OCular Phys Flashcards
• eyelid closure is the result of the orbicularis oculi muscles, NOT relaxation of the levator muscle
Blinking
Spontaneous blinking
◦ The most common type of blinking
◦ Resutls from the contraction of the palpebral portion of the orbicularis oculi in the absence of an external stimulus and occurs at an average rare of 12-15 blinks per minutes
◦ Spontaneous blinking helps to maintain the optics and comfort of the eye but stabilizing the tear film. During a spontaneous blink, new tears are secreted and spread across the ocular surface while old tears are pushed medically towards the nasolacrimal drainage system
Decreased rate in spontaneous blinking
resutls in decreased tear secretion and an increase in tear evaporation, resulting in dry eye sybdrome and secondary epiphora. A decreased blink rate commonly occurs when reading, watching TV, or after LASIK surgery due to a decrease in corneal sensitivity
Reflex blinking
◦ Caused by sensory stumiuli, including ◦ auditory: loud noises sensed by CN VIII ◦ Touch or irritation: CN V ◦ dazzle: CN II ◦ Menace: CN II
2,5,8
Cotton swab testing
evaluates the health of V1 by determining its ability to initiate reflex blinking in repsosne to an irritating stimulus
What type of reflex blinking does NOT involve the cortex
• The efferent loop of reflex blinking in repsosne to auditory, touch/irritation, and menacing stimuli begins in the frontal lobe. The dazzle reflex is the only reflex blink that does NOT involve the cortex. Remember that the efferent loop involves stimulation of the orbicularis oculi via CN VII
DAZZLE
Difference between reflex and spontaneous blinking
• Unlike reflex blinking, spontaneous blinking occurs in the absence of an external stimulus. The palpebral portion of the orbicularis is responsible for both spontaneous and reflex blinking
Voluntary blinking
◦ The amplitude and duration of voluntary blinking is varied and more prolonged compared to spontaneous and reflex blinking
◦ Winking: a form of voluntary blinking that requires simultaneous contraction of the orbital and palpebral portions of the orbicularis oculi
Eyelid spasm
• includes the condition benign essential blepharospasm
◦ Characterized by bilateral, involuntary, sustained twitching and/or closing of the eyelids
◦ Resutls from spasms of the orbicularis oculi, procerus, and corrugator musculature
Tight or forced eyelid closure rewuire
contraction of the orbital portion of the orbicularis oculi.
Bells phenomenon
normal defense reflex present in about 75% of the population, occurs after forced eyelid closure and is characterized by an upwards and outwards rotation of the globe
Orbital portion of the orbicularis
◦ Forced closure
◦ Bells phenomenon
◦ Voluntary blinking
Palpebral portion of the orbicularis oculi
◦ Forced closure
◦ Reflex and spontaneous blinking
◦ A. Horner
‣ Shorten canaliculi
‣ Enlarge lacrimal sac
◦ B. Riolan
‣ Row, Tight, Divide ant/post (gray lineMeibomian glands
sebaceous glands located within the upper (25) and lower (20) tarsal plates of the eyelids that are responsible for secreting the anterior lipid layer of the tears. Blinking stimulates lips release via holocrine secretions
Accessory lacrimal glands
tubuloacinar exocrine glands that contribute to the aqueous layer of the tears
‣ Glands of Krause: more numerous and are located in the fornices
• Krause=crease
‣ Glands of Wolfring: less numerous and are found in the tarsal conjunctiva
Glands of Krause
Accessory lacrimal gland
In the crease
Glands of wolfring
Accessory lacrimal gland
Found in the tarsal conjunctiva, less numerous
Distribution of tears
◦ The upper eyelid closes lateral to medically during a blink, spreading the mucin layer of the tears evenly across the corneal epithelium and bulbar conjunctiva to aid in proper tear film formation
Drainage of tears
The lacrimal pump theory summarizes how eyelid closure affects tear drainage
‣ When the eye is OPEN, tears passively drain into the puncta via capillary attraction
‣ When the eyelids close during a blink, the muscle of Horner contracts, causing the canaliculi to shorten as they move medially towards the lacrimal sac. This action helps “pump” the tears into the lacrimal sac
‣ as the eyelids close, the orbicularis oculi also contracts, stretching the temporal wall of the lacrimal sac away from the nose. This action crease a negative pressure that helps to draw the tears into the lacrimal sac
