13-10-23 - Visual pathways (special senses) Flashcards
Learning outcomes
- Understand how to perform visual acuity, pupil, direct ophthalmoscopy and eye movement examinations.
- Know the underlying physiology and anatomy of clinical examination techniques
- Know definitions of blindness and driving vision criteria
- Know the critical sign of a relative afferent pupillary defect (RAPD) and some common causes
- Know the different retina refection appearances and associated causes
- Know the normal appearance of the optic nerve and the abnormal appearances and underlying causes
- Know the causes of blindness in rich and poor countries and the differences and why
- Know the inverse care law
What are the 2 segments of the eye?
What are 5 connections involved in vision?
What 4 parts of the brain are involved in vision?
- 2 segments of the eye:
1) Optical front end
2) Retina/optic disc at the back - 5 connections involved in vision:
1) Optic nerve
2) Optic chiasm
3) Optic tract
4) LGN (lateral geniculate nucleus)
5) Radiation - 4 parts of the brain are involved in vision:
1) Occipital
2) Temporal
3) Parietal
4) Frontal lobes
What does the optic nerve consist of?
Where does it exit the eye?
Where can the optic nerve be seen?
Is the optic nerve myelinated?
What happens if myelination occurs where it shouldn’t?
What are unmyelinated retinal nerve fibres connected to?
- The optic nerve is a collection of all the ganglion cells
- It 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 disc)
- The optic nerve is myelinated, with myelination stopping as the nerve enters the eye
- If myelination of the optic nerve occurs in the, this can block vision, as myelin is not opaque
- Unmyelinated retinal nerve fibres are connected to many photoreceptors
Label these parts of the retina (in picture)
Why is spontaneous venous pulsation in ophthalmoscopy a good sign?
- Spontaneous venous pulsation is a good sign in ophthalmoscopy, as it indicates there is no sign of swelling of the nerve of vein, as pulsations wouldn’t transmit otherwise
What do afferents of each eye supply?
Where are pupillary pathways located?
How does the autonomic nervous system affect the pupils?
- Afferents of each eye (optic nerve) supplies the efferent for both eyes
- Pupillary pathways are located in the midbrain
- The parasympathetics cause pupillary constriction
- The sympathetics come from the thalamus to the spinal cord and along the cervical ganglion to cause pupillary dilation
Describe the technique for testing pupils (in picture)
(in picture) What 6 conditions cause small pupils?
What condition causes asymmetric pupils?
What 4 conditions cause large pupils?
What is glaucoma?
What is it characterised by?
What is it usually caused by?
What is it often associated with?
What condition does glaucoma lead to?
How does this affect field of vision?
How does glaucoma appear on ophthalmoscopy?
- Glaucoma is an optic neuropathy
- It is a neurodegenerative disease of the optic nerve (optic neuropathy) and is characterised by the progressive, irreversible loss of retinal ganglion cells resulting in irreversible visual impairment and eventual blindness
- It is usually caused by dying axons of the optic nerve
- Glaucoma is usually associated with an increase in pressure in the eye (intraocular pressure)
- Glaucoma is characterised by a loss of field of vision
- In glaucoma nerve fibers begin to die and atrophy, creating a “cupped” or curved shape to the normal disc shape of the optic nerve seen on fundoscopy.
Glaucoma cupped disc (in picture)
What are the 2 main causes of visual impairment and blindness worldwide?
Which sides of the eyes are innervated by which primary visual cortex?
Which visual field are these responsible for?
What are the 2 hemifields of each?
- The right side of both eyes are innervated by the right primary visual cortex, which is responsible for the left visual field
- The left side of both eyes is innervated by the left primary visual cortex, which is responsible for the right visual field
- The 2 hemifields of eye are the nasal and temporal hemifield
Where does information go after the optic nerve?
Where do fibres crossover?
Where does it go after this?
How many layers are there in the LGN?
Which layers are ipsilateral, contralateral eyes?
Which layers are magnocellular and parvocellular?
Describe the route from the eye to the LGN (in picture)
- After the optic nerve, information goes along the optic tract
- There is then the crossover of nerve fibres in the optic chiasm
- Fibres then go the LGN in the thalamus
- The LGN has 6 layers
- Layers of the LGN for the ipsilateral eye are 2, 3 and 5
- Layers of the LGN of the contralateral eye are 1, 4 and 6
- Magnocellular layers of the LGN are 1 and 2
- Parvocellular layers of the LGN are 3, 4, 5, and 6
- Route from the eye to the LGN (in picture)
Magnocellular and parvocellular ganglion cells:
1) Axon type
2) Information received
3) Function
4) Projection
Projection from the occipital lobe to the parietal and temporal lobe (In picture)
- Magnocellular ganglion cells (LGN layers 1 and 2)
1) Large axons – lots of myelin – respond quickly
2) Receive information from large number of photoreceptors
3) For Movement, brightness, depth perception (bigger field)
4) Project to parietal lobes – the ‘where’ stream (dorsal stream) - Parvocellular ganglion cells (LGN layers 3, 4, 5 and 6)
1) Thin axons – less myelin – respond slowly
2) Receive information from small number of photoreceptors
3) Detail of objects assisting in recognition e.g faces words
4) Project to temporal lobes – the ‘what’ stream (ventral stream) - Projection from the occipital lobe to the parietal and temporal lobe (In picture)
Where is the next synapse after the LGN?
