Visual Physiology Flashcards
How is light transmitted through the eye in basic terms?
transmission of light through eye structures (in ocular bulb) to reach the photoreceptors in the retina
excitation of these photoreceptors stimulates (or not) glutamate release which then activates APs in the basal ganglia
these travel along the optic nerve to the primary visual cortex (occipital lobe) to be interpreted as sight
What important structures aid visual capture in the eye?
CHOROID
this provides a highly specialised blood supply to the outer retina
RETINAL PIGMENT EPITHELIUM
located between the choroid and the neural retina, this RPE is also known as the non-neural retina
has multiple functions including light absorption, phagocytic regulation of the outer segments of rods/cones and maintaining the blood-retinal barrier
What are the different layers of the peripheral retina?
outermost (furthest away from vitreous body)
- photoreceptors
- interneurones (bind glutamate and generate APs to transduce signal)
- ganglion cells (strip out the noise and select the salient info for transmission to the brain via APs)
innermost (closest to vitreous body)
What are the 2 main types of photoreceptors?
RODS
night vision
system is more sensitive and phototransduction response is sustained for longer
CONES
day vision
three types: RGB (red, green, blue)
What is the structure of a cone photoreceptor?
2 main sections of cell:
- outer segment (membrane discs containing multilayered phospholipid bilayers like the PM)
- inner segment (cell body and therefore nucleus of cell
Below the cell body of the photoreceptor, there is an “axon” and a synaptic terminal
the “axon” is so-called due to its anatomical structure, but does not fire APs like neurons do
the synaptic terminal is responsible for releasing GLUTAMATE vesicles (upon stimulation)
What is the normal resting membrane potential of a neuron?
-70mV
due to the leakage of K+ ions out of the cells
What is the resting membrane potential of a cone photoreceptor cell?
-45mV
caused by leakage of Na+ into the cell by non-specific cation channels in the outer segment of the cone cells
What is the consequence of having a resting potential of -45mV in cone cells?
much less negative than resting potential for neuron
but not an issue as these cone cells do not fire APs
however, does mean that even at rest, there will be some basal release of glutamate vesicle from the presynaptic termini
What occurs to cone cells when exposed to increased light (brightness)?
cone cell becomes HYPERPOLARISED
(more negative in potential)
this causes closure of the Na+ channels in the outer segments
less glutamate is released from the presynaptic termini
What occurs to cone cells when exposed to reduced light (brightness)?
Cone cell becomes DEPOLARISED
(less negative in potential)
causes opening of Na+ channels in outer segment)
more glutamate is released
What is opsin?
protein component of the colour pigment in the retina
sits on the membrane of membrane disc in the outer segment of cone/rod cells
functions in the initiation of the light response: TRANSDUCTION pathway
in rods, this is rhodopsin
cones: 3 types of opsin proteins (one for each type of colour pigment RGB)
All opsin proteins have a binding site for retinal
What is retinal?
= 11-cis-retinaldehyde (commonly known as retinal)
chromophore derivative of vitamin A
can be in a cis- or trans- conformation
cis-retinal is less stable and therefore more likely to react or be broken down
What is photopigment vs. activated photopigment?
PHOTOPIGMENT
opsin bound to cis-retinal
ACTIVATED PHOTOPIGMENT
opsin bound to trans-retinol
What occurs in the ‘photoisomerisation of retinal’ of light transduction?
Retinal = cis conformation which is less stable that in a trans conformation
(minority of molecules, most are in trans)
when light hits a cis-isomer, the unstable bond is ruptured and the molecule is reformed in its trans conformation
this forms the ACTIVATED PHOTOPIGMENT
What occurs in the ‘amplifying cascade’ of light transduction?
Opsin bound at the membrane disc is a GPCR
binding of retinal at the opsin = activated photopigment
light activation causes conversion to all-trans-retinol (end product) and activation of the attached G-protein
this increases GMP phosphodiesterase activity (leading to increased cGMP hydrolysis/breakdown)
Leads to a fall in [cGMP], which means less are bound to the Na+ channels in open conformation
Na+ channels close, causing change in membrane potential
What is the mechanism regulating the amplifying cascade of light transduction?
= DENSENSITISATION
need the amplification to be transient, so that it can be stimulated from baseline for the next cycle without an after-image
- opsin is inactivated (conformational change due to loss of Ga subunit)
- loss of binding site means that the retinal will fall off
- removal of retinal derivative and destruction
- termination of GPCR cascade
- intracellular [cGMP] rebuilt (by guanylate cyclase)
- allows re-opening of Na+ channels
How does the light transduction cascade differ in rods (vs. cones)?
In rods, the same process happens
just happens more slowly and the GPCR cascade is sustained for longer
more pixels per photon, greater quality ?