Most common bacterial cause of canaliculitis
Actinomyces israeli
How does blinking occur
from the lateral to the medial canthus and helps to move tears towards the puncta. Blinking also lowers the pressure in the canaliculi, creating a pressure difference between the atmosphere and the lacrimal sac that promotes tear drainage
Protective function of eyelids
- cilia (eyelashes): screening and sensing the environment and induce blink reflexes. 150 UL, 75 LL
- Glands of eyelid: produce the tear film and help move debris away from the cornea in concern with spontaneous blinking
Functions of tears
Optical Nutritional Mechanical Antibacterial Corneal transparency
Optical function of tears
the primary role of the tear film is to create a smooth optical surface of clear vision. Remember, the largest change in RI occurs between the air/tear film interface
Nutritional role of tears
the primary source of O2 for the corneal epithelium is form diffusion of atmospheric O2 through the tear film
Mechanical role of tears
the tear film collects debris and moves it away from the cornea during blinking. It also helps to remove metabolic waste products from the corneal epithelial cells
Antibacterial role of tears
the aqueous layer of the tear film contains lysozyme, lacrtoferrin, IgA, and other proteins of the immune system
Corneal transparency role of tears
the tear film has a specific osmolarity (308) and pH (7.45) that is maintained by the secretory glands and the corneal epithelial cells, thus helping to prevent corneal edema
Total tear film thickness
3um
Anteiror lipid layer of tears
◦ Composed of free fatty acids, cholesterol and waxy esters. It is secreted by the meibomian glands. The main purpose of the anteiror lipid layer is to slow the evaporation of the aqueous layer of the tears. It also helps to maintain optical clarity
‣ Although blinking is the primary method for releasing lipids from the glands, parasympathetic innervation from nerves surrounding the glands may also increase lipid secretion
‣ Meibomian and zeiss=holocrine secretions and moll=apocrine
‣ Blinking=increased lipid layer
Awusous layer of tears function
‣ Provides protection through antibacterial proteins
‣ Provides nutrition by supplying glucose to the corneal epithelium
‣ Adds thickness to the tear film
Aqueous portion of tear film contains the following components
‣ Water (the main component of tears)
‣ Electrolytes: Na+, K+, and Cl-
‣ Antimicrobial components: IgA, lactoferrin, lysozyme, beta-lysin, and interferon
• Lysozyme cleaves peptidoglycan bonds in bacterial cell walls
• Lactoferrin chelates Fe2+, an essential nutrient for bacterial cell growth and metabolism
• Beta lysin destroys bacterial cytoplasmic membranes and acts in concert with lysozyme
• Lipocalins: decrease the surface tension to enhance spread abiltiy and act as a carrier for all-trans-retinol. Also blocks Fe2+ from binding to the surface of bacteria
• Vit A: present within the tears in the form of all-trans-retinol. Necessary for goblet cells
• Enzyme cofactors: help maintain membrane permeability of corneal epithelial cells
• HCO3-: buffer for tears
• Solutes: glucose, urea, lactate, citrate, ascorbate, and AA
• Additional protein including albumin, growth factors, interleukins, and VEGF
Composition of the aqueous layer of the tears and age
decreased levels of lysozyme and lactoferrin proteins within the tears, as well as an overall decrease in aqueous secretion
Aqueous layer of the tear film and CL wear
increase in electrolytes and protein concentration due to increase evaporation of the tears
Aqueous layer of the tear film under closed eye conditions
has a higher concentration of IgA and serum albumin compared to open eye conditions. Lysozyme and lactoferrin levels are essentially the same. Decreased pH
What secreted the aqueous layer of the tear film
◦ the main lacrimal gland and the accessory lacrimal glands of Krause and Wolfring secrete the aqueous layer of the tear film
Main lacrimal gland innervated by
parasympathetic fibers from CN VII (VIdian nerve), sympathetic fibers, and sensory nerves of V1
Accessory lacrimal gland innervated by
thought to be innervated by parasympathetic Nerves; however, neural control of the accessory lacrimal glands secretions is not well understood and conclusive researach is unavailable
