The Eye Flashcards
Three layers of the eye
- Schleroid
- Choroid
- Retina
Schleroid
-White of the eye
-Continuous with cornea
Choroid
-Blood vessel layer
-Iris and ciliary body
Retina
-Back and sides only
-contains photoreceptors
Vitreous humour
-Gelatinous
-Maintains shape of eye so that the lens focuses on retina
Aqueous humour
-Provides nutrients to the cornea (has no blood vessels)
-5 mL/day
Where does the aqueous humour drain?
-Canal of Schlemm and eventually enters the blood
Glaucoma
-Blocked drainage duct
-Aqueous humour builds up, building pressure that can damage the nerve
Pupil
Eye opening for light
Iris
-Colour of the eye
-Controls amount of light entering eye
-Circular muscles and radial muscles
Circular muscles of the iris
-Constrict (they relax) pupil in light conditions
Radial muscles of the iris
-Dilate pupil in dark conditions
Structures contributing to light refraction:
Cornea:
-Contributes the most because of liquid
-Curvature never changes so refractive ability remains constant
Lens:
-Refractive ability can be adjusted by changing curvature as needed for near or far vision
Refraction
Bending of light to focus on the retina
Accommodation
-Changing shape of lens to adjust to light coming into eye
Accommodation for far vision
-Light rays are coming in almost parallel because they have traveled so far
-They need less bending
-Ciliary muscles relax, pulling suspensory ligaments taut. The lens becomes flatter/weaker
Accommodation for near vision
-Light rays are coming in diverging (going away from each other)
-They need more bending
-Lens should be rounder
-Ciliary muscles contract, suspensory ligaments go slack. The lens becomes rounder/stronger
Convex lens
-Convergent
-Bends light rays inwards
-Lens of the eye
Concave lens
-Divergent
-Bends light rays out
Emmetropia
Normal vision
Myopia
-Near sightedness
What is wrong with the lens or eye in myopia?
-The lens is too strong (bending light too much) or the eye is too long (can’t reach the back of the eye)
Where is the focal point in myopia?
In front of the retina
How do you correct myopia?
Concave lens diverge the light rays outwards
Hypermetropia or hyperopia
Far-sightedness
What is wrong with the lens or the eye in hyperopia?
Lens is too weak (not bending light enough) or eye is too short (going too far past retina)
Where is the focal point in hyperopia?
Behind the retina
How do you correct hyperopia?
Convex lens to bend the light more
Presbyopia
-Loss of near vision with age due to stiffening of lens (harder to become round)
-Correct with reading glasses
Where does visual transduction occur?
On the retina
Layers of retina from deepest to superficial
- Outer layer of eye: Sclera and Pigmented choroid coat
- Retina: pigmented epithelium, receptor cells, horizontal cells, bipolar neurons, amacrine cells, ganglion cells
- Nerve fibres (to optic nerve)
- Light waves
- Vitreous humour
Layers within retina
- Fibers of optic nerve
- Ganglion cells
- Amacrine cells
- Bipolar cells
- Horizontal cells
- Cones
- Rods
What is the blind spot in our eye?
Where optic nerve leaves eye
Name the eye structure that controls the amount of light entering the eye.
Iris
Fovea
-Pinhead-sized depression in centre of retina
-No bipolar or ganglion cells on top
-Most distinct vision
-Has only cones (no filtering)
Macula lutea
-Area immediately surrounding fovea
-High acuity
-Has only cones, but is overlaid with bipolars and ganglions
Macular degeneration
-Loss of cones in macula and fovea
-Lose central vision
-Periphery ends up having better vision
-Leading cause of blindness in western hemisphere
-Wet - more blood vessels and bleeding
-Dry - atrophy of pigment
What are the two photoreceptors in the eye? How are they different?
Rods:
-100 million per retina
-Vision in shades of grey
-High sensitivity (work well in low light)
-Low acuity
-Periphery
-Much convergence in retinal pathways
Cones:
-3 million per retina
-Colour vision
-Low sensitivity
-High acuity
-Concentrated in fovea
-Little convergence in retinal pathways
What’s the wiring of rods to bipolars? How does this affect acuity and sensitivity?
100:1
-Low light needed to stimulate 1 bipolar because even though the light is low, its being fed to 100 rods (GPs) who will all give that information to the bipolar (summate). It is highly sensitive to light.
-Because there are 100 rods, the receptor field for the bipolar is quite large which means poor acuity
What is the wiring of cones to bipolars? How does this affect acuity and sensitivity?
1:1
-Need a lot of light into one cone to stimulate the bipolar for an AP. It is not sensitive.
-Because there is only 1 cone, the receptor field for the bipolar is quite small which means better acuity. We know exactly which cell that signal is coming from.
What is the rod pigment?
Rhodopsin
What are the three types of cones?
Red Green and Blue
In the dark, are the rods and cones polarized, hyperpolarized, or depolarized?
-Depolarized
-Na+ gates are open: they release the inhibitory NT glutamate
-Bipolar cells are inhibited (hyperpolarized)
-No APs to ganglions/optic nerve
In the dark, is rhodopsin active or inactive?
Inactive
What happens when light hits the retina?
-Photopigment absorbs light, activating it
-Activates transducin (g protein) and decreases cyclic GMP
-Closes Na+ gates: stops the release of glutamate
-Hyperpolarizes membrane (K+ leaves cell) to -70mV
-Closes calcium channels in synaptic terminal
-Decreases inhibition of bipolars: bipolars are excited and send GPs to ganglions, who decide to send APs to the optic nerve
Why are rods good for night vision?
They only need a small amount of light to work
What is colour blindness?
-Poor or lack of function in one or more colour cones
Dark adaptation
-Reform more photopigment, which lowers the threshold to send an AP with more pigment
-Eyes become more sensitive
-More rods used
-Vitamin A needed for regeneration of rhodopsin
Light adaptation
-Sudden break-down of photo-pigment from too much info coming in
-Results in bleaching
-Eyes become less sensitive with less photopigment (leave it broken down)
Images on the medial side of the optic nerve…
-Cross over to the other side of the brain
Why do we need to keep the bipolars inhibited?
-They get depolarized really easily (basically anytime they’re not being inhibited)
List the structures that light passes through to get to the photoreceptor cells
Cornea, aqueous humour, pupil, vitreous humor, ganglion cells, bipolar cells, photoreceptor cells
what lies between the lens and the retina?
vitreous chamber
Circular muscles of the iris are controlled by the [dropdown 1] nervous system and cause the
pupil to [dropdown 2]. Radial muscles of the iris are controlled by the [dropdown 3] nervous
system and cause the pupil to [dropdown 4]. The circular muscles are controlled specifically by
the [dropdown 5] cranial nerve.
1: parasympathetic
2: constrict
3: sympathetic
4: dilate
5: oculomotor
The first step in the visual transduction process that occurs in the retina is
the absorption of light by the photopigments
