Vision Flashcards
What ar ethe two funcamental protective mechanisms of the eye?
blinking and tear production
What neurotransmitters on what types of muscles are involved in blinking?
ACh on nicotinic receptors for the straited muscle
Norepinephrine on A1 adrenergic receptors for the smooth muscle
What part of the nervous system is involved with tear production? With what NT on what receptors?
the parasympathetic nervous system
ACh on muscarinic receptors
What is epiphora, and what are the two general causes?
overlow of tears - crying
cam be due to overproduction or blocked drainage
What three muscles are involved in blinking?
orbicularis oculi (striated)
levator palpebrae superioris (striated)
superior tarsal muscle (smooth)
In Horner Syndrome, which of the three blinking muscles is impaired, leading to ptosis?
the superior tarsal muscle - it’s smooth muscle innervated by sympathetic fibers in the sympathetic chain which are damaged in Horner’s syndrome
What are the states of the three muscles under the following conditions:
maintaining ocular opening
adjusting to changes in globe position
blinking/firm cloising of eyes
open: levator and superior tarsal tonically activated, inactivation of orbicularis
adjusting: activation/inactivation of levator, inactivation of orvicularis
blinking: inhibition of levator, actiation of orbicularis
How does spontaneous blinking differ from the blink reflex?
It’s periodic, precisely ocnjugated, symmetrical, brief and occurs in the asbence of external stimuli or internal effort - not as fast as the blink reflex
What is the typical blink rate in adults? Slower or faster in children?
10-20 blinks per minuts in adult, lower in children
Where in the brain does spontaneous blinking originate? Through what NT pathway?
premotor brainstem structures
descending dopaminergic activity
If blinking is increased with higher dopamine levels, what alteration in blinking will occur in Parkinsons’s disease, schizophrenia, and Huntington’s disease?
Parkinsons: decrease in dopamine, so decrease in blinking
Schizophrenia and Huntinton’s: increase in dopamine, so increase in blinking
What stimuli will initiate the blink refelx?
touch to the cornea (afferents in the trigeminal)
bright light or rapidly approaching objects (afferents in optic nerve)
What are the three layers of the tear film?
- lipids from oil glands in the eye
- acqueous-based solution from lacrimal gland (with lysozymes)
- mucous from the conjunctiva
How is tear production affected by situation and age?
emotional tears contain more hormones like prolacting, ACTH and enkephalin
tear production decreases with age, which means dry eye is an issue in the elderly
In what two ways does tear flow occur?
evaportation
drainage through nasolacrimal ducts into the nasal cavity
What branch of the nervous system will lead to tear production? How?
parasympathetic
it increases tear production by the lacrimal gland and decreases outflow by facilitating closure of the lacrimal duct passage
What nerve will stimulate epiphora during stimulation of the cornea?
What brain regions will mediate epiphoria as an emotional response?
trigeminal nerve
limbic system, especially hypothalamus
What is refraction? What does it?
focusing the light on the retina by the cornea and the lens
Which does most of the work for refraction - the cornea or the lens?
the cornea (has stronger refractive power)
What special thing can the lens do in terms of refraction? What’s the special term for this?
It can adjust its focusing power to deal with near vision- this is called accomodation
Focusing power is 1/focal length. What is the normal range for focusing power in the eye, with cornea and lens combined?
cornea = +44 D
lens = +15-29 D
so total range is +59-75 D
What happens to the image as it passes through the lens?
it was inverted and reversed
What is the significant modification to the maintenance of the topographical map established in the retina?
information of the two nasal retinas crosses in the optic chiasm, while information from the two temporal retinas goes to the ipsilateral side
this ensures that information from the elft visula field is processed in the right visual cortex
Is the lens designed to be better at far sight or near sight? Why?
far sight - it does not need to change itself for far information to hit the retina - it’s already designed to do so
evolutionarily we were better served to be able to see things when they were still far away so we could have time to run away
What is the distance from the lens ot the focusing point? What is the focusing point?
the focal length
the fovea
What has to happen to the lens when looking at near objects? Why?