What glands are responsible for reflex, emotional, and basal tearing
Main lacrimal gland
What glands are responsible for basal tearing only
Main lacrimal gland and accessory lacrimal gland
Blink reflex
‣ The sensory nerves (V1) of the cornea are involved in a reflex arc that causes lacrimation (through parasympathetic stimulation of the lacrimal gland via CN VII), miosis, and a protective blink. The dazzle blink reflex can also stimulate a lacrimal gland secretion
Mucous layer of the tears
◦ Consist of an outer mucin layer that interacts with the glycocalyx of the corneal epithelium and helps to spread the tears across the corneal surface, as well as trap debris, bacteria, and sloughed corneal epithelial cells
How are mucin molecules in the mucous layer of the tear film unique
they are capable of mixing with lipid AND water, this property allows the mucous layer of mix with the aqueous layer and spread it evenly over the hydrophobic corneal epithelial surface
How is the mucous layer of the tear film produced
by the goblet cells of the conjunctiva and the squamous cells of the cornea and conjunctiva
‣ Goblet cells are predominately found in the inferonasal fornix and the bulbar conjunctiva
‣ Goblet cells require vitamin A for development, which is found in the aqueous layer of the tears as all-trans-retinol. Vit A deficiency results in keratinization of the conjunctiva and cornea
Vit A deficiency
causes Bitot’s spots (foamy build up of keratin) on the conjunctiva
Nerves and goblet cells
◦ Sensory nerves in the corneal and conjunctival epithelium stimulate sympathetic and parasympathetic nerve endings surrounding goblet cells; parasympathetic stimulation causes an increase in mucous secretions.
Mucous fishing syndrome
occurs as a result of patients fishing for and removing excess mucous in the conjunctiva. This results in damage to the conjunctival epithelium and a subsequent increase in mucous production, creating an unfortunate cycle that exacerbates symptoms. Dry eye syndrome is the msot common cause of mucous fishing sybdrome
Tear film distribution, structure, and stability
• recall that the eyelids play an important role in spreading the mucous layer evenly over the corneal epithelium. The mucin layer interacts with the glycocalyx of the corneal epithelium, allowing the tear film to be evenly spread across the corneal and conjunctival epithelium
How can the stability of the tears be examined
by analyzing the TBUT. NaFL is instilled in the eye and spreads evenly through the tears. Over time, the aqueous layer evaporates as a result of an insufficiency lipid layer, resulting in a break up of tears. Blinking promotes secretion of the anterior lipid layer and restores the tear film. A TBUT less than 10s is considered abnormal
Elimination of tears
• appx 25% of secreted tear volume is continually lost via evaporation. The remaining 75% of the tear volume drains through the nasolacrimal system or into the systemic circulation via absorption into the conjunctival and/or nasolacrimal vasculature
Total volume of the ocular surface
appx 7-9uL. The max amount of fluid the eye can hold within the tear film and the fornices is 20-30uL. Normal tear production is appx 1 uL/minute and the average eye drop contains 50uL. Drop instillation or tear production greater than 1uL/min results intear overflow onto the cheeks (epiphora)
Tear film osmolarity
308
What can tear film osmolarity depend on
Tear film osmolarity can vary depending on the blink rate, humidity, CL wear, and ocular pathology
What are the main contributors to tear film osmolarity
Na+ and Cl- ions within the aqueous portion of the tears are the main contributors to tear osmolarity.
-Ca2+ and K+ are also important components of the aqueous portion of the tears
Ca2+ in the tears
essential for hemidesmosome formation in the BM of the corneal epithelium,. Excess calcium can deposit on CL, forming “jelly bumps” that may decrease VA
K+ in the tears
helps to maintain the health of the corneal epithelium and has a 4X greater concentration in the tears compared to blood plasma
Dry eyes and osmolarity
Dry eye syndrome causes an increase in tear osmolarity. Hypotonic eye drops (osmolarity of 150 mOsm/L) are often utilized in treatment. There is an increase pH in dry eye so we use hypotonic drops to decrease it.
