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• nervous system biology
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Cranial Nerves

Cranial nerves provide sensory and motor innervation to the head. They leave the skull in groups.

• 1 - olfactory
• 2 - optic
• 3 - oculomotor
• 4 - troclear
• 5 - trigeminal
• 6 - abducens
• 7 - facial
• 8 - vestibulocolear
• 9 - glossopharyngeal
• 10 -vagus
• 11 - accessory
• 12 -hypoglossal

Cranial Nerve nuclei

 

 

 

I - Olfactory

function

smell

 

path

associated with forebrain

testing

give familiar and nonirritating odors with eyes closed; close one nostril at a time

 

 

 

II - Optic

function

• vision!

path

The optic nerve is an extension of the diencephalon.

 

testing

• visual acuity: Snellen eye chart
• confrontation field test in four quadrants
• pupillary reflex
  • equal
  • direct and consensual pupillary reflex
  • swinging flashlight
  • accomodation
• fundoscopy to examine optic nerve

 

 

III - Oculomotor

function

• pupillary constriction (parasympathetic)
• opening eye - levator palpebrae superioris
• most eye movements, save those of CN IV and VI

 

path

• oculomotor nucleus is in the anterior periacqueductal gray of the midbrain
• several nerve bundles leave and
• Edinger-Westphal nucleus is beside it, projecting to the ipsilateral ciliary ganglion and controlling pupillary constriction
• CN III leaves the anterior midbrain between the cerebral peduncles and gathers with CN IV, VI, and V1 of CN V in the cavernous sinus before exiting via the superior orbital fissure.

 

testing

• inspect size and shape of pupils, comparing one another
• test cardinal eye movements
• look for nysagmus
• look for ptosis

problems

• characterized by diplopia (double vision) due to lateral strabismus
• CN III palsy: eye is down and out (exotropia), dilated pupil (mydrosis), ptosis
• pupil sparing syndromes can accompany diabetes
• uncal herniation can push down on CN III, leading to blown pupil

 

 

IV - Trochlear

function

• downward, inward movement of eye via innervation of the superior oblique (LR6 SO4)

path

• the contralateral trochlear nucleus is small and at the level of the inferior colliculus, jutting into the medial longitudinal fasciculus
• leaves the midbrain-pons junction posteriorly, just below the inferior colliculus

testing

• test dowward, inward gaze
• look for nysagmus
• look for ptosis

problems

• palsy of CN IV will cause hypertropia and head tilt to compensate

 

V - Trigeminal

 

function

• general sensory fibres from face, mouth, gums, teeth, and a portion of the tongue
• motor outflow to muscles of mastication, and a few muscles of the palate, inner ear, and upper neck

 

path

afferent

• sensory root passes into trigeminal ganglion in floor of skull, adjacent to sella turcica
• three branches emerge - opthalmic (V1), maxillary (V2), and mandibular (V3) divisions.

sensory nuclei are in a column extending from the midbrain to the spinal cord

• principle sensory nucleus is homologous to dorsal column nuclei, processing touch and proprioreceptive information
  • sends information via the medial lemniscus to ventral posteromedial (VPM) nucleus of the thalamus
• spinal trigeminal nucleus extends projects pain and temperature information down the spinal trigeminal tract and up the contralatreral spinothalamic tract to the ventral posteromedial nucleus in the thalamus
  • becomes continuous with substantia gelatinosa
  • has three histological types
• mesencephalic nucleus: deals with proprioreception

trigeminal motor nucleus is located in the midpons.

testing

sensory

• cotton wool (light touch) and pinprick (pain) are useful, with temperature used to confirm
• corneal reflex: sensory is CN V, while the motor is CN VII

motor

• palpate temporal and masseter muscles while asking person to clench teeth
• ask them to keep jaw open as you attempt to close it with two hands (tests pterygoid)
• move jaw from side to side (pterygoid)
• jaw reflex

problems

if loss of sensation to face is reoprted, check inside of mouth too to rule out psychosomatic concerns

 

