Vision Flashcards by Freya Millar

What are the cranial nerves involved?

3rd= oculomotor
4th= trochlear
6th= abducens

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What are the three broad aspects of vision?

Visual field
Visual acuity
Colour vision

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What is the visual field?

‘Field of view’= essentially optical definition

‘True VF’= spatial array of visual sensation available to subject

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Describe the normal human visual field

-From vertical meridian
= 60* nasally
= 100* temporally (lateral)
-From horizontal meridian
= 60* above
= 75* below

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What colour objects have a narrower visual field?

Red (compared to white)

smaller radius

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What is visual acuity?

A measure of the clarity of vision

- Spatial resolution

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What is colour vision?

Different frequencies of EMR are perceived as different colours

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What are the components of the visual pathway?

Retina
Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation
Visual cortex

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What is the retina?

-Layers of neuronal cells
=only neurones that are directly light sensitive are photoreceptor cells
-Connected by synapses

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What are the types of photoreceptor cells in the retina?

Rods
Cones
Photosensitive ganglion cell

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What are the layers of the retina?

Nerve fiber layer
Ganglion cell
Inner plexiform
Inner nuclear
Outer plexiform
Outer nuclear
External limiting membrane
Rods and cones
Pigmented epithelium
Lamina vitrea
Choroid
Sclera

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What happens when light hits the retina?

Light stimulates rods and cones

Signal to other cells that go to surface of retina (ganglion)
Axons to optic nerve

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Describe rods

Function mainly in dim light
Black and white vision
-120 million

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Describe cones

Function in bright light
Perception of colour
=L-cones Red 564 nm
=M-cones Green 533 nm
=S-cones Blue 437 nm
-6-7 million

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What are photosensitive ganglion cells important for?

Reflexive responses to light

Constriction of pupil

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Describe the pathway from retina to nerve

Signals from rods and cones
Processed by other retinal neurones
Axons of the retinal ganglia cells form the optic nerve

What are the special areas on the retina?

Macula= yellow oval spot near retina centre, specialised for high acuity vision (6mm) (13* degrees of central field)
Fovea= a pit in the macula centre, the area of greatest VA and best colour vision- many cones (1.5 mm) (3* of central field)

Describe the fovea

Occupies 0.01% of VF
3* of visual angle
10% of optic nerve axons

How are rods and cones distributed?

Rods= periphery
Cones= fovea
None in blind spot

How can visual acuity be affected?

Light is impaired in transmission through eye
Disease of cornea and lens
Refractive (optical) problems
Developmental anomalies/ changes in elasticity in lens in age

Which VA problems are correctable?

Refractive errors correctable with glasses/ lens

Neurological VA problems (difficulty in neural structures) not correctable

What are visual fields based on?

Angle of view
Enlarge with distance
Depend on object size (small= small field) and colour

What are blind spots?

Temporal field side of fixation
Optic nerve head/ disc has no light sensitive cells
Displaced from point of fixation
Not noticed in normality

Describe blind spot in disease

BS reflects optic nerve
BS size reflects optic nerve size
Enlarged= papilledema, optic neuritis

How does visual information cross in the visual pathway?

Light from right visual space goes to both left and right eyes Right visual space light enters right eye and hits nasal retina, enters left eye and hits temporal retina So nasal retinal half has to cross at optic chiasm

Describe how the chiasm is involved in the visual pathway

``` Pre-chiasm= all data from one eye Chiasm= half the data crosses (from nasal retina) Post-chiasm= all left visual space data on right, all right visual space data on left ```

What are the types of visual field defects?

- Major visual field defects (loss of substantial segment of visual field) - Constriction of peripheral field - Enlarged blind spots - Scotomata (holes in visual field)

What are the classifications of major visual field defects?

- Pre-chiasmal (trauma, removal of one eye, optic nerve)= uni-ocular loss - Chiasmal (bitemporal hemianopia so affects temporal half of field, problems with pituitary gland like tumour) - Post-chiasmal (homonymous hemianopia, so left hemisphere loss of right visual space so left nasal field and right temporal)

What are quadrantopias?

- Lesion to optic radiation where fibers fan and spread out | - Upper quadrant= superior

What are other major visual field defects?

Altitudinal VFDs= upper half of visual field loss

What are the colour vision defects?

Congenital or acquired (optic neuritis)

What are the extraocular muscles?

- Superior, inferior, lateral and medial rectus | - Superior (down) and inferior (up) oblique

What does the third cranial nerve innervate?

All the recti but lateral rectus Innervates inferior oblique Opens eye lid and parasympathetic pupil (diameter)

What does the 4th cranial nerve innervate?

Superior oblique

What does the 6th cranial nerve innervate?

Abducts eye via Lateral rectus

What are the movements caused by the recti muscles?

``` Straight Superior= up Inferior= down Medial= in Lateral= out ```

What are the movements caused by the oblique muscles?

``` Superior= down Inferior= up ```

What are ocular palsies?

Eye takes on abnormal position because of unopposed action of remaining intact muscles

What is diplopia?

Results from failure to align eyes (two eyes) - Double vision may mean blurred vision - Did it resolve if you closed/ covered one eye