Ph, buffering and temperature of tear film
HCO3- within the aqueous layer of the tears are an excellent buffer and can tolerate ophthalmic solutions with a pH ranging from 3.5-10.5. The average pH of the tears is 7.45
‣ The pH of the tears during sleep decreases (becomes more acidic) due to the byproducts of anaerobic respiration
‣ Sleep=decreased O2-increased LA-decreased pH
Most ophthalmic drops are _____
Weak bases
Because the pH of the tears is 7.45, most ophthalmic drugs present in the non-ionized form within the tear film, promoting drug absorption across the hydrophobic corneal endothelium.
Middle ear
separated from the external ear by the tympanic membrane. Sound waves are amplified 10-20x by the tympanic membrane (eardrum) before being converted into mechanical vibrations and sent into the inner ear. The middle ear contains
Tympanic cavity
the space internal to the tympanic membrane (middle ear)
Auditory ossicles
the small ear bones, including the malleus, incus, and stapes. These bones are located in the series between the tympanic membrane and the oval window. The malleus is first in the series and is attached to the oval window. The auditory ossicles amplify and transmit vibrations received by the tympanic membrane
Stapedius and tensor tympani muscles
dampen the amount of vibrations placed on the auditory ossicles (STRAPEDIUS STOPS SOUNDS)
‣ The stapedius muscle is innervated by a branch of CN VII just before it exits the skull via the styalomastoid foramen
‣ The tensor tympani muscle is innervated by a branch from the mandibualr division (v3) if CN V.
Chords tympani
‣ the chorda tympani nerve of CN V and the tympanic nerve plexus (CN IX) travel within but do not innervate the middle ear cavity
Inner ear
converts mechanical vibrations into neural signals
Vestibulocochlear organ of inner ear
help to maintain balance, receive sound, and contribute to ocular reflex actions
Bony labyrinth of inner ear
consists of three parts that are innervated by CN VII:
‣ Cochlea
-vestibulae
-semi circular canals
Cochlea
Inner ear
shell shaped portion of the inner ear. Contains the organ of corti that contains hair cells that control hearing
Vestibulae
Inner ear
consists of the utricle and saccule that help maintain balance. These organs detect static linear acceleration and cause reflex eye movements (linear VOR) that are equal and opposite to the motion of the head
• The utricle detects horizontal linear movements
• The saccule detects vertical linear movements
Semi circular canals
Inner ear
communicate with the vestibule and contain the ampullae that detect angular acceleration (rotational) and cause the reflex eye movement known as the angular VOR
Tympanic membrane the oval window
• The tympanic membrane separates the external and middle ear. The oval winded separates the middle and inner ear. The TM is much larger, allowing amplification of sound. The malleus, incus, and stapes bones lie between the TM and OW.
