Vision - chemoreceptors

vision

• large complex sensory structure

• signal transduction: light to AP

• integration of info begins at level of receptor cells

• information sent to brain for additional processing

photoreception

light

• light

• we can only see light in a tine area of wave legnth

why cant we use x-rays for vision?

x-rays contain too much energy to damage rhodopsin

• light outside of visable range has too much energy (damaging rhodopsine) or not enough energy to excite retinal

human eye

light comes in through cornea to lens to other critical parts of the eye to retina to optic nerve

• the lens in your eye flips the image upside down and is sent to brain. once at the brain it flips this image back up to the way we see it now

• Light passes to back of retina and the signal is then sent back to the front near the optic nerve

what forms the optic nerve?

ganglion cells

accomodation of eye

lens changes optical power to mainatin clear vision

• near: cillary muscle contracted causing lens to be more rounded for close vision

• far: cillary muscle relaxed causing lens to be flattened for distant vision

retina

• important cells and photoreceptors

• light moves through many different neurons in the retina to get to the photoreceptors

• light cant go THROUGH cells in the front, these cells block the light

  • it has to find a way to get to the photoreceptors

• Amacrine cells, bipolar cells, horizontal cells also part of processing

  • Bipolar cells directly connect photoreceptors to ganglion cell

photoreceptors

• cones and rod cells

• send electrical signals to the brain

rod cells

detect absense of light (black and white)

• more sensitive, dont distinguish color

cone cells

• less sensitive, distinguish color

How do we get pigment?

• Retinal + Opsin protein = Rhodopsin = Photopigment

  • Opsin changes the amino acid sequence

• Rod cells: 1 type of opsin

  • Black and white

  • Can still absorb SOME color (see photo)

• Cone cells: 3 types of opsin

  • Red, green, blue

• Light changes shape of retinal

  • opens ion channels

  • affects pigment that we see

effect of light on retinal

• Retinal when light hits it it changes the shape of the rod cells

  • Cis vs trans this is what changes that causes an AP down the line

• cis isomer: kinked retinal form bound to opsin

• trans isomer: straight retinal form detached from opsin 

• trans recycled back to cis: this takes time

• light outside of visable range has too much energy (damaging rhodopsine) or not enough energy to excite retinal

recycling of photopigment

takes time!

• ex: temporary blindness after bright light

from light reception to receptor potential

• Light hits rhodopsin, causing retinal to change from cis to trans shape.

  • This shape change is crucial for activating the phototransduction pathway.

• Phosphodiesterase (PDE):

  • Activated by the change in retinal shape.

• PDE breaks down cGMP (cyclic guanosine monophosphate).

• Normally, cGMP keeps sodium channels open.

• When cGMP levels drop, sodium channels close

• Closing sodium channels prevents sodium ions from entering the cell.

• The inside of the cell becomes more negative (hyperpolarized).

How does your vision get AP’s in light

1. photoreceptor cell hyperpolarized

2. Ca channels closed

3. No IPSP; bipolar cell depolarizes spontaneously

4. Bipolar cell releases excitatory NT

5. EPSP in ganglion cell

6. Ganglion cell produces APs

How does your vision get AP’s in dark

1. in dark, photoreceptor depolarizes

2. Ca channels open

3. photoreceptor cell releases inhibitory NT

4. IPSP in bipolar cell

5. Bipolar cell fails to release NT

6. No EPSP in ganglion cell

7. Ganglion cell does not produce AP’s

color mixing

• primary colors: cyan, magenta, and yellow

• black is the absence of light

wavelegnth sensitivity of rods and 3 types of cone cells

• each type of cone cell has a different opsin

• patterns of firing of the 3 types of cones to diff colors. Size of cone symbolizes size of receptor response

  • blue: short cone has a larger response

  • green: medium cone has a larger response

  • red: large cone has a larger response

  • yellow: medium and large cone has a larger response

  • white: all cone sizes have a large response

color blindness

• some people are missing certian cone colors that cause them not to be able to see certian colors

neural pathways for vision

• info of object on the right visual field goes to brain on left side

  • same for left to right side of brain

    • info crosses over

• visual processing in back of brain, visual cortex

  • ur brain can put things together and fill in info that was never there