Visual Pathway Flashcards
Where is the retina located?
What are the first 3 neurones of the visual pathway located in the neural layer of the retina?
- the retina is located at the very inferior aspect of the eye
- the first 3 neurones of the optic pathway are located in the neural layer of the retina:
- photoreceptors (rods & cones)
- bipolar cells
- ganglion cells
- the photoreceptors absorb light and connect to bipolar cells
- bipolar cells then connect to ganglion cells
- the axons of the ganglion cells collect together at the back of the retina as the optic nerve
Where is the optic disc located?
What vessels are found here? What cells are not found here?
- the optic disc is the point at which the optic nerve exits the retina
- the central retinal artery and vein are found at the optic disc
- it is called the “blind spot” as there are no photoreceptors located here
- these are the cells that absorb light - so we are blind to any light information that falls on the optic disc
What is the macula lutea and where is it located?
What is the centre of it called and why is this significant?
- it is an area adjacent to the optic disc that has a high concentration of photoreceptors
- it is located along the visual axis
- the fovea is the area at the very centre of the macula that contains a high concentration of cone cells
- the fovea is the centre of the visual field and is the point of highest visual acuity
What is significant about the optic nerve being an extension of the diencephalon?
- as the optic nerve is an extension of the diencephalon, it is surrounded by meninges
- there is a subarachnoid space containing CSF surrounding the optic nerve
- if there is an increase in CSF pressure at the level of the brain, this can translate to the optic nerve and cause it to swell (papilloedema)
What is papilloedema?
What typically causes it and what are the symptoms?
- papilloedema is swelling of the optic disc
- it is caused by increases in CSF pressure by space-occupying lesions or a failure to drain CSF
- the increase in CSF pressure compresses the central retinal vein, preventing drainage of blood from the eye and increasing the pressure even further
Symptoms:
- headaches
- drowsiness
- blurred vision
- vomiting
What are the structures involved in the visual pathway starting from the optic nerve?
- the optic nerve travels inferiorly to reach the optic chiasm
- at the chiasm, some fibres will cross and some will not
- from the chiasm, fibres travel to the lateral geniculate nucleus (LGN) of the thalamus via the optic tract
- this is underlying the lateral geniculate body (LGB)
- the LGN gives rise to optic radiations, which travel to the primary visual cortex in the occipital lobe
Locate the structures of the visual pathway
- the optic tract radiates around the midbrain to reach the LGN of the thalamus
- the primary visual cortex is the area of grey matter that surrounds the calcarine sulcus
What is an alternative name for the optic radiations?
geniculocalcarine tract
- they are passing from the geniculate nucleus to the area surrounding the calcarine sulcus
- the area of grey matter surrounding the calcarine sulcus is the primary visual cortex (striate cortex)
Identify the following structures
In which part of the internal capsule do the optic radiations travel in?
- the optic radiations travel mostly through the retrolenticular part of the internal capsule
- this is a radiation away from the posterior limb
- some fibres also travel in the sublenticular parts of the internal capsule
What are the 3 main parts of the internal capsule and their anatomical relationships?
Anterior limb:
- located lateral to the head of the caudate nucleus
Genu:
- named as it bends like a knee
Posterior limb:
- located lateral to the thalamus and medial to the lentiform nucleus
How do the sublenticular and retrolenticular limbs of the internal capsule travel and what fibres do they contain?
Sublenticular limb:
- fibres travel underneath / below the lentiform nucleus
- contains auditory radiations and some fibres that are part of the optic radiations
Retrolenticular limb:
- fibres arch behind the lentiform nucleus
- contains the majority of the optic radiations
What is labelled 1-5 in this diagram?
What must be removed to expose the internal capsule?
- sublentiform part
- retrolentiform part
- posterior limb of IC
- genu
- anterior limb of IC
- the lentiform nucleus must be removed to allow for exposure of the entire internal capsule
How can the primary visual cortex be further subdivided?
What is the role of this area and what is its BA?
- the primary visual cortex (BA 17) is where we consciously perceive visual information, but we do not interpret it
- it can be further subdivided into:
- upper bank of calcarine sulcus (superior to sulcus)
- lower bank of calcarine sulcus (inferior to sulcus)
- occipital pole (tip of occipital lobe)
What are the roles of the visual association cortex?
How can this be divided into 2 streams?
- visual association areas (BA 18, 19**) surround the primary visual cortex and **give meaning to the visual information we perceive
dorsal “where” stream:
- involved in analysis of motion and spatial relationships
ventral “what” stream:
- involved in analysis of letters, shapes, faces, colours, etc.