Supranuclear control of saccades
◦ Rapid eye movements that maintain fixation (aka foveation) on the object of regard
◦ Horizontal saccades are controlled by the contralateral FEF in the frontal lobe and the superior colliculus. For example, the right frontal lobes controls saccades towards the left
Supranuclear control of pursuits
◦ Smooth tracking movements that maintain foveation on slow moving objects
◦ Controlled by the ipsilateral parietal lobe. For example, the right pursuit is controlled by the right parietal lobe, and the left pursuit is driven by the left parietal lobe
Supranuclear control of vergences
◦ Control of vergences is presumably located at the level of the brainstem
◦ Divergence and convergence eye movements are likely driven by retinal disparity and help to maintain sensory fusion and stereopsis
If spinning the OKN drum clockwise, what parts of the brain are being used
‣ Left FEF and superior colliculus for right saccades
‣ Left parietal lobe for left pursuits
The order of the ear structure
External ear Tympanic membrane Middle ear (malleus, incus, stapes) Oval window Inner ear (VOR)
Corneal epithelium permeability characteristics
contains zonula occludens junctions (tight junctions) that force molecules to travel THROUGH the cells rather than passing between them. The epithelium is highlight lipophilic (hydrophobic), limiting the absorption of hydrophilic, ionized molecules
Permeability of the corneal stroma
highly hydrophilic. Hydrophilic, ionized substances can easily pass through the corneal structures
Permeability of the corneal endothelium
contains macula occludens junctions. The endothelium is highly lipophilic (similar to epi) and allows only lipophilic, non-ionized substances to pass through
UV-C
100-280
UV-B
280-315
UV-A
315-400
What layers protect against UV C
Corneal epithelium and bowmans
Anything below 300 so some UV-B too
What structures protect against UV B
Lens and vitreous
The cornea transmits light with a wavelgnth of
300-2500 (infrared)
The visible wavelengths of light and the cornea
tranmistted through the cornea with a high degree of precision. More than 99% of light above a wavelength of 400nm is transmitted through the cornea
What range of UV light is the cornea most sensitive to
most sensitive to radiation in the UV-C range (particularly 260-280nm); snow blindness, welder’s keratitis, and tanning sun lamps can cause UV keratitis
What contributes to minimal light scattering, allowing for optical transparency of the cornea
Corneal crystallines Ascorbate (vit C) Glutathione Precise spacing of collagen fibrils in stroma Avascular High water content
Corneal crystllines
located in the cytoplasm of epithelial and endothelial cells and help to maintain corneal transparency by limiting light scattering, similar to crystallins in the lens
Ascorbate (vit C) in the cornea
and glutathione are located within the epithelial cells and help to protect the cornea from UV rays and free radical scavengers
Precise arrangement of collagen fibrils in the corneal stroma
The corneal stroma contains appx 200-250 layers of 30nm lamellae composed of collagen fibrils (n-1.55) that lie within a network of GAGs (n=1.345). Collagen fibrils have a uniform size and are precisely spaced less than one half the wavelgnth of visible light from one another
‣ Proteoglycans (PGs) are present within the ground substance that fills the space between the corneal cells and collagen fibrils and lemellae. The glycosaminoglycan side chains of PGs help to maintain appropriate collagen spacing by forming negatively charged bonds with water molecules
‣ Precise spacing of the collagen fibrils increases destructive interference, thereby minimizing light scattering and increasing transparency
Proteoglycans in the corneal stroma
present within the ground substance that fills the space between the corneal cells and collagen fibrils and lemellae. The glycosaminoglycan side chains of PGs help to maintain appropriate collagen spacing by forming negatively charged bonds with water molecules
Precise spacing of the collagen fibrils does what (cornea)
increases destructive interference, thereby minimizing light scattering and increasing transparency
High water content of the cornea
helps maintain a regular spacing of collagen
‣ Sclera has lower concentration of GAGs compared to cornea, and is thus dehydrated and less transparent compared to the corneal stroma
Proteoglycan composition
proteoglycans are composed of a core protein with one or more covalently linked GAG side chains. Sulfonation of the GAG side chains in the corneal stroma allows PGs to bind to water, creating a hydrophilic environment that’s helps maintain the precise spacing of collagen. The major PGs is keratin sulfate
Major proteoglycan in the cornea
Keratin sulfate
The most important factors that influence corneal thickness (hydration) and maintain corneal deturgesnce
epithelial pump mechanisms, endothelial pump mechanisms, and aquaporins
Corneal deturgescenc
state of relative dehydration maintained by the normal cornea that is necessary for transparency; 75-80% stromal water content is optimal. Deturgescence relies on the endothelial (main contributor) and epithelial transports mechanisms
What kind of pumps are on the epithelium
NAK
NA/K/Cl cotransporter
Corneal epithelial pump mechanisms
NAK ATPase, Na/K/Cl co
-Na+ passively enters the epithelial cell from the tear film covering the anterior surface of the cell. The Na+/K+ ATPase pump actively moves Na+ from the epithelial cell into the corneal stroma, creating a higher Na+ concentration in the stroma compared to the epithelium
-The Na+/K+/Cl- cotransporter utilizes the Na+ concentration gradient to passively move Na+, K+, and 2Cl- from the stroma into the epithelial cells. Cl- and K+ each have their own channels that allow for passive diffusion back into the tears and towards the aqueous humor, respectively
-Movement of K+ into the aqueous humor will stimulate Cl- to move into the tears. Water will follow Cl-, contributing to the dehydration of the cornea.
