Lecture 15 Outline Flashcards
The eye is the…
sensory organ for vision
List & describe the mechanisms that help protect eyes from injury
- EYEBALL
- is sheltered by bony socket in which it is positioned - EYELIDS
- act like shutters to protect eye from environmental hazards (keep objects out of your eyes) - EYELASHES
- trap fine, airborne debris such as dust before it can fall into eye - Tears
- continuously produced by lacrimal glands & conjunctiva
- lubricate, cleanse, bactericidal (chemicals within tears that help prevent the growth of bacteria & fungus within your eye)
What is conjunctiva?
is a mucus membrane
- runs the surface of your eye as well under your eyelids
- secretes very thin mucus & tears
What is conjuctivitis?
either a viral or bacterial infection AKA pink eye
Spherical, fluid-filled structure enclosed by…
3 tissue layers
What are the 3 tissue layers?
- Sclera/cornea
- Choroid/ciliary body/iris
- Retina
Describe Sclera/cornea
a. Sclera - tough outer layer of CONNECTIVE TISSUE; forms visible white part of the eye
b. Cornea - anterior TRANSPARENT outer layer, allows passage of light rays
Describe Choroid/ciliary body/iris
a. Choroid - MIDDLE layer underneath sclera which contains blood vessels that nourish retina
b. contains dark pigment (melanin) under retina
- purpose: to absorb straight photons of life
c. choroid layer forms ciliary body, suspensory ligaments & iris
Choroid
continuous with 2 other structures: ciliary body & suspensory ligaments
Ciliary body
- has # of diff types of muscles
- what you see from the outside
- what see see of it at from the outside really is the iris of your eye
“The ciliary body is a circular structure that is an extension of the iris, the colored part of the eye. The ciliary body produces the fluid in the eye called aqueous humor. It also contains the ciliary muscle, which changes the shape of the lens when your eyes focus on a near object.”
Suspensory ligament
very, fine thread like structures that connect the ciliary body to the lense
Describe Retina
INNERMOST layer under choroid
Consists of:
- outer pigment cells
- rods & cones (photo receptors)
- axons of visual nerve fibres (exiting the eye)
RD
Compare & contrast the Posterior & Anterior Cavity
- Posterior cavity
a. b/t LENS & RETINA
b. (fluid that fills this the most, the main part of the eye is:) VITREOUS HUMOR, (gelatinous - b/c it helps the eye maintain its rounded structure)
- basically most of the volume of the eye - Anterior cavity
a. b/t CORNEA & LENS
b. (filled by) AQUEOUS (water, basically salt solution) HUMOR (similar to normal extracellular fluid)
Iris
controls amount of light entering eye
contains 2 sets of smooth muscle:
- Circular (=constrictor)
- Radial (=dilator)
Pupil
opening through which light enters the eye (before it strikes the lense)
- can constrict & dilate depending on the actions of the 2 muscles
Lens
focuses light
- can change shape & allow you to focus on objects that are near or objects that are distant depending how that lens changes shape
What is Tapetum Lucidum
many vertebrates have less melanin, but an additional reflective layer within the choroid, called a tapetum lucidum (humans DON’T have this)
- when light hits it, if a photon of light hit it, that means that it hasn’t hit one of the photoreceptor cells, but if it hits this reflective layer it can be:
reflects light back towards retina
- & gets a 2nd shot at activating 1 of the rods or cons
improves sensitivity of vision under low light, but may cause some blurriness
- b/c its reflective off of this layer & then it can strike a near by photoreceptor
Dilating/constricting the pupil
- Changing the pupil size controls the amount of light entering the eye
- Optimization for light & dark conditions
* if its very bright (in conditions with a lot of light) the pupil can constrict to reduce the amount coming in so it doesn’t saturate the photo receptors)
* when you’re in a very dark condition the pupil can dilate to open & it will allow more light to enter your eye & hopefully that will allow you to gather up enough light to activate photoreceptors & have vision (be able to see what’s in your environment)
- Gives the eye a wide “dynamic range” (range of useful light conditions) - Changing pupil size also controls “depth of field”
As with photography,
- a small aperture gives large depth of field
- a large aperture reduces depth of field
- SMARTPHONE DOES THIS DIGITALLY WITH “PORTRAIT MODE”
Close objects viewed with constricted pupils (gives more depth of field, so close can move a little bit & still be in focus), so if object moves short distance, still in focus