How is termination of the transduction cascade terminated?
can be terminated at several levels
e.g. phosphorylation of rhodopsin by rhodopsin kinase
then binding of phosphorylated rhodopsin by arrestin
binding causes quenching of signal (produced by GPCR cascade)
What kind of light changes do photoreceptors respond to?
range of light sensitivity is massive
but will primarily respond to minute changes in brightness from one moment to the next
able to do this by casting aside the absolute quantification of the overall brightness in that situation
due to adaptation process: amends resting potential between light transduction cascades
if the overall light brightness remains the same throughout then the photoreceptor cell will return to it OG baseline (-45mV)
but if the brightness changes, then the basal resting potential will also change to adapt
What mechanisms need to occur in photoreceptors with utmost biochemical precision?
- transduction of the light stimulus
- termination of the light response
- regeneration of photopigment
- controlling the rate and sensitivity of light response (adaptation)
How do photoreceptors control the nature of the light response produced?
Presence of CONTROLLER PROTEINS
these can regulate transducer/enzyme activity
each type of light stimulus will therefore result in a different level of response
e.g. in darker environments, want each photon to produce a bigger response (and vice versa in lighter environments)
What is retinitis pigmentosa?
degenerative disease of the retina, which can lead to blindness
monogenic disorder causes by point mutations in single gene
(>60 causal genes identified in 2013)
causes erosion of the (peripheral) retina and release of photopigment from the photoreceptor cells
Rod death causes cascade effect for surrounding tissue
What is the rate of light response we can see?
can see a stimulus that is flickering 70x per second
What is needed in order for outer segments to produce a very fast response to light?
- v. high metabolic rate
- supply of O2 and nutrients to produce ATP/GTP
much higher metabolic demand than the brain
How is higher resolution sampling supported in photoreceptor outer segments?
- dense packing into arranged hexagonal array
- hence why there are no capillary beds seen near the retina
- gaps between photoreceptors would create gaps in vision
Where is the choroid located in relation to the peripheral retina?
on top of the retinal pigment epithelium
What is the function of the choroid?
contains massive vessels with a massive blood flow
maintains arterial-level partial pressures of O2 at the outer segments of photoreceptors
Where is the RPE located?
RPE = retinal pigment epithelium (non-neural retina)
forms interdigitations between the outer segments of the photoreceptors
What is the function of the RPE?
RPE = retinal pigment epithelium (non-neural retina)
- pumps out fluid from junction between the 2 cell types
- glycoproteins in junction provide structure to cells
- RPE is also a blood barrier between choroid and outer segments
- transport all-trans-retinol and for it to be converted back to 11-cis-retinal
- phagocytosis of the terminal 10% of outer segments (helps to minimise photo damage)
What damage occurs to excessive photo-exposure at the outer segments of photoreceptors?
repeated exposure to light plus high O2 environment
causes increased rate of protein and cellular damage (radical formation also)
therefore, cells must have mechanisms in place to replenish the damaged molecules
phagocytes in RPE also help to remove damaged outer segments
What can occur if this renewal process/damage control of photoreceptors can’t occur?
disease - often occurs with increasing age
damaged proteins/intracellular debris (= LIPOFUSCIN) from within photoreceptors aren’t broken down and eventually get secreted on to the basement membranes of RPE
this attracts further lipid accumulation from the surrounding blood vessels in the choroid creating fatty plaques (= DRUSEN)
these appear as white spots on fundoscopy
What are the supply roles that the RPE does?
- holds the retina in place
- acts at the blood-retinal barrier between outer segments and choroid
- regenerated 11-cis-retinal
- helps to renew outer segment membranes
dysfunction of the RPE is a feature of many retinal diseases
What determines ability to see fine detail?
- precise focus
- detailed sampling
- small pixels
Why does most of the retina not have ideal condition for visual capture?
LIGHT SCATTERING
occurs due to structures that obscure the direct path for light reaching the retina
this causes impaired focusing power and the image blurs as it passes through the retina
SPARSE SAMPLING
there are big gaps in the sampling matrix
LARGE PIXEL SIZE
many photoreceptors direct signals to one retinal ganglion cell causing big pixels
the greater the convergence of multiple signals, the greater the pixel size
What is the ‘receptive field centre?’
Part of the visual world that is detected by a particular ganglion cell
as you venture into the peripheral retina, the quality of visual acuity reduces
Which area of the retina is the most specialised for high resolution vision?
Centre of retina
this is called the macula lutea (contains yellow pigment)
in the centre of the macula lutea is the fovea centralis
Why is high resolution imaging possible at the fovea?
foveal pit: 0.35mm has high focussing power
there are much fewer cells and virtually no blood vessels
hence less light scattering
only cones (RG) are present
- > provides cleanest sampling matrix
- > highly densely packed cones as each is v. thin
there is no signal convergence
signal from each cone is separate
What kind of vision loss is present in Glaucoma?