• The K+ channel has been shown to respond to pH changes within the cornea. A hypoxia cornea will have high acidity and increased thickness due to corneal swelling
• The K+ channel responds by moving more K+ into the aqueous causing more Cl- and H20 to move into the tear film to restore normal corneal thickness
Corneal endothelial pump mechanisms
NAK ATPase
‣ Na+ enters the endothelial cell from the corneal stroma via ion exchangers. The Na+/K+ ATPase pump, located on the basolateral membrane of the endothelial cell, pumps Na+ out of the endothelial cell into the aqueous humor, establishing a higher Na+ concentration in the aqueous humor compared to the endothelium
‣ The Na+/H+ pump utilizes the Na+ concentration gradient to move H+ ions out of the endothelial cells into the aqueous in exchange for the transfer of Na+ ions back into the endothelial cells. Movement of H+ ions into the aqueous humor results in a decrease in extracellular pH, causing CO2 to diffuse into the endothelial cell
‣ CO2 is combined with H20 for form H2CO3, which then dissociated into H+ and HCO3- (bicarbonate ions)
‣ Bicarbonate and Cl- move out of the endothelial cell and into the aqueous humor. H20 will then follow, contributing to the dehydration of the cornea
Major factors for water transport across the corneal epithelium and endothelium
Cl- excretion and Na+ absorption
Aquaporins
◦ Proteins embedded within the apical and basal membranes of corneal epithelial and endothelial cells that regulate bi-directional waters transport
Where does cornea get O2 from
• The entire cornea receives oxygen primarily from the atmosphere. The aqueous humor, limbal vasculature, and palpebral conjunctiva caps provide a minor contribution of O2 to the cornea under open eye conditions
ppO2 of the atmosphere
155
PpO2 within the tears
155
During closed eye conditions, ppO2 of tears
55
‣ The superior palpebral conjunctiva (primary contributor) and the limbal vasculature supply the epithelium and the anteiror stroma
‣ The aqueous humor supplies the posterior stroma and endothelium
Corneal edema in the AM
• Mild cornea edema occurs after awakening in all healthy individuals; in fact the cornea is always thickest in the AM. Mile corneal edema is due to a build up of lactate from anaerobic respiration and the limited supply of O2 when the eye is closed
Critical ppO2 for the cornea
10-20mmHg. A CL that is worn while sleeping must maintain a PP)2 above the critical value. Remember that minus lenses are thinner in the center and thus are likely more capable of transporting O2 compared to plus lenses
Oxygen to the cornea open eye
Tears for all layers
Nutrients to the cornea, open eye
AA, glucose, Vit C from tears
O2 for cornea during closed eye
Lids for the outer layers
aqueous for endothelium
Nutrients for cornea under closed eye
AA, glucose, Vit C from aqueous
Formula used for O2 flow to cornea and CL wear
J/A=DK/t (P1-P2)
◦ J/A=how much oxygen flows over a certain time
◦ Dk=oxygen permeability of the lens
◦ DK/t=transmissibility, measure of how much oxygen will diffuse thougha contact lens with a given thickness
Proper control of pH within the cornea
pH-7-7.3) is essential for maintaining corneal transparency. Decreased levels of O2 (hypoxia) can lead to accumulation of H+ ions produced in glycolysis, resulting in increased acidity of the corneal cells
Decreased corneal pH
causes a change in K+ channels, resulting in a massive reflux from the keratocytes with subsequent collagen damage and scar formation
Glucose for the cornea
produced for the cornea via anaerobic glycolysis, aerobic glycolysis, and the HMP manophosphate shunt
Glucose cxn in tears
low in the tears, but is high inthe aqueous humor. As a result, the aqueous humor is the primary contributor of glucose to all corneal layers.
