S4) The Retina and Central Visual Pathways Flashcards
Identify the structures observed in the retina in a cross-section of the eyeball
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
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)
found in the macula
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
help to form the optic nerve
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
- lateral inhibition prevents too many neural impulses being sent back to the brain
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
Identify the seven structures observed in a fundoscopy of the eye
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
Briefly, describe the layout of the visual fields
- Nasal fibres are responsible for our temporal field of vision
- Temporal fibres are responsible for our nasal field of vision
Lesions at any point in the pathway will correspond to a pattern of visual loss
How are visual field defects named?
These are named based on the area of visual loss rather than the site of the lesion
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 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 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 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 is a quadrantanopia?
A quadrantanopia is an anopia affecting a quarter of the field of vision, associated with a lesion of an optic radiation
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
⇒ Middle cerebral artery supplies the occipital pole (represents the macula)
⇒ Macular function (central vision) will be spared
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 are stimulated to cause direct and consensual pupillary constriction (parasympathetic fibres)
Describe the three aspects of the accommodation reflex as well as the structures involved
- Convergence (medial rectus)
- Pupillary constriction (constrictor pupillae)
- Convexity of the lens to increase refractive power (ciliary muscle)
Briefly, explain how the cerebral cortex is involved in the accommodation reflex (image analysis)
The reflex follows the visual pathway via the lateral geniculate nucleus to the visual cortex
what are the three main layers of the eye?
- Outermost Sclera (tough and continuous with dural sheath if optic nerve)
- Uvea (pigmented vascular layer)
- Retina (neural layer)
what are the layers of the Uvea
- Choroid (deep to sclera) full of blood vessels
- Ciliary body and iris sitting anterior
what are the layers of the retina from superficial to deep
→ retinal pigment (stop light bouncing around)
→ photoreceptor cells
→ Bipolar cells (1st order neurones receiving input from horizontal cells that enhance edges via lateral inhibition)
→ Ganglion cell layer (receives input from ganglion cells)
→ nerve fibre layer formed from ganglion cells
→ REMEMEBR: light will travel from nerve fibre layer to the photoreceptor cells
what does the pigmented layer do?
- absorbs some light rays / modulates about of rays eye receives
- in people with albinism ( they don’t have this layer → photophobia)
- anchors the photoreceptor cells
what pathologies can fundoscopy of the retina highlight?
- retinopathy (hypertension, diabetes)
- macula (central vision) → degeneration with age
- optic disc swelling (papilloedema)
what is amaurosis fugax
- occlusion of the ophthalmic artery
label this fundus, where does the macula sit in comparison with the optic disc?
the macula sits lateral to the optic disc
what type of imaging do you use to look at the retina
OCT
(Optical coherence tomography)
what are the two sides to the eye referred to as
temporal and nasal/ lateral and medial
→ light from the temporal field is detected from the nasal retinal fibres and vice versa
which nerves decussate in the optic chiasm
nasal fibres
→ optic tracts from after the nerves [ass the optic chiasm
→ beyond the chiasm fibres to both eyes will be effected
what is the lateral geniculate nucleus?
→ ganglion cells in the axon project to a part of the thalamus called the LGN which projects to the primary visual cortex (occipital lobe) via optic radiations
what two types of radiation are there for signals to travel from the LGN to the primary visual cortex
SUPERIOR: optic radiations will travel through the parietal lobe (Barums loop)
INFERIOR: optic radiations will travel through the temporal lobe (Meyers loop)
visual fields
what is monocular blindness
→ damage to the optic nerve
how is bitemporal Hemianopia caused?
→ damage to nasal retinal fibres on both sides so temporal vision is lost on both sides
= tunnel vision (lose outer peripheral vision)
→ pituitary adenomas can cause this issue as its very close to the chiasm
how is contralateral homonymous hemianopia caused?
→ damage to the optic tract
this is left homonymous loss as you are describing the visual fields that are lost NOT anatomy that the issue lies
what are our superior retinal fields detected by?
- inferior retinal fibres and vice versa
- superior radiations are responsible for our inferior quadrant field of vision
what does lesion of the right superior optic radiation cause
causes a left homonmous inferior quadrantanopia (lost a quarter of the visual field)
what does lesion of the right inferior optic radiation cause?
→ left homonmous superior quadrantanopia
if you had a blockage to the middle cerebral artery blocking the the superior and inferior optic radiations what would happen
left homonyomous hemianopia
what is macula sparing?
- phenomenon to do with damage to the posterior cerebral artery
- If its infarcted there is a dual blood supply from the MCA
- still blood supply to macula
- so if lesion in the visual cortex then there will be macula sparing
- if there is a lesion in the optic tract there will be no macula sparing
what does non-vascular damage to the occipital lobe cause
- contralateral homonymous hemianopia without macula sparing
what is the pupillary light reflex
- afferent arm: optic nerve
- processing centre: pretectal nucleus projecting bilaterally to Edinger Westphal nuclei (parasympathetic preganglionics)
- efferent arm: oculomotor nerve
- effect: direct and consensual pupillary constriction (consensual reflex mediated by bilateral rejections from pretectal nucleus)
what is the accommodation light reflex
afferent arm: optic nerve
processing centres: visual cortex, allowing processing of visual image → oculomotor and edinger Westphal nuclei / cerebral cortex must be involved as it relates to image analysis
efferent: oculomotor
effect: focus on nearby object = pupillary constriction, convergence of eyes and thickening of lens
three C’s in accommodation
what are three diagnoses for right homonymous inferior quandrantanopia
meningitis
brain abscess
metastasise
Which lobe does the lateral geniculate nucleus project to?
it is part of the thalamus relaying visual info from optic tracts to visual cortex
so frontal lobe
Internuclear opthalmoplegia can be caused by damage to which pathway
medial longitudinal fascia
- Difficulty moving the affected eye horizontally towards the opposite side of the lesion
- Nystagmus, which is an involuntary eye movement, in the abducting eye (the eye that moves outward from the nose)
- Diplopia, or double vision, especially when looking in the direction of the affected eye
photoreceptor cells synapse with which retinal cell predominantly
bipolar
tumour arises from the right cavernous sinus, invades medially towards optic chiasm, which visual field defect may this cause
right nasal hemianopia
what is the medial longitudinal fasciculus
The MLF is a bundle of nerve fibers that connects the oculomotor and abducens nuclei in the brainstem, and is responsible for coordinating eye movements. It is located in the brainstem and is involved in the control of horizontal eye movements.
