PBL 3 Flashcards
(48 cards)
What are the refractive structures that light passes through to reach to retina?
- Cornea
- Aqueous Humor
- Lens
- Vitrous Humor
Explain what happens in the accommodation reflex for near vision?
- Ciliary muscles contract, ciliary body moves towards lens
- tension is released is suspensory ligaments
- lens becomes more spherical
- rounder shaped lens can refract more light, to focus near image
Explain what happens in the accommodation reflex for distant vision?
- Ciliary muscles relax, ciliary body moves away from lens
- suspensory Ligaments pull at the lens
- lens becomes flatter
- flat shape allows lens to bring in more light from distant objects
What is visual acuity and how is it measured?
The ability to distinguish two nearby points
Measured using a snellen eye chart
Name the two layers of the retina
Pigmented part - outer layer
Neural part - inner layer
List the order of layers in the neural layer of the retina from outside to the inside
- photoreceptor outer segment
- outer nuclear layer
- outer plexiform layer
- inner nuclear layer
- inner plexiform layer
- ganglion cell layer
- nerve fibre layer
In which layer of the neural layer in the retina are the cell bodies for horizontal and amacrine cells located?
Inner nuclear layer
What is the area of the retina where there are only cones present?
What is the structure in the middle of this called?
Macula
Fovea
What is the difference between horizontal and amacrine cells?
Horizontal - forms network at outer plexiform layer at the level of synapses between photoreceptor and bipolar cells
Amacrine - forms network at inner plexiform layer at the level of synpases between bipolar cells and ganglion cells
What is the role of horizontal and amacrine cells?
Facilitate or inhibit communication at the plexiform layers, thereby altering the sensitivity of the retina
Play important role in eye’s adjustment to dim or brightly lit environments
What is the difference in convergence in rod cells and cone cells?
Rod cells - high convergence (lots of rod bipolar cells synapse onto one ganglion cell)
Cone cells - low convergence (one cone bipolar cell contacts one ganglion cell)
What is rhodopsin?
What is its structure?
Visual pigment present in rod and cones cells
Made up of:
- Opsin: 7 transmembrane G protein coupled receptor
- Retinal: light absorbing chromophore
How does the shape of retinal change when it absorbs light?
Converted from 11-cis form to all-trans form
Describe the process of photoreception
- Activation of opsin: due to change in retinal shape
- Opsin activates transducin
- Transducin activates phosphodiesterase (PDE)
- PDE breaks down cGMP to GMP
- removal of GMP from sodium channels causes inactivation
- membrane is hyperpolarised as neurotransmitter release declines
- as photoreceptor is turned off, bipolar cell is turned on
What is bleaching?
After a photon has been absorbed by rhodopsin, it has reduced sensitivity to light.
The whole molecule must be broken down and reassembled before it can absorb light again.
What are the 3 forms of opsin in cones and their wavelengths?
Blue: 420-550nm (short wavelength)
Green: 450-630nm (medium wavelength)
Red: 480-690nm (long wavelength)
What is the difference between on centre and off centre ganglion cells?
On centre ganglion cells - excited by light arriving in the centre of their receptive field
Off centre ganglion cells - excited by light arriving in the edges of their receptive field
What glutamate receptors do on centre bipolar cells have?
Explain how on centre bipolar cells respond in light and dark
- on centre bipolar cells have mGluR6 metatropic receptor which when bound to glutamate, causes hyperpolarisation of the cell
- In dark: they are inhibited (hyperpolarised) by glutamate
- In light: they are activated (depolarised) as glutamate release from photoreceptors is terminated
What glutamate receptors do off centre bipolar cells have?
Explain how off centre bipolar cells respond in light and dark?
Off centre bipolar cells have ionotropic (AMPA/kainate) glutamate receptors which are depolarised by glutamate
- In dark: they are depolarised by presence of glutamate
- In light: they are hyperpolarsied as glutamate is no longer present
What is the difference between M and P type ganglion cells?
M type - monitor rods (large receptive field)
P type - monitor cones (specific location monitored due to 1:1 ratio)
What two types of occupancy can be seen in colour-opponent ganglion cells?
Red vs Green
Blue vs Yellow
What happens to the medial (nasal) ganglion fibres when they reach the optic chiasm?
They decussate to the opposite side
What are the different neurons present in the different layers of the lateral geniculate nucleus?
Which type of ganglion cells do they receive information from?
Magnocellular neruons (layers 1+2) - receive information from M type ganglion cells
Parvocellular neruons (layers 3,4,5,6) - receive information from P type ganglion cells
What are the two types of pupillary muscles and where are they located in the eye?
Which part of the autonomic nervous system are they controlled by?
- pupillary dilators: located radially (controlled by sympathetic nervous system)
- pupillary constrictors: form a sphincter around the pupil (controlled by autonomic nervous system)
