Orthoptics I - the visual pathway

What is the retina?

neurosensory, light-detecting tissue

What are the 10 layers of the retina?

pigment epithelium
rods and cones
external limiting membrane
outer nuclear layer
outer plexiform layer
inner nuclear layer
inner nuclear layer
ganglion cell layer
nerve fibre layer
internal limiting membrane

What are the 3 main types of neurone found in the retina?

photoreceptors
bipolar cells
ganglion cells

What are the photoreceptors?

rods and cones at the back of the retina which propogate light signal

What are the bipolar cells?

interneurons
On and Off types

What are the ganglion cells?

non-myelinated cells that collect information from the bipolar cells

What are the support cells of the retina?

horizontal cells and amacrine

What are the horizontal cells?

laterally interconnecting neurons
integrate and regulate multiple photoreceptors

What are the amacrine cells?

major carrier of rod signals to ganglion cells, speeding up the slow potential

What cells make up the optic nerve?

axons of ganglion cells that run in arc-like patterns

What are the 4 regions of the optic nerve?

intraocular - at OD, where fibres move to the retro-orbital region
intraorbital - posterior eye to optic canal (surrounded by meninges)
intracanalicular - inside optic canal of the sphenoid
intracranial - travels superior to diaphragma sellae and cavernous sinus, forming the optic chiasm

What is the diaphragma sellae?

flat dura mater with a circular hole allowing the vertical passage of the pituitary stalk

What is the optic chiasm?

point at which optic nerve fibers cross in the brain

Where does the optic chiasm sit?

in/around the circle and Willis and pituitary gland

How can a pituitary adenoma impact the chiasm and the visual field?

susceptible to compression
- medial axons impacted most (nasal fibres as they are contralaterally projecting)
- temporal VF defect

What are the optic tracts?

axons project posterolaterally from the chiasm, winding around the lateral margin of the cerebral penduncle
- adherent to the midbrain, overlapped by the uncus and parahippocampal gyrus
- heading towards the lateral geniculate nucleus

What is the cerebral penduncle?

two stalks that attach the cerebellum and brainstem

What is the parahippocampal gyrus?

grey matter cortical region of the brain that surrounds the hippocampus and is part of the limbic system

What is the lateral geniculate nucleus?

ovoid swelling found in the thalamus, connected to the superior colliculus via superior brachum
- 6 layers

What can the LGN be divided into?

magnocellular and parvocellular layer

What layers do contralateral (nasal) nerve fibres terminate at in the LGN?

1, 4 and 6

What layers do ipsilateral (temporal) nerve fibres terminate at in the LGN?

2, 3 and 5

What is another name for the optic radiations?

geniculocalcarine tract

What do the optic radiations do?

transmit visual symptom to visual cortex, allowing for retinotopic organisation

What is retinotopic organisation?

neurons with receptive fields close together in visual space have cell bodies close together in the visual cortex ?

What can the optic radiations be divided into?

ventral - superior retina
medial - macula
dorsal - inferior retina

Where is the visual cortex?

occipital lobe

Where is the primary visual area of the visual cortex?

calcarine sulcus
- Brodmans area 17

Where is the secondary visual area of the visual cortex?

Brodmans area 18 + 19

Describe Brodmans area 17

located in and on either side of the calcarine sulcus, on the medial surface of the occipital lobe
- 6 layers that discern shape, size and location of objects

In which layer of Brodmans area 17 does the LGN terminate?

4th

What is the intracortical loop?

layers 2-3 of brodmans area 17 being transferred to layer 5 and then 6

How is the primary somatosensory cortex organised?

columnar cell organisation, where each column corresponds to specific visual stimuli
e.g. inferior calcarine sulcus = superior VF

Where does a disproportionate amount of the cortex respond to?

macula

What is the striate cortex responsible for?

colour, motion, stereopsis

What is the occipital cortex responsible for?

spatial recognition and object identification