What is Refraction?
Fact or phenomenon of light, radio waves, etc., being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density
What part of the eye is the first involved in refraction of light and what other structure in the eye also plays a role; which is fixed and which is variable?
- The cornea is the first site of refraction (is fixed) = 2/3's of light bending
- The lens adds variable amount of bending, depending on the curvature of the lens
How does a round vs. flat lens affect the refraction of light?
Round = more refraction
Flatter = less refraction
What is involved in increasing the curvature of the lens of eye and what kind of vision is this useful for?
- Ciliary muscle contracts, which allows suspensory ligaments to loosen
- Lens assumes a more rounded shape by natural recoil
- Used for near vision
What is involved in decreasing the curvature of the lens of eye and what kind of vision is this useful for?
- Ciliary muscle relaxes, which causes the suspensory ligaments to tighten
- The lens is pulled tight, flattening it.
- Used for far vision
When an object is far away, what happens to the light rays entering our eyes and how does this play a role in adjusting our vision?
- Light rays have diverged far enough that only parallel light rays enter the eye
- They don't need to be bent a lot to focus on retina, so the lens is best off flat
When an object is nearby, what happens to the light rays entering our eyes and how does this play a role in adjusting our vision?
- Light rays are still diverging and need a lot of bending to focus on the retina
- Requires our body to increase the curvature of the lens to compensate
What is the purpose of the increased curvature of the lens of the eye and what occurs with age?
- Allows light rays from a nearby object to be bent sufficiently to bring them into focus on the retina
- With age, the lens stiffens and loses elasticity = presbyopia = near vision becomes more difficult
Near visions requires 2 additional changes on top of contraction of the ciliary muscle, what are they?
1) Convergence of the the eyes to the point of focus
2) Constriction of the pupil
How does constriction of the pupil contribute to near vision?
- Reduce the opening for light to enter
- Eliminates some of the diverging light rays, so we don't need to bend as many to focus on the retina
- Allows us to focus better
Which 3 cells are in the vertical pathway of the retina and what is this pathway used for?
2) Bipolar cell
3) Ganglion cell
- This is the pathway for relaying the visual info to the brain
Glutamate release within the retina is highest when?
- HIGHEST in the dark
- No stimulation by photons
Glutamate release within the retina is lowest when and why?
- LOWEST when there is light
- Stimulation by photons causes the cells to hyperpolarize
What are the 3 steps in the activation of the bipolar cells by cone photoreceptors?
1. Photon stimulates photoreceptor
2. Photoreceptor hyperpolarizes
3. Glutamate release onto the bipolar cell DECREASES
What are ON-center bipolar cells, how are they activated, and what increases their discharge rate?
- Activation of a photoreceptor in the center of this bipolar cell's receptive field causes depolarization of this bipolar cell
- Activation in the periphery of this bipolar cell's receptive field causes hyperpolarization of this bipolar cell
- These cell increase their discharge rate to luminance increments in the receptive field center
What are the OFF-center bipolar cells, how are they activated, and what increases their discharge rate?
- Activation of a photoreceptor in the center of this bipolar cell's receptive field causes hyperpolarization of this bipolar cell
- Activation in the periphery of this bipolar cell's receptive field causes depolarization of this bipolar cell
- These cell increase their discharge rate to luminance decrements in the receptive field center
What is the glutamate receptor in depolarizing bipolar cells and what is its function?
- A metabotropic receptor (Gi, GPCR) that is activated by glutamate
- Closes cGMP-gated ion channels similar to light transduction in photoreceptors = hyperpolarizes cell
- Channels open when less glutamate is present
Explain the process of activation of an ON-center bipolar cell by cone photoreceptors in light conditions.
- Photons decrease the presence of glutamate
- Less glutamate = less activation of Gi-GPCR metabotropic receptor on the ON-center bipolar cell
- Less Gi signaling
- Results in an increase in cation influx into the bipolar cells = DEPOLARIZATION
What type of glutamate receptor found on OFF-center bipolar cells and what is the effect of light and dark on them?
- An AMPA receptor is activated and glutamate causes an INCREASE in cation influx (Na+ and some Ca2+)
- In the dark, the OFF-center cell is depolarized since glutamate release is high
- With light, glutamate release from the photorecptor decreases and AMPA receptor is NOT activated = OFF-center cell hyperpolarized
Explain the activation that occurs to the OFF-center bipolar cells when light hits cone photoreceptors?
- Light decreases the presence of glutamate
- Less glutamate = less activation of AMPA receptor on the OFF-center bipolar cell
- Results in decrease in cation influx into the bipolar cell
- Hyperpolarizes the cell
Explain the activation that occurs to the ON-center bipolar cells in the dark by cone photoreceptors?
- Glutamate is high and activates the Gi GPCR-metabotropic receptor on the ON-center bipolar cell
- Results in a decrease in cation influx into the bipolar cell
- Hyperpolarizes the cell
Explain the activation that occurs to OFF-center bipolar cells by cone photoreceptors in the dark?
- Increased glutamate would activate AMPA receptor on the OFF-center bipolar cell
- Results in an increase in cation influx into the bipolar cell (Na+ and Ca2+)
- Depolarizes the cell
How are ganglion cells linked to bipolar cells, how are they activated, rely on what NT?
- Ganglion cells are also ON-center and OFF-center varieties (whichever kind of bipolar cell its connected to)
- When bipolar cell is activated, releases glutamate to depolarize the ganglion cell
- Depolarization ---> threshold ---> AP
Ganglion cell axons become ______
the fibers of the optic nerve
How is a bipolar cell activated by rod photorecptors?
- Many rods converge on one ON-center Bipolar cell
- Which then synapses on an A11-amacrine cell and rod-bipolar cell
- Which then synapses on a Cone ON-Center bipolar cell
- Then activates a Ganglion cell
Why do we require many rods to coverge on a bipolar cell?
- Rods do vision in low light situation
- Many rods converging on a bipolar cells increases our chanches of seeing what we need to see
What is the specific function of ON-center and OFF-center cells in refining the signal to the retina (i.e., tells us what)?
- The ON-center cell tells us where something is
- The OFF-center cell tell us where it ends
What is the function of amacrine cells and horizontal cells in refining the signal in the retina?
Work by providing inhibitory (GABA/glycine) modulation that modifies the activity of neighboring photoreceptors, bipolar cells, or ganglion cells
Where is the AP related to vision and activation of the retina actually occuring?
Ganglion cells - axons of these cells become fibers of optic nerves
Light from the right visual field hits what part of each retina?
- The left temporal retina of left eye
- The right nasal retina of right eye