VI - Abducens

function

• lateral deviation of eye by lateral rectus muscle

 

path

• nucleus is in pons in the floor of the third ventricle, lateral to the MLF; genu of CN VII wraps around it
• abducens nucleus and facial nerve genu form the facial colliculus of the pons
• nucelus also projects to contralateral oculomotor nucleus via internuclear neurons of the MLF, coordinating lateral gaze
• has the longest intracranial pathway, making it more sensitive to raised intracranial pressure
• it passes through the arachnoid space, petrous apex, cavernous sinus, and through the orbit (check)

testing

• test lateral eye movement

problems

paralysis of CN VI is called esotropia.

sixth nerve palsies can be caused by:

• diabetes
• arteriosclerosis
• multiple sclerosis
• raised intracranial pressure

 

VII - Facial

function

• innervates muscles of face, save muscles of mastication and eye movement
• small sensory area by ear
• taste of anterior two-thirds of tongue
• mediates the motor component of the corneal reflex
• parasympathetic: lacrimation, salivation

Five branches (to Zanzibar by motorcar)

• temporal
• zygomatic
• buccal
• mandibular
• cervical

path

Leaves the pons, often as separate sensory and motor fibres

large facial nerve nucleus is in anterolateral pons

CN VII wraps around abducent nucleus

• sensory ganglion, the geniculate nucleus, is near middle ear
• upper motor neurons (corticobulbar neurons) in brain stem; brainstem problems result in lower facial paralysis; LMN problems result in complete hemiparalysis

testing

• look for assymetry of face at rest and while talking, including eye blinks
• ask person to raise eyebrows, clench eyelids, puff out mouth, purse lips, and bare teeth
  • with UMN problems, forehead is relatively spared, while in LMN problems (Bell's palsy) affects upper face as well as lower face
• corneal reflex: sensory is CN V, while the motor is CN VII

problems

Bell's palsy is an isolated CN VII problem, often due to viral infections

• look for vesicles in the ear canal

 

VIII - Vestibulocochlear

function

• carries information from the auditory system and the vestibular system. It also carries efferent information to outer hair cells.

 

path

• two bundles of sensory axons, with vestibular and auditory ganglia in bone
• enter the pons

testing

• test hearing; if hearing loss is suspected, test for lateralization and air/bone conduction
• in clinic, can whisper (from about 0.8 m) or rub fingers together
• ask about vertigo, ringing

problems

vestibular schwannomas are one of the most common intracranial tumours. patients complain mainly of hearing loss as gradual loss of vestibular information is adapted to.

 

 

 

 

IX - Glossopharyngeal

function

• taste from posterior one-third of tongue
• sensation from palate, tonsillar fossae, carotid sinus
  • pain information travels through spinal trigeminal nucleus
• motor: stylopharyngeus muscle (swallowing)
• parasympethetic: controls parotid salivary gland
• visceral carotid bodies for chemoreceptors - synapse in solitary nucleus in medulla
• afferent arc of gag reflex

path

• motor neurons, together with those of CN X, leave the nucleus ambiguus in the medulla

testing

• gag reflex

 

 

 

 

 

X - Vagus

function

• principle parasympathetic nerve of the body, innervating smooth muscle and glands of GI, pulmonary, and cardiovascular systems
• sensory: posterior pharynx; visceral sensation from pharynx, larynx, viscera
• motor: striated muscles of larynx and pharynx

 

aordic bodies for baroreceptors - synapse in solitary nucleus in medulla

 

path

most parasympathetic fibres arise in the dorsal motor nucleus, though some come from nucleus ambiguus as well

nucleus ambiguus provides motor innervation

 

testing

• listen to voice for hoarseness (recurrent laryngeal branch)
• ask patient to say 'aah', look for midline palate elevation
• test gag reflex; palate deviates to intact side

 

 

 

XI - Accessory

function

• innervates sternocleidomastoid and upper trapezius

path

testing

• ask person to rotate head against resistance
• shoulder shrug

 

 

 

 

XII - Hypoglossal

function

• innervates intrisic muscles of tongue

path

• hypoglossal nucleus extends throughout medulla, adjacent to midline and just beneath floor of fourth ventricle
• hypoglossal nerve emerges as a series of rootlets in grove between pyramid and olive

testing

• get them to keep their tongue inside
• look for tongue fasciculations; ignore restless movements
• look for assymetry, atrophy, or deviation
  • in a unilateral problem, the tongue will deviate away from problem

 

Cranial Nerve Nuclei

Neurons of the cranial nerve nuclei are arranged into longitudinal columns.