Ocular Physiology (Done) Flashcards
Which muscles control spontaneous blinking?
Palpebral portion of orbicularis oculi
Reflex blinking caused by touch is mediated by which CN?
CN V
Reflex blinking caused by loud noises is mediated by which CN?
CN VIII
How many blinks/minute on average?
12-15
Which muscle is associated with reflex blinking?
Palpebral portion of orbicularis oculi
When winking, which muscles are controlling this?
palpebral and orbital portions of orbicularis oculi
Which muscles are involved with Blepharospasm?
Procerus, corrugator, and orbicularis
How many meibomian glands located on upper and lower lids, respectively?
30-40 and 20-30
Blinking stimulates lipid release from meibomian glands via which type of secretion?
Holocrine secretion
Where are glands of Krause located?
Fornices
Where are glands of Wolfring located?
Tarsal conjunctiva
What happens to the canaliculi during a blink?
Muscle of Horner contracts, causing canaliculi to SHORTEN as they move medially towards the lacrimal sac
Where is the muscle of Horner located?
It is part of the palpebral portion of the orbicularis and surrounds the canaliculi
What happens to the lacrimal sac during a blink?
as orbicularis contracts, lacrimal sac is pulled away from the nose, which creates a negative pressure that helps draw tears into the lacrimal sac
How many eyelashes on upper and lower lid, respectively?
150 and 75
Which components of the aqueous layer of the tears help in anti-bacterial measures?
IgA, lactoferrin, and lysozyme
What is the anterior lipid layer composed of?
fatty acids, cholesterol, and waxy esters
List as many things as you can think about what is found in the aqueous layer of the tear film…GO!
Na, K+, Cl-, IgA, lactoferrin, lysozymes, Vitamin A (all-trans retinol), enzyme co-factors, HCO3-, glucose, urea, lactate, citrate, Vitamin C, albumin, growth factors, and VEGF
Under closed eye conditions, is IgA higher or lower in concentrations?
HIGHER
As age increases, what happens to lactoferrin and levels of lysozyme within the tears?
THEY DECREASE!
What innervates the main lacrimal gland?
Parasympathetic fibers from CN VII, sympathetic fibers, and sensory nerves of V1
What makes the mucin layer of the tear film unique?
It is capable of mixing with lipid AND water»therefore, able to spread tears evenly over hydrophobic corneal surface
What produces the mucous layer of the tear film?
goblet cells, as well as squamous cells of the cornea & conjunctiva
Where are the greatest concentrations of goblet cells found?
Inferonasal fornix & bulbar conjunctiva (mostly temporally)
What do goblet cells require in order to develop properly?
Vitamin A»no Vit. A means keratinization of conjunctiva and cornea»BITOT SPOTS
Which type of stimulation can cause an increase in goblet cell mucous secretions?
Parasympathetic nerve stimulation
Which part of the corneal epithelium does the mucin layer interact with in order to provide an evenly spread tear film?
Mucin layer interacts w/ the GLYCOCALYX of the corneal epithelium
What is the total tear volume on the ocular surface?
7-9 uL
What is the maximum amount of fluid that the eye can hold?
20-30 uL
What are the main contributors to tear osmolarity?
Na+ and Cl-»Ca2+ and K+ also importantn in aqueous portion of tears
Which mineral is essential for hemidesmosome formation in the BM of the corneal epithelium?
Calcium»jelly bumps on contact lenses
How much more potassium is found in the tears compared to the blood?
4 X as much
Would Hypotonic or Hypertonic eye drops be used for Dry Eye?
Hypotonic because dry eye causes increased tear osmolarity (hypotonic solution is about 150 mOsm/L)
What happens to the pH of tears during sleep?
tears become more ACIDIC»b/c of the byproducts of anaerobic respiration
What happens to the pH of tears in dry eye syndrome?
tears become more BASIC
The auditory ossicles (i.e. malleus, incus, then stapes) are located between which 2 structures?
located between the tympanic membrane and oval window
What is the function of the tympanic membrane (i.e. eardrum)
amplifies the sound waves by 10-20X
What does the stapedius muscle do?
dampens the amount of vibrations placed on the auditory ossicles»stapedius innervated by CN VII, just before exiting the skull via stylomastoid foramen
Which nerve innervates the tensor tympani muscle?
branch from V3 of CN V
Which section of the ear do mechanical vibrations get converted into neural signals?
