Vision Flashcards
What are the cranial nerves involved?
3rd= oculomotor 4th= trochlear 6th= abducens
What are the three broad aspects of vision?
- Visual field
- Visual acuity
- Colour vision
What is the visual field?
‘Field of view’= essentially optical definition
‘True VF’= spatial array of visual sensation available to subject
Describe the normal human visual field
-From vertical meridian = 60* nasally = 100* temporally (lateral) -From horizontal meridian = 60* above = 75* below
What colour objects have a narrower visual field?
Red (compared to white)
smaller radius
What is visual acuity?
- A measure of the clarity of vision
- Spatial resolution
What is colour vision?
Different frequencies of EMR are perceived as different colours
What are the components of the visual pathway?
- Retina
- Optic nerve
- Optic chiasm
- Optic tract
- Lateral geniculate nucleus
- Optic radiation
- Visual cortex
What is the retina?
-Layers of neuronal cells
=only neurones that are directly light sensitive are photoreceptor cells
-Connected by synapses
What are the types of photoreceptor cells in the retina?
Rods
Cones
Photosensitive ganglion cell
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
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
Describe rods
Function mainly in dim light
Black and white vision
-120 million
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
What are photosensitive ganglion cells important for?
Reflexive responses to light
Constriction of pupil
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
