Flashcards in Chapter 3: Cranial Nerves Deck (117):
Where does cranial nerve 1 originate? what is its path of travel? Are the fibers myelinated or unmyelinated?
The olfactory nerve originates from the small olfactory receptors located in the mucous membrane of the nose. The fibers are unmyelinated, and travel through the cribiform plate to the olfactory bulb, and then travel posteriorly on the inferior surface of the cerebrum as olfactory tracts
How many axons of the gcl originate in the retina?
1x10^6, or 1 million axons
What are the 4 regions of the optic nerve?
1- intraocular region
2- intraorbital region (located within muscle cone)
3- intracanalicular region (located within optic canal)
4- intracranial region (ends in the optic chiasm)
T/F: is the optic nerve surrounded by schwann cells?
False. As the optic nerve is considered part of the brain for developmental purposes, they have oligodendrocytes, not schwann cells, surround it.
How long is the intraorbital portion of the optic nerve? How does this effect it course of travel?
The optic nerve is 25-30 mm within the orbit, making it long than the distance b/w the back of the globe and the orbit). Therefore, it has a sinuous path to compensate for the increased distance
What is the average size of the optic disc?
1.76 mm wide x 1.92 mm tall
What is the physiologic cup? Where is it located?
represents and "axon free" zone, where the branches of the central retinal artery and vein pass through its center.
It is located slightly temporally from the geometric center
What are the 4 parts of the optic nerve?
-the superficial nerve fiber layer
- prelaminar area
- laminar area
- retrolaminar area
How are the fibers arranged in the superficial nerve fiber layer?
the fibers from the upper retina are located above, the lower retina below, the nasal retina medially and the temporal retina laterally.
Macular fibers, which constitute 1/3 of ON, are laterally placed (foveal fibers located peripherally, peripapillary fibers centrally)
What is the Prelaminar Area?
area where mullers cells are replaced by astrocytes in the ILM and ganglion cells are separated into "fascicles" by the astrocytes.
New astrocytic ILM is continuous with the previous Muller cell ILM
What is pathologic cupping of the disc?
when the ON is damaged, axons and glial cells may be lost, resulting in pathologic enlargement of the cup.
What is the laminar area?
the area where the nerve is trasmitted through pores in the lamina cribosa, which consists of approximately 10 CT sheets
What is the lamina cribosa made up of?
Collagen types I and III, elastin, fibronectin, and laminin
What are the functions of the lamina cribosa?
1- scaffold for ON axons
2- Point of fixation for the CRA/CRV
3- Reinforcement of the posterior segment of the globe
What is the retrolaminar area?
The area where the diameter of the ON increases to 3 mm due to myelination of the nerve fibers and presence of oligodendroglia/surrounding meningeal sheaths
What is the intraorbital region?
In this region, the ON passess through the muscular cone, where the superior and medial rectus muscles share a CT sheath with the ON.
At the optic canal, the dural sheath of the nerve fuses wtih periosteum, completely immobilizing it (however, makes it susceptible to traumatic shearing forces)
What is the Pia Mater?
the innermost portion of the ON sheath
sends numerous septa into the ON, dividing its axons into bundles (this provides mechanical support as well as nutrition for the axons and glial cells)
What is the arachnoid mater?
Middle layer of the meningeal sheath, is connect to the Pia across the subarachnoid space via vascular trabeculae
Where does the subarachnoid space end anteriorly? Posteriorly?
Anteriorly, the SAS ends at the level of the lamina cribosa. Posteriorly, it is continuous with the SAS of the brain
What is the Dura mater? What is it made of?
the outer layer of the meningeal sheath, measuring 0.3-0.5 mm thick
The dura is made of dense bundles of collagen and elastic tissue that fuse anterioly with the outer layer of the sclera
What is the blood supply of the ON in the intracanalicular region?
blood supply is derived from pial vessels that are branches from the ophthalmic artery
In the intracanalicular region, what structure are the ON and sheath tethered to?
the periosteum of the optic canal
What is indirect traumatic optic neuropathy?
blunt trauma that causes shearing of the blood supply of the ON in the intracanalicular region
What is the pathway of the optic nerve to the brain after leaving the optic canal?
