midline are thosee that are mostly characterised only by GSE components, while the furthest from the midline also have SSE components — (see book-like opening in NS development)

Cranial nerves “recap”

• Cranial nerves emerge ventrolaterally except for the 4th (trochlear) nerve — emerges dorsally

• Nerves emerging close to midline are mostly characterized by GSE components, while furthest from midline also have SSE components

• Cranial nerves can be grouped by their components —

  • Visceromotor Parasympathetic (GVE) —

    • III, VII, IX, X

  • Motor Somatic (GSE) —

    • III, IV, VI, XII

  • Motor Branchial/Visceromotor (SVE) —

    • V, VII, IX, X, XI

  • Sensory Somatic General (GSA) —

    • V, VII, IX, X

  • Sensory Somatic Special (SSA) —

    • VIII

  • Sensory Visceral general (GVA) —

    • IX, X

  • Sensory Visceral Special (SVA) —

    • VII, IX, X