Cranial Nerves Flashcards
What are the 3 major cranial nerves serving special sense organ?
I- olfactory
II- Optic
VIII - Vestibulocochlear
They develop in part from epidermal neurogenic placodes
are not parts of other cranial sensory-motor nerve
What are the 4 somatic motor nerves that move the eye and tongue?
III - Occulomotor (exits from vental midbrain)
IV - Trochlear (exits dorsally and decussates i.e crosses)
VI - Abducens (emerges caudal to VII)
XII - Hypoglossal (work close with C1)
- have no affiliated sensory ganglia, thus they have only efferent components (innervated striated skeletal m.)
- sensory fibers in the peripheral parts of these nerves are “hitch-hikers” from nearby mixed nerves
- are more (XII, VI, III) or less (IV) comparable to ventral roots of spinal nerves
What are the 4 mixed branchial nerves that move and sense jaws, face, pharynx, and larynx?
V - trigeminal (no taste fibers)
VII - Facial
IX - Glossopharyngeal
X - Vagus
Pharyngeal Arch Nerves
One or more sensory ganglia
This nerve is the only special motor nerve that is cranial only in a roundabout way and innervates the sternocleidomastoid and trapezius muscle
XI - Spinal Accessory
Motor nuclei in CNS
What is the Otic Placode?
(located around medulla)
produces auditory and vestibular sensory hair cells and ganglion cells (receive input and send axons centrally as nerve)
CT for inner ear
Involves Cranial nerve VIII
Placodes are regions of the surface ectoderm
What is the Optic Placode?
Produces Lens
Involves CN II (brain tract)
(not neurogenic)
What is the olfactory placode?
Central Projecting sensory cells
(produces olfactory sensory neurons (also known as the ganglion cells))
This structure is located at the diencephalon wall. What is it and what does it contain?
This is the Optic Vesicle
contain sensory cells and ganglion cells
What does CN I (Olfactory Nerve) consist of?
consists of the central-projecting axons of bipolar olfactory sensory neurons (= olfactory receptor cells) situated in the olfactory epithelium.
The axons collect as fine bundles that traverse the olfactory foramina in the cribriform plate of the ethmoid bone to reach the olfactory bulbs where they synapse on second order olfactory neurons.
Chemosensory
Receptors for Odarent molecules
Easily disrupted
This “Cranial Nerve” is actually a CNS tract, not peripheral nerve and covered by meningies. Located in the nasal cavity?
CN I (Olfactory)
It is covered with dura, arachnoids, and pia mater
The nerve is just the axons coming through cribiform plate. Bulb and tract are part of the brain
Patient comes in reporting that he has difficulty smelling after doing some drugs. What is one possible diagnosis that can be considered?
Anosmia (loss of sense of smell)
This occurs when there is Damage to the olfactory sensory epithelium or disruption of the fibers at the cribiformplate
To exam the function of CN I you will expose the patient to test odorants
Head injury or frontal lobe surgery can lead to damage
What is the path of the Optic nerve (CN II)
begin at the retina and extend to the optic chiasm (outside orbit), where approximately half of the optic fibers cross to the other side. The fibers continue as the optic tracts and most of them terminate by synapsing in the lateral geniculate nuclei (LGN is a big nucleus of the thalamus) of the thalamus.
What is different about the Optic nerve compared to some other cranial nerves?
The retina and optic nerves arise from the walls of the embryonic diencephalon. Accordingly, the optic nerves are brain tracts, not peripheral nerves.
Most of the axons that make up the optic nerve come from where?
axons of retinal ganglion cells.
This nerve is actually made of two nerves. It enters the brain at the pontomedullary junction, just dorsal to the root of the VIIth nerve.
CN: VIII
(Vestibulocochlear Nerve)
The VIIIth nerve fibers pass peripherally to into the _____, accompanied by the VIIth nerve, via the __________.