After leaving the canal, the ON lies above the ophthalmic arteries, above and medial to the ICA's. Then passes posteriorly over the cavernous sinuses and forms the optic chiasm, which then divides into the left and right optic tracts, which then travel to their respective lateral geniculate bodies. From here, they leave as geniculocalcarine pathways (optic radiations), and pass to the primary visual cortex.
When do the CRA and long posterior ciliary arteries branch off from the ophthalmic artery?
When the OA enters the muscular cone
What is the blood supply of the retrolaminar optic nerve?
supplied by pial vessels and short posterior ciliary vessels, with some help from the CRA and recurrent choroidal arteries.
What is the blood supply of the laminar optic nerve?
supplied by short posterior ciliary arteries or branches of the arterial circle of hall and zinn (arises from branches of short posterior ciliary arteries, and is usually embedded in the sclera around the ON head)
What is the blood supply of the prelaminar optic nerve?
supplied by short posterior ciliary arteries and recurrent choroidal arteries (NFL is supplied by CRA)
WHat is the blood supply of the intraorbital region of the ON?
Proximally supplied by pial vessels and neighboring branches of the ophthalmic artery
Distally by intraneural branches of the CRA
Anteriorly, supplied by short posterior ciliary arteries and occasionally peripapillary choroidal arteries
What is the blood supply of the intracanalicular region?
supplied almost exclusivelu by ophthalmic artery
What is the blood supply in the intracranial region?
supplied primarily by branches of the ICA and ophthamic artery
What is the difference between the CRA and retinal arterioles?
The CRA contains fenestrated internal elastic lamina and an outer layer of muscle cells surround by a thin basement membrane
What is the optic chiasm? What are its dimensions?
the decusation of the two optic nerves. 12 mm wide x 8 mm long x 4 mm thick
Where do extramacular fibers from the inferonasal retina cross in the chiasm?
They cross anteriorly in the chiasm, at "Wilbrands knee"
Where do the extramacular superonasal fibers cross in the chiasm?
They cross directly opposite to their tracts
Where do the temporal fibers cross?
Trick question--the temporal fibers remain uncrossed
What is the course that the temporal macular fibers run?
They pursue a direct course through the chiasm, as a bundle of uncrossed fibers
What is the course that the nasal macular fibers run?
The nasal fibers cross in the posterior aspect of the chiasm
What percentage of of the fibers are crossed? Uncrossed?
53% crossed, 47% uncrossed
What kind of fibers does the optic tract contain?
ipsilateral temporal and contralateral nasal fibers
upper retinal projections travel medially in the tract; lower projections travel laterally
What kind of orientation do the macular fibers adopt in the optic tracts?
The macular fibers adopt a dorsolateral orientation
What is the lateral geniculate body?
a synaptic zone for higher visual projections, receives 70% of the optic tract fibers within 6 alternating layers of white and gray matter
What layers of the LGN synapse with fibers from the ipsilateral nerve? Contralateral nerve?
Layers 1, 4, & 6 synapse with contralateral fibers
Layers 2,3, & 5 synapse with ipsilateral fibers
What are the optic radiations? By what name are they also known?
aka geniculocalcarine pathways, they connect the LGN to the primary visual cortex
What is the pathway from the LGN to the visual cortex?
radiations leave the LGN and wind around the temporal horn of the lateral ventricles (called "meyers loops), swee backward toward the visual area of the occipital love
Facts about visual cortex?
Thinnest area of the human cerebral cortex
Has 6 layers
occupies the superior and inferior lips of the calcarine fissues
What is the blood supply of the visual cortex?
The visual cortex is supplied by the posterior cerebral artery, which is a branch of the basilar artery.
Note: In some people, the MCA also contributes
What does CN III supply?
Innervates all of the EOM except the SO and LR
Carries cholinergic fibers to the pupillary sphincter and ciliary muscles
How many nerve fibers does CN III have?
24,000 nerve fibers
Where does CN III originate?