What is the primary contributor of glucose to all layers of the cornea
AQUEOUS
Also serves as the primary source for AA and vitamins for all layers of the cornea
Why are corneal epithelial cells unique
because they can store large amounts of glycogen for basal cell mitosis and epithelial wound healing. The endothelium also requires large stores of energy in order to maintain the function of the Na+/K+ ATPase pumps that contribute to corneal transparency
Maintenance epithelial regeneration
The entire corneal epithelium replaces itself every 7-14 days
‣ Basal cells are the only mitotic cells in the epithelium. They are derived from differentiating limbal stem cells from the palisades of Vogt
‣ Basal cells differentiate into wing cells and then squamous cells before reaching the corneal surface. Old superficial corneal cells are shed as this process occurs
Traumatic epithelial regeneration
- Basal mitosis is stopped
- Scaffold
- Rapid basal cell mitosis
If the BM remains intact, corneal regenerate occurs quickly. If the BM is damaged, corneal regeneration occurs more slowly
‣ Complete healing of the BM (with creation of intact hemidesmosomes) takes appx 8 weeks
Complete healing of the BM (intact hemidesmosomes) takes
8 weeks
MMPs and hemidesmosomes
‣ Matrix metalloproteinases can degrade hemidesmosome formation. Because corticosteroids and tetracylclines have been shown to decrease th activity of metalloproteinases, the yare often included in the treatment regimen of RCEs
‣ Corneal abrasions result from trauma. RCEs occur in eyes with poor adhesions between the corneal epithelium and BM from previous abrasions or corneal dystrophies
Which corneal layers can regenerated
Epithelium
Descemets
Which corneal layers cannot regerenate
Bowmans
Endothelium
Stroma regeneration
will replace itself if damaged, but with a very different textured tissue. The new collagen is larger and less organized, resulting in a scar.
Which layer of the cornea is always growing throughout life
Descemets
Can triple in thickness
Corneal nerves
there are NO NERVEs in Descemets membrane or the endothelium. Corneal nerves enter at the level of the mid stroma and travel through Bowmans layer to the corneal epithelium
Tropic function of corneal nerves
sensory innervation is essential for epithelial cell maintenance and regeneration
◦ Reduced corneal sensitivity is typical after LASIK and with aging
Neurotrophic keratitis
characterized by CN V damage and decreased corneal sensitivity and can be diagnosed with the cotton swab test. HSV and HZV, stroke, and DM are common causes of neurotrophic keratitis
Aging changes of the cornea
- the vertical meridian flattens, resulting in an increase in ATR astigmatism
- light scattering increases
- Corneal sensitivity decreases
- The BM thickens
- The degree of corneal arcus in the peripheral stroma increases
- Descemets membrane thickens, causing an increase in the number of Hassal-Henle bodies in the corneal periphery
- The endothelium cell density decreases as the endothelium becomes thinner with age
Function of the lens
• provides 1/3 of the total dioptric power of the eye and allows for accommodation to near objects.