- if your pupils were dilated & an object that was very close to you moved just a little bit - it will no longer be in focus - you would have to refocus it through another physiological process
Larger aperture (open pupil) *fully dilated
- sharper focus
- trade off - nothing else in focus
Small aperture (constricted pupil) *opened a very small amount
- items in background are also in focus
- more “depth of field” where more items are in focus (small aperture, even though its less sharp)
Pupil size is controlled by the…
Autonomic NS
Circular muscle of iris
When these muscles contract - the pupil will constrict (make the hole SMALLER)
- therefore, CONSTRICTOR MUSCLES
The muscles are arranged in a CIRCLE all through this iris
Radial muscle of iris
When these muscles contract they pull the pupil open (dilate)
- therefore, dilators
- point radially
Describe Parasympathetic stimulation CN III (ACh)
using neurotransmitter Acetylcholine will cause contraction of those circular muscles & they’ll get shorter & as they get shorter, the pupil will constrict
Pupillary constriction
- as these muscles contract, that will make the pupil smaller
Sympathetic stimulation Superior Cervical ganglion (Nor-epi)
uses neurotransmitter norepinephrine
- it will cause contraction of these radial muscles & as they contract & those muscle fibres get shorter - they will pull the pupil open
Pupillary dilation
Parasympathetic stimulation (pupillary constriction) & Sympathetic stimulation (pupillary dilation) are an example of…
antagonistic control (opposing each other) by ANS
- we have these 2 branches the para & sympa branch of the ANS & the action of 1 branch serves to directly oppose the action of the other
- regulation of homeostasis (normal body functions) requires those 2 sides (branches of ANS) but still are acting in direct opposition to each other
Focusing requires a…
lens
Focusing requires a lens. For clear vision, the focal point must fall on the retina. Describe
Parallel light rays pass through a flattened lens, & the focal point falls on the retina
- will converge on a single point & focus on 1 specific spot
- if you are looking at object that is very far way with those parallel light rays it can focus directly on the retina
For close objects, the light rays are no longer parallel. The lens & its focal length have not changed, but the object is seen out of focus b/c the light beam is not focused on the retina
- if the object moves - the object is no longer in focus so its image distance no longer lies directly on the retina & in this case the image will be blurry
- you need to correct this - move this image distance, shorter so that you can move this back towards the retina
- focal length of lens becomes LONGER b/c those light rays are no longer parallel but diverging this way
To keep an object in focus as it moves closer, the lens becomes STRONGER & ROUNDED
- bend light rays more & that will shorten that focal light & bring that point of focus back onto the retina
- our eyes need to control the shape of the lens to account for whether or not something that you’re looking at is distant or close
- in order to do it - we will make the lens in some situations stronger & more rounded
What is the focal length of lens
the point at which that object is gonna focus HAS MOVED FARTHER AWAY
- becomes longer b/c those light rays are no longer parallel but diverging this way
Describe Accommodation (focusing the eye)
process of focusing
- change in strength of lens
- FAST: takes only 350 ms to change focus from far to near
- accomplished by action of autonomic nervous system on CILIARY MUSCLE (actions by PARAsympathetic; ACh)
- the natural shape of the lens is strong & rounded
- the ciliary muscle pulls the lens to a flatter, weaker shape
The lens is attached to the ciliary muscle by…
suspensory ligaments (zonules)
The lens is attached to the ciliary muscle by…
suspensory ligaments (zonules)
Compare & contrast ciliary muscle, lens & ligaments
ciliary muscle: ring shaped muscle, made of smooth muscle & it sits all around the lens
- connected to the lens by these ligaments
lens: is connected to the ciliary muscle by these ligaments
ligaments: attach ciliary muscle to lens suspensory ligaments (zonules)
When ciliary muscle is RELAXED (& therefore the ring shaped (ciliary muscle) is larger)…
the ligaments pull on & flatten the lends
DISTANT objects
When ciliary muscle CONTRACTS (ring muscle gets smaller)…
it releases tension on the ligaments & the lens becomes more rounded (returns to stronger, round shape)
FAST ~ 350 ms to go from a flattened lens to a rounded lens
CLOSE objects
Describe accommodation of the eye
main action of accommodation is via PARAsympathetic NS (ACh) ONLY THIS 1 branch of ANS!
