week 3 - anatomy of the eye + photoreceptors Flashcards
What is the main stimulus of vision?
light
light
exists on a huge spectrum but humans are only able to see a small portion of all light rays
Can humans see UV rays?
no, but bees can
infrared rays
perspective view (heat vs. cold)
How can you right right eye from left eye when looking from above?
the optic nerve will move towards the middle/nose area
cornea
the initial, transparent layer that light passes through
In what order does light go through the eye?
- cornea
- iris
- pupil
- lens
- vitreous humor
- retina
iris
colored part of eye
pupil
- hole at the center of iris
- controls amount of light coming into eye (dilates and shrinks)
lens
refracts light with the help of the cornea
What is the purpose of refraction?
allows us to focus light on retina
vitreous humor
- clear gel that fills up space between lens and retina
- makes up majority of eyeball
- filled with nutrients
What is transduction in simple terms?
Visual receptors are turning external stimulus (light waves) into something the brain can understand (electrochemical patterns).
What are the 3 main layers of the retina?
- ganglion cells
- bipolar cells
- photoreceptors
How does light information enter into the retina?
- light enters from the ganglion cells and into the photoreceptors where it it goes through transduction
- information from photoreceptors is sent to the bipolar cells and then the ganglion cells
- the axons of the ganglion cells (optic nerve) send that information to the brain
How do our receptive fields capture information?
- Everything is projected onto the opposing side of our back wall
- Our left visual field goes to right side and vice versa.
- Our top visual field is projected onto the bottom of wall and vice versa
Why do we not perceive everything based on how it is projected into our retina?
our brain codes and fills in the gaps of missing information
optic disk
- lacks photoreceptors and retina
- natural blind spot
Why are we not able to notice our blind spot?
- brain compensates for missing information
- information is projected onto opposite side of retina
What would happen if part of the optic nerve was destroyed?
- you would begin to lose visual information from that line
- aka Glaucoma
- encroachment of the visual field
fovea
- most fixated and focused portion of the eye
- better acuity
- sensitive to detail
What are the two types of photoreceptors?
rods and cones
rods
- 120 million
- found in peripheries
- better sensitivity to light
cones
- 6 million
- found in fovea
neural convergence and perception
the distribution of a number of rods and cones
neural convergence in rods
- multiple rods converge upon a single neuron
- found in temporal or nasal peripheries
- less detailed visoin
neural convergence in cones
- a single cone converges on a single neuron
- found in central retina
- allows for better acuity/more detail
How does the wiring of rods determine detailed vision?
- multiple light rays are sent to a single neuron only causes one action potential
- only one form of information is sent to brain
How does the wiring of cones determine detailed vision?
- one single light ray going into one neuron causes one action potential
- multiple forms of information is sent to brain
Why does the fovea have better acuity?
Cones located in the fovea allows for the one-to-one ratio of information
macular degeneration
- fovea and small surrounding area are destroyed
- creates blind spot on retina
- most common in older individuals
retinitis pigmentosa
- genetic disease
- rods are destroyed first
- foveal cones can also be attacked
- several causes result in complete blindness
photopigment reaction
- when light hits retinal opsin (photopigment), the retinal bends away and releases from opsin, causing the pigment to bleach
- when light activates photoreceptors in rods, we have less pigment
rhodopsin
- photopigment in rods
- results in action potential
iodopsin
- photopigment in cones
- results in action potential
How does photopigment result in hyperpolarizaiton
- the cleavage/release of photopigment causes sodium channels to close.
- cell membrane becomes more negative
- light essentially causes hyperpolarization
How does the dark cause depolarizatoin?
- the photoreceptor is actively releasing neurotransmitter
- the neurotransmitter released by the photoreceptor inhibits the bipolar cell
What is the relay of information from photoreceptors to brain in the dark?
Photoreceptors (inhibitory neurotransmitter) → bipolar cells (actively inhibited) → ganglion cells (not activated; does not send action potential) → brain (no signals send
What is the relay of information from photoreceptors to brain in the light/hyperpolarization?
Photoreceptors (photoreceptors become more negative and less inhibitory neurotransmitters are released) → bipolar cells (excited and can depolarize) → ganglion cells (excited by bipolar cells) → brain (action potentials are sent)
How does hyperpolarization produce an action potential?
- hyperpolarization reduces neurotransmitter release, but the neurotransmitter is inhibitory. So reducing the neurotransmitter excited and depolarizes bipolar cell
- Responses of photoreceptors and bipolar cells are graded (either big or small)
- Ganglion cell will produce an action potential and the signal exits the eye via the optic nerve
dark adaptation
- We become increasingly more sensitive to the dark
- We are better able to see out of our peripheries
- cones take a shorter time to regenerate
perception and physiology
- Our sensitivity to light depends on the concentration of a chemical
- The speed at which our sensitivity increases in the dark depends on a chemical reaction
spectral sensitivity
sensitivity of rods and cones to different parts of the visual spectrum
rod spectral sensitivity
- More sensitive to short-wavelength light
- Most sensitivity at 500 nm
cone spectral sensitvity
- most sensitivity at 560 nm
Purkinje shift
enhanced sensitivity to short wavelengths during dark adaptation when the shift from cone to rod vision occurs
