5 –The visual system Flashcards
How many pairs of cranial Nerve do we have?
12 pairs
What makes cranial nerves 1 and 2 different?
They develop from the CNS
What is cranial nerve 1 an extension of?
Our telencephalon
What is cranial nerve 2 an extension of?
Our diencephalon
Describe the cornea
The clear outer portion of the eye
What is the role of the cornea?
It filters light first
Describe the sclera
It is a continuation of the cornea
What part of the eye does the sclera form?
It is responsible for the white part of your eye
What are the responsibilities of the sclera and cornea?
They both protect the eye
What is the role of the sclera
It is the attachment point of all the muscles to be able to move the eye
Name the structures that form the outer layer fo the eye
The cornea and sclera
What is the middle layer of the eye composed of?
- The choroid
- the iris
- Ciliary body
Describe the choroid
It is very vascular
Why is the choroid very vascular
It supplies both the cornea and the retina
What is the iris responsible for?
Iy is a pigmented layer responsible for our eye colour
What is the Iris composed of?
Different muscles the are in a circular shape
What do the muscles surrounding the iris do?
They can either contract or dilute to change the diameter of the pupil
What is the pupil?
The central opening of the iris
What does the Ciliary body attach via?
Attaches via the suspensory ligament to the lens
What does the Ciliary body do?
By pulling on the suspensory ligaments it can stretch and change the shape of the lens
This ultimately control the amount of light reaching the retina
What is the inner most layer?
The retina
What is the retina full of?
Neurones that receive light
What is found on the visual axis of the retina?
The macula
Describe the macula
It Is an area where there is a high density of cones only
What is the centre of the macula called?
The fovea
What is special about the fovea?
It is the area where we have the highest visual acuity
Name cranial nerve 2
The optic nerve
Where does the optic nerve exit the retina from?
The optic disk
What is social about the otic disk
There are no photoreceptors in that area (the human blindspot)
What is the cornea a major area of?
Refraction
What is the retina an extension of?
The diencephalon
What can the retina be split into?
a non-neuronal and a neuronal layer.
What does the non-neuronal layer consist of?
PIgmented epithelium
Where is the non-neuronal layer found?
It Sits against the choroid and is light absorbing.
What does the pigmented epithelium of the retina do?
It absorbs light first
What does the neural layer consist of?
Photoreceptors, bipolar cells, ganglion cells
What do the cells of the pigmented epithelium connect to ?
the first Neurones:
The photoreceptor part of the neural layer
What are the 2 types of photoreceptors called?
Rods
Cones
What do the photoreceptors cells connect to ?
Bipolar cells
What do the bipolar cells connect to ?
Ganglion cells
What do the axons of the ganglion cells exit as?
The optic nerve
What is an interneurone
A neurone that connects one neurone to another
Name the interneurones present in the retina
- Amacrine
2. Horizontal
What are all the neurone connections in the retina essentially doing?
Helping transmit the electrical information from light into electrochemical information of action potentials
Where is the blind spot in our eye?
At the optic disk
What are photoreceptors?
They are neurones specialised in interpreting light information that comes through our eyes
How are rods and cones distributed in our eyes?
They are heterogeneously expressed across the retina
Name an area of the eye where only cones are expressed
The macula
Which type of photoreceptor is the most common?
Los are 20times more common than cones
Describe rods
They are Sensitive to light
Good Vision in dimlight
Have a High level of convergence
Describe cones
Give us colour vision
Have a high visual acuity
Have a lower level of convergence than rods
What are cones important for
Giving is colour vision and high visual acuity
Where are cones concentrated?
At the macula
Where is the macula found?
Middle of the fovea
How many neurones make up the pathway from the photoreceptors to the cortex?
3 Neurone chain
Where are the 1st and second neurones of the visual pathway found?
In the retina
Name the 1st neurone of the visual pathway
Bipolar cells
Name the 2nd neurone of the visual pathway
Ganglion cells
Where does the optic nerve project after leaving the retina?
