IOA2 Exam 3 - Visual Pathway Pt 1

What is the visual pathway?

A group of cells and synaptic connections that carry visual information from the environment to the brain for processing

What is the first step of the visual pathway?

Light from the external world is converted into electrical signals by photoreceptors in the retina

After photoreceptors convert light into electrical signals, how does the signal travel?

Through RGCs, which form the optic nerve

Where does the signal travel thru after reaching the optic nerve?

Optic chiasm, where some fibers cross over

Which fibers cross over at the optic chiasm?

Fibers from the nasal retina cross to the opposite hemisphere, while temporal fibers remain uncrossed

Which area is the continuation of optic nerve fibers after the optic chiasm, and where does this carry visual information to?

Optic tract carries visual info to the thalamus (LGN)

Where do optic tract fibers synapse in the brain?

LGN of the thalamus (diencephalon)

After passing through the LGN, where does visual information go?

Visual cortex (primary and association areas), for processing

What role does the primary visual cortex play in vision?

It processes vision

Which is the first area to receive visual input, and is where initial processing occurs?

Primary visual cortex (V1)

Visual information processing doesn't stop at V1! Which other visual areas or associated areas process more complex visual information?

V1 --> V2 --> MT/V5 --> VIP, MST, or FST

V1 --> V3 --> V4 --> FST, IT, or TEO

Which main structures are involved in the primary visual pathway? (ie summarize the route visual information takes from the eye to the brain for processing)

-Retina

-Optic nerve

Optic chiasm

-Optic tract

-Laternal Geniculate Nucleus (LGN)

-Optic radiations

-Primary Visual Cortex (V1)

-Visual association areas

Which part of the brain is V1 located in?

Occipital lobe

Which artery is V1 located in?

Calcarine artery

What is the process of forming the neural tube called?

Neurulation

At what week during nervous system development does neurulation occur?

3-8 weeks

In the early embryo (3-4 weeks), the neural tube forms

3 primary brain vesicles

List the 3 primary vesicles formed by the neural tube

-Prosencephalon (forebrain)

-Mesencephalon (midbrain)

-Rhombencephalon (hindbrain)

Forebrain =

Prosencephalon

Midbrain =

Mesencephalon

Hindbrain =

Rhombencephalon

Which of the 3 primary vesicles are considered part of the brainstem?

-Midbrain or Mesencephalon

-Hindbrain or Rhombencephalon

What occurs in the embryo at 5 weeks?

The 3 primary brain vesicles differentiate into 5 secondary brain vesicles

Which secondary brain vesicles differentiate from the prosencephalon?

-Telencephalon

-Diencephalon

The two areas of the forebrain (telencephalon and diencephalon) are separated by the ________ in the adult brain.

optic chiasm

Which structures does the telencephalon give rise to?

-Optic tracts

-Optic radiations

-LGN

-Cerebral cortex (cerebellum)

Which structures does the diencephalon give rise to?

-Optic cup

-Optic stalk

-Optic chiasm

-Epithalamus (pineal gland)

-Thalamus --> LGN + MGN

-Hypothalamus

Which secondary brain vesicle develops from the mesencephalon?

Mesencephalon (stays the same)

Which structures does the mesencephalon give rise to?

-Midbrain

-Tectum

-Superior colliculi

-Inferior colliculi

Which secondary brain vesicles differentiate from the rhombencephalon?

-Metencephalon

-Myelencephalon

Which strucutres does the metencephalon give rise to?

-Pons

-Cerebellum

Which structures does the myelencephalon give rise to?

Medulla oblongata

What structure does the optic nerve develop from?

Optic stalk

Which cells form the optic nerve?

Axons of ganglion cells

What develops at the center of the optic cup?

Optic fissure develops into the optic disc

What penetrates the optic fissure, and what does this eventually develop into?

Hyaloid vessels, which develops into the CRA

What does the neural ectoderm develop?