INNER EAR
What are the 3 parts of the bony labyrinth?
Cochlea, Vestibule, and Semi-circular canals
Which structure separates the external ear and middle ear?
Tympanic membrane
Which brain structure controls horizontal saccades?
CONTRALATERAL frontal eye fields in the frontal lobe and superior colliculus
Which structure controls Pursuit movements?
IPSILATERAL parietal lobe
If a pt. is having problem with saccades to the RIGHT side, which side is affected?
deficient saccades to right side would indicated right frontal eye field damage
What types of junctions are found in corneal epithelium that limit the absorption of hydrophilic, ionized molecules through?
Zonula occludens
Which junctions are found in the corneal endothelium?
Macula occludens
What are the ranges of UV-A, UV-B, and UV-C light?
200-290, 290-320, and 320-400nm
Which layers block UVC and UVB light?
epithelium and Bowman’s
More than 99% of light above ______ wavelength is transmitted through the cornea.
99% of light over 400nm is transmitted
What are some factors contributing to DECREASED light scattering in cornea?
Corneal crystallins (similar to crystallins in the lens), Vitamin C & Glutathione (protect cornea from UV rays and free radical scavengers), avascular nature of the cornea, proteoglycans (which fill space b/w corneal cells and collagen fibrils), high water content of cornea helps maintain regular spacing between collagen too
What is the major proteoglycan found in the corneal stroma?
Keratin Sulfate»proteoglycans help maintain spacing between collagen fibrils
What are the 2 main transport mechanisms in the corneal epithelium?
Na+/K+ ATP ase pump & Na+/K+/Cl- co transporter
Explain the mechanism of movement of ions between the epithelium and the stroma..
- Na+ enters epithelial cells from tear film
- Na+/K+ pump moves Na+ into stroma»aka Na+ higher [ ] in stroma compared to epithelium
- Na+/K+/Cl- utilizes higher [ ] to move these ions from stroma into epithelial cells
- Cl- and K+ have own channels that allow for passive diffusion back into tears and towards aqueous humour
- Movement of K+ into aq. humour
- Cl- moves into tears; water follows»dehydration of cornea
A hypoxic cornea will produce what change in pH?
Higher acidity
Explain mechanism of movement of ions between stroma and endothelium…
- Na+ enters endothelial cell from stroma
- Na+/K+ pump pumps Na+ out of endothelium into aqueous humour»therefore, higher Na+ [ ] in aq. compared to endothelium
- Na+/H+ pump move H+ ions out of endothelium into aqueous in exchange for transfer of Na+ back into the endothelium
- Movement of H+ into aq.»decrease in extracellular pH causing CO2 to diffuse into endothelium
- CO2»H2CO3»H+ and HCO3- ions
- Bicarbonate and Cl- move from endo to aq. Water follows contributing to water dehydration
What are the major factors for water transport across epithelium and endothelium?
Na+ absorption and Cl-excretion
Which structures supply oxygen to the cornea under closed eye conditions?
- superior palpebral conj & limbal vasculature supply epithelium and anterior stroma
- aq. humor supplies post. stroma and endothelium
Why is the cornea slightly edematous upon awakening in the morning?
Build-up of lactate from anaerobic respiration and limited supply of O2 when eye is closed
Why are minus CL’s more capable of transporting O2 compared to plus CL’s?
minus lenses are THINNER in the center compared to plus lenses
What does Dk/t mean?
how much oxygen will diffuse through a contact lens of a given thickness
Why does the cornea become acidic when is edematous?
decreased levels of O2 can lead to an accumulation of H+ ions produced by glycolysis, resulting in increased acidity of corneal cells
What is the primary glucose contributor for the cornea?
Aqueous humor
What is the main mechanism of production of glucose for the cornea?
Anaerobic glycolysis (85%)»minor contribution from aerobic glycolysis and hexose monophosphate shunt
The entire corneal epithelium replaces itself every _______ days.
7-14 days
What are the steps in epithelial regeneration?
- Basal cell mitosis is INHIBITED
- Fibronectin released–serves as scaffolding for epithelial cells to migrate over the wound
- Hemidesmosomes are then created
- Basal cell mitosis resumes at rapid rate (but only once wound is closed w/ a single layer of cells and cell to cell junctions are created)
Complete healing of the BM takes _______ time.
