Lecture 6– The retina and the visual pathway Flashcards
purpose of the pigmented layer of the retina
- Acts as the site of absorption of light- to modulate amount of light being received by the photoreceptors
- People with albinism (lack of pigment- melanin) have photophobia
- Anchors photoreceptor
function of Rods and cones
- Rods- black and white vision
-
Cones – colour vision- high acuity vision
- Found in the fovea (macula densa)
function of bipolar cells
- Act to connect photoreceptor cells to the ganglion cells
- Ganglion cells axons come together to form the optic nerve
- Optic nerve exits via the optic disc
- Blind spot- not photoreceptor cells
function of Horizontal cells
Lateral inhibition- detects area where image is coming from and detects the photoreceptor that is most in line to pick up that signal and inhibits photoreceptors next to it prevents too many neural impulses
fundoscopies can be used to look at the….
- Retinopathies
- E.g. hypertension, DM
- Vascular occlusion
- E.g. branch of central artery or vein
- ‘amaurosis fugax’- like a curtain coming down
- Macula
- E.g. degeneration
- Optic disc
- E.g. papilledema
Optical coherence tomography (OCT)
label the retina
thin layer of tissue that lines the back of the eye on the inside
the central visual pathway is comprised of:
- The optic nerve (CN II)
- The optic chiasm
- The optic tracts
- Optic radiations
the optic nerve can be split into…… divisions
4 different fibres of the retina
- Temporal (lateral)- orange
- Nasal (medial)- green
- Also have up and down
nasal fibres
medial
temporal fibres
lateral
The optic chiasm
- Nasal fibres decussate
- Temporal fibres remain ipsilateral
The optic tracts runs from the
From optic chiasm to lateral geniculate nucleus (LGN
what do optic tracts contain
Contains temporal fibres from the ipislateral side
Contains nasal fibres from tbe contralateral side
Optic radiations run from the
From LGN to primary visual cortex (x2 ) in the occipital lobe (x2 lobes)
the optic radiations have
2 routes to the occipital lobe
- Superior route via the parietal (superior optic radiations)
- Continuation of superior quadrant fibres (temporal and nasal)
- ‘Baums loop’
- Inferior route via the temporal (inferior optic radiations)
- Continuation of inferior quadrant fibres (temporal and nasal)
- “meyes loop’
summary of the visual pathway
Visual fields
Visual fields relate to peripheral vision (also called temporal and nasal). Each eye has its own set of visual filed
- These overlap to form binocular vision
- Good for depth perception
nasal fibres are responsible fore …….. visual fields
temporal
temporal fibres are responsible for
nasal visual fields
If we want to detect something in the temporal visual field, light will travel through the pupil straight to
the nasal retinal fibres (temporal visual field detected by nasal retinal fibres)
- If we want to detect something in the nasal visual field, light will travel through the pupil straight to the
temporal retinal fibres (nasal visual field detected by temporal retinal fibres)
Visual field defects
Named based on the area of visual loss rather than site of lesion
- monocular blindness
- bitemporal hemianopia
- homonomous hemianopia
CN II lesion: Monocular blindness
- Temporal and nasal fibres on the ipsilateral side affected
- Therefore the nasal and temporal visual field are lost on the ipsilateral side
Bitemporal hemianopia
- lesion at the chiasm where the nasal fibres decussate
- Nasal fibres on both sides affected
- Temporal visual field loss on both sides
- ‘tunnel vision’
- Cause e.g. pituitary adenoma
Homonomous hemianopia
- Lesion of the optic tract on the right hand side
- Left nasal retinal fibres (contralateral) and right temporal nasal fibres (ipsilateral) affected
- Left temporal (contralateral) visual field lost and right nasal (ipsilateral) field loss
- ‘left homonomous hemianopia’- even though lesion is on the right- due to decussation
- Name the visual defect on visual loss not lesion
i.e. homo- loss of both left or loss of both right
Optic radiation lesion
- Superior visual fields are detected by inferior retinal fibres
- Inferior visual fields are detected by superior retinal fibres
Quadrantanopia’s
Affects left/right superior quadrant fibres and inferior quadrant fibres
Homonomous inferior quadrantanopia (left)
x
Lesion on right superior optic radiation (parietal lobe)
- Superior ipsilateral temporal fibre affected
- Loss of inferior nasal visual field
- Superior contralateral nasal fibre affected
- Loss of inferior temporal visual fields
Homonomous superior quadrantanopia (left)
- Inferior temporal fibres on ipsilateral side affected
- Loss of superior nasal visual field
- Inferior nasal fibre on contralateral side is affected
- Loss of superior temporal visual field
What if superior and inferior radiations are affected? E.g. stroke
- Superior and inferior ipsilateral temporal fibres affected
- Superior and inferior nasal fibres contralateral affected
- Homonomous hemianopia
macula sparing strokes
Occipital lobe has dual blood supply
- Middle Cerebral Artery (occipital pole)
- Posterior Cerebral Artery
In stroke affecting the posterior cerebral artery…
- Most occipital lobe will be lost
- However middle cerebral supplies the occipital pole (represents the macula)
- Therefore macular function (central vision) will be saved
The light reflex (i.e. light shone in left eye)
- Sensory afferent from left retina (optic nerve)
- Some branches leave the optic nerve and enter the midbrain and synapse in pre-tectal nucleus
- Synapse with EDW nuclei (left and right)
- Pre-ganglionic parasympathetic fibres from EDW leave brainstem with oculomotor (left and right)
- Pre-ganglionic synapse in the ciliary ganglion, becoming post-ganglionic
- Reach sphincter pupillae muscle of iris
- Direct and consensual light reflex
accomodation reflex required for
near vision
accomodation reflex requires 3 C’s
- Convergence (medial rectus)
- Pupillary constriction (constrictor pupillae)
- Convexity (becomes thicker) of the lens to increase refractive power (ciliary muscle)
why must the cerebral cortex be involved in the accomodation reflex
- Cerebral cortex must be involved because its relating to image analysis
- Therefore the reflex follows the visual pathway via the lateral geniculate nucleus to the visual cortex
- Midbrain receives info from visual cortex
- Sends off information via cranial nerve 3 nuclei (MR) and the Edwinger Westphal nuclei (EWN) (pupillary constriction)