If the lens doesn’t accomodate, the focal length won’t change which means the image will be projected to an area behind the eye and will therefore be out of focus
this means the lens has to becomes shorter and fatter, allowing it to increase its focusing power
Relaxation of the ciliary muscles to will relax the lens - better for far vision. What branch of the autonomic nervous system does this?
Constriction of the ciliary muscles will make the lens fatter for near vision. What branch of the autonomic nervous system?
sympathetic = far vision
parasympathetic = near vision
If there are two systems: sympathetic and parasympathetic involved in refraction, how many drug targets are there?
4
agonsit and antagonist on sympathetic
agaonist and antagonist on parasympathetic
What is hyperopia?
farsightedness
so the axial length is less than the focal length because the eyeball is too short or the lens is too week
What lens do you use to correct hyperopia?
a convex lens to increase the refractive power
What is myopia?
nearsightedness - the axial length is longer than the focal length
What type of lens is used to correct myopia?
a concave lens
What is presbyopia?
when the lens becomes less flexible, so you have a decrease in accommodation - leading to farsightedness
happens to virtually everyone in middle age, so they need to wear reading glasses
What is an astigmatism?
an unevenness in the lens, meaning part of the visual field will be out of focus
How is the amount of light entering the eye regulated?
the pupillary light reflex
What are the two directions of the pupillary light reflex and what branches of the nervous system to which?
miosis (constriction) - parasympathetic stimulation of the muscarinic recpetors on sphincter pupillae muscles
Mydriasis (dilation) from sympathetic stimulation of alpha1 receptors on dilator pupllae muscles
What is normal IOP? What IOP and above is considered glaucoma?
normal is 20 mmHg
anything over 30 mmHg is glaucoma
What physics law controlls aqueous humor flow?
Ohm’s law
pressure = flow x resistance
In terms of acqueous humor PRODUCTION…how is it controlled by the sympathetic system?
sympathetic activity on B2 receptors will INCREASE flow
sympathetic activity on alpha2 receptors will DECREASE flow
Besides sympathetic innervation, what major drug target is involved in aqueous humor production?
carbonic anhydrase creates bicarbonate, which is used to drive Cl- secretion into the eye
water then follows the Cl- through osmosis, creating more aqueous humor
What is the most important factor in terms of aqueous humor OUTFLOW?
the dilation of the canal of Schlemm
How is flow through the canal of schlemm regulated?
by parasympathetic function on the sphincter pupillae
if you contract the sphincter pupillae, you will increase the flow (dilates the canal)
Dilation will decrease flow - this is why they check for glaucoma before they dilate the eyes in an exam
Besides the canal of schlemm, what factor controls aqueous humor outflow? How is it a drug target?
contraction of the ciliary muscles will decrease flow
(this is the uveoscleral system)
it’s less important than the canal of schlemm, but prostaglandins work on this arm of the system, so it’s an important drug target
Describe the worsening symptoms of glaucoma in order…
- increasing IOP impacts the cornea first, such that it bulges out and becomes distorted - lead to trouble focusing so you see halos, blurring, etc.
- Pressure affects the photoreceptors
- Pressure compresses the optic nerve
- pressure blocks arterial supply to the eye
Overal: you get a restruciton in the visual field leading to progressive tunnel vision
What are the cell layers of the retina? Which layer does light hit first?
- photoreceptors
- bipolar cells
- ganglion cells
light actually hits the ganglion cells first, which is counterintuitive
What sort of potentials are generated by each of the cell layers in the retina?
the photoreceptors and bipolar cells have graded potentials and the ganglion cells are what do the action potentials
What are the two types of additional cells that allow for lateral inhibition in the retina?
horixontal cells and amacrine cells
For visible light, frequency defines the light’s ___ while the intensity describes the light’s _____
color
brightness
What photoreceptors are responsible for scotopic vision, and which are responsible for potopic vision?
scotopic = black/white = rods
photophic = color = cones
How many types of cones are there and what are they?