From a complex group of cells located in the rostral midbrain (mesencephalon) at the level of the superior colliculus
Where is the CN III nucleus located in relation to the CN IV nucleus?
CN III nucleus is located immediately rostral to the CN IV nucleus
What does the CN III nucleus consist of?
It consist of several distinct pairs of large nuclei, each subserving the muscle they innervate
However, LPS only has one nucleus that subserves the muscles bilaterally
Where do the fibers of the dorsal nucleus decussate?
the caudal aspect of the nucleus, and supply the SR muscles
Where is the Edinger-Westphal nucleus located? What is its function?
Located cephalic and dorsomedially
Provides parasympathetics to the ciliary muscle and iris sphincter
What is the most ventral group of nuclei in the CN III nucleus?
Where do aneurysms that affect CN III commonly occur?
At the junction of the posterior communicating arteries and the ICA
What is the pathway of CN III to the globe?
It pierces the dura on the lateral side of the posterior clinoid, runs along the lateral wall of the cavernous sinus (and above CN IV), enters the orbit via the superior orbital tissue and then separates into the superior and inferior divisions after passing through the annulus of Zinn
Which division of CN III is larger?
The inferior division, as it supplies three muscles compared to two.
What path does the parasympathetic aspect of CN III take to the orbit?
It winds around the periphery of the nerve, enters the inferior division, travels through to the IO branch, joins the ciliary ganglion, emerges as short ciliary nerves, pierces the sclera, travels through the choroid, and innervates the ciliary muscle and pupillary sphincter
What is the Light Reflex?
consists of simultaneous and equal constriction of pupils in response to illumination of one eye
What is the afferent pathway of the light reflex?
Coincides with that of the afferent visual pathway, including decussation of nasal fibers at optic chasm
What is the efferent pathway of the light reflex?
At the posterior optic tract, pupillary fibers leave the visual tract and pass to the pretectal nuclei (located at level of superior colliculus of lateral midbrain), and then pass to the Edinger-Westphal nuclei (where they partially decussate). They then exit the EW nucleus and travel with CN III as it leaves the brainstem, and then travel with the inferior division after entering the orbit. Fibers synapse on ciliary ganglion, exit as postganglionic short ciliary nerves (3-5% of total nerve are pupillomotor fibers)
What is the Near Reflex?
the synkinesis that occurs when visual focus is changed from distance to near, including accommodation, convergence, and pupillary constriction
What is the pathway of the near reflex?
Originates in the occipital association cortex, impulses descent down the corticofugal pathway to the pretectal nuclei +/- tegmentum, after which the fibers pass to the EW nucleus and motor nuclei of MR and nuclei of CN VI
Efferent Fibers for accommodation follow the same pathway as the light reflex (however, final destination via the short ciliary nerves is the ciliary muscle)
What is Cranial Nerve IV?
The trochlear nerve, which supplies the SO muscle.
Contains the fewest nerve fibers of all the cranial nerves (3400)
Travels the longest course of any cranial nerves (75 mm)
Where is the nucleus located for CN IV?
the caudal mesencephalon at the level of the inferior colliculus
is continuous with the caudal end of CN III nucleus (only difference is that its cells are smaller in size)
T/F: CN IV is the only nerve to completely decussate?
True. After leaving its nucleus, the nerve fascicles curve dorso-caudally around the peri-aqueductal gray matter and completely decussate in the superior medullary velum.
What affect does retrobulbar anesthesia have on CN IV?
It usually does not affect CN IV (b/c of its location outside the muscle cone inside the orbit)
Where does CN IV exit the nervous system? What is unique about its pathway?
It exits the nucleus/nervous system dorsally, and is the only cranial nerve to do so.
What does CN V (Trigeminal Nerve) supply?
Provides motor innervation to the muscles of mastication (pterygoid, masseter, and temporal is)
Sensory innervation to scalp, forehead, face, eyelids, eyes, lacrimal glands, EOM, ears, dura mater, and tongue
How many nuclei does CN V have? Where is its nucleus located?
CN V has 4 nuclei
The CN V nuclear complex extends from the midbrain to the upper cervical segments (C4)
What are the nuclei of CN V?
