S4) The Retina and Central Visual Pathways Flashcards
Identify the structures observed in the retina in a cross-section of the eyeball
The eye has 3 layers.
What are they?
- Outermost sclera
- Uvea (pigmented vascular layer)
- Retina (neural layer)
Describe the outermost sclera.
tough and continuous with dural sheath of the optic nerve
Describe the uvea
(pigmented vascular layer)
- Choroid sitting just deep to sclera
- Ciliary body and iris sitting anteriorly
What cells can be seen in the retina (neural layer) from superficial to deep.
- Retinal pigment epithelium (prevents light from ‘bouncing around’ in the eyeball, causing glare)
- Photoreceptor cells
- Bipolar cells (first order neurones receiving input from photoreceptors).
- Bipolar cells are connected by horizontal cells which assist with enhancing edges through a process called lateral inhibition •
- Ganglion cell layer (receives input from bipolar cells. Axons of ganglion cells form the optic nerve)
- Nerve fibre layer
- Interestingly, our retina is the ‘wrong way around’, since light has to pass through the nerve fibre layer and other layers before getting to the photoreceptors. Cephalopods have a much better arrangement…!
What are rod cells and what do they do?
Rod cells are photoreceptor cells in the retina which function in low intensity light and are responsible for black and white vision (night vision)
What are cone cells and what do they do?
Cone cells are photoreceptor cells in the retina which function in high intensity light and are responsible for high acuity vision (detects colours)
Macula = highest acuity vision (central vision)
Which part of the eye has the highest density of cones?
Fovea - found in the macula densa
Has high resolution vision
It is a pit - doesn’t have any ganglion cell axons .: less interruption for light to reach the cones
What is the pigmented layer?
Contains melanin – prevents excessive reflections within the eyeball itself
They also maintain/ anchor the photoreceptor layer.
prevents light from ‘bouncing around’ in the eyeball, causing glare
clinical correlate: Albinism - don’t have melanin pigment present .: have to wear sunglasses if too much light reflections (everything seems to bright for them)
What are bipolar cells and what do they do?
Bipolar cells are cells which exist between photoreceptors in the retina and act indirectly/directly to transmit signals from the photoreceptors to the ganglion cells
first order neurones receiving input from photoreceptors
What are horizontal cells and what do they do?
- Horizontal cells are the laterally interconnecting neurons which help integrate and regulate the input from multiple photoreceptor cells
- They also allow the eyes to adjust to see well in both bright & dim light conditions
Bipolar cells are connected by horizontal cells which assist with enhancing edges through a process called lateral inhibition
What are amacrine cells and what do they do?
Amacrine cells are inhibitory neurons and project their dendrites to the inner plexiform layer to interact with retinal ganglion cells and/or bipolar cells
What are ganglion cells and what do they do?
A retinal ganglion cell is a type of neuron in the retina which receives visual information from photoreceptors via bipolar cells and amacrine cells
Receives input from bipolar cells. Axons of ganglion cells form the optic nerve
What is a blind spot?
Region where there are no photoreceptors .: no detection of light or imaging
e.g. optic disc
What is Amaurosis Fugax?
- Sudden transient loss of vision
- Caused by TIA - where small embolus has temporarily blocked the ophthalmic artery
- Ppl often describe it as a ‘curtain coming down over their vision’
- Vision then returns again after a couple of hours
- Often caused by hypoxia of the retina so photoreceptors and the neurones aren’t able to function
- Occlusion of the central retinal artery (a branch of the ophthalmic artery) causes sudden visual loss known as amaurosis fugax
Identify the labels of the retina
Examination of the retina by fundoscopy can detect signs of many diseases such as
hypertensive retinopathy, diabetic retinopathy and macular degeneration .
Identify the seven structures observed in a fundoscopy of the eye
The normal appearance of the fundus, with the macula (point of highest acuity)
sitting lateral to the optic disc (point of exit of ganglion cell axons). Branches of
central retinal artery and vein are visible on the macula.
Why should we do fundoscopy?
Retinopathies
◦ e.g. hypertension, diabetes
Vascular occlusions
◦ Branch of central retinal artery or vein – ◦“Amaurosis fugax” (can be a symptom of stroke)
Macula (region of central vision)
◦ e.g. degeneration ◦ Optic disc
◦ e.g. papilloedema (optic disc swelling due to raised intracranial pressure)
What specialist technique can be used to visualise the layers of the retina?
