Task 2: The Retina Flashcards
Visible light
energy within the electromagnetic spectrum that humans can perceive
Electromagnetic spectrum
continuum of electromagnetic energy that is produced by electric charges and is radiated a waves
Wavelength
the distance between peaks of electromagnetic waves, associated with different colours of spectrum
blue = 400 nanometers red= 700 nanometers
Light is reflected from objects to the ___ and is focused by the ___ and ___ to form sharp images on the ____
pupil, cornea, lens, retina
Where does light enter the eye?
cornea
How much focusing power does the
a) cornea
b) lens
have
a) 80%
b) 20%
Why is the cornea transparent?
no blood vessels therefore photons
Why is the lens transparent?
no blood supply
What does the ciliary muscle due?
changes the shape of the lens to adjust the eyes focus by increasing the curvature of the lens (this process is called accommodation)
a) Define accommodation
b) What does accommodation prevent?
a) change in len’s shape that occurs unconsciously when ciliary muscles tighten and increase curvature of lens
b) blurring
Define retina
network of neurons that covers the back of the eye and contains photoreceptors responsible for receiving image froms lens and sending it to the brain via optic nerve
What are photoreceptors?
rods and cones containing light-sensitive visual pigments, transduces light energy to neural energy
What part of the eye brings light into focus?
retina
What is the role of the retina?
to detect light and communicate to brain the aspects of light related to objects in the environment
What part of the eye gives it its colour?
iris
What does the iris do?
the iris controls the size of the pupil and the amount of light that reaches the retina, attached to ciliary muscles
Pupillary light reflex
automatic expansion or contraction when level of light increases or decreases, that allows either more or less light to enter the eye
What happens to the lens and pupil when the ciliary muscle
a) contracts
b) relaxes
a) lens thickens for close vision, pupil small for less light
b) lens thins for distant vision, pupil large for more light
Role of pupil
allows light in to reach lens
Aquous humor
a) define
b) function
a) fluid filled space behind cornea
b) supply of O2 and nutrients to cornea and lens and; removing waste from cornea and lens
Vitreous humor
a) define
b) function
a) gel-like fluid that fills the vitreous chamber
b) light refractor - to reach retina, light goes through vitreous chamber, where it is refracted by vitreous humor
What is the longest part of the journey through the eyeball for light?
vitreous chamber
Fovea
a) location
b) function
a) part in the center of the retina
b) - responsible for producing the highest visual acuity and point of fixation - seeing fine detail, reading, identifying objects
Light energy from stimulus is converted into
neural energy
Fundus
back layer of retina, only place where veins and arteries can be seen directly
Optic disc
- point where arteries and veins responsible for feeding the retina, enter eye
- point where axons of ganglion cells leave the eye via optic nerve
Discuss how the optic disc contains no photoreceptors
- part of field of vision is invisible
- blind spot: area in retina with no receptors, occurs in place where ganglion fibers are collected into optic nerve and leave the eye
How does the brain substitute missing information from blind spot?
based in surronding detail and info from other eye
Differentiate between
a) near point
b) far point
a) distance at which lens can no longer accommodate to bring closer ojects into focus
b) distance at which light becomes focused on the retina
Presbyopia
a) define
b) cause?
c) approx age
d) solution
a) a condition that occurs when a person gets older,
where the distance of the near point increases
b) lens hardens with age and ciliary muscles weaken
c) 45
d) hold reading material further away or reading glasses to replace focusing power
Myopia/nearsightedness
a) define
b) reason
c) solutions x 3
a) a condition where people are unable to see distant objects clearly
b) myopic optical system brings parallel rays of light into focus at point in front of retina, 2 factors for this
- refractive myopia: corena and/or lens bends light too much (curvature)
- axial myopia: eyeball is too long
c) 1. move object closer as it pushes point further back
2. corretive glasses/lens that bend incoming light focusing it as if it were at far point
3. laser which sculpts the corena
Hyperopia/farsightedness
a) define
b) cause
c) symptoms
d) solution x1
a) a condition where people can see distant objects clearly but have trouble seeing nearby objects
b) focus point is located behind retina as the eyeball is too short
c) eyestrain and headaches due to effort needed to accommodate
d) corrective lens bringing focus point forward on retina
Name the 5 classes of neurons in the retina
- photoreceptors
- horizontal cells
- amacrine cells
- bipolar cells
- ganglion cells
On what dimensions do rods and cones differ?