-Optic cup (retina, iris, CB)

-Optic stalk

The neural crest cells (NCCs) interact with the _______ and ________ to contribute to the development of the optic nerve and optic. stalk.

optic vesicle and surface ectoderm

The neural crest cells (NCCs) interact with the optic vesicle and surface ectoderm to contribute to the development of the _______ and _______.

optic nerve and optic stalk

The neural crest cells (NCCs) also develop the _________ to contribute to the formation of the corneal endothelium, eyelids, iris, and ciliary body.

optic nerve meninges

The neural crest cells (NCCs) also develop the optic nerve meninges to contribute to the formation of the (4):

-corneal endothelium

-eyelids

-iris

-CB

Defects in NCCs migration and differentiation can lead to eye diseases like...

-Axenfeld-Rieger Syndrome

-Peters Anomaly

Both Axenfeld-Rieger Syndrome and Peters Anomaly are:

A: Autosomal dominant

B: Autosomal recessive

C: X-linked dominant

D: X-linked recessive

Autosomal recessive

3 multiple choice options

What part of the eye is primarily affected in Axenfeld-Rieger Syndrome?

Anterior segment of the eye

Which abnormalities in the anterior segment of the eye can lead to Axenfeld-Rieger Syndrome?

-Aniridia

-Glaucoma (50% of cases)

-Childhood

-AR

Which gene mutations are associated with Axenfeld-Rieger Syndrome?

-PITX2

-PAX6

-FOXC1

-RIEG2

What are the underlying NCCs defects in Axenfeld-Rieger Syndrome?

Defects in NCCs

-migration

-differentiation

-cell cycle arrest in the anterior chamber, facial bones, and CV system

What is Peters Anomaly?

Rare congenital disorder characterized by central corneal opacity with a clear peripheral cornea

What is the tx for Peters Anomaly?

Surgery of corneal transplantation

What happens during weeks 6-8 of embryological development?

Increase of differentiation of visual structures from the neuroectoderm and NCCs

What visual structures differentiate from neuroectoderm and NCCs during weeks 6-8?

-Neural retina

-Future RPE

-Sclera

-Choroid

-Optic nerve sheaths

-Ganglion cell axons in optic nerve

When does myelination of the axons within the optic nerve begin?

Shortly before birth and continues after birth

What are the first cells in the visual pathway that convert light energy into neuronal signals?

Photoreceptors

Name the types of photoreceptors in the retina

-Blue cones

-Green cones

-Red cones

-Rods

Photoreceptors convert light energy into neuronal signals that is passed to the:

bipolar cell --> ganglion cell layer --> ON

What cells modulate signals between photoreceptors and bipolar cells?

Horizontal cells

What cells modulate signals between bipolar cells and retinal ganglion cells?

Amacrine cells

What are the output neurons of the retina whose axons form the optic nerve?

Retinal ganglion cells

Where are all the cells and synapses of the visual pathway located?

Within the retina

Where do retinal ganglion cells project, and where do their fibers exit?

-Project into the NFL

-Fibers exit retina via the ON

What is Meyer's Loop?

A bundle of white matter fibers that runs from the thalamus to the occipital cortex, looping around the temporal horn of the lateral ventricle

Meyer's loop runs from the _______ to the _______

thalamus --> occipital cortex

What does Meyer's loop loop around?

Temporal horn of the lateral vesicle

Part of Meyer's loop which is a broad white band of white matter that carries visual info from the retina

Optic radiation

What type of cell is the first order neuron in the visual pathway connected to?

Bipolar cells

What type of cell is the second order neuron in the visual pathway connected to?

Ganglion cells

Where is the third order neuron in the visual pathway located?

Lateral geniculate body

What are the steps of the flow of visual information, for light entering the right eye?

