Visual Pathway Flashcards
Give the anatomical overview of the visual pathway
- Light information reaches the back of the eyes in the retina.
- The optic nerves (CN II) emerges from back of retina and travels back until it reaches the optic chiasma (where the 2 optic nerves converge).
- The optic tracts then pass towards the back of the midbrain until they hook upwards to connect with the lateral geniculate body of the thalamus (this includes the lateral geniculate nucleus).
- From the LGB, there are the optic radiations; the fibres that bring visual information from the thalamus to the primary visual cortex in the occipital lobe.
Where do the optic nerves emerge from?
The retina
After converging at the optic chiasma, where do the optic tracts then pass?
To the lateral geniculate body of the thalamus (this includes the lateral geniculate nucleus)
What is the lateral geniculate nucleus important for?
This is an important nucleus in the thalamus; important in relaying visual information.
From the lateral geniculate body of the thalamus, how is information brought to the primary visual cortex?
From the LGB there are the optic radiations
Where is the primary visual cortex?
Occipital lobe
There are 3 layers encircling the eye. What are they composed of?
- Outer layer; cornea and sclera (these are continuous)
- Middle (vascular) layer; choroid
- Inner layer; retina
Describe the cornea. Main function?
- Description:
- Transparent part of the eye that covers the front portion of the eye (pupil and iris)
- Thick, transparent, avascular.
- Function: major area of refraction
Describe the sclera. Main function?
- Description:
- Covers most of the ocular surface, “white of the eye”.
- Continuous with the cornea
- Tough, white, fibrous tissue
- Function: Insertion point for muscles that move the eyeball.
Describe the choroid. Function?
- Description:
- Vascular
- Continuous with the lens and ciliary muscle.
- Functinon: Nourishes the cornea and retina
Describe the iris. Function?
- Description: Pigmented, vascular.
- Function:
- Eye color is defined by that of the iris
- Muscles of the iris control the amount of light entering the eye by controlling the diameter of the pupil (the central opening of the iris).
Function of the ciliary body?
Controls the shape of the lens by pulling on the suspensory ligaments.
Describe the lens. Function?
- Description: Biconvex, avascular. Light passes through the lens after passing through the pupil.
- Function: changes shape (due to ciliary body)
- When rounded, can focus on objects which are near.
- When flattened, can focus on distant objects
When rounded, what can the lens focus on?
can focus on objects which are near
When flattened, what can the lens focus on?
distant objects
What is the retina an extension from?
Diencephalon
How many layers is the retina composed of?
2
What are the 2 layers of the retina?
-
Non-neuronal layer:
- Consists of pigmented epithelium.
- Sits against the choroid and is light absorbing
-
Neural layer:
- Nerve tissue; photoreceptors, bipolar cells, ganglion cells
- Ganglion cell axons form the optic nerve –> This allows transmission of light information (photons) into electrochemical information
What is the optic disk?
Where the optic nerve exits –> this is a blind spot
Why is the optic disk a blind spot?
we do not perceive any light information that falls in this region as there are no photoreceptors
What is the macula? Where is it located? Function?
- Region at very back of retina that falls right in middle of visual axis (peripheral vision not as clear)
- Found medial to optic disk
- Any light rays that travel directly in this region and fall upon the macula, this information is interpreted with high visual acuity –> as the macula contains a high density of photoreceptors
Why does the macula have a high visual acuity?
as the macula contains a high density of photoreceptors
Where is the fovea located? Function?
- Found in centre of macula
- Contains some specialised photoreceptors called cones –> highest point of visual acuity
What are ‘cones’? Where are they located?
Cone cells, or cones, are photoreceptor cells in the retinas (in the fovea) of vertebrate eyes
What is the yellow spot?
Optic disk; corresponds to exit of optic nerve (surrounded by arteries)
Why is the macula lutea region darker?
As contains higher density of photoreceptors
What is the function of the non-neuronal layer of the retina? What does it contain?
- Pigmented epithelium
- Contains melanin which absorbs light; provides nutrients to photoreceptors
- Function: maintains the metabolic activity of the photoreceptors, bipolar neurones and ganglion cells (found in neural layer)
What does the non-neuronal layer of the retina sit against?
The choroid of the eye
How does the non-neuronal layer of the retina protect the retina?
Absorbs excess light which is not passed on to the photoreceptors to reduce degeneration of the retina.
What does the neuronal layer of the retina contain?
The neuronal layer contains photoreceptors, primary bipolar neurons, secondary ganglion neurons and interneurons
What are the 2 photoreceptors of the retina?
Rods and cones
After light information is passed to the neuronal layer of the retina, describe the order of the visual pathway in this layer
- After the non-neuronal layer, light information is then passed onto our photoreceptors (rods and cones) which transmit this information to the neurons that make up the visual pathway:
- 1st order neurons (bipolar cells) link photoreceptors (rods and cones) to 2nd order neurons
- 2nd order neurons (ganglion cells) axons of ganglion cells combine to form optic nerve
(optic nerve then leaves retina to travel to thalamus)
What are the 1st order neurons of the visual pathway?
Bipolar cells; these link photoreceptors (rods and cones) to 2nd order neurons
What are the 2nd order neurons of the visual pathway?
Ganglion cells
What forms the optic nerve?
Axons of ganglion cells combine to form optic nerve
Interneurons are also present in the neuronal layer of the retina. What is their function? What are the 2 types of interneurons found?
Purpose is to modulate transmission of information:
- Horizontal interneurons
- Amacrine interneurons
Where are the horizontal interneurons found?
between bipolar cells and rods and cones (photoreceptors)
Where are the amacrine interneurons found?
Between bipolar cells and ganglion cells
Describe the organisation of rods and cones across the retina.
They are heterogeneously expressed across the retina with only cones being located at the macula. Rods are 20x more common than cones.
Individual advantages of rods and cones?
Rods:
- Sensitive to light; allow vision in dim light
- High level of convergence
Cones:
- Colour vision
- High visual acuity –> at the macula, one cone to one ganglion cell
- Lower level of convergence
What is the black spot?
Optic disk is black spot; absence of rods and cones
Identify this
Calcarine sulcus (can only appreciate in medial view of brain)
When the optic nerve exits through the optic canal, what does it take with it? Why?
- Exits through optic canal, taking layers of meninges with it.
- CN II is an outgrow from the diencephalon (is part of the CNS)
Which fissure transmits the optic nerve?
Optic canal (the optic canal opens into the skull base at the optic foramen)