- mesencephalic nuclei (Most superiorly located)
- main sensory nucleus
- spinal nucleus and tract
- motor nucleus (most inferiorly located)
What is the function of the mesencephalic nucleus?
Mediates proprioception and deep sensation from the masticatory, facial, and EOM
Extends inferiorly into the Pons as far as the Main Sensory Nucleus
What is the function of the Main Sensory Nucleus? Where is it located?
Located in the Pons next to the motor nucleus; is continuous with the mesencephalic nucleus (above) and the spinal nucleus (below)
serves light touch from the skin and mucus membranes
From where does the main sensory nucleus receive input? What happens to the fibers after entering the Pons?
It receives input from the ascending branches of the sensory root
After entering the Pons, the fibers divide into the ascending tract (terminates in the main sensory nucleus) and descending tracts (terminates in the spinal nucleus)
Where is the spinal nucleus located?
Its extends from the medulla to C4
Where does the spinal nucleus receive fibers from?
Receives pain, temperature, and cutaneous component of CN VII/IX/X from ear/external auditory meatus from the descending tracts
Where do the fibers terminate in the spinal nucleus?
- Sensory fibers from V1 end in most ventral portion of nucleus
- Fibers from V2 end in the mid portion
- Fibers from V3 end in the dorsal aspect
- Fibers from the peri-oral region thought to terminate most rostrally; fibers from peripheral face and scalp most caudally
Where do the axons of the mesencephalic, main sensory, and spinal nuclei travel to?
They relay sensory information to the higher sensory ares of the brain
Where do the axons leaving the CN V nuclei decussate on their way to the brain?
The fibers leaving the CN V nuclei decussate in the Pons en route to the thalamus
What pathway do the axons from CN V take en route to the brain?
The fibers cross in Pons, and ascend to the thalamus via the ventral and dorsal trigeminothalmic tracts, and terminate in the ventral posteromedial nucleus of the thalamus.
Thalamic nucleus then sends axons through the internal capsule to the post-central gyrus of the cerebral cortex
What nerve mediates the oculocardiac reflex?
the Trigeminal nerve mediates the afferent pathway of the oculocardiac reflex
Where is the motor nucleus of CN V located?
medial to the main sensory nucleus in the Pons
Where does the motor nucleus of CN V receive fibers from?
It receives fibers from both cerebral hemispheres, the reticular formation, the red nucleus, the tectum, the MLF, and mesencephalic nucleus
Where does the motor nucleus of CN V send fibers to?
Sends efferent fibers to the muscles of mastication (masseter, pterygoid, and temporal is), tensor tympani, tensor veil palatini, mylohyoid muscle, and the anterior belly of the digastric muscle
What is the pathway of CN V?
The intracranial portion emerges from the superior lateral portion of the ventral points, passes over the petrous apex and forms the trigeminal ganglion, where is divides into its 3 branches
What are other names for the trigeminal ganglion? Where is it located?
aka gasserian or semilumar ganglion, is located in the Meckel Cave
Where is the Meckel Cave?
located near the apex of the petrous part of the temporal bone in the middle cranial fossa.
What is the pathway of the Ophthalmic branch of CN V?
Enters the cavernous sinus lateral to the ICA and courses beneath CN III and IV, and passes into the orbit via the superior orbital fissure, where it divides into its three branches
What are the branches of the Ophthalmic branch of CN V?
Within the cavernous sinus, it gives off a tenurial-dural branch.
Within the orbit, it gives off the Frontal nerve, the lacrimal nerve, and the nasociliary nerve
What does the frontal nerve innervate? Does it have any branches?
Divides into the supraorbital and supratrochlear nerves, which provide sensation to the medial portion of the upper eyelid, conjunctiva, forehead, scalp, frontal sinuses, and side of nose
What does the lacrimal nerve innervate? Does it have any branches?
Supplies sensation to the lacrimal gland and neighboring conjunctiva.
Parasympathetic innervation to retro-orbital plexus
What does the nasociliary nerve innervate? Does it have any branches?