ACh causes contraction of ciliary muscle, & constriction of the ring (increase in the curvature to allow for focusing on close objects)
- leads to increasing lens curvature for close objects
- decreasing parasympathetic tone relaxes muscle, causes a flatter lens for distant objects
- SYMPAthetic NS does NOT play a major role (unlike pupil dilation/constriction process)
Describe Myopia
or NEAR-sightedness (can’t see far away)
- occurs when the focal point falls in front of the retina
How is Myopia corrected?
corrected with a conCAVE lens
- SPLITS LIGHT - causes light to diverge a bit which would allow the focal point to move backwards & onto the retina
Describe Hyperopia
or FAR-sightedness (can’t see up close)
- occurs when the focal point falls behind the retina
How is Hyperopia corrected?
corrected with a conVEX lens
- want to bend the light a little bit more before it actually gets in the eye in order to shorten that focal point
- put convex lens infront of eye it bends light (converges light a little before it even gets to the lens)
- -> that will allow that focal point to be right on the retina itself
Light has to travel through a # of cell layers before it even gets to the photoreceptors itself…
last cell to be impacted by the light is the photoreceptor cell
- info travels this way from the photoreceptor cells to the ganglion cells
- light must 1st pass through a # of diff cell types before it actually gets to the photoreceptors (due to the way eyes are constructed)
Direction of light starts with fibers of the optic nerve & ends with the photoreceptor cells, the direction of retinal visual processing…
starts with the photoreceptor cells & ends with the fibers of the optic nerve
Describe the bipolar cell
- primary sensory neuron in visual system
- biplolar shape has cell body:
- 1 area is dendrite & 1 is axon
photoreceptor cells are specialized neural epithial cells, will make synapses onto this primary SN (bipolar cell)
- bipolar cell then inturn, is gonna make synapses on these ganglion cells
- these ganglion cells will send their axons out of the retina & will automatically become the optic nerve & send their info to the thalamus
- from the thalamus, visual info will get processed in the visual cortex
Where are photoreceptor cells located?
in the INNERMOST part of the retina
Rods & Cones are NOT neurons, they are…
specialized neural epithelial cells
Describe Rods
more (90 million)
GRAYscale vision
- prioritize for situations with low light
- larger/bigger therefore able to gather more light
high sensitivity
- night vision
- low acuity
more convergence (of rods) onto ganglion cells - large # of rod for every ganglion cell
mainly located in peripheral retina
Describe Cones
fewer (5-10 million)
- smaller than rods
COLOUR vision (at least 3 varieties) - specialized to detect light of certain frequencies
lower sensitivity
- high acuity (much more spatial recognition)
- day vision
less convergence (=higher acuity)
mainly located in the fovea
- the spot of our retina that we use for the most sensitive vision has increased concentration of cones
- ideal b/c even though cones have a low sensitivity they have a much higher acuity each one is physically smaller but there is less convergence onto ganglion cells
*closer ratio of cones: ganglion cells
What is the Fundus?