shape, distribution , function
Discuss rods in relation to
a) sensitivity
b) night
c) photopigment
d) colour
e) convergence and acuity
a) very sensitive, can only be activated by a single photon, slow response
b) depolarize at night, specialised for night vision as it releases an inhibitory neurotransmitter at night
c) rhodopsin
d) only black and white due to one type of photopigment therefore cannot signal colour differences
e) high convergence and low acuity
Discuss cones in relation to
a) location
b) why specialised for daylight vision?
c) 3 photopgimens
d) convergence
a) focused in fovea, the further away from it the lower the density
b) low sensitivity but high acuity
c)
1. blue/shortwavelengths/s-cones/none in fovea as it only have 2 cones
2. green/mediumwavelengths/m-cones/fewest no. of cones
3. red/longwavelengths/cones most prevalent
d) little
Retinal eccentricity
the further from the fovea, the lower the density f cones
Which has a biggger receptive field?
a) rods
b) cones
b
Discuss the 3rd type of photoreceptor
contains melanopsin which is sensitive to ambient light, involved in adjusting bioloical rhythms, sending signals to suprachiasmatic nucleus (= circadian clock)
How is information passed to bipolar cells from photoreceptors?
via graded potential (electrical potential that varies continuously in amplitude)
Light is captured by photoreceptors, chemical changes produce neural events leading to ____ _____
visual sensation
Name and describe the 3 components of photoreceptors
Outer segment: contains photopigment molecules and is adjacent to pigment epithelium
Inner segment: between outer segment and cell nucleus, filled with mitochondria and produces visual pigment molecules
Synaptic terminal: place where axons terminate for transmission of information by release of chemical transmitters, contains connections from neurons to which photoreceptors communicate
Pigment epithelium
pigmented cell layer just outside the neurosensory retina that nourishes photoreceptors
Discuss the 4 steps involved in light transduction by receptors
- photoactivation: photoreceptors are activated by light initiating a chain of biochemical events that lead to an overall hyperpolarization of the cell
- hyperpolarization: increase in membrane potential that makes inner membrane surface more negative than the outer one
- Calcium channels close at synaptic terminals
- Glutamate concentration decreases signalling to the bipolar cells that rod has captured a photon, hyperpolariztion in photoreceptor leads to depolarization in bipolar cell (due to less gluatamate
What is the purpose of horizontal and amacrine cells?
they are specialized retinal cells, vertical to photoreceptors, they form a lateral pahway in the retina that enhance information
Discuss horizontal cells
- produce inhibitory GABA
- form lateral connections between photoreceptors and cause lateral inhibition
- important for center-surrond organization of ganglion receptive fields and for perceiving visual contrast
- in contact with photoreceptors and bipolar cells
Discuss amacrine cells
- many different types producing various neurotransmitters, mostly inhibitory
- found in inner synaptic layer connecting bipolar with ganglion cells
- present in cone and rod pathway thus sometimes combines input from these pathways
- involved in contrast enhancement and temporal sensitivitiy, detection of movement and temporal sensitivity changes in light patterns over time
Lateral inhibtion
antagonistic neural interaction between adjacent regions of the retina
-> important in visual perception and illusion, as well as for perceiving contrast
Bipolar cells
synapses with rods,cones and horizontal cells, receive information is passed to ganglion cells, function like a “mediator”
a) Diffuse bipolar cells
- > sensitivity
- > acuity
processes are spread out so they receive info from multiple cones
- > sensitivity increased by gathering information from many photoreceptors
- > acuity falls off rapdily with eccentricity
b) Midget bipolar cells
in fovea, receive input from single cones, pass it on to single ganglion cell
-> in fovea, no convergence (one-to-one pathway) causing high acuity and low sensitivity to light
(midget bipolar cells contd.) Discuss how each foveal cone contacts 2 bipolar cells (divergence of information)
- ON bipolar cell: responds to increase in light captured by cones
- OFF bipolar cell: responds to decrease in light captured by cones
- > presence of both provides info about whether retinal illumination is increased or decreased
Ganglion cells
responsible for the process of visual information received from photoreceptors to the visual cortex and thalamus
From what do ganglion cells receive information?
