1.7 The Visual System Flashcards
what does the visual system do
provide long and short range sensory information about the layout of surrounding
how does the visual system work
o Converts photons into electrical signals
o Processes the signals using several pathways
Analyses visual scenes, identifies objects and faces
Provides info about threats
Estimates self-motion and motion of external objects
how is the optical apparatus is suited to focus light onto the photoreceptors
photoreceptors in the eye
o By adjusting the curvature of the lens, visual objects located at different distances are focussed onto the retina.
what does the retina consist of
- photoreceptor cells (PRCs)
- bipolar cells
- retina ganglion cells
- horizontal cells
what are photoreceptor cells
Outermost layer
Absorb light and convert to a neural signal (phototransduction)
what are bipolar cells
Receive synaptic signals from PRCs
what are retina ganglion cells
Innermost layer
Input from bipolar cells
Output from retina, with axons forming optic nerve
what are horizontal cells
Provide lateral connections
where is the vision sharpest
The area of retina near the optical axis, the fovea, is where vision is the sharpest
o Corresponds to the centre of gaze when we look at something
o Density of photoreceptors, bipolar cells and ganglion cells is highest at the fovea.
o In the centre of the fovea, the foveola, cellular layers are pushed aside to allow more immediate access to PRCs
what are the 4 regions in RPCs
outer segment
inner segment
cell boy
synaptic terminal
what is the outer segment
at distance surface of retina
what is the inner segment
located proximally
what are the 2 types of photoreceptors
rods and cones
what is the shape of a rod
Long, cylindrical outer segment within which the stacks of discs are separated from the plasma membrane
what is the sensitivity of rods
Highly sensitive, even to a single photon so dominate vision at low light levels
o Saturate at higher light levels
where are rods found
Only a single type present in primates
100 million in human retina
o None in fovea
We can’t see what we are directly looking at during the night
what is the shape of a cone
Shorter, tapered outer segment and the discs are continuous with the outer membrane
what is the sensitivity of a cone
Much less sensitive to light o Make no contribution to night vision Considerably faster response
what are the types of cones primates have
L, M and S cones
where are cones found
6 million in human retina
o Concentrated in fovea
o S-cones make up 10% but none in fovea
what type of vision is fovea specialised for
daytime vision
what controls the membrane potential o RPC
controlled by conductances of 𝐾+ and 𝑁𝑎+ ions whose transmembrane gradients are maintained by pumps
what happens to the ions in the dark
In the dark, 𝑁𝑎+ ions flow into the photoreceptor through nonselective cation channels that are activated by the second messenger cGMP
what triggers the photocascade
Absorption of a photon triggers a biochemical cascade that lowers the concentration of cGMP, closing the gated channels and hyperpolarising the cell to the 𝐾+ potential
whats the visual pigment in rod cells
rhodopsin
where is the opsin component
embedded in disc membrane
what is retinal
the light absorbing moiety
11-cis isomer covalently linked to a lysine residue of Opsin
Absorption of a photon causes it to flip from 11-cis to the all-trans configuration
what does the activated rhodopsin do
diffuses within the disc membrane where it encounters a G-protein (transducin in rods)
what does the inactive form of rhodopsin do
Inactive form binds GDP but interaction with rhodopsin promotes binding of GTP instead This causes dissociation into active subunits
what does the active transducin subunit do
subunit T𝛼-GTP complexes with cGMP phosphodiesterase increasing its activity
o Enzyme hydrolyses cGMP -> 5’-GMP
what controls the activity of the cGMP-gated channels in the plasma membrane of out segment
conc of cGMP
o Lowered cGMP upon photon interaction causes 𝑁𝑎+ channels to close and reduce the inward current into the outer segment
what happens with the dark current
There is an inward 𝑁𝑎+ current and an outward 𝐾+ current in darkness
o PRC membrane potential ~−40 𝑚𝑉
o Cell’s synaptic terminal continually releases neurotransmitter glutamate
what happens to Na current flow when there is light
o PRC membrane potential about that of potassium potential ~−70 𝑚𝑉
o Hyperpolarisation slows release of glutamate from photoreceptor terminal and so initiates a neural signal
what does the photoreceptor layer produce
(Low level visual processing)
produces a simple representation of the visual scene
o Neurones in bright regions are hyperpolarised and those in dark regions depolarised
what does the retinal circuit do
must edit the information before it is conveyed to the brain
o This can be seen physically as the optic nerve has 1% as many axons as there are receptor cells
what are the local interneurons is the retina
they generate graded membrane potentials
- horizontal cells
- bipolar cells
- amacrine cells
local interneuron - horizontal cells
collect signals from several neighbouring PRCs
local interneuron - bipolar cells
send signals to amacrine or ganglion cells
local interneuron - amacrine cells
horizontal processing layer
retinal ganglion cells
axons form optic nerve and send AP to brain
what do retinal ganglion cells (RCGs) have
receptive fields that are physically represented by a compact area on the retina
o Comprises of a centre region and a surround region
o These two regions produce antagonistic responses
RGCs centre-sound anagonism is due to…
lateral connections by horizontal cells
what happens when RCGs are on-centre
Increased firing rate when light incident on centre of receptive field
Decreased firing rate when light incident on surround of receptive field
If both parts of receptive field covered, a weaker response occurs
what happens when RCGs are off-centre
Decreased firing rate when light incident on centre of receptive field
Increased firing rate when light incident on surround of receptive field
RGCs transmit signals to the bain via three visual pathways…
o Pretectum Pupillary reflex o Superior colliculus Control of eye movements o Lateral geniculate nucleus (LGN) Visual perception
what does the LGN act as
the primary relay for the visual system
- part of the thalamus
what is LGN
lateral geniculate nucleus
how are the pathways that process light intensity and colour information organised
retinotopically organised
outputs to the primary visual cortex
where are simple cells
primary visual cortex
what do orientation selective neurones have
have receptive fields divided into ON and OFF sub-regions
o Larger than those of RGCs
o Respond optimally if a light bar of specific orientation enters the ON region
what is the characteristic response to a moving bar
o Discharge briskly when a bar of light leaves an OFF region and enters ON
o Highly selective for the position of a line or edge in space
what does convergent synaptic connections
o Individual simple cell may receive inputs from three LGN cells whose receptive fields are arranged in space
o The receptive fields are summed to create an orientation selective receptive field.
how are orientation-selective cells are arranged in…
orientation columns
o These contains cells that are selective for a specific orientation
o Together they cover the entire range of 360°
cortex is retinotopically arranged so…
each column can be seen as a set of orientation filters analysing contours within a certain patch of the visual field.
o This information is passed to higher centres
how do we perceive faces
Certain combinations of contours are processed by cells in the inferior temporal cortex to be perceived as faces
o Recordings show how the contours of a stimulus need to be arranged to cause an increase in cell spiking activity.
what is responsible for the cell’s response to faces
Selective combination of simple cell outputs converging onto a cell in the inferior temporal cortex