Once it exits the retina via the optic disk the optic nerve projects to thalamus
What area of the thalamus does the optic nerve connect to?
The lateral geniculate nucleus (LGN)
What happens to the lateral geniculate nucleus?
It is where the optic nerve synapses with the third neurone of the pathway
After the synapsing the the lateral geniculate nucleus where does the impulse travel?
From the thalamus to the cerebral cortex via optic radiation
What is the optic nerve?
It is an outgrowth from the diencephalon so it is actually part of the CNS.
Since the optic nerve is an extension of the CNS what is it surrounded by?
Meninges surrounding it to the eyeball.
Name the 3 layers of meninges
Dura
Arachnoid
Pia
What does a rise in cerebral spinal fluid pressure lead to?
Papilloedema
What is Papilloedema?
A swelling of the optic disk due to the increase in CSF pressure
What does Papilloedema lead to?
Increase in pressure compresses the central retinal vein preventing venous drainage from the eye
resulting in major swelling
What are some symptoms of papilloedema?
Head aches
Drowsiness
Blurred vision
Vomiting
After exiting the retina what do the pair of optic nerves do?
They merge at the optic chiasma
Where do the 2 optic nerves merge once exiting the retina?
The optic chiasma
What happens at the optic chiasma?
The 2 optic nerves meet and some of the fibres from the left and right nerves cross over to the other side
Some fibres will not
What is the crossing over of SOME fibres called and where does it occur?
It is called Hemidecussation
of fibres
and it occurs at the optic chiasma
Where do the fibres travel after leaving the optic chiasma?
The fibres travel along the optic tract to our Lateral
geniculate
nucleus of
thalamus
What happens to the fibres after reaching the lateral geniculate nucleus?
From the thalamus the fibres travels the primary visual cortex vis the optic radiations
How is the visual pathway organised?
It os retinotopically organised
What does it mean when we say the visual pathway is retinotopically organised?
Visual info is going to be segregated into different specific ways all along the pathway
Where does the left half of the visual field go?
To the right hemisphere
Where does the RIGHT half of the visual field go?
To the left hemisphere
Where does the upper visual field go?
To the lower bank of calcarine sulcus
Where does the lower visual field go?
To the upper bank of calcarine sulcus
Where does the centre of the visual axis (macula) go?
Goes to the occipital pole
What happens to our visual field in the retina?
The images from the visual field are inverted onto the retinal fields: upside down and mirror reversed
Which fibres cross at the optic chiasm?
Nasal fibres cross
Which fibres don’t cross at the optic chiasm?
Temporal don’t
Where are your nasal fibres?
They are the fibres Medial to the fovea
Where are your temporal fibres?
They are the fibres Lateral to the fovea
Where are the upper and lower visual fibres separated?
At the Lateral geniculate nucleus (LGN)
Where do the fibres from the lower visual field travel?
Along the superior trajectory to the upper bank of the Calcarine sulcus
Where do the fibres from the superior visual field travel?
Inferior trajectory/Meyer’s loop to the lower bank of the Calcarine sulcus
What is another name for the inferior trajectory?
Meyer’s loop
Where are fibres fro the macula represented in the primary visual cortex?
Most posteriorly (towards the tip of the occipital pole)
How can vision be disrupted?
Vision can be disrupted due to damage to the visual pathway at it passes from the retina to the cerebral cortex.
What is a scotoma?
localised patch of blindness
What is an anopia?
Refers to the loss of one or more quadrants of the visual field
What are the 2 types of anopia called?
Hemianopia
Quadrantanopia
What is Quadrantanopia?
A type of anopia where by a quarter of the visual field is lost
What is Hemianopia?
A type of anopia where by half of the visual field is lost
What is Homonymous?
When Visual field losses are similar for both sides
What is Heteronymous?
When Visual field losses are on different sides
What is the clinical condition associated with a localised patch of blindness?
Scotoma
What is the clinical condition associated with losing Half of the visual field?
Hemianopia which is a type of anopia
What is the clinical condition associated with losing a quarter of the visual field?
Quadrantanopia which is a type of anopia
