vision

light

electromagnetic radiation traveling in waves

wall of eye

3 types: fibrous, vascular, inner layer

fibrous

sclera + cornea

vascular

posterior choroid

inner

retina

posterior choroid

membrane that supplies all of the layers w/ blood

iris

circular muscle that controls the diameter of the pupil; helps to restrict light rays to focus; controls how much light enters the retina

Two systems of the iris

1. parasympathetic

2. sympathetic

parasympathetic iris

causes the sphincter pupillae muscles in iris to contract (makes pupils smaller) in response to certain conditions like bright light or focusing on close objects

sympathetic iris

causes dilator pupillae muscles in iris so iris can lead to pupil dilation (widens pupil) so more light is allowed in; helpful in low light conditions

pupil

opening that allows light to reach the retina

aqueous humor

1. fluid behind cornea

2. in anterior chamber

sclera

1. outermost layer that forms eyeball

2. only layer in anterior chamber

extraocular muscles

attached to the eye and skull and allow movement

conjunctiva

membrane inside the eyelid attached to the sclera

optic nerve

axons of the retina leaving eye

cornea

transparent surface covering the iris and pupil

ophthalmoscopic appearance

1. optic disk

2. macula

3. fovea

optic disk

blind spot; no vision possible

1. blood vessels originate here (shadow retina)

2. optic nerve fibers exit here

3. no photoreceptors (rods and cones)

macula

area of retina responsible for central vision (vs. peripheral)

fovea

center of retina

1. where most cones are

2. inside macula

lens

1. transparent surface that contributes to the formation of images

2. focuses the light that comes through the cornea and pupil and projects it onto retina

ciliary body

1. attaches to the fibers of the lens

2. curves / changes shape of the lens

3. needs to contract too refract lens

4. allows for focusing

vitreous humor

1. provides eye spherical shape

2. lies between the lens and retina in posterior chamber

retina

1. inner most layer of cell wall in posterior chamber

2. transduces light energy into neural activity

anterior chamber

has aqueous humor

posterior chamber

has vitreous humor

parts of the eye

1. pupil

2. iris

3. aqueous humor

4. sclera

5. vascular layer: posterior choroid

6. retina

7. cornea

8. lens

9. ciliary body

10. zonule fivers

11. vitreous humor

12. retina

general sequence of events

1. light entering the eye is focused on the retina

2. retina coverts light energy into neural activity

3. axons of the retinal neurons are bundled to form the optic nerves

4. visual information is distributed to several brain structures that perform different functions

process of vision

1. light energy is transduced into neural activity

2. neural activity is processed by the brain

process of image formation

1. refraction by the cornea

2. accommodation by the lens

3. pupillary light reflex

refract

change direction

refractive surface

where light bends

ie. cornea

focal point

where light rays finally cometogether after refraction

refraction by the cornea

1. distant objects

2. light rays slow

3. light rays bend

4. focal distance

distant objects

light rays run in parallel

light rays slow

aqueous humor and cornea slow light down; light hits the cornea so light changes direction

slowing light down makes light bend = allows for back of eye to focus light to see clearly

light rays bend

1. still parallel but light diverges away when distance is closer to eye

2. light bends when it hits the cornea

3. bends at perpendicular angle to the curve (radius) of cornea

focal distance

distance between cornea to the image / retina; distance between the surface of cornea (where light bends + refractive surface) and the point where the light rays finally come together (focal point)

1. depends on how curved the cornea is

2. avg focal point is 2.4cm

accommodation by lens

1. objects within 9 meters; light rays do not travel parallel, diverge

2. lens adds refractive power by changing shape of lens

3. contraction of ciliary muscles

contraction of ciliary muscles

1. tension of suspensory ligament is released

2. which then cause lens to round

3. which causes refraction

   greater the curve = greater the refraction

pupillary light reflex

lets us absorb divergent rays

cell types for retina

1. ganglion cells

2. amacrine cells

3. bipolar cells

4. horizontal cells

5. photoreceptors

photoreceptors

1. the only light sensitive cells in retina

2. transduces light energy into neural signals

ie. rods + cones

bipolar cells

connect photoreceptors to ganglion cells

ganglion cells

fire action potential and send axons to the brain

horizontal cells

receive inputs from photoreceptors and project laterally to bipolar cells

amacrine cells

receives inputs from bipolar cells and project laterally to ganglion cells

cell layers in retina

In order from superficial to deep (relative to incoming light)

1. ganglion cell layer

2. inner plexiform layer

3. inner nuclear layer

4. outer plexiform layer

5. outer nuclear layer

6. pigmented epithelial

ganglion cell layer

1. has ganglion cells

2. only cells with action potential

3. all ganglion cells leave through optic disk to form optic nerve

inner plexiform layer

1. has amacrine cells

2. takes signal from 1 bipolar cell and activates multiple ganglion cells

inner nuclear layer

1. has bipolar cells

outer plexiform layer

1. had horizontal cells

2. takes signaling from 1 photoreceptor to multiple bipolar cells

outer nuclear layer

1. has photoreceptors

2. has outer segment

outer segment: pigmented epithelial layer

absorbs light that doesn’t get absorbed by retina

(the pigment does the absorption)

characteristics of layer

1. photoreceptors are only cells that respond to light

2. ganglion cells are the only output cells

3. light travels through other cell layers to reach photoreceptors

4. pigmented epithelium located at back of the eye

rods

long; many disks

1. photopigment used rhodopsin (found in disk)

2. higher pigment concentration / more sensitive to light

3. used in scotopic system / conditions

rhodopsin

1. photopigment in rods

2. when hit with quick light the rhodopsin inactivates (can’t see light anymore) “bleaches” can reactivate with another molecular change

cones

shorter; less disks

1. used in photopic system / conditions

2. photopigment: opsin

3. more dense in macula

opsin

1. used in cones

2. has 3 ranges: blue, green, red

scotopic system / retina

1. see in the dark

2. only uses rods

3. convergent: where multiple rods signal to a single retinal ganglion cell

photopic system / retina

1. sees in light

2. primarily uses cones

3. 1:1:1 - one cone goes to one cell

mesoptic

have ability to see between both systems: photopic and scotopic

phototransduction w/ no light

1. no light starts the rod

2. cGMP rises when rod receives no light

3. rise of cGMP causes sodium channel to open

4. sodium influx enters rod

5. causes rod to depolarize

6. which causes voltage gated calcium channels to open

7. calcium influx

8. initiates the NT to release

ophthalmoscopic appearance

1. optic disk

2. macula

3. fovea