3:
blue (short wavelength)
green (medium wavelength)
red (long wavelength)
Which type of photoreceptor has better temporal and spatial resolution?
cones
Which photoreceptors work better under low light conditions?
rods
Which photoreceptor takes only 1 photon to activate?
rods (if dark adapted)
cones take many
When do the two types of photoreceptors saturate?
rods = normal daylight
cones = very intense light
Which are more sensitive, rods or cones?
rods
Given that cones have better temporal resolution, are their responses more or less integrated?
less integrated - you can’t keep precision if you integrate the information too much
Which photoreceptors are more prevalent at the fovea?
cones - rods are only located at the periphery of the retina
Which have more convergence onto the bipolar cells?
the rods - they’re less precise so the information can be integrated without causing any issues
cones have very little convergence onto bipolar cells - their acuity is higher
Why do ships use red light on their decks at night?
the wavelengths that rods and red cones respond to do not overlap at all
this means that by using red light, the sailors won’t saturate either their cones or their rods, so their vision can still be precise even though it’s dark
Which have more pigment - rods or cones?
It’s a little counterintuitive, but it’s actually rods
cones have 3 types of pigment while rods only have 1, but rods have more of it
What are the three types of visual acuity?
spatial
temporal
spectral
Visual acuity is a function of what photoreceptor system?
cones
How do you measure spatial acuity?
it’s where something is in space
measure with the snellen eye chart
What is the difference between someone who has 20/30 vision and someone who has 20/10 vision?
the person with 20/30 vision sees at 20 feet what a normal person sees at 30 feet
the person with the 20/15 vision sees at 20 feet what a normal person sees at 15, so their vision is the better of the two.
Where on the retina is spatial acuity strongest?
at the fovea - this is why it’s easier to tell exactly where something is when you look directly at it
What is temporal acuity?
the ability to distinguish two events as being separate
What is spectral acuity?
this is basically the ability to distinguish differences in wavelength, so color
Describe the biochemical cascade involved in activation of rods. In other words, what are the steps of phototransduction in rods?
Light hits the rod
in the rod, the rhodopsin is activated and it will join with a g-protein called transducin
activated transducin will convert GTP to GDP
this causes an activation of phosphodiesterase
the phosphodiesterase breaks down cGMP
the cGMP had been holding sodium channels open, so without it, the sodium channels close
this causes a hyperpolarization in the rod, leading to a decrease in NT release, but counterintuitively a depolarization in the bipolar cells
What happens in retinitis pigmentosa?
decreased response of the photoreceptors either because they’re dying or malfunctioning
Rods usually go first, then cones
so symptoms start with nictalopia (night blindness) and tunnel vision first, then you have a decrease in central vision and color vision later
In general, what is retinopathy?
it’s a non-inflammatory condition - usually from a block in the blood supply such that the patient has reduced vision and reduced acuity in particularly
What is the visual recycle/
it’s a way to recycle the retinol pigment in the photoreceptors
How does the visual cycle work in rods?
rhodopsin is composed of opsin and 11-cis retinol
with exposure to light, the rhosopsin is cleaved into it’s two parts and the 11-cis-retinol is ocnverted to all-trans retinol
the all-trans retinol is transported out of the rod into the retinal pigment epithelium
in the RPE the all-trans is converted back to 11-cis-retinol
the new11-cis-retinol is then transported back into the rod to be combined with opsin to form rhodopsin again
Which step in the visual cycle is known as “bleaching”
why does this inactivate the pigment?
it’s the cleavage of the rhodopsin into the retinol and opsin
it’s nonfunctional because it can no longer interact with transducin
Where does the visual cycle for cones happen?
it’s all within the cone - this is why cones are faster than rods
What vitamin is retinol derived from?
vitamine A
If retinol is a derivative of vitamin A, what is the first symptoms of vitamin A deficiency?
night blindness