back part of the eye; where light may strike the retina
- if you looked directly into someone’s eye through the lens & you can take a pic - then what you would take a pic of is the part of the eye called the fundus
Macula
the macula is an area (on the retina) of a HIGH concentration of CONES
images FOCUSED here to ensure HIGHEST resolution (remember cones have less convergence…)
- general area in the retina where light images will strike & then be focused
Why is the macula generally darker?
b/c there’s a higher concentration of photoreceptors there (mostly CONES)
Optic Nerve (disk)
where blood vessels can enter & leave the eye & also the spot where all of the axons from the ganglion cells are going to exit the eye
- NO photoreceptors there (clearly then not an ideal spot to focus an image)
Why is the optic nerve (disk) not an ideal spot to focus an image?
NO photoreceptors there
The ______ is enriched with cones, & has very less convergence of photoreceptors to ganglion cells
Macula
The macula is directly opposed so…
light thru pupil strikes there
At the _____ light strikes the photoreceptors DIRECTLY, b/c overlying neurons are PUSHED ASIDE
Fovea
(light is able to be focused almost directly upon the photoreceptor cells - very few other kinds of cells on top of the photoreceptors at this point of the fovea)
The VERY HIGHEST DENSITY OF CONES (& bipolar & ganglion cells) is found at the…
fovea
There is very LITTLE CONVERGENCE (=higher acuity) of sensory neurons at the…
fovea
Why is there very little convergence of sensory neurons at the fovea?
single photoreceptor synapse with bipolar neurons that synapse onto single ganglion cells
- ratio is more 1:1 for the cones to the neurons here
What is phototransduction
how our photons of light actually converted into APs
Photoreceptors are “________ ____”
neuroepithelial cells
Compare & contrast the outer & inner segment for phototransduction (also what happens at the synaptic terminal?)
OUTER segment
- visual pigments in membrane disks
- business part of photo transduction
- where the molecules that ultimately allow for photo transduction are located & their located in these structures called DISKS
INNER segment
- location of major organelles & metabolic operations such as photopigment synthesis & ATP production
Synaptic terminal
- synapses with bipolar cells
Which have more disks rods or cones? And why?
RODS have more disks
- b/c they’re physically LONGER
What is the Rhodopsin molecule within the disks?
converts light into some other signal
- made up of opsin protein & retinal
- membrane spanning protein within the membrane disks
What happens when light strikes this retinal molecule?
it changes the shape (structure) from cis-retinal to trans-retinal
- small conformational change in this molecule but its significant as it activates this opsin molecule
dark (=cis-retinal) INACTIVE
light (=trans-retinal) ACTIVE
What is opsin?
a modified GPCR
- integral membrane protein sitting in this membranous disk (like a phospholipid bilayer)
What is retinal?
light sensitive molecule
- can think of it as a ligand for a GPCR
(vitamin A derivative)
What is rhodopsin?
opsin & retinal together
*special thing: always bound by rhodopsin, but in order to activate rhodopsin it has to be converted from the cis-retinal to the trans-retinal (inactive GPCR –> activate GPCR)
Describe Phototransduction in DARKNESS
Rhodopsin is INactive
Levels of cGMP are HIGH (in an inactive photoreceptor)
- cGMP is a cyclic nucleotide, similar to cAMP; INTRACELLULAR 2nd messenger
CYCLIC NUCLEOTIDE GATED (CNG) Na+/Ca++ channels & K+ channels are OPEN (leaking K+ all the time will hyperpolarize the MP of these photoreceptor cells)
- cGMP binds to CNG channels & causes them to OPEN
Cells are CONSTANTLY DEPOLARIZED by entry of Na+ & Ca++: they TONICALLY (constantly) RELEASE transmitter onto BIPOLAR cells
Describe Phototransduction in Light
Light bleaches rhodopsin & ACTIVATES it
Activated rhodopsin ACTIVATES transducin (a G-protein)
Transducin ACTIVATES phosphodiesterase (PD), (amplifier) enzyme that DEGRADES cGMP (reduces it)
The decrease in cGMP (the ligand) causes CNG channels to CLOSE, HYPERpolarizes the cell
- no longer allow the diffusion into the cell with Na+ (stop that depolarizing input into the cell, so these photoreceptor cells now HYPERpolarize therefore light causes HYPERpolarization of a photoreceptor cell)
This causes LESS transmitter to be released onto bipolar cells
So many difference from phototransduction in darkness & light is…
In darkness, photoreceptor cells are releasing n.t. onto bipolar cells
In Light, this causes LESS transmitter to be released onto bipolar cells
Describe the Information flow in visual pathway
- Photoreceptors
- Bipolar cells (primary SN)
- Ganglion cells (optic nerve)
- Optic Chiasm
- Optic tract
- Lateral geniculate nucleus of thalamus
- Optic radiation
- Occipital Lobe
Describe the steps to get to the visual cortex?
