Week 11: Visual Pathway Lecture Flashcards
How does light information travel from the retina to the cortex?
- Light information reaches retina at back of eye
- From there, the optic nerve emerges and carries information to the optic chiasma
- The optic tract travels to the lateral geniculate nucleus in the thalamus
- Optic radiations bring information from the thalamus to the primary visual cortex in the occipital lobes
What is the order of layers of the front of the eye that light passes through?
- Cornea and sclera
- Choroid
- Pupil
- Lens
- Retina
What structures is the choroid continous with?
lens and ciliary muscle
What is found in the neural layer of the retina?
- photoreceptors, bipolar cells, ganglion cells
- ganglion cell axons form the optic nerve
Where does the optic nerve exit the retina?
at the optic disc
What is another word for the optic disc?
blind spot –> any light information that falls in this region is invisible to us because there are no photoreceptors in this region
What does the non-neuronal layer of the retina consist of?
- pigmented epithelium
- pigmented epithelium contain melanin
- sits against the choroid
- is light absorbing
- provides nutrients to photoreceptors
Explain how light information goes from the pigmented epithelium to the optic nerve ?
- pigmented epithelium absorbs light and transmits information to rods and cones
- horizontal interneuron
- these photoreceptors pass information to bipolar cells (1st order)
- amacrine interneuron
- bipolar cells link to ganglion cells (2nd order)
- these axons combine to form the optic nerve
What is the function of the horizontal and amacrine interneurones?
to modulate the transmission of information
Give 4 features of each:
- rods
- cones
Rods:
- 20x more common
- sensitive to light
- vision in dim light
- high level of convergence
Cones:
- responsible for colour vision
- high visual acuity
- lower level of convergence
- at the macula, one cone to one ganglion cell
Why does the optic nerve have meninges?
it is an outgrowth from the diencephalon so it is actually part of the CNS
What is papilloedema?
- a swelling of the optic disc
- increase in CSF pressure in head is transmitted to the layers surrounding the optic nerve
- increases pressure and compresses the central retinal vein preventing venous drainage from the eye
What are the symptoms of papilloedema?
- headaches
- drowsiness
- blurred vision
- vomiting
Where is the primary visual cortex?
above and below the calcarine sulcus in the occipital lobe
Explain the 3 neuron visual pathway from photoreceptor to cortex
- Bipolar cells
- Ganglion cells
- Axons of ganglion cells run over retina to optic disc to form optic nerve
- Optic nerve projects to LGN
- Thalamocortical neurones (3rd order) travel in optic radiations to visual cortex
How is the visual pathway retinotopically organised?
- left half of visual field goes to right hemisphere
- right half of visual field goes to left hemisphere
- upper visual field goes to lower bank of calcarine sulcus
- lower visual field goes to upper bank of calcarine sulcus
- macula goes to occipital pole
How are images from the retina percieved?
mirror reversed and flipped upside down
What is the fixation point?
- when 4 quadrants transect and are divided by a horizontal line
- corresponds to fovea
Which fibres cross at the optic chiasma?
nasal fibres
Which fibres do NOT cross at the optic chiasma?
temporal fibres –> they remain ipsilateral
How do lower visual field fibres reach the upper bank of the calcarine sulcus?
through the superior trajectory
How do upper visual field fibres reach the lower bank of the calcarine sulcus?
through the inferior trajectory (Meyer’s loop)
Where are nasal and temporal fibres found?
nasal are more medial / further into the nose
temporal field is further into the periphery/ more lateral
How is the primary visual cortex retinotopically organised?
- the macula is represented most posteriorly (toward the tip of the occipital pole)
- the peripheral fields are represented more anteriorly
