Eyelids, Tears, & Eye Movements

what prompts eyelid closure

contraction of orbicularis oculi muscles

not the relaxation of the levator muscles

3 types of eyelid closure

blinking, winking, spasm

3 types of blinking

spontaneous, reflex, voluntary

most common type of blinking

spontaneous

rate of spontaneous blinking

12-15 blinks a minute

spontaneous blinking is a contraction of the palpebral orbicularis muscle in the absence of what

absence of external stimulus

role of spontaneous blinking

stabilizes the tear film

what happens to tears during a spontaneous blink

new tears are secreted and spread across the ocular surface

old tears are pushed towards the nasolacrimal drainage system

what happens when there is a decreased rate of spontaneous blinking

decreased tear secretion and increased tear evaporation

causes dry eye syndrome and secondary epiphora (tearing)

what causes a decreased blink rate

decreased corneal sensitivity

often when watching TV, reading, or after LASIK

what is a reflex blink

blink caused by sensory stimuli, including auditory, touch, bright light, or external threat

what cranial nerve mediates reflex blinking in response to loud sounds

CN VIII (auditory)

what CN mediates reflex blinking in response to touch

CN V (responsible for corneal sensitivity)

what does cotton swab testing check for, and what CN is it evaluating

checks for corneal sensitivity by assessing reflex blinking in response to irritation

assesses CN V1 (responsible for corneal sensitivity)

what 2 reflex blinks does CN II cause

dazzle & menace

dazzle

reflex blink in response to bright light detected by CN II

menace

reflex blink in response to an unexpected object threatening the eye detected by CN II

where does the efferent loop of reflex blinking in response to external stimuli begin

frontal lobe

what is the only reflex blink that does not involve the cortex

dazzle blink

afferent stimulus and efferent response loop remains in the eye

difference between reflex and spontaneous blinking

both are involuntary but reflex blinking is in response to an external stimulus, while spontaneous blinking is in the absence of external stimulus

the efferent loop of blinking involves stimulation of what CN

CN VII

stimulates palpebral orbicularis to close eye

difference between voluntary blinking and spontaneous or reflex blinking

voluntary blinking has a more prolonged duration

what is winking

a form of voluntary blinking

what muscles are used to wink or forcefully close the eye

simultaneous contraction of orbital and palpebral orbicularis oculi

benign essential blepharospasm

condition characterized by bilateral, involuntary, sustained twitching of eyelids

what muscles cause benign essential blepharospasm

spasms of the orbicularis oculi, procerus, and corrugator muscles

what muscle is used in tight or forced eyelid closure

requires contraction of the orbital orbicularis oculi

bell’s phenomenon

normal defense reflex in 75% of the population

upwards and outwards globe rotation after forced lid closure

location of meibomian glands

upper and lower tarsal plates of the eyelids

role of meibomian glands

secrete anterior lipid layer of tear film

what kind of secretion do meibomian glands have

holocrine secretion (secrete their entire cell content as a product)

blinking stimulates release of what through holocrine secretion

release of lipids via the meibomian glands

what kind of glands are the accessory lacrimal glands

tubuloacinar exocrine glands

name and location of accessory lacrimal glands

glands of krause: fornices

glands of wolfring: tarsal conjunctiva

are there more glands of krause or wolfring

krause

in which direction does the eyelid close during a blink and why

laterally to medially

helps move tears toward puncta

what layer of the tear film is being spread evenly across the eye during a blink

mucin layer

how do tears drain when the eye is open

passive drainage into the puncta via capillary attraction

when the eyelids close, which 2 muscles contract to help with tear drainage

Horner’s and orbicularis oculi

how does the contraction of horner’s help with tear drainage

when horner’s contracts, the canaliculi shorten and move medially towards the lacrimal sac

this action pumps tears into the lacrimal sac

how does the contraction of the orbicularis help with tear drainage

when orbicularis contracts, temporal wall of the lacrimal sac is stretched away from the nose

this creates a negative pressure that forces tears into the NLD

the orbicularis causes lacrimal sac (compression/dilation)

compression

forces tears into the NLD

role of blinking in tear drainage

moves the tears medially towards the puncta

lowers the canaliculi pressure, creating a pressure difference that promotes tear drainage

number of cilia on UL and LL

150 lashes on UL

75 lashes on LL

role of cilia

screen the environment and induce blinking when necessary to protect eyes

2 roles of eyelid glands

produce tear film

move debris away from cornea

5 functions of tears

optical, nutritional, mechanical, antibacterial, corneal transparency

optical function of tears

creates smooth optical surface for clear vision

primary role of tear film

largest change of refractive index of the eye

air/tear film interface

nutritional function of tears

primary source of oxygen for the corneal epithelium

from diffusion of atmospheric O2 from the tear film

mechanical function of tears

tear film collects debris during a blink and removes waste from corneal epithelial cells

antibacterial function of tears

aqueous layer has immune cells (lysozymes, lactoferrin, IgA, etc)

how tears help with corneal transparency

has specific osmolarity and pH that is maintained by secretory glands and corneal epithelial cells

helps prevent corneal edema

what is the tear film thickness (according to recent non-invasive techniques)

