2 - RETINA Flashcards
ophthalmoscope
instrument for looking into ones eye through the pupil
- used by optometrists and physicians
- can see optic disk and macula
- dark patch of macula is the fovea
- area of eye visible = FUNDUS
optic disc
the roughly circular yellow region with long branching tendrils (blood vessels) over the surface of the retina coming out
- where the blood vessels leave and enter the eye
- where ganglion axons leave the eye to the brain (head of the optic nerve)
no photoreceptors or neurons = blindspot
sclera
tough, fibrous outer ‘skin’
order of cell layers in retina
thin ganglion layer closest to retinal surface
intermediate cell layer
bottom layer (photoreceptors) - rods and cones
pigment epithelium
(all are transparent)
why stain retina samples
because they’re transparent to let light in to the photoreceptors
- but blood vessels aren’t transparent (on the retinal surface)
- so can block and prevent light from reaching the photoreceptors
cells in the retina are neurons
they are formed from nervous tissue
- all cells in figure 8 (wk1, pg10) are all neurons
- including photoreceptors
- retinas are considered part of the brain
- only part of the brain that doesn’t require opening the skull to see
neurites
the axons and dendrites of the neuron cell bodies
found in the layers between the cell bodies
THEY DO NOT TRANSMIT ACTION POTENTIALS
- they communicate via passive conduction of ion currents to the neurons in the next layer
- ‘neurites of the photoreceptors make synaptic connections with the neurites and cell bodies in the intermediate cell layer and the neurites of those cells make synaptic connections with the cells in the top layer’
📣SIGNAL TRANSMISSION FROM BOTTOM TO TOP
☀️LIGHT TRANSMISSION FROM TOP TO BOTTOM
bipolar cells
transmit signals between photoreceptors and ganglion cells (ganglion cell layer is 1 cell thick)
amacrine cells
connects to ganglion cells (kinda horizontally)
photoreceptors
sensory neuron
relatively long and thin cells
light sensitive part is the outer segments (contains opsins)
cell bodies aren’t light sensitive
rods
long thin photoreceptors
- only respond when light stimulation is low
cones
shorter and fatter photoreceptors
opsins
light sensitive proteins found in the outer segments of photoreceptors
inner segment of photoreceptors
part that is between cell body and outer segments
order of structure of photoreceptors
neurites (axons)
cell bodies
inner segments
outer segments
in a micrograph of a horizontal slice through the receptor level, the dots with the circular gap around are cones
- this is seen about 6-10mm from the macula centre
-
daytime vision (photopic)
cones
they need a lot more stimulation so require more light to respond
nighttime vision (scotopic)
rods
respond to low light stimulation
- due to few rods in the fovea = cannot see fine details at night
- ‘night blind at the very centre of our vision’ (foveola)
macula
contains lots of photoreceptors (vast majority of all cones)
- fovea seen through ophthalmoscope
- darkest region in centre = FOVEOLA (only cones)
parafovea
fuzzy region that extends the macula
- extends about 0.5mm from the foveal boundary
- lines on graph of cone and rod density cross at about 1mm from retinal centre
- densities equal (about 40,000/mm^2)
order of macula structures
foveola > fovea > parafovea
foveola
the very bottom of the depression in the centre of the macula
- 0.3mm in diameter
- only the bottom layer (photoreceptor layer) and their neurites
- the cell bodies above have been swept away and are found distributed around the foveola
- no retinal cell bodies over the photoreceptors
- no blood vessels
- only CONES
- high density of about 200,000 per square mm
- NO RODS
- cones here are rod shaped
- means nearly half the cones in the retina aren’t even cone shaped
- ‘highest density of photoreceptors than anywhere in the retina’
fovea
the depression in the macula
- includes the sides and bottom of the depression
- 1.5mm across
- mostly cones
- few rods (most near or at the foveal boundary)
- ‘no blood vessels to prevent light from reaching the photoreceptors’
- the clearest image
- need to position eyes so that the image falls on the fovea
clinical macula
seen through an ophthalmoscope
- central part of the anatomical macula
- dark patch on retina
anatomical macula
defined by cellular anatomy rather than observed through a clinical eye exam
- the depth of cells in the ganglion layer is used to distinguish the anatomical macula
- twice as wide as the clinical macula
ganglion cell layer depths
1 cell = outside anatomical macula
two or more cells = inside anatomical macula
- none in foveola
- thickest in parafovea
perifovea
completely outside the clinical macula
- ‘annular region of the anatomical macula that surrounds the parafovea’
- cone density of around 10,000/mm^2
- around 1.3mm from macula centre
peripheral retina
‘region of the retina outside of the anatomical macula’
- mainly rods
- few cones
- cone density outside the macula = 3-4000/mm^2
- around 5mm from macula centre
- but a very cone rich zone at the edge of the retina
rod density
peak density = 150,000/mm^2
temporal side = 4mm
nasal side = 6mm (just beyond the optic disk)
- edges of retina = 50,000-70,000/mm^2
- no rods in centre of macula
- peaks around 4mm from macula centre
how many rods and cones in the human retina?
90 million rods
6 million cones
- area under the density cure for rods is about 20x that of the cones
graph of densities on pg 17 of week 1
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what makes a cell a rod or cone
the sensitivity to light stimulation
not the shape!!
as some cones are rod shaped
how to see stars at night
- don’t look directly at them because it’ll fall on the foveola which doesn’t have night vision (has no rods)
- so look out the corner of your eye at it and you’ll see it better
cone density
around 200,000 per mm^2 in centre of macula
- highest density in retina ^
- drops to about 1000-2000 per mm^2
- cones seem smaller in centre of macula
there is a vascular zone surrounding the fovea
- having no blood vessels and no cell bodies above the photoreceptors means light can access the photoreceptors in the fovea easier
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