The following occur during accommodation (lens)
◦ 1. Parasympathetic stimulation causes contraction of the ciliary muscle, resulting in a decrease in the tension on the lens zonules
◦ 2. The anterior pole of the lens moves forward and the anterior curvature increases
◦ 3. The posterior pole of th lens moves back slightly and the posterior curvature increases
◦ 4. The lens thickness (anterior-posterior dimension) increases and the anterior chamber depth decreases
◦ 5. The lens diameter decreases
◦ 6. The lens power increases
IOP and accommodation
accommodation causes a temporary decrease in IOP because the ciliary muscle contraction pulls the scleral spur posteriorly and opens up the pores of the TM. Accommodation may also result in a temporary IOP increase due to a decrease in the depth of the AC because the anterior pole of the lens moves forward, the anterior lens curvature increases and the lens thickness increases. In patients with narrow angles, these changes may induce pupillary block and result in significantly elevated IPO, which are important adverse effects of miotic drugs such as pilocarpine
How does pilocarpine work
increases uveoscleral output
◦ Long muscle of CB pulls SS down, open up TM
◦ IOP decreases
Lens and nutrients
although the lens is avascular, it has the largest concentration of proteins of any structure in the body and this requires glucose and oxygen from the aqueous in order to maintain the following functions
◦ 1. Production of new lens fibers and protein synthesis
◦ 2. Maintenance of the Na+/K+ ATPase pump that helps to establish a balance between H20 and ions within the lens to maintain lens transparency
‣ The Na+/K+ ATPase pump on the epithelial cells constantly move Na+ into the aqueous humor (and K+ into the lens). H20 ultimately follows Na+ into the aqueous, contributing to lens dehydration and transparency
What do the NAK pumps on the lens do
On the epithelium
constantly move Na+ into the aqueous humor (and K+ into the lens). H20 ultimately follows Na+ into the aqueous, contributing to lens dehydration and transparency
How does the lens get glucose
70% of the glucose required by the lens is produced through anaerobic glycolysis. Aerobic metabolism is limited to the lens epithelium
Hexokinase and the lens
The first step in both aerobic and anaerobic respiration involves the conversion of glucose to glucose-6-phosphate via th enzyme hexokinase. If hexokinase is not available, glucose is converted to sorbitol via the enzyme aldose reductase
Excess sorbitol in the lens
can accumulate in the lens, creating an osmotic gradient that favors the movement of H20 into the lens, ultimately causing lens swelling, lens fiber damage, and cataract formation
Diabetes and the lens
‣ Excessive levels of glucose in diabetes leads to the accumulation of sorbitol within the lens, ultimately leading to early cataract development and an acute shift in refractive error secondary to lens swelling
What is a lot of needed for lens metabolism
A lot of ATP
Effects of glutathione on lens clarity
the primary protector against oxidative damage in the lens
‣ Acts as a reducing agent and detoxifies hydrogen peroxide
‣ Transported into the lens from the aqueous but can also be synthesized from lens epithelial cells and superficial fiber cells
‣ Deep fiber cells ad nuclear cells produce minimally glutathione and thus rely on diffusion of glutathione from the superficial fibers and lens epithelial cells
What is the primary protector against oxidative damage in the lens
Glutathione
Glutathione and age
diffusion diminishes with age and is a factor in the formation of cataracts
Ascorbic acid and lens clarity
helps to protect the lens from oxidative damage. Ascorbic acid is found in a much higher concentration in the lens compared to the aqueous humor
What do we want in the lens to have lens clarity
Increased crystalline
Increased glutathione
Increased vit C
Decreased Ca2+ (but balanced)
What helps maintain lens transparency
- NAK pumps on the epithelium (NA in, K out)
- avascular
- cells lack membrane bound organelles
- lens fibers packed very close
- cytoplasm have tons of crystllines
- very well balanced Ca2+ in lens
Growth of the embryonic nucleus
formed from primary lens fibers of the posterior lens epithelium during embryological development. All remaining growth of the lens is due to the production of secondary lens fibers by the anterior lens epithelium
Mitosis of lens fiber cells
occurs in the germinative zone of the anterior lens epithelium. After mitosis, lens fiber cells gradually migrate through the transition zone and into the equator, where fiber elongation occurs. During this process, lens fibers lose their membrane bound organelles and acquire crystallins
◦ The anterior lens epithelium has the greatest metabolism demand of all lens components and thus contains a significant amount of mitochondria to produce energy for mitosis. Remember that aerobic respiration is limited to the anteiror lens epithelium!
What has the greates metabolic demand of lens components
anterior lens epithelium has the greatest metabolism demand of all lens components and thus contains a significant amount of mitochondria to produce energy for mitosis. Remember that aerobic respiration is limited to the anteiror lens epithelium!
Where is the ONLY place in the lens that aerobic respiration occurs
Anterior lens epithelium
Crystlines in the lens and age
Alpha cryslines in the lens decrease with age. There are NONE by age 45 in the lens nucleus
What do alpha crystallines do
function as molecular chaperones by preventing the degradation of other crystallins. A reduction in alpha crystallins results in an increase in degradation of lens fiber cells and ultimately contributes to cataract formation