Optical coherence tomography (OCT)
What can be seen through an ophthalmoscope?
The eye is a pinhole camera.
Illustrate what this implies.
This implies that light from a lateral visual field is detected by the medial retina (lateral retina = medial visual field) and that light from an upper visual field is detected by the inferior retina (lower visual field = superior retina)
What 4 features make up the visual pathway?
- The optic nerve (CNII)
- The optic chiasm
- The optic tracts
- The optic radiations
The optic nerve contains two types of fibres.
What are they?
The medial retina is referred to as nasal. Light from the temporal field is detected by
the medial retina
The lateral retina is referred to as temporal. Light from the temporal field is detected by the nasal retina
In the optic chiasm, describe the path of the nasal and temporal retinal fibres.
Ganglion cells supplying the temporal retina project to the ipsilateral cerebral
hemisphere whereas ganglion cells from the nasal retina project to the contralateral
hemisphere via the optic chiasm (i.e. they decussate)
→ This implies that the left binocular visual field projects to the right hemisphere and vice versa
In the optic tract, what does it connect and what does it contain?
Ganglion cell axons project to a part of the thalamus called the lateral geniculate nucleus via the optic tract
Where are the optic radiations found?
From the Lateral Geniculate nucleus to Primary visual cortex (occipital lobe)
The lateral geniculate nucleus projects to the visual cortex through the optic radiations
We have two types of optic radiations. What are they and how do they differ?
- Ganglion cells from the superior retina (i.e. inferior field) project through the superior optic radiation running through the parietal lobe
- Ganglion cells from the inferior retina (i.e. superior field) project through the inferior optic radiation running through the temporal lobe
Illustrate the layout of the visual pathway
- Temporal fibres run ipsilateral
- Nasal fibres decussate at the optic chiasm
- Optic tract runs to the lateral geniculate nucleus
- Optic radiations split into superior and inferior and runs to the primary visual cortex – Superior via parietal lobes and Inferior via temporal lobes
What are visual fields?
These relate to peripheral vision
Each eye has its own set of visual field:
–These overlap to form our binocular vision
–Good for depth and perception
Visual fields also called temporal or nasal
Briefly, describe the layout of the visual fields
- Nasal fibres are responsible for our temporal field of vision (sees peripheral vision)
- Temporal fibres are responsible for our nasal field of vision (sees central fields)
Lesions at any point in the pathway will correspond to a pattern of visual loss
What is a scotoma?
A localised defect in the retina can cause a small patch of visual loss called a scotoma
How are visual field defects named?
These are named based on the area of visual loss rather than the site of the lesion
we always refer to the lost part of the field, not the lost part of the retina
What are some examples of visual field defects?
- Monocular blindness
- Bitemporal hemianopia
- Homonomous (affecting both eyes equally) hemianopia
Describe the pattern of visual field loss at the following locations:
- Before the optic chiasm
- At/after the optic chiasm
- After the optic chiasm
- Before the optic chiasm: signs are unilateral and ipsilateral
- At/after the optic chiasm: signs are bilateral
- After the optic chiasm: signs will be bilateral and contralateral
What happens in a CNII lesion (on one side)?
Damage to the optic nerve can lead to monocular blindness
What is the general cause of monocular blindness?
Monocular blindness is caused by a lesion of the optic nerve (right)
Identify some of the underlying pathological causes of monocular blindness in children and adults
- Optic nerve glioma or retinoblastoma (children)
- Optic sheath meningiomas (middle aged)
What happens in a lesion at the optic chiasm?
Damage to the optic chiasm can cause bitemporal hemianopia.
What is the general cause of bitemporal hemianopia?
- Bitemporal hemianopia is caused by a lesion at the optic chiasm
- It affects both nasal fibres, and thus, both temporal fields lost
What happens in an optic tract lesion?
Damage to the optic tract causes a contralateral homonymous hemianopia
What is the general cause of left homonymous hemianopia?
- Left homonymous hemianopia is caused by a lesion of the right optic tract
- It affects the right temporal and left nasal fibres
What is the general cause of right homonymous hemianopia?
