Chapter 3: Cranial Nerves Flashcards
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