3 microns

what is the anterior lipid layer of the tear film composed of

fatty acids, cholesterol, waxy esters

what secrets the anterior lipid layer of the tear film

meibomian glands, zeis & moll glands

what is the main role of the anterior lipid layer of the tear film

to slow the evaporation of the aqueous layer of the tear film and maintain clarity

what is the main method of releasing lipids from the glands

blinking

what type of innervation is able to increase lipid secretion

PS innervation

increases lacrimation

3 functions of the aqueous layer of the tear film

protection through antibacterial proteins

nutrition by supplying glucose to the corneal epithelium

adding thickness to the tear film

what nutrient does the aqueous provide to the corneal epithelium

glucose

what is the main component of tears

water

components of the aqueous layer of the tear film

water, electrolytes, antimicrobials, lipocalins, vitamin A, enzyme cofactors, HCO3-, solutes, proteins

what 3 electrolytes are in the tear film aqueous

Na+, K+, Cl-

5 antimicrobial components in aqueous tear film

IgA, lactoferrin, lysozyme, beta-lysin, interferon

role of lysozyme in aqueous tear film

cleaves peptidoglycan in bacterial cell wall, killing bacterial

antimicrobial

role of lactoferrin in aqueous tear film

chelates Fe2+, which is an essential nutrient for bacterial cell wall growth and metabolism

antimicrobial

role of beta-lysin in aqueous tear film

destroys bacterial cytoplasmic membrane and works together with lysozyme

antimicrobial

role of lipocalins in aqueous tear film

decrease surface tension of tears to enhance spreadability

role of vitamin A in aqueous tear film

development of goblet cells of the conjunctiva

in what form is vitamin A present in aqueous tear film

all-trans retinol

role of enzyme cofactors in aqueous tear film

maintain membrane permeability of corneal epithelial cells

how does the composition of the aqueous layer of the tear film change with increasing age

decrease in levels of lysozyme and lactoferrin proteins (antimicrobials) in the tears

overall decrease in aqueous secretion

how does contact lens wear change the aqueous composition of the tear film

increase in electrolyte and protein concentrations because of increased tear evaporation

how do closed eye conditions change the aqueous composition of the tear film

increased concentration of IgA and albumin

lysozyme and lactoferrin levels remain the same

what secretes the aqueous layer of the tears

main lacrimal gland

accessory lacrimal glands of krause and wolfring

what innervates the main lacrimal gland

PS fibers of CN VII

sympathetic fibers

sensory nerves of V1

what innervates the accessory lacrimal glands

PS nerves

role of main vs accessory lacrimal glands

main: reflex and emotional tearing

accessory: maintenance/basal tearing

recent theories that both glands are responsible for basal tearing

reflex arc of corneal CN V1

when a reflex stimulates CN V1, it causes lacrimation, miosis, and a protective bling

the dazzle reflex also causes lacrimal gland secretion

innermost layer of the tear film

mucous layer

outer layer of the mucous layer

mucin layer

role of mucin layer

interacts with glycocalyx of corneal epithelium to spread tears across the corneal surface

traps debris, bacteria, and sloughed corneal epithelial cells

what is unique about mucin molecules

can mix with lipid and water

allows mucous layer to mix with the aqueous layer and spread it evenly across hydrophobic corneal surface

what produces the mucous layer of the tear film

goblet cells of the conjunctiva

squamous cells of the cornea and conjunctiva

where are goblet cells mostly found

inferonasal fornix

nasal bulbar conjunctiva

what do goblet cells need for development and where is it found

vitamin A

found in the tears in the aqueous layer as all-trans retinol

what does vitamin A deficiency cause in the conjunctiva and why

causes keratinization of the conjunctiva and cornea because without vitamin A goblet cells cannot develop

can cause bitot’s spots

bitot’s spots and what cause them

foamy build up of keratin on the conjunctiva

caused by vitamin A deficiency

innervation of corneal and conjunctival mucous secretion

sensory nerves in the corneal and conjunctival epithelium stimulate sympathetic and PS nerve endings surrounding goblet cells

PS stimulation causes an increase in mucous secretion

what kind of innervation causes increased mucous secretion

PS

disease process of mucous fishing syndrome

when patients fish for mucous, they further damage conjunctival epithelium

this further increases mucous production and creates a cycle that worsens the mucous

most common cause of mucous fishing syndrome

dry eye syndrome

new research suggests that the tear film has what kind of composition

instead of 3 separate layers, aqueous and mucin layers are intermixed

there is a greater mucin concentration towards the corneal surface of the tears

how does TBUT work

once the anterior lipid layer becomes insufficient, the aqueous layer evaporates and the tears break up

with a blink, the anterior lipid layer is secreted again and restores the tear film

a TBUT of less than _____ seconds is considered abnormal

10

how are 25% of tears eliminated

evaporation

how are 75% of tears eliminated

drain through the nasolacrimal system or into systemic circulation by absorbing into conjunctival or nasolacrimal vessels

total tear volume of ocular surface

7-9 microns

max amount of fluid the eye can hold in the tear film

20-30 microns