- Cross over
- Make 1st synapse in thalamus
- Those neurons with a cell body in a thalamus will project to the visual cortex
80% cross over. What is the reason why not all of those ganglion axons are gonna cross over?
is b/c part of your eye is divided left to right & depending on exactly what you’re looking at, sometimes that image is gonna focus on the right part of the right retina & if your looking at the same image it’s also gonna focus on the right part of the left retina
- therefore, same part of your visual image even though its being observed by 2 diff eyes on 2 obvi diff retinas it’s gonna get pieced back together at the level of your visual cortex
Optic nerve
axons of ganglion cells
- many of them will project out & cross over from 1 side to the other
Describe Binocular vision
the left visual field of each eye is projected to the visual cortex on the right side of the brain, & the right visual field is projected to the left visual cortex
- objects seen by both eyes fall within the binocular zone & are perceived in 3-D
- objects seen with only one eye fall outside the binocular zone & are perceived in only 2-D
Compare & contrast the binocular zone to the monocular zone
binocular zone - is where left & right visual fields overlap
- fact that you’re eyes are pointing straight ahead & that your visual fields are overlapping, that is what allows us to have binocular vision & allows us to have very good DEPTH PERCEPTION
monocular zone - is the portion of the visual field associated with only 1 eye
Simultaneously processing from both eyes is what allows…
depth perception
If you lose use of 1 eye, than you lose…
effective depth perception
Describe eyes of animals like deer, horses, cows etc
have eyes on side of head - wider visual field & can see predators coming from more directions than we can
- eye on each side of their head (hard time seeing things directly infront of them)
Describe eyes of animals like deer, horses, cows etc
have eyes on side of head - wider visual field & can see predators coming from more directions than we can
- eye on each side of their head (hard time seeing things directly infront of them)
Pathways for vision & the Pupillary Reflex
- Midbrain has neurons that send axons back out to the eyes
- & they do things like: control gaze, & pupil reflexes
- shine light in 1 eye & both pupils constrict if they don’t then there is damage in the midbrain - Some of these neurons will project from thalamus into the midbrain
What is Amblyopia: “Lazy Eye”
normal development of visual cortex relies on synchronous input from BOTH eyes (related to the exact same object
For young children with Amblyopia, proper development of visual cortex WILL NOT OCCUR!!!
- not as strong in 1 eye (b/c your eyes are intended to both be able to gaze simultaneously at the same object)
- if you grew up with a serious deviation in 1 eye then your depth perception can never ever develop
- -> condition that depends on synaptic plasticity (proper dev. of the visual cortex (plastic event) & requires simultaneous input from both eyes
What is the treatment like for Amblyopia: “Lazy eye”?
treatment is often to simply patch the good eye, forcing the good eye, forcing the brain to use the info from the weaker eye
strengthen inputs from the weak eye & proper VC development
- only works for younger people not adult
- treat early or else it is almost impossible to correct
