eye Flashcards
cornea
– main (fixed) refractory
mechanism
Lens
secondary (adjustable) refractory
mechanism
Ciliary body
adjusts lens curvature (i.e.
refractory power)
Iris
adjusts light levels through lens
Retina
sensory (neural) elements
Fovea centralis
retinal zone of highest
acuity
Optic nerve (CN II) –
– transmits visual data
to brain for further processing
Rods and cones
rods - low level light - distributed over the retina
cones - colours - densely located over fova
Refraction and lenses
Refraction
– Bending of light rays
* Due to change in speed when light
passes from one transparent medium to
another
* Occurs when light meets surface of
different medium at an oblique angle
* Greatest change occurs when difference
in densities between two media is
greatest
* – e.g. air/water, air/glass
* Curved lens can refract light
How does the brain interpret visual information?
Flipped image
* Integration within
cortical areas
* Interpretation based on
previous experience
Depth perception
Both eyes view same image from slightly
different angles
* Depth perception (three-dimensional vision)
results from cortical fusion of slightly different
images
* Requires input from both eyes
Convex lenses
Light passing through convex lens (as in eye) is
bent so that rays converge at focal point
* – e.g. magnifying glass
* – Image formed at focal point is upside-down and
reversed right to left
The retina
Outpocketing of the
forebrain during
embryological development
* Useful model for studying
CNS development
* Transduces light energy
into chemical energy, then
electrical energy.
Retinal layers and cells
Pigmented epithelium of the retina
Contains many dark brown melanin pigment
granules
* Absorbs light that passes through the neural part
Prevents light from bouncing back and producing
visual “echoes”
* Cells have important biochemical interactions
with photoreceptors in neural part
– Only loosely connected to photoreceptors
Possibility of detached retina
* Photoreceptors degenerate
Colour blindness
Defined as an inability to distinguish certain colours (red-green
most common)
* Occurs when one or more types of cones are non-functional
(protein of red cones are “missing”)
* Cause?
– Genetics - Sex-linked trait involving one or two pigments
– Disease such as MS and Diabetes
– Normal ageing.
* 1 in 12 males / 1 in 200 females
* Protanomaly / deuteranomaly / tritanomaly
* Most common type is red-green colour blindness
* Not seeing any colour is extremely rare
Visual processing
Lateral geniculate nuclei of thalamus
* Process for depth perception, cone input emphasised,
contrast sharpened
* Primary visual cortex (striate cortex)
* Neurons respond to dark and bright edges, and object
orientation – help with navigation (think sharp corners)
* Provide form, colour, motion inputs to adjacent visual
association areas (pre-striate cortices)
* More complex processing in other brain regions – “what”,
“where”, related activities
Visual pathway
Axons of the retinal ganglion cells form
the Optic Nerve CN2
* Medial fibres of the optic nerve
decussate at the optic chiasm
* R visual field = left hemisphere
* L visual field = right hemisphere
* Most fibres of the optic tracts continue
to the lateral geniculate of the thalamus.
* Optic radiation fibres connect to
primary visual cortex in occipital lobes
* Other optic tract fibres send branches to
midbrain nuclei (initiating visual
reflexes)
Refraction problems
Hyperopia
* Farsightedness
* Focal distance too long
* Image focuses beyond the retina
* Cause may be:
– Eyeball is too shallow
– Lens is too flat
– Corrected with converging (convex) lens in front of eye
– Provide additional refraction to focus on retina
Focusing for close vision
Light from close objects (<6 m) diverges as approaches eye
* Moves focal point further from lens (i.e. behind retina)
* Requires eye to make active adjustments using three simultaneous
processes
– Accommodation of lenses
– Constriction of pupils to prevent divergent light in
– Convergence of eyeballs medially towards object
* Determined by degree of elasticity in lens
* Increases with age as lens becomes less elastic
– In children 7–9 cm, young adults 15–20 cm
– By age 60, increases to about 83 cm
Focusing for distant vision
Eyes best adapted for distant vision
* Far point of vision
* Distance beyond which no change in lens shape needed for focusing
* 6 metres for emmetropic (normal) eye
* Cornea and lens focus light precisely on retina
* Ciliary muscles relaxed
* Lens stretched flat by tension in ciliary zonule