- bipolar cells
- amacrine cells
Name the 3 types of ganglion cells
- p ganglion cells
- m ganglion cells
- koniocellular cells
P ganglion cells
a) input
b) receptive field
c) temporal response
d) information transmitted
a) receive excitatory input from single midget bipolar cell in central retina
b) small because they do not converge and are connected to one photoreceptor, have finer resolution therefore a great acuity
c) respond with sustained firing when light shines on excitatory regions
d) transmit info about contrast in retinal image
M ganglion cells
a) input
b) receptive field
c) temporal response
d) information transmitted
a) receive excitatory input from diffuse bipolar cells
b) large because of convergence, more sensitive under low-light conditions
c) respond transently, with brief bursts of impulse when the spot of the receptive field is turned on, then quickly return to spontaneous state
d) transmit info about how this image changes over time
Koniocellular cells
located in koniocellular layer (layer between 2 layers of LGN)
Receptive field
region on retina in which visual stimuli influence a neuron’s firing rate
-> excitatory or inhibitory
Name and describe the 2 types of receptive fields ganglion cells can have
- on-center cell: depolarizes in response to increase in light intensity in receptive-field cener
- off-center cell: depolarizes in response to decrease in light intensity in receptive-field center
- > firing increases when light shines on surrond
Lateral inhibition of receptive fields
there is interaction between antagonistic center and surronding
What do optic nerve fibers monitor?
small areas of the retina and take in information about what is happening over the entire retina
Center-surrond organization
a) define
b) name 2 types
a) area in “center” of receptive field responds differently to light than area in the “surrond” of the field
b) excitatory-center, inhibitory-surrond receptive field
inhibtory-center, excitatory-surrond receptive field
Center-surrond antagonism
when light becomes large enough to cover the inhibitory area as it counteracts the excitatory response decreasing the firing rate
Name and explain the 2 parts of visual pigments
- opsin: long protein
2. retinal: small light-sensitive component
Isomerization
change of shape in retinal that occurs when visual pigment molecule absorbs on phonton of light
Dark adaptation
increasing sensitivity in the dark
Why do rods and cones adapt to light at different rates?
different visual pigments
Macular degneration
condition common in older people that destorys the fovea and small area surronding it, creates blind region in central vision making person lose sight of something if they look directly at it
Peripheral retina
includes all of retina outsie fovea, more rods than cones
Retinis pigmentosa
hereditary condition where the retina degenerates, results in poor vision in peripheral visual field
Dark adaptation curve
shows the function relating sensitivity to light to time in the dark
Discuss stages of adaptation curve
- Light turned off, sensitivity of rods and cones increases therefore higher sensitivity to light
- First stage/initial rapid stage: cones reach max sensitivity and rods continue adapting
- Second stage/slower stage: rods become more sensitive thean cones and rod adaptation is visible
Rod-cone break
place where rods begin to determine dark adaptation
Discuss visual pigment regeneration
in weak light, there are lots of pigments available therefore rods and cones absorb and respond to them as much as they can
Convergence
increased sensitivity of ganglion cells, adding together of inputs
Amount of light hitting photoreceptors determines amount of ___ released
glutamate
Photoreceptors and bipolar cells produce ___ ___
Ganglion cells works with ______ _____
graded potentials, all-or-none responses