Supplies sensation to the middle and inferior turbinates (via nasal branches), septum, lateral nasal wall, and tip of nose
Its infratrochlear branch supplies the lacrimal drainage system, conjunctiva, and medial canthal region
What does the Maxillary division of CN V innervate?
Inferior Palpebral branch--lower eyelid
Nasal Branch- side of nose
Superior Labial Branch- upper lip
Also supplies teeth, maxillary sinus, roof of mouth, and soft palat
What is the pathway of the Maxillary Division of CN V?
leaves the trigeminal ganglion to exit through the foramen rotundum, courses through the pterygopalatine fossa and into the orbit via the inferior orbital fissure. Then travels through the infraorbital canal as the infraorbital nerve
What does the mandibular division of CN V innervate?
provides motor input to the muscles of mastication
Provides sensory input to the mucosa and skin of mandible, lower lip, tongue, external ear, and tympanum
Where is CN VI (Abducens Nerve) located?
Its situated on the floor of the fourth ventricle
The MLF lies medially to the CN VI nucleus
What is the pathway of CN VI to the Orbit?
Travels vertically along the ventral face of the Pons, continues through the subarachnoid space, perforates the dura mater 2 cm below the posterior clinoid process, passes intradurally into the cavernous sinus, where it runs below and lateral to the ICA. Then enters the orbit through the superior orbital fissure within the annulus of Zinn and innervates the LR on its ocular surface
What does CN VI innervate>
It provides motor input to the lateral rectus muscle
What makes this nerve susceptible to stretch injury in situations of increased ICP?
The long route that it travels makes it susceptible
What is the function of CN VII? Developmentally, where does it come from?
Provides with motor and sensory input to a variety of structures of the face/head
Developmentally comes from the 2nd branchial arch
What does the motor root of CN VII contain?
It contains special visceral efferent fibers that innervate the muscles of facial expression
What does the sensory root of CN VII contain?
contains the nervus intermedius, which itself contains the special visceral afferents, general somatic afferents, and general visceral efferent fibers.
What are the special visceral afferent fibers of CN VII?
the convey taste from the anterior 2/3 of the tongue
terminate centrally in the nucleus of the tracts solitarius
What are the general somatic afferent fibers of CN VII?
convey sensory information from the external auditory meatus and retro-auricular skin
centrally enters the spinal nucleus of CN V
What are the general visceral efferent fibers of CN V II?
Pre-ganglionic parasympathetic innervation via the sphenopalatine ganglion to the lacrimal, submandibular, and sublingual glands
Where is the motor nucleus of CN VII located?
its located in the caudal 1/3 of the Pons
Note: is cigar-shaped and 4 mm long, and contains 4 distinct groups that innervate specific facial muscles
Note 2: the lower half of the face is influenced by the corticobulbar fibers from the opposite cerebral cortex
Where is the sensory nucleus of CN VII located?
located in the Rostral portion of the tractus solitarius
What is the sensory nucleus of CN VII known as? What does it innervate?
also known as the "Gustatory Nucleus", taste from the anterior 2/3 of the tongue are carried here via special visceral efferent fibers
What does the temporofacial division of CN VII innervate?
It gives rise to temporal, zygomatic, and buccal branches
Temporal--upper half of orbicularis oculi, frontalis, corrugator supercilii, and pyramidal is muscles
Zygomatic- supplies lower 1/2 of orbicularis
Where does parasympathetic outflow of CN VII originate from?
the superior salivary nucleus and lacrimal nucleus
What controls reflex lacrimation?
Sensory nuclei of CN V
What does the Greater superficial petrosal nerve supply?
gives rise to unmyelinated postganglionic fibers that innervate the globe, lacrimal glands, glands of palate, and nose
What is the cavernous sinus?
an interconnected series of venous channels located posteriorly to the orbital apex and lateral to sphenoidal air sinus and pituitary fossa
What structures does the cavernous sinus contain?
the ICA (surrounded by sympathetic carotid plexus), CN III, CN IV, CN V1, CN V2, and Cn VI
Where do the cavernous sinus and other venous sinuses drain into?
they carry blood away from the brain and drain into the internal jugular veins