Task 2: The Retina Flashcards
Wavelenght
Distance between peaks of electromagnetic waves.
Physiological process of vision (steps)
- light reflects from an object to the tear film
- lights enters through pupil
- lights is focused by the lens
- sharp images form on the retina
- transduction
- energy emerges the optic nerve
- energy transmitted to the brain through the optic chasm.
Role of the Retina
- place where light is brought into focus (by the lens)
- detect light
- light energy from stimulus is converted into neural energy that can be interpreted by the brain
- the process of seeing begins here
Accomodation
change in lens’ shape
The retina contains 5 major classes of neurons:
1- photoreceptors 2- horizontal cells 3- bipolar cells 4- amacrine cells 5- ganglion cells
Types of photoreceptors
Cones and Rods
Cones
- daylight vision (cones need light)
- mostly present in the fovea + periphery
- little convergence
- high acuity
- low sensitivity (responds fast)
- three photopigments
a) S-cones = blue
b) M-cones = green
c) L-cones = yellow, orange, red
Convergence
increased sensitivity of the ganglion cells
- more convergence => bigger receptive field
- retinal convergence => sharing of a single nerve fibre by multiple rods
Rods
- night vision
- sensitive photoreceptor => activated by one photon
- high convergence
- low acuity
- high sensitivity (low response)
- one type of rods = one photopigment Rhodopsin
- in the periphery
Photoreceptors do not respond in an
all-or-nothing fashion
Photoreceptors consist of
- outer segment: photopigment molecules, adjacent to pigment epithelium
- inner segment: visual pigment molecules
- synaptic terminal: release of chemical transmitters
Isomerization or photoactiavation
activation by light that initiates a chain of biochemical events => overall hyperpolarization of cell => retinal changes from bent to straight.
Hyperpolarization
increase in membrane potential => inner surface more negative => decrease of glutamate concentration
Hyperpolarization in the photoreceptor causes
depolarization in bipolar cells (because less glutamate)
Depolarization
less negative charge inside cell => visual pigment regeneration = reattachment of the retinal
Visual pigment bleaching
retinal separates from opsin after isomerization
Each foveal cone contains
two bipolar cells:
1- on bipolar cell (increase in light captured by cones)
2- off bipolar cell (decrease in light captured by cones)
Each ganglion cell has
its own receptive field
+ on-centre ganglion cell (depolarises to increase in light)
and off-centre ganglion cell (depolarises to decrease in light)
Antagonistic centre-surround organisation in the ganglion cell receptive field occurs because
of inhibitory action of horizontal cells => release GABA
If horizontal cell is hyperpolarized then
it produces less GABA => less inhibition and bipolar cell is then depolarized.
Excitatory area
where there is an increase of firing when presented light
Inhibitory area
where there is a decrease in firing when presented light
On-Center
cone hyperpolarized=>less glutamate=>on-center bipolar cell is depolarized=>transmitter release increases=>firing on-center ganglion cells increases
Off-Surround
cone depolarized=>horizontal cells active=> increase in GABA=>off-center bipolar cells are hyperpolarized=> transmitter release decreases=> firing off-center ganglion cells decreases
Off-Center
cone depolarized=> more glutamate=>on-center bipolar cell is hyperpolarized=> transmitter release decreases=> firing on-center ganglion cells decreases
On-Surround
cone hyperpolarized=>less glutamate=>horizontal cells less active=>off-center bipolar cells depolarise=> transmitter release increases=>firing off-center increases
Process of transduction
Light -> photopigment activation -> enzyme cascade -> less Na+ channels are open -> less Ca+ channels are open -> hyperpolarization -> less Glutamate
Dark adaptation
process of increasing sensitivity in the dark
threshold intensity decreases
Dark adaptation steps:
1- light turned off = > sensitivity of both cones and rods increases => higher sensitivity to light
2- initial rapid stage = cones reach maximum sensitivity, rods still adapting
3- slower stage = rod become more sensitive than cones (rod-cone break)
Hermann grid
- excitatory centre of this receptive field between squares has stronger response
- receptive fields in central fovea are smaller than in rest of retina
