Visual pathways (special senses) Flashcards
What makes up the eyeball?
- Optical front end
- Retina
- Optic disc at back
What 3 things make up vision?
Eyeball, connections and brain
What makes up the connections in vision?
- Optic nerve
- Chiasm
- Optic tract
- Lateral Geniculate Nucleus (LGN)
- Radiation
What parts of the brain are used in vision?
- occipital
- temporal
- parietal
- frontal
What is the optic nerve ?
- Collection of all the ganglion cella
- Exits the back of the eye through a hole in the sclera
- The optic nerve head can be seen at the back of the eye (aka optic disk)
What are the different parts that you would see in a healthy eye ?
Optic nerve, macular, fovea
What is wrong in this image ?
Swollen optic nerve = Worry increase pressure
What is wrong in this image ?
Cupped optic nerve = glaucoma
What is wrong in this image ?
Pale optic nerve - Nutritional deficiency
If pupils are same size then where would the problem you are looking for be?
If pupils are same size problem is in the sensory afferent pathway
If pupils are different in size then where would the problem you are looking for be?
motor efferent - sympathetic innervation of the pupil
How would you test pupils ?
Technique for Testing Pupils;
- Measure pupil diameter in light and dark conditions
- The less reactive pupil is the abnormal pupil
- Test the direct response
- Shine the light in one eye and examine the
response in the same eye - Test the indirect response
- Shine the light in one eye and examine the
response in the same eye - Check for a relative afferent pupillary defect (RAPD)
- Shine the light on one eye for 2-3 seconds, then
rapidly move to the fellow eye - Normal response is either no change in size, or a
brief constriction and returning to the same
state (“hippus”) - A pupil with an RAPD will paradoxically dilate
when the light moves towards it - Check for accommodation — alternating fixation on a distant then a near target
What are the pathologies associated with small, asymmetrical and large pupils ?
Small pupils;
*Horner’s syndrome
* Uveitis
* Drugs (i.e pilocarpine)
* Neurosyphilis (ii.e Argyl Robertson0
* Long-standing Holmes-Adie pupil(s)
* Corgenital miosis or microcoria
Asymmetric pupils;
* Physiological anisocoria (20% of population)
Large pupills;
* 3rd nerve palsy
* Sphincter damage
* Drugs
* Holmes-Adie pupil(s)
What does Glaucoma look like and how would you test for it?
Massive cup
RAPD - one on right would paradoxically dilate as it isn’t as good as the one on the other side
What are the features of the Lateral Geniculate Nucleus (LGN)?
Lateral Geniculate Nucleus (LGN);
- First synapse after info leaves retina joins brainstem at LGN
- Big afferent centre in brain
- Has lots of layers; some take info from same side and some takes info from other side (temporal)
- Has Magnocellular (moving big info) and Parvocellular (static small info) different types of visual information
What would the cause for a mild ptosis with a small pupil?
sympathetic palsy - horners
What would the cause for a more obvious ptosis with a big pupil?
parasympathetic problem, if down and out 3rd nerve palsy
What are the features of Magnocellular ganglion cells?
Magnocellular ganglion cells;
* Large axons — lots of myelin — respond quickly
* Receive information from large number of photoreceptors
* Movement, brightness, depth perception
* Project to parietal lobes — the ‘where’ stream
What are the features of Parvocellular ganglion cells?
Parvocellular Ganglion Cells;
* Thin axons — less myelin — respond slowly
Receive information from small number of photoreceptors
* Detail of objects assisting in recognition
* Project to temporal lobes — the ‘what’ stream
After the Lateral Geniculate Nucleus (LGN) what is the next synapse?
After the Lateral Geniculate Nucleus (LGN) the next synapse is the Occipital lobe
What is the line of Gennari?
The line of Gennari are myelinated fibres running from LGN to synapse in layer 4 of the ‘striate’ cortex
What is the function of Occipital Lobe in vision?
- Relay station
- Retinotopic map - direct physical relationship between spots on the retina and spots in the occipital lobe - so if you damage part of occipital lobe will lose certain part of vision (see image)
Neural tuning is present - specific populations of cells will respond to;
- colour
- spatial frequency
- orientation
What is Ocular Dominance and Amblyopia ?
Ocular dominance occurs where one eye is stronger than the other and the poorer eye is deprived of a clear image usually due to a uncorrected refractive error or a squint (Amblyopia/’lazy eye)
We can patch the good eye to force other to develop more (can only really do when young due to plasticity as this is a critical period of visual development - do screening for this in UK)
How can we treat Ocular dominance/amblyopia ?
- Patching
- Glasses
- Correct squint
- Improve input into the lazy eye
Give a summary of where lesions in the visual tract can occur and the symptoms of them?
See image
What are the different causes of a Retinal Lesion and what would this look like?
Retinal Lesion;
- Age related Macular Degeneration (AMD)
- Diabetic eye disease
(missing middle of vision in these 2 - image)
- Retinal detachment (everything turns upside down)
What would a lesion of the right optic nerve cause?
Total blindness of the right eye
What are the 2 types/causes of chiasmal lesion and how may vision in these eyes look?
Chiasmal lesions give Bitemporal Hemianopia so below have this, just different extents;
Pituitary tumour;
- Grows from underneath the chiasma and presses on the bottom part chiasm - because of this worse vision in upper quadrant so may only see hand movements there but in lower quadrant can see counting fingers
Craniophayngioma;
- Grows from above the chiasma and presses on the top part chiasm - because of this worse vision in lower quadrant so may only see hand movements there but in upper quadrant can see counting fingers
What type of tumour is this and how may your patient present?
Pituitary tumour;
- Grows from underneath the chiasma and presses on the bottom part chiasm giving a Bitemporal Hemianopia
- Worse vision in upper quadrant so may only see hand movements there but in lower quadrant can see counting fingers
What type of tumour is this and how may your patient present?
Craniophayngioma;
- Grows from above the chiasma and presses on the top part chiasm causing a Bitemporal Hemianopia
- Worse vision in lower quadrant so may only see hand movements there but in upper quadrant can see counting fingers
What are the most common causes of Post Chiasmal Lesions?
- Stroke
- Space occupying lesions (tumours and bleeds)
What symptoms would Post Chiasmal Lesions give ?
Remember PIST
- A lesion in the parietal lobe would give a quadrantanopia with the opposite side and inferior quadrant lesion
- A lesion in the temporal lobe would give a quadrantanopia with the opposite side and superior quadrant lesion
- A lesion in the occipital lobe would give you the opposite side homonymous hemianopia.
What part of the visual system “sees”
The brain as it creates conscious vision
How and where is vision made?
Vision is made in 2 separate functional streams that complement each other creating the overall sense of vision - The ‘what’ and the ‘where’ streams