What is the Line of Gennari?
Where do they synapse?
What is the striate cortex?
What can damage to the striate cortex cause?
- The net synapse after the LGN is the occipital lobe
- The Line of Gennari is made of Myelinated fibres running from LGN to synapse in layer 4 of the ‘striate’ cortex
- The striate cortex is the primary sensory cortical area for vision.
- Damage to striate cortex causes blind regions, called scotomas, in the field of vision.
What is the blood supply to the occipital lobe?
How can central vision be maintained in occipital stroke?
- Most of the occipital lobe is supplied by the posterior cerebral artery (PCA)
- The tip of the occipital lobe is supplied by the middle cerebral artery (MCA), and corresponds to the fovea
- If there is an occipital stroke concerning the PCA, the area responsible for the fovea will be unaffected hence, central vision will be maintained
Where is the primary visual cortex (V1) located?
What does it receive?
How can areas of V1 be divided up?
- The primary visual cortex, also known as V1 or Brodmann area 17, surrounds the calcarine sulcus on the occipital lobe’s medial aspect.
- It receives the visual information from the retina via the thalamus.
- Areas of V1 can be divided up into ocular dominance columns
What is amblyopia?
How are these ocular dominance columns affected in amblyopia?
What is it commonly caused by?
Does this condition affect both eyes?
How does amblyopia affect the ocular dominance columns of V1?
- Amblyopia, or lazy eye, is impaired or dim vision without obvious defect or change in the eye, causing the eye to be deprived of a clear image
- Is it commonly due to an uncorrected refractive error or a squint during the critical period of visual development
- Usually only one eye is affected
- In amblyopia, the ocular dominance columns in the amblyopic eye will be much smaller than in the dominant eye
How doe pre- and post-chiasmal lesions differ from each other?
Describe pre- and post-chiasmal lesions (in picture)
- Post-chiasmal lesions usually affect both eyes
- Pre-chiasmal lesions usually only affect one eye
- Pre- and post-chiasmal lesions (in picture)
Age related Macular Degeneration (AMD)
Diabetic eye disease
Retinal detachment
(3 on lesions diagram - chiasmal lesions)
What are the 2 types of chiasmal lesions?
How do they affect the field of view (in picture).
What field defects do chiasmal lesions cause?
- (3 on lesion diagram - chiasmal lesions)
- 2 types of chiasmal lesions:
1) Pituitary tumour
* Tumour from below
2) Craniopharyngioma
* Tumour from above
* Causes bitemporal problems
- Chiasmal lesions cause bitemporal hemianopia
(number 5 and 6 on lesion diagram - quadrantanopia).
What are quadrantanopia?
What is it usually caused by?
What are the 4 types of quadrantanopia?
- (number 5 and 6 on lesion diagram - quadrantanopia).
- Quadrantanopia is a loss of vision in a quarter of the visual field.
- The defect is usually bilateral as it is typically caused by a lesion past the optic chiasma.
- 4 types of quadrantanopia:
1) Homonymous (binasal, bitemporal, upper or lower)
2) Crossed (one upper and the other lower)
3) Congruous (equal size of the defects)
4) Incongruous (unequal size of the defects).
What is Meyers loop?
Where does it travel?
What does it transmit?
What causes Right Superior Quadrantanopia?
- The inferior or ventral bundle, known as Meyer’s loop, travels around the temporal horn.
- This bundle makes a wide anterior and lateral loop around the temporal horn of the lateral ventricle before curving around the posterior atrium to reach the occipital cortex.
- The Meyer’s loop transmits visual information from the contralateral superior field of the eyes,
- Right Superior Quadrantanopia is caused by Lesion of the Left Temporal Lobe – CVA (cerebrovascular accident – stroke)
Right Superior Quadrantanopia is caused by Lesion of the Left Temporal Lobe – CVA (cerebrovascular accident – stroke)
(number 7 and 8 on lesion diagram - occipital lobe lesions).
What do occipital lobe lesions lead to?
- (number 7 and 8 on lesion diagram - occipital lobe lesions)
- Occipital lobe lesions lead to homonymous hemianopia
- Lesions on the left occipital lobe will cause right visual field defects
(number 7 and 8 on lesion diagram - occipital lobe lesions).
Describe the technique for testing visual fields (in picture)
How do the what and where streams interact?
What are they each responsible for?
- There are two separate functional streams of visual information that complement each other creating the overall sense of vision – the ‘what’ and the ‘where’ streams
- The ventral ‘what’ stream is for faces, word etc. and goes to the temporal lobe
- The dorsal ‘where’ stream is for motion, spaciousness etc. and goes to the parietal lobe