1) Light enters the Right Eye

2) Bipolar Cells and Ganglion Cells process the signal

3) First and Second Order Neurons transmit the signal

4) Optic Nerve carries information from the eye

5) Optic Chiasma: Nerves cross over

6) Optic Tract carries information onward

7) Lateral Geniculate Body (LGB) processes information (Third Order Neuron)

8) Optic Radiations carry information to the cortex

9) Meyer's Loop (part of radiations) loops around the temporal horn

10) Occipital Cortex interprets the visual information

How are corneal and retinal quadrants anatomically oriented relative to the optic disk?

-Nasal (towards nose)

-Temporal (towards temporal bone)

-Superior

-Inferior

OD:

2 = Temporal

3 = Nasal

4 = Superior

5 = Inferior

OS:

2 = Nasal

3 = Temporal

4 = Superior

5 = Inferior

Label #2-5 in a right eye vs left eye:

What happens to the nasal information from RGC axons at the optic chiasm?

Contralateral nasal information of RGCs axons crosses at the level of the optic chiasm (optic nerve decussation)

Where do nasal RGCs axons synapse?

At the opposite (contralateral) side of the brain

What type of information does the optic nerve contain?

Info ONLY from one eye (ipsilateral nasal and temporal)

What type of information does the optic tract contain?

Mixed info from both eyes

-ipsilateral from temporal RGC axons

-contralateral from nasal fibers of the opposite eye

Where in the forebrain do the RGC fibers in the optic tract synapse?

-LGN

-Pulvinar

-Superior colliculus (SC)

-Tectal areas

Where do the fibers that project from the LGN synapse?

As optic radiations, they synapse at the PRIMARY VISUAL CORTEX (V1) in the occipital lobe

2) Dorsal bundle

3) Central bundle

4) Meyer's loop

Label the components of optical radiation (#2-4)

How do retinal nerve fibers exit the eye?

Retinal nerve fibers make a 90 degree turn at the optic disc and exit as the optic nerve

Which type of fibers does the optic nerve consist of?

RGCs fibers

Where do 90% of optic nerve fibers synapse?

LGN

Where do 10% of optic nerve fibers project to?

Areas controlling pupil responses or the circadian rhythm

How many nerve fibers are in the optic nerve?

1.0 - 1.3 million

What is the length of the optic nerve?

5-6 cm long

What is the diameter of the optic nerve when IOP <20mmHg?

3.5-5 mm

The optic nerve extends from the _______ to the _______.

eye --> optic chiasm

What 4 segments is the optic nerve divided into?

-Intraocular

-Intraorbital

-Intracanalicular

-Intracranial

What is the length of the intraocular (disc) segment of the optic nerve?

0.7 - 1 mm

What are the regions of the intraocular segment of the optic nerve?

-Prelaminar

-Laminar (lamina cribosa)

-Retrolaminar

1) Prelaminar region

2) Laminar region

3) Retrolaminar region

Label the regions of the intraocular segment of the optic nerve

What does the prelaminar region contain?

Unmyelinated axons

What does the laminar region contain?

Unmyelinated axons

What does the retrolaminar region contain?

Myelinated axons

What is the length of the intraorbital segment of the optic nerve?

25 - 30 mm

What is the length of the intracanalicular segment of the optic nerve?

6 - 10 mm

What is the length of the intracranial segment of the optic nerve?

10 - 16 mm

What is the optic nerve head aka?

Blind spot

Why is the optic nerve head called the blind spot?

There are no rods or cones (photoreceptors) overlying it

Name the common diseases related to the ONH

-Optic nerve glioma

-Glaucoma

A: Optic nerve head

B: Retina

C: CRA / CRV

D: Bundles of RGC axons forming optic nerve

E: Lamina cribosa

F: Retina

G: Choroid

H: SPCA / SPCV

I: Pial sheath

J: Arachnoid sheath

K: Dural sheath

L: Intervaginal spaces

M: RGCs

Label each component of the optic nerve head

A: Surface NFL and physiologic cup

B: Prelaminar region

C: Laminar cribosa region

Label each histological component of the longitudinal section of the optic nerve head (A, B, C)