Temporal bone (Petrous Part)
Internal Acoustic meatus
What does the cochlear nerve (portion of CN VIII) comprise of?
afferent axonal processes of cells in the spiral ganglion which innervate the auditory hair cells (mechanosensory) of the spiral organ (Organ of Corti - sensory hair cells + galnglion)
as well as,
efferent axons of central neurons located in the superior olivary/periolivary nuclei that form the olivocochlear bundle (modulate sensitivity - cochlear amplifier)
The Vestibular nerve (part of CN VIII) comprises of?
afferent axonal processes of cells in the vestibular ganglion (Scarpa’s) which innervate:
vestibular hair cells of the ampullae of the semicircular canals (3), sensing angular acceleration
vestibular hair cells of the maculae of the saccular and utricular otoliths, sensing linear acceleration
as well as,
efferent axons of central neurons located in two vestibular efferent nuclei
Where do the afferent fibers of the cochlear nerve project?
project to the lowest order brainstem auditory centers, the dorsal and ventral cochlear nuclei
The vestibular nerve afferent fibers project to where?
to the vestibular nuclei (superior, medial, lateral and descending) and to the vestibulocerebellum
The vestibular nuclei connect to what other structures besides the Vestibular nerve afferent fibers?
connect with brainstem, spinal and cerebellar centers involved in postural and gaze control (among others)
MLF and Vestibulospinal tracts are what?
Major central vestibular pathways
They lead to extrocular and cervical motor nuclie controlling eye and head movements
How do you test Vestibular function?
Routine auditory test (done with a tuning fork)
or
asking the patient to walk a straight line (balance)
or
observing eye movements during introduction of cool or warm water into the external ear canal.
What is the Caloric Test?
Observing eye movement during introduction of cool/warm water.
induces alternating eye movements (Nystagmus) by generating convection in the endolymph of the horizontal semicircular canal. The caloric vestibulo-ocular reflex is especially useful in testing brainstem function in comatose patients.
What is a complication of the CN III?
This is the oculomotor nerve and it also has parasympathetic components - to smooth muscle of the eye
What are the Embryonic Origins of Striated Cranial Muscles Innervated by Somatomotor Cranial Nerves III, IV, VI and XII?
- Dorsal head mesoderm (pre-otic mesoderm) in front of the ear (otic vesicle) produces the extraocular muscles, innervated by cranial nerves III, IV and VI
- Occipital somites 1-4 give rise to a migratory stream of myocytes that travel ventrally into the floor of the oral-pharyngeal region to produce the tongue muscles innervated by CN XII.
Which ganglion does CN III innervate?
parasympathetic ciliary ganglion.
Side note: Innervates most of the striated and smooth muscles of the eye, thus has both somatic and visceral (parasympathetic) motor fibers
The parasympathetic ciliary ganglion and the occular smooth muscles innervated by CN III are derived from what?
Neural crest
The CN III roots emerge from where?
emerge from the ventral midbrain, on either side of the posterior perforated substance, and between the mamillary bodies and the pontine fibers
Edinger-Westphal Nucleus* (GVE)
Preganglionic parasympathetic neurons in the Edinger-Westphal Nucleus innervate post-ganglionic neurons in the ciliary ganglion (located in orbit) where they synapse. Ciliary ganglia send axons into the front of the eye
Oculomotor Nucleus (GSE)
Motor neurons innervate:
4 striated muscles that move the eye
- superior rectus
- inferior rectus
- medial rectus
- inferior oblique
1 striated muscle that elevates the upper eyelid
-levator palpebrae superioris
Ciliary ganglion neurons innervate 2 smooth muscles in the eye?
- the pupillary sphincter muscle which constricts the pupil (red)
- the ciliary muscle (active when you look at things close) which regulates the curvature of the lens (blue) - recieve 95% of neurons in ciliary ganglio.
What are the characteristics signs of a unilateral occulomotor nerve palsy?
“down out syndrome”
Ptosis (drooping) of the upper eyelid, due to lack of action of the levator palpebrae superioris
Dilation of the pupil due to loss of sphincter pupillae function
Depressed and abducted resting eye position due to the IVth and VIth nerve eye muscles acting unopposed
CN IV (Trochlear) innervates only what?
ONLY the superior oblique muscle
(contains somatic motor fibers)
The trochlear nerve received that name because the superior oblique muscle passes through a pulley (L., trochlea) before inserting on the globe.
What is the direction of the Trochlear nerve roots?
roots exit dorsally at the midbrain/hindbrain junction and cross to the opposite side (right to left, left to right)
Where is the trochlear nuclei located?
trochlear nuclei are located near the midline beneath the most superior part of the fourth ventricle
(green arrow)
*look at picture*
What are the two main actions of the superior oblique muscle?
Depresses and medially rotate (in-tort) the eye