8 weeks
What do matrix metalloproteinases do?
degrade hemidesmosome function»prescribe tetracyclines to decrease MMP activity
Does the stroma regenerate?
Kind of. It replaces itself when damaged, but with a very different textured tissue»new collagen is larger and less organized, resulting in a scar.
What is neurotrophic keratitis?
CN V damage and decreased corneal sensitivity
What are some changes that occur in the cornea with age?
more ATR astigmatism, BM thickens, DM thickens, corneal arcus increases, endothelial cell density decreases
List all of the changes that occur in the lens w/ accommodation?
- decrease in tension in lens zonules
- ant. pole of lens moves forward & ant. curvature increases
- post. pole moves back slightly; post. curvature increases
- lens thickness increases
- ant. chamber depth decreases
- lens diameter decreases
- lens power increases
Over 70% of glucose required by the lens is produced by _______________.
Anaerobic Glycolysis
What happens in the lens if hexokinase enzyme is not present? (enzyme used to convert glucose to glucose-6-phosphate)
glucose will be converted to sorbital via the enzyme aldose reductase»excess sorbital creates osmotic gradient favoring H2O, ultimately leading to lens swelling, lens fiber damage, and cataract formation
What is the primary protector against oxidative damage in the lens? and what is its purpose?
Glutathione»acts as a REDUCING AGENT
How is glutathione associated with cataract formation?
Glutathione DECREASES with age–a factor in cataract formation
Is Vitamin C. concentration higher in the aqueous humor or the lens?
LENS
Where are ions pumped by Na+K+ pump between the lens epithelium and aq. humor?
Na+ pumped into the aqueous humor and K+ into the lens
What produces the secondary lens fibers of the lens during adult growth?
Anterior lens epithelium
What produces the embryonic nucleus of the lens during embryological development?
formed from primary lens fibers of the posterior lens epithelium
Where does mitosis of fiber cells occur?
Germinative zone of the Anterior lens epithelium
In the lens, where does aerobic respiration occur?
Limited to the anterior lens epithelium
Where in the lens is the greatest metabolic demand?
Anterior lens epithelium
Where is a majority of the mitochondria in the lens located?
Anterior lens epithelium
What is the purpose of alpha crystallins?
Act as molecular chaperones by preventing degradation of other crystallins
What happens to the lens when there is a decrease in crystallins?
increase in insoluble proteins, causing change in collagen cross-linking, which promotes increased H20 in the lens»cataract formation
Does the ant. lens capsule or posterior lens capsule increase in thickness during life?
Anterior lens capsule
What type of collagen comprises the lens capsule?
Type IV»it is the thickest BM in the entire body too!
What happens to the anterior and posterior radius of curvature throughout life?
Ant. & Post. both decrease»lens becomes more convex throughout life
In respect to glutathione, Ca2+, Na+, and H20–do these increase or decrease in value throughout life?
Glutathione DECREASES, everything else INCREASES
Where in the lens does nuclear sclerosis begin?
Begins in the embryonic nucleus
Where is the greatest concentration of crystallins in the lens found?
Embryonic nucleus»also the highest refractive index of 1.41
Does ciliary muscle contraction decrease with age?
NO»loss of accommodation with age is secondary to changes in the lens
Bruch’s membrane ________ in thickness with age.
increases
Choriocapillaris _________ in thickness with age.
decreases
Overall choroidal thickness _________ with age.
decreases
What effect does the gel-like consistency of the vitreous have on drug absorption?
It decreases the bioavailability of topical drugs
What makes up the vitreous substance? What type of collagen?
H20, Type II collagen, and hyaluronic acid (GAG)
What is the purpose of Hyaluronic acid?
proves support to collagen fibers, maintains proper collagen fibril spacing, and maintains viscosity of the vitreous
Vitamin C is how many times higher in concentration in the vitreous than in the blood plasma?
40X greater!
What is the most common cause of a PVD?
Vitreous syneresis
Where is collagen concentration the highest and lowest in the vitreous, respectively?
Vitreous Base and vitreous center
What happens to the concentration of hyaluronic acid throughout life?
20-50 yr old stable; after 50 years, INCREASES
What is the mean arterial pressure of the arteries entering the eye
65 mmHg
What is the mean arterial pressure in the episcleral veins leaving the eye?
