The Eye and Retina Flashcards
Major anatomical structures in the eye
- Ciliary body
- Cornea
- Iris
- Retina
- Optic nerve
- Choroid
- Sclera
Tunica of the eye
- Tunica fibrosa
- Tunica vasculosa
- Tunica interna
Cell types in the retina
- Ganglion cells
- Amacrine cell
- Bipolar cell
- Horizontal cell
- Cone
- Rod
- Retinal pigment epithelium (RPE)
Retinal pigment epithelium
Maintains photoreceptor health by transporting nutrients and recycling shed photoreceptor parts
Bruch’s membrane
Extracellular matrix structure at the interface between the choroid and retina
Rods and cones
Light-sensing photoreceptors
Amacrine, bipolar and horizontal cells
Connecting neurons
Ganglion cells
Transmit signals originating in the photoreceptor layer through the optic nerve and into the brain
What type of stem cell have RPE, photoreceptors and ganglion cells been differentiated from?
Pluripotent stem cells
What does the iris do?
Regulates light levels
What is the pupillary constrictor?
Smooth circular muscle
What is the pupillary dilator?
Smooth radial muscle
Photopupillary reflex
Light reflex which changes pupil diameter
Occurs in both eyes even when only one is stimulated
What is the pupil?
A hole in the retina
What is a cataract?
A cloudy lens
How do rods and cones differ?
Rods simply detect light whereas cones detect colour
Where are cones concentrated?
Fovea
How does the eye focus light?
- Light is partially focused when it passes through the cornea
- Then travels through the aqueous humour to the crystalline lens, which lets the eye focus on different depths
- The light converges in the lens and travels out the other side flipped, traveling through the vitreous humour to the retina on the inner back surface of the eye, where rods and cones detect light
- Then your brain presents a coherent, correctly-oriented image
Resting potential of photoreceptors
- 40mV
What type of polarisation occurs when light stimulates photoreceptors?
Hyperpolarisation
Phototransduction steps
- Light entering the eye activates the opsin molecules in the photoreceptors
- Activated rhodopsin causes a reduction in the cGMP intracellular concentration
- The photoreceptor is hyperpolarized following exposure to light
How is phototransduction terminated?
(1) Inactivation of rhodopsin occurs through phosphorylation by the opsin kinase, followed by the binding of arrestin to phosphorylated rhodopsin.
(2) Inactivation of transducin occurs through the hydrolysis of bound GTP to GDP (Tα-GTP to Tα-GDP) via an intrinsic GTPase activity that is accelerated by the GTPase activating protein RGS9 (regulator of G-protein signaling).
(3) Inactivation of phosphodiesterase (PDE) is coupled to the inactivation of transducin. Inactivated transducin (Tα-GDP) dissociates from PDE, resulting in a cessation of PDE-mediated cGMP hydrolysis.
(4) Activation of guanylate cyclase by guanylate cyclase activating protein (GCAP) restores cGMP levels and thus promotes the re-opening of cGMP-gated channels.
A single photoactivated rhodopsin catalyses the activation of ____ transducin molecules
500
a single activated rhodopsin can cause the hydrolysis of more than ____ molecules of cGMP per second.
10^5
Duplicity theory
Can’t have high sensitivity and high resolution in a single receptor, hence separate systems for colour and monochrome
Types of bipolar cells
- ON
- OFF
ON bipolar cells inhibition
Glutamate inhibits ON bipolar cells via metabotropic glutamate receptors
OFF bipolar cells excitation
Glutamate excites OFF bipolar cells via inotropic glutamate receptors
ON bipolar cell
- Hyperpolarised in the dark
- Depolarised by central illumination
- Metabotropic glutamate receptors
- Conductance increases in light
- Light response leads to loss of inhibition by glutamate
OFF bipolar cells
- Depolarised in dark
- Hyperpolarised by central illumination
- Ionotropic glutamate receptors
- Conductance decreases in light
- Light response leads to loss of excitation by glutamate
Retinal ganglion cell types
- W
- X
- Y
- Melanopsin RGCs
W cells
- Mainly from rods
- Slow conducting
- Broad fields, dendrites spread
- Directional movement
- Vision in low light
X cells
- Medium conduction
- Small fields, dendrites not spread
- Image formation
- Colour vision
Y cells
- Fast conduction
- Broad fields, dendrites spread
- Rapid responses to changes in visual fields
Melanopsin RGCs
- Large-slow-light responsive
- Independent of rods and cones
- Circadian rhythm
Why is vitamin A important in our diet?
- It is the precursor for 11-cis-retinal which is important in light transduction
- Vitamin A cannot be synthesised by humans
Vitamin D on vision
- Preventing angiogenesis (new blood vessel formation)
- Anti-inflammatory, inflammation plays a role in the development of age-related macular degeneration (AMD)
- Vitamin D may help prevent dry eyes by inducing cathelicidin, an anti-microbial protein that can be produced by cells in the eyes and heal eye wounds
