The Retina and Central Visual Pathways Flashcards
What are the important layers of the retina?
- Choroid
- Pigemented layer
- Neural layer
What cells are contained within the neural layer of the retina?
- Photoreceptor cells
- Bipolar cells
- Horizontal cells
What does the pigemented layer of the retina contain?
Melanin
What are the functions of the pigemented layer of the retina?
- Reduces refraction of light
- Acts as an anchor for photoreceptor cells
Why is it important that the pigmented layer of the retina reduces the refraction of light?
Prevents the light from being too bright
What are the types of photoreceptor cells?
- Rod cells
- Cone cells
What are rod cells good for?
- Black and white
- Low levels of light
What are cone cells good for?
- Colour vision
- Higher acuity vision
What is the function of horizontal cells?
They are inhibitory cells that are employed so you don’t get too many conflicting messages from photoreceptors
Which part of the retina is responsible for central vision?
The macula lutea
What is found within the macula?
The fovea centralis
What is significant about the fovea centralis?
It has the highest concentration of cone cells, and therefore allows you to see details when looking straight ahead
How can the retina be visualised?
Fundoscopy
What can fundoscopies be useful to visualise?
- Retinopathies
- Vascular occlusions
- Macula
- Optic disc
What retinopathy can be caused by vascular occlusions?
Amaurosis fungax
What is amaurosis fugax?
A usually transient condition characterised by a ‘curtain coming down’ over vision
What optic disc pathology can be seen on fundoscopy?
Swelling of the optic disc, called papilloedema
What is papillodema a sign of?
Raised ICP
Other than fundoscopy, what investigations can be done on the eye?
Optical coherence tomography
Which fibres of the optic nerve run ipsilaterally?
The temporal fibres
Which fibres of the optic nerve decussate at the optic chiasm?
The nasal fibres
Where do the optic tracts run?
From the optic chiasm to the lateral geniculate nucleus
What happens to the optic radiations?
They split into superior and inferior portions, and run into the primary visual cortex
What field of vision are the nasal fibres responsible for?
The temporal
What field of vision are the temporal fibres responsible for?
The nasal
What is the clinical relevance of knowing the pathway of the optic nerve?
Becasue lesions at any point in the pathway will correspond to a pattern of visual loss, and so the pattern of loss can localise the lesion
What are visual fields named based on?
The area of visual loss, not the site of the lesion
What does a visual loss that is unilateral and ipsilateral suggest?
The damage has occured before the optic chiasm
What does a bilateral visual loss suggest?
The damage occurs at or after the optic chiasm
What does a visual loss that is bilateral and contralateral suggest?
The damage occurs after the optic chiasm
What is monocular blindness?
Blindness in one eye
What is monocular blindness caused by?
A lesion of the optic nerve
Give two examples of things that might cause a lesion of the optic nerve in children
- Optic nerve glioma
- Retinoblastoma
Give an example of a condition that can cause an optic nerve lesion in adults
Meningiomas
What is bitemporal hemianopia?
A loss of vision in both temporal fields, also known as tunnel vision
What causes bitemporal hemianopia?
A lesion at the optic chiasm, therefore affecting both nasal fibres and so loss of both temporal fields
What can cause a lesion at the optic chiasm?
Pathology of the pituitary gland or anterior communicating artery
What causes a left homonomous hemianopia?
A loss of the right temporal and left nasal fibres due to a lesion of the right optic tract
What can cause a lesion of the optic tract?
- Stroke
- Neoplasia
- Trauma
What causes a right homonomous hemianopia?
Loss of the left temporal and right nasal fibres, caused by a lesion of the left optic tract
What is the blood supply of the occipital lobe?
- Posterior cerebral artery
- Occipital part of the middle cerebral artery
Has a dual blood supply
What happens in a stroke affecting the posterior cerebral artery?
Most of the occipital lobe will be lost, however the middle cerebral artery supplies the occipital pole, which represents the macula. Therefore, macula function will be spared
Describe the pathway of the light reflex
- The light stimulates the afferent nerve - CN II
- The nerve synpases in the pretectal area, which gives rise to neurones supplying the Edinger-Westphal nuclei bilaterally
- The pre-ganglionic parasympathetic fibres and then post-ganglionic parasympathetic fibres of both oculomotor nerves are stimulated to cause direct and consenual pupillary constriction
What is the accomodation reflex required for?
Near vision
What aspects does the accommodation reflex consist of?
- Convergence
- Pupillary constriction
- Change in convexity of the lens to increase its refractive power
What muscle is responsible for convergence in the accomodation reflex?
Medial rectus
What muscle is responsible for pupillary constriction in the accomodation reflex?
Constrictor pupillae
What muscle is responsible for the change in convexity of the lens to increase the refractive power in the accomodation reflex?
The ciliary muscle
Why must the cerebral cortex be involved in the accomodation reflex?
Because it is related to image analysis
What is the result of the accomodation reflex involving the cerebral cortex on the pathway it must take?
The visual pathway goes via the lateral geniculate nucleus to the visual cortex
Where does the accomodation reflex pathway go from the visual cortex?
It is common with the light reflex pathway, via the Edinger-Westphal nucleus