15 mmHg
What is the approximate perfusion pressure in the eye?
50 mmHg
How do you calculate ocular perfusion pressure?
OPP= diastolic blood pressure-IOP
Are glaucoma patients more likely to have a increased or decreased OPP?
LOW OPP’S; Remember, if IOP increases (like in glaucoma), OPP decreases!
What are most likely responsible for autoregulation within the blood supply of the retina and the optic nerve?
Pericytes
What is the immediate threat to vision in acute angle closure? and why?
CRAO»increased IOP, leads to decreased OPP»retinal vessels increased vessel diameter through autoregulation»if IOP remains high long enough, CRA reaches critical closing pressure
What effect does the sympathetic system have on blood vessels?
Causes vasoconstriction–especially when BP increases»resulting in compensatory reduction in blood flow
The IOP must be _________ than the episcleral venous pressure so aq. humor can drain
GREATER
The IOP must be _________ than the intracranial pressure in order to maintain an axoplasmic gradient that flows from the ON towards the brain.
GREATER
Why does Papilledema occur?
Results from a reversal in axoplasmic gradient between the eye and the brain due to an increase in intracranial pressure.
Therefore, CSF spills from subarachnoid space onto optic disc margins and surrounding RNFL.
What % of blood flow in the ocular vessels occurs in the choriocapillaris?
60%
What makes up the Major Arterial Circle of the Iris?
anterior ciliary arteries + long posterior ciliary arteries
What forms the Minor arterial circle of the iris?
formed by anastomoses of the iris radial vessels
Where does the central fovea receive its blood supply from?
The underlying choriocapillaris
The CRA forms 2 networks of capillaries»where are they located?
RNFL (superficial) and INL (deep capillary network)
The blood-retinal barrier is formed by tight junctions in 2 locations:
- Between endothelial cells lining retinal vessels.
- Between RPE cells
* Remember, blood is toxic to the retina
What are some of the histological changes that occur in Diabetic Retinopathy?
- increase blood glucose damages pericytes
- Basement membranes of retinal capillaries also damaged, allowing blood and plasma to leak into surrounding retinal tissue
Vitamin A is an alcohol retinol that is oxidized in the RPE to form __________.
11-cis retinal
What is the first transformation when light is absorbed by a photorecptor?
11-cis-retinal to all-trans-retinal
What is the membrane potential of a photoreceptor in the dark?
(-50mV)
What type of potentials do photoreceptors produce?
Graded potentials
Which types of cells release GABA and Glycine?
Horizontal and Amacrine cells
Which retinal cells release Glutamate?
rods, cones, bipolar cells, and most ganglion cells
In the dark, photoreceptors are ____________ and constantly release __________ to bipolar cells and cGMP keeps ______ channels open to promote depolarization.
DEPOLARIZED, GLUTAMATE, Na+
When rhodopsin is dissociated, the activation of which enzyme leads to decrease in cGMP [ ]?
Transducin
What happens to sodium channels when cGMP decreases?
Na+ channels CLOSE, resulting in increased cell membrane charge (-65mV)»This hyperpolarization results in a decrease in release of glutamate to bipolar cells
What controls whether or not the photoreceptors decrease their release of glutamate?
Closure of Na+ channels
Which type of potentials do Bipolar cells produce?
Graded potentials
What happens to a on-center bipolar cell when light is present?
less glutamate is released, resulting in DEPOLARIZATION; on-center BP cells are inhibited by glutamate and are thus HYPERPOLARIZED in the dark
What happens to off-center bipolar cell when light is present?
less glutamate is released, resulting in HYPERPOLARIZATION; off-BP cells are excited by glutamate and are thus depolarized in the dark
Horizontal cells respond with __________ potentials and _____________ in response to light
Graded potentials, Hyperpolarized
What is the purpose of horizontal cells?
Impact the surround response of BP cells by providing inhibitory feedback to PR cells
»they also provide lateral inhibition, which fine-tunes the neural signal
PR’s, horizontal cells, and Off-center BP cells are unique because they ___________ in response to light.
HYPERPOLARIZE
Amacrine cells have ______ potentials and always _______ in response to light.
Action potential, depolarize
What do amacrine cells do?
Fine-tune the signal between BP and ganglion cells.
Ganglion cells respond to BP cells with _______ potentials.
Action
ON-center/OFF-surround Gang. cells synapse w. ON-center BP cells and _______ in response to light.
