Retina to Brain Visual Pathway Flashcards
The visual pathway is the: optic nerve, optic chiasm, optic tract, and optic radiation (geniculocalcarine tract)
What are each parts of the pathway composed of?
The optic nerve, optic chiasm and and optic tract are all composed of retinal ganglion cell axons.
The otptic radiation is composed of LGN cell axons. This carries the signal from the LGN to the primary visual cortex.
Why is the ON not a “True” peripheral nerve?
- It is an extension of the brain tract
- It is myelinated by oligodendrocytes (CNS) and not schwann cells (PNS)
- Most of it is covered by the 3 layers of the meninges, including the subarachnoid with surrounding CSF
Location of retinal nerve fibers comprising the optic nerve from the optic disc to orbit
Macular fibers make up the largest wedge temporally. Superior temporal fibers- superior wedge Inferior temporal fibers- Inferior wedge Superior nasal fibers- SN wedge Inferior nasal fibers- IN wedge
Location of ganglion cell axons comprising the optic nerve from the orbit to the cranium (anterior to pituitary gland)
Macular fibers- Central Superior temporal fibers- ST quad Inferior temporal fibers- IT quad Superior nasal fibers- SN quad Inferior nasal fibers- IN quad
Along its path, the ON is divided into which 4 sections?
Intraocular (1 mm)
Intraorbital (25-30mm)
Intracanalicular (6-10mm) In the optic canal
Intracranial (10-16mm)
Intraocularlly, the nerve fibers (The ON) are separated from retinal layers by ___
A ring of glial tissues called the intermediary tissue of kuhnt.
Continuation of the intermediary tissue of Kuhnt (Same glial tissue)
The border tissue of Jacoby continues posteriorly. This separates the nerve fibers from the choroid.
Border tissue of Elschnig
A ring of collagenous tissue derived from sclera.
Lies outer to glial tissue (Intermediary of kuhnt and border tissue of jacoby)
Has two regions: Prelaminar and Laminar
Glial tissue that surrounds ON
Intraocularly. The intermediary tissue of Kuhnt separates the retina from the ON.
The Border Tissue of Jacoby separates the choroid from the ON.
Same glial tissue. A continuation.
The two divisions of the border tissue of Elschnig that is intraocularly
Prelaminar is intraocular. Extends from the optic disc to the lamina cribrosa. Not myelinated. Sheathes of astrocytes bundle the nerve fibers into ~1,000 fascicles.
Laminar region is intraocular. Includes fibers that extend through the lamina cribrosa. At the posterior portion of the lamina cribrosa, the fibers become myelinated.
(lamina cribrosa may be a barrier to oligodendrocytes)
The infraorbital portion of the ON extends from __ to ___
The posterior margins of the sclera to the optic canal. Courses medially to reach the optic canal.
Why does the length of the ON within the infraorbital part exceed the distance from the globe to the optic canal?
Slight sine wave shaped curve. Allows for full eye movement without stretching of the nerve.
The infraorbital region of the ON is covered by 3 layers of
Meninges that fuse and merge with the sclera.
Pia- innermost. Loose, vascularized connective tissue. Separates fiber bundles until the optic chiasm (takes over for the astrocytes)
Arachanoid- Web of collagen with CSF in the subarachnoid space. Surrounds ON, but stops once it reaches posterior sclera on the back of the eye.
Dura- Tough, dense connective tissue with elastic fibers.
Pia septa in the intraorbital space has a similar role to which cell from the intraocular space?
Astrocytes in the intraocular space (mostly the prelaminar division) bundle the nerve fibers into fascicles.
The pia in the infraorbital space separate the fiber bundles.
Difference between the intermediary tissue of Kuhnt and the border tissue of Jacoby
Intermediary tissue of Kuhnt- Separates ON from retina
Border tissue of Jacoby- Separates ON from choroid.
Intracanalicular division of the ON
Is the portion of the ON within the optic canal. Accompanied by the ophthalmic artery.
mm in length of each division of the ON
Intraocular- 1mm (shortest)
Intraorbital- 25-30mm (longest)
Intracanalicular- 6-10mm
Intracranial- 10-16mm
The optic chiasm is usually located where and lies within what
Located anterior to the pituitary gland. in 10% of people, it is displaced more anterior, and in 15% of people, it is displaced posteriorly.
Lies within the circle of willis.
At the optic chiasm, what do nasal fibers do?
SOME Inferior nasal fibers loop contralaterally to form the anterior knee of wildebrand and then continue down the contralateral side.
SOME Superior nasal fibers loop ipsilateral to form the posterior knee of hildebrand and then continue down the contralateral side.
Others will make a direct and efficient cross.
Big picture. The optic tract is divided into
The small medial root (10% of fibers head here)
The large lateral root (90% of fibers head here)
The small medial root, which is where 10% of ON fibers travel along the optic tract, has 4 terminations where
- Several - Superior colliculus, eye/head movements
- People - Pretectal nuclei, pupillary light reflex
- Purchased - Paracellular reticular formation, arousal
- Houses - Hypothalamus, circadian rhythm
The large medial root, which contains 90% of the optic tract fibers, synapses where?
With neurons in the LGN of the thalamus.
Optic radiation. Where do superior fibers and inferior fibers travel and terminate?
Superior fibers travel through the parietal lobe and then terminate at the cuneus gyrus, which is above the calcarine sulcus.
Inferior fibers travel anteriorly into the temporal lobe at first, which is called meyer’s loop, and then travels posteriorly to terminate at the lingual gyrus, which is below the calcarine sulcus.
Macular fibers are generally situated between superior and inferior fibers.
Primary visual cortex can also be called
Striate cortex, Area 17, Brodmann 17, V1
The primary visual cortex is located where
Located at the most posterior aspect of the occipital lobe. It extends into the medial surface of each hemisphere, where it is largely buried within the calcarine fissure.
Fibers representing central retina terminate where at the primary visual cortex?
At the most posterior aspect. Accounts for 1/3 of the visual cortex.
Fibers representing peripheral retina terminate where at the primary visual cortex?
Terminate at the anterior aspect of the cortex, medial in the calcarine fissure.
