The visual system Flashcards
What is cranial nerve III?
Oculomotor nerve
What does cranial nerve III control?
Movement of the extraocular muscles
What is cranial nerve II?
Optic nerve
What is the sclera?
Tough outer protective wall of the eyeball
What does binocular vision allow?
Depth perception
What is the fovea?
Thinnest part of the retina with the highest visual acuity because it contains many cone cells
What is the optic disk?
- Blind spot
- Origin of blood vessels and optic nerve which block vision
What is the macula?
- Region of the retina for central vision, not blocked by blood vessels to improve vision quality
- Has fovea in the centre
What are zonal fibres?
- AKA suspensory ligaments
- Suspends the lens by attaching to ciliary muscles which enables stretching of the lens
What is aqueous humour?
- The fluid between the cornea and the lens which provides nutrients to the cells of the cornea
- Clear to allow refraction
Which fluid is located between the cornea and the lens?
Aqueous humor
What is vitreous humour?
Fluid inside the eyeball which maintains the shape and outward pressure
Which fluid fills the eyeball?
Vitreous humor
How much refraction happens at the cornea?
80% (most of it)
How much refraction happens at the lens?
20%
What is the refractive index?
- Measure of the speed of light within a certain media
- Liquids have a higher refractive index than gases because liquids are more dense
What does a large difference in refractive index between two media mean?
More refraction
What is the degree of refraction determined by?
- Difference in refractive index between the two media
- The angle at which light hits the interface i.e. the cornea (perpendicular means no refraction)
What is the focal distance?
Distance from the refractive surface (i.e. cornea) to convergence of parallel light rays (i.e. retina)
Are light rays parallel when coming from distant objects?
Yes
Are light rays parallel when coming from near objects?
No
How does the lens accommodate to focus light from distant objects?
- The light rays are parallel so the cornea provides enough refraction do focus them on the retina
- Ciliary muscle relaxes, suspensory ligaments contract so lens is flattened
How does the lens accommodate to focus light from near objects?
- Non-parallel light rays require more refraction than the cornea can provide so the lens fattens
- Ciliary muscle contracts, suspensory ligaments relax so lens is fat
What is emmetropia?
Normal sight
What is hyperopia?
Far-sightedness
What does far-sighted mean?
Can see far away but not close up
What happens in hyperopia?
- The eyeball is too short so the point where the light rays converge would be behind the retina
- More refraction needed from a convex lens in glasses
Which lens is needed for glasses in hyperopia?
Convex
What is myopia?
Short-sightedness
What does short-sighted mean?
Can see close up but not far away
What happens in myopia?
- The eyeball is too long so light rays converge before they hit the retina
- Refraction decreased by a concave lens in glasses which makes the rays more divergent
Which lens is needed for glasses in myopia?
Concave
What are ganglion cells?
- The output from the retina
- The only cells which produce action potentials, other cells produce graded potentials
What are bipolar cells?
Connect the photoreceptors to the ganglion cells
What are photoreceptors?
Rods and cones
What are amacrine cells?
Modify energy transfer between bipolar cells and ganglion cells
What are horizontal cells?
Modify information transfer between photoreceptors and bipolar cells
What is the pigmented epithelium?
Epithelial layer underneath the photoreceptors which nourishes them
What is the duplicity theory?
Can’t have high sensitivity and high resolution in a single receptor
What do rod cells perceive?
Black and white
What do cone cells perceive?
Colour