- Right homonymous hemianopia is caused by a lesion of the left optic tract
- It affects the left temporal and right nasal fibres
Identify some of the underlying pathological causes of right/left homonomous hemianopia
- Vascular causes (stroke – common)
- Neoplasia
- Trauma
What does damage to the lateral geniculate nucleus result in?
Damage to the lateral geniculate causes a contralateral homonymous hemianopia
What is the relationship between the optic radiations and their role in terms of field of vision?
What is a quadrantanopia?
A quadrantanopia is an anopia affecting a quarter of the field of vision, associated with a lesion of an optic radiation
What happens in a superior optic radiation lesion?
Damage to the superior optic radiations (in the parietal lobe) causes contralateral homonymous inferior quadrantanopia
What happens in an inferior optic radiation lesion?
Damage to the inferior optic radiations (in the temporal lobe) causes contralateral homonymous superior quadrantanopia
What happens if both the superior and inferior radiations are affected e.g. in a stroke?
Damage to both optic radiations causes a contralateral homonymous hemianopia
Localise a quadrantopia observed in the following visual fields:
- Left field
- Right field
- Superior field
- Inferior field
- Left field → Right hemisphere
- Right field → Left hemisphere
- Superior field → Inferior radiations
- Inferior field → Superior radiations
The occipital lobe has dual blood supply.
Identify these arteries
- Posterior cerebral artery
- Middle cerebral artery (occipital pole)
In four steps, explain the concept of macular sparing after a stroke
⇒ Stroke can affect the posterior cerebral artery
⇒ Most of occipital lobe will be lost
⇒ But, Middle cerebral artery supplies the occipital pole (represents the macula)
⇒ Macular function (central vision) will be spared
If there is non vascular damage to the occipital lobe, what can happen?
Can cause a contralateral homonymous hemianopia without macular sparing
If there is an occlusion of the posterior cerebral artery, what can happen?
Causes a contralateral homonymous hemianopia with macular sparing
This is due to the fact that the area of visual cortex that supplies the macula receives blood from the deep branch of the middle cerebral artery
In four steps, describe the nervous pathways involved in the light reflex
⇒ Light stimulates the afferent nerve in the pathway (CN II)
⇒ Afferent nerve synapses in pretectal area
⇒ Gives rise to neurones supplying Edinger Westphal nuclei bilaterally
⇒ Both CN III (oculomotor nerve) are stimulated to cause direct and consensual pupillary constriction (parasympathetic fibres)
What is the purpose of the accommodation reflex?
Required for near vision
Needed for focussing on something very close to you - eyes need to change to accommodate this i.e. lens need to thicken, refraction occurs more
Describe the three aspects of the accommodation reflex as well as the structures involved
- Convergence (medial rectus) (pulls eye medially/adducted)
- Pupillary constriction (constrictor pupillae) (constricts pupil to help focus light)
- Convexity of the lens to increase refractive power (ciliary muscle) (thickens the lens)
Briefly, explain how the cerebral cortex is involved in the accommodation reflex (image analysis)
Cerebral cortex must be involved because it is relating to image analysis
The reflex follows the visual pathway via the lateral geniculate nucleus to the visual cortex
Difference between Light reflex and Accommodation reflex
In accommodation reflex →
oculomotor nucleus → mediates convergence response by acting on medial rectus → adduction of eye .: convergence
edinger westphal nucleus → stimulates ciliary ganglion → stimulates sphincter pupillae → pupils constrict. Also contracts ciliary muscle → lens thicken
Illustrate how the medial longitudinal fasciculus and its relationship with how we get co-ordination of eye movements e.g. when looking to the left: we use the LEFT lateral rectus and the RIGHT medial rectus.
If we get a problem/ disorder of the medial longitudinal fasciculus pathway, what can arise?
Internuclear ophthalmoplegia → results in un-coordinated eye movement! - can result in diplopia
What is sensory ataxia?
- caused by involvement of somatosensory nerve → leads to interruption of sensory feedback signals and body incoordination is caused
- loss of co-ordination of due to loss of sensory input for motor output
LABEL THE MAJOR RETINAL LAYERS IN THE OCT IMAGE.
If we get a problem/ disorder of the medial longitudinal fasciculus pathway, what can arise?
Internuclear ophthalmoplegia →
If we get a problem/ disorder of the medial longitudinal fasciculus pathway, what can arise?
Internuclear ophthalmoplegia →
look over session 3 groupwork questions