- peripheral visual field loss
- can still read and recognise faces
What vision loss is present in ace-related macular degeneration?
- loss of central vision
What elements of brightness do photoreceptors report on?
changes in brightness from one MOMENT to another
will discard info about absolute brightness
What changes do retinal ganglion cells report on?
changes in illumination from one LOCATION to another
What is a bipolar cell?
intermediate neuron between photoreceptors and basal ganglia
these cells can fire APs in response to glutamate secreted from photoreceptor cells
How might a basal ganglion cell receive visual info?
Via BIPOLAR CELL: which may be activated in response to photoreceptor cell
Via INHIBITORY INTERNEURONES: these will have inhibitory responses to the photoreceptor activation in periphery
What is the nature of the receptive field surround?
it is antagonistic
=> if the primary visual stimulus from one photoreceptor is mimicked by its surrounding photoreceptors (there is no change in brightness in location) and so there is no net effect
How are cone receptors arranged?
only one array of cone receptors, but each provides input to several types of ganglion cell
What are the different types of cone photoreceptors?
“OFF” CELLS
excited by DECREASED illumination of their photoreceptors
“ON” CELLS
excited by INCREASED illumination of their photoreceptors
What are the different types of ganglion cell in the retina?
PARVOCELLULAR GCs
high resolution
colour (R/G colour distinction)
MAGNOCELLULAR GCs
detect fast moving objects
low contrast objects
How does brightness relate to light wavelength in vision?
brightness = light wavelength
What is the nature of PARVOCELLULAR ganglion cells in the retina?
- small receptive field centre
- strong receptive field surround inhibition
- provide spatially precise info about contrast
- slower pathways, but better resolution
DAMAGE: reduce VISUAL ACUITY
What is the nature of MAGNOCELLULAR ganglion cells in the retina?
- bigger receptive field centres
- weaker receptive surround inhibition
- provide coarse resolution
- good for fast-moving objects
- really fast signalling pathways to the brain
DAMAGE: reduces sensitivity to RAPID MOVEMENT
What are the 3 types of cone photoreceptors?
Red Green Blue
Red: can respond to a large proportion of visible light wavelengths
However, not possible to gain wavelength info from a single cone
Response of a single cone is ambiguous
What systems exist to build a colour picture for an image?
System 1: compare red and green cells
System 2: compare blue cells to the R/G combo. This is known as the blue-yellow system)
What is the nature of blue cone colour blindness?
usually acquired not genetic
What is the nature of the visual acuity at the central acuity?
useful resolution and good colour discrimination
[this decreases as you venture into peripheral retina]
Fovea is a specialised area for highest visual resolution
What is the primary visual pathway responsible for?
conscious vision
visual info is sent to the lateral geniculate nucleus (located in dorsal thalamus)
from here, it is sent to the primary visual cortex (V1) in the posterior occipital lobe as optic radiations
What info do the retina and lateral geniculate cells encode?
- contrast e.g. edges of things
- wavelength of light
What do primary visual cortical cells all encode?
- orientation of edges
- continuity of edges (e.g. corners)
- direction of motion
- binocular disparity (depth)
More complex features of vision)
Where is info sent beyond the primary visual cortex?
can be sent to ~20 higher visual areas (of 2 main types)
however, V1 is fussy when deciding if APs will be fired
so this is a specific response
What do the higher visual cortical areas process?
different features of an image depending on the area its sent to:
- PARIETAL: location tasks
- TEMPORAL: facial recognition
In which specific areas of the temporal cortex is facial recognition processed?
these tasks will activate the
- VENTRAL OCCIPITAL
- INFERIOR TEMPORAL CORTEX
How may visual cortex processing be measured/detected?
by PET-CT (scan)
-> due to increased blood flow to these areas
What is associative agnosia?
“a normal percept stripped of meaning”
- caused by lesions in the occipital-temporal regions
- causes interruption of the ventral pathway
Patients can copy images but cannot recognise what they are
What is needed in order for (higher) construction of an image?
prior experience or exposure to an object or concept
can visualise this by PET-CT: will see areas of the frontal cortex lighting up
What can occipito-parietal lesions cause?
problems with spatial vision and visuo-spatial coordination
given a photo, pts will be able to identify isolated objects but will have difficulty in describing the relationships between objects
What problems can arise from parietal lesions?
inability to distinguish self-movement from outside movement
e.g. occipital junction may respond to a movement that doesn’t actually exist
movement will also activate the INFERIOR PARIETAL cortex