DEPOLARIZE
OFF-center/ON-surround Gang. cells synapse w/ OFF-center BP cells and _______ in response to light.
HYPERPOLARIZE
Does rod density decrease with age? If so, how does this affect scotopic function?
Yes, they decrease, but scotopic function DOES NOT decline.
RNFL fibers decrease, resulting in an increase in the diameter of the ______ cup.
Vertical
A lesion above the medulla, will lead to problems with motor control on the __________ side.
Contralateral side
Where do fibers in the tectospoinal pathway originate?
Superior colliculus
What type of information does the spinothalamic pathway carry?
Pain and temperature information from the body
What does the trigeminothalamic pathway carry?
Pain and temperature info from the face
The medial lemniscus pathway carries what type of information?
Info about touch, pressure, and vibration
What side will be affected if there is a lesion above the crossover point in the medial lemniscus pathway?
contralateral side»below crossover would be ipsilateral side
Where are pre-ganglionic neurons of the sympethetic system located?
thoracic and lumbar sections of spinal cord
Pre-ganglionic sympathetic fibers release ___________.
acetylcholine
Post-ganglionic sympathetic fibers release _______________
NE
Where are pre-ganglionic neurons located in parasympathetic system?
2nd-4th sacral segments of spinal cord
Pre-ganglioniparasympathetic fibers release ________________.
Ach
Post-ganglioniparasympathetic fibers release ________________.
Ach
Where do the axons of the retinal ganglion cells terminate?
LGN
Axons that leave the LGN are called the ____________________.
optic radiations
Which location in the brain is where monocular input is combined for binocular processing?
V1!»LGN neurons are still monocular!
Which layer of the visual cortex receives primary visual input from the LGN?
Layer 4
What does the inferotemporal cortex role have?
“what” identification of objects
What role is middle temporal cortex (MT) for?
identification of spatial relationship of the object»aka “where” system
What are the 2 primary functions of the frontal eye fields?
- Pupillary response to near objects.
2. Activates during initiation of voluntary and reflex eye movements
What is considered to be a normal Arden ratio?
1.8
What do the A,B, and C waves represent, respectively, in the ERG?
A= PR's B= bipolar and Muller cells C= RPE cells
What is the best electrical potential test of choice for analyzing the RPE function?
EOG!
In early cases of RP, what does the ERG look like?
scotopic ERG is abnormal»late stages of RP, ERG is completely extinguished due to poor function of rods and cones
What is the pathway for the pupillary response? beginning with the pupillary fibers leaving the optic tract…
Optic tract»brachium of superior colliculus»synapse at pre-tectal nucleus in midbrain»ipsilateral & contralateral Edinger Westphal nucleus
During sleep or anesthesia, what happens to the pupils?
they become MIOTIC!
When is IOP found to be the highest?
During nocturnal hours (midnight-6AM)
What are some of the drugs that DECREASE aqueous production?
B-blockers, alpha-2 agonist, CAI’s, hyperosmotic agents, uveitis, decline in blood pressure
How many uL/min does the corneoscleral pathway drain?
2.25 uL/min
Through which structures does the aq. humor flow through Corneoscleral pathway? is it pressure dependent or independent?
aqueous flows from ant. chamber»across TM into Schlemm’s canal»episcleral veins drain aq. from Schlemm’s canal
»pressure dependent!
What is drainage of the uveoscleral outflow?
What amount in uL/min?
Aq. drains through ciliary stroma into surrounding vessels of the venous system\
»0.25uL/min
Since total amount of aw. outflow is 2.5uL/min and ant. chamber volume is 250uL, how often is aq. humor fluid replaced?
Every 100 minutes
What effect will increased episcleral venous pressure have on IOP?
INCREASED IOP!
Active secretion accounts for what % of aqueous humor formation?
80-90%
Bicarbonate ions are believed to increase aq. production by increasing __ and ____ flux into the posterior chamber.
Cl- and Na+
»Na+/K+ pump located within the NPCE pumps Na+ out of cell into post. chamber (water follows)»Na+ constantly moves from ciliary stroma into PCE
How many times more Vit. C is in the aq. humour compared to blood plasma?
20 X more
The blood-aqueous barrier consists of tight junctions in what 3 locations?
the iris vessels, enothelium of Schlemm’s canal, and the NPCE
