6720 - Accommodation and Pupillary Function

accommodation definition

a dioptric change in optical power of the eye due to ciliary muscle contraction

basic accommodation mechanism

1. ciliary muscle contraction moves the apex of the CB towards the eye's axis

2. releases resting zonular tension around lens equator

3. elastic lens capsule molds lens into a more spherical form

accommodation definition

a dioptric change in optical power of the eye due to ciliary muscle contraction

basic accommodation mechanism

1. ciliary muscle contraction moves the apex of the CB towards the eye's axis

2. releases resting zonular tension around lens equator

3. elastic lens capsule molds lens into a more spherical form

how does lens diameter change during accommodation?

decreases

how does lens thickness change during accommodation?

increases

what happens to the anterior lens surface during accommodation?

moves anteriorly

what happens to the posterior lens surface during accommodation?

moves posteriorly

what happens to the anterior and posterior surface curvatures?

they increase

what happens to the optic power of the lens during accommodation?

increases

what muscle fiber types are found in the ciliary muscle?

longitudinal fibers, radial fibers, circular fibers

longitudinal fibers

runs along inside of sclera

what is the function of the anterior longitudinal/radial muscle fibers?

applies force to the scleral spur and opens TM

what is the function of the posterior longitudinal/radial muscle fibers?

applies force to pars plana moving it anteriorly

how are the radial fibers attached posteriorly?

elastic tendons of the choroid

radial fibers

fan out from chamber angle toward ciliary processes

what anatomical changes happen to the ciliary muscle upon contraciton?

1. increase in thickness of the circular portion

2. decrease in thickness of the radial and longitudinal portions

3. contraction pulls the anterior choroid forward, moves the apex of the ciliary processes towards the lens equator

ciliary muscle relaxation

radial fibers are attached to elastic choroid so CB relaxes through the elasticity of the choroid and posterior ciliary zonules

circular fibers

runs parallel to schlemm's canal

CB smooth muscle characteristics

1. one stimulus causes near-simultaneous contraction

2. longitudinal fibers similar to fast twitch muscles

3. larger neuron size than typical smooth muscle

4. from ectodermal origin

parasympathetic CB innervation

M3 receptors mediate contraction of CB

sympathetic CB innervation

beta 2 adrenergic receptors mediate relaxation

what type of fibers are longitudinal muscle fibers?

similar to fast twitch muscles, fewer mitochondria and more myofibrils are present

where are fast twitch fiber proteins most commonly found in the eye?

orbicularis oculi/LPS

which muscles of the CB have more mitcohondria?

radial/circular portions

2 primary functions of ciliary zonules

1. hold lens in place

2. transmit tensile forces for lens shape change

ciliary zonule composition

1. primarily made of glycoproteins

2. elastic but non-collagenous

3. beads on a string structure

what glycoproteins make up ciliary zonules?

fibrillin and MAGP-1

where are zonules embedded?

superficial lens capsule

when does zonule formation occur?

occurs late in embryonic development

what creates ciliary zonules?

non-pigmented ciliary epithelial cells

characteristics of the lens capsule

very elastic, thin, transparent membrane

what is the lens capsule composed of?

collage IV, laminin, heparin sulfate proteoglycans, lens integrins

what proteoglycans are found in the lens capsule?

perlecan, nidogen, collagen XVIII

what secretes the lens capsule?

lens epithelial and fiber cells

how does accommodation change as you age?

gradual loss of accommodative amplitude as you age?

what is the linear decline seen with presbyopia?

about 2.5 diopter loss per 10 years

what happens to the lens as you age?

increased mass, increased axial thickness, increased anterior/posterior curvature

what happens to the lens capsule as you age?

increased thickness, reduced elasticity

what happens to the zonules as you age?

increased insertion distance from lens equator

what happens to the ciliary muscles as you age?

loss of muscle fibers, increased connective tissue, reduced lens shape-changing ability

lens paradox

aged lens appear similar to accommodated lens

ectopia lentis

dislocation of the lens due to zonule failure due to a mutation in fibrillin gene

what is the purpose of the lens capsule during zonule relaxation?

increases anterior/posterior curvature of lens, increases lens thickness, increases optical power

reasons for near vision loss

1. thickness differences in cortical layers

2. decreased refractive index near equatorial region

accommodative triad

1. ciliary muscle contraction

2. pupil constriction

3. convergence

what stimulates an accommodative reflex?

blur cues, convergence

blur cues

blur presented to one or both eye induce both eyes to accommodate, induces bilateral pupil constriction

convergence cues

isolating convergence with base out lenses, induces both eyes to accommodate and bilateral pupil constriction

afferent accommodative reflex

-optic nerve axons project to and synapse in LGN which project to the visual cortex

-interneurons from visual cortex to FEF to EW nucleus and oculomotor nucleus

efferent accommodative reflex

1. from EW to ciliary ganglion along CN3

2. from ciliary ganglion to ciliary muscle

3. parasympathetic stimulates M3 receptors in muscle, fast twitch for fine control

convergence definition

simultaneous and synchronous adduction in both eyes

what stimulates convergence?

contraction of medial rectus muscle

convergence pathway

1. oculomotor nerve receives signals from the oculomotor nucleus to stimulate MR muscles

2. supranuclear signals from FEF and visual cortex couple ciliary muscle and medial rectus contraction

pupil function

1. controls retinal illumination

2. reduces optical aberrations

3. improves depth of focus/field

dim light

dilation of pupil maximizes number of photons reaching the retina

bright light

pupil constriction reduces illumination by 1.5 log units within 0.5 seconds

dilator muscle

radially oriented, associated with pigmented epithelium

what does contraction of the dilator cause?

pulls pupillary margin toward ciliary body peripherally

sphincter muscle

encircles pupillary margin, separated from pigmented epithelium

what does contraction of the sphincter cause?

reduces pupil size

what are the major excitatory neurons of the sphincter muscle?

parasympathetic

what are the major excitatory neurons of the dilator muscle?

sympathetic

how does lens capsule thickness change over time?

becomes thicker anteriorly and thinner posteriorly

what are the 2 stimuli for pupillary reponse?

light/brightness and near-reflex (accommodation)

lens capsule elasticity

non-enzymatic glycation of collagen IV which increases stiffness as you age

pupil near reflex

1. miosis occurs without change in retinal luminance

2. efferent pathway is the same as pupil light reflex

3. supranuclear neurons come from cortical areas surrounding visual cortex and from the FEF and goes to EW

afferent pupil light reflex

retinal ganglion cells axons project to pretectal olivary nucleus

interneuron pupil light reflex

from pretectal olivary nucleus to EW nucleus

efferent pupil light reflex

from EW to ciliary ganglion to iris sphincter

what are the two competing signals regulating pupil size?

1. illumination induced increase in parasympathetic nerve stimulation

2. continuous supranuclear inhibition of parasympathetic nerves

ipRGCs

contain melanopsin, can trigger action potential

what are the cell turnover rates like in the lens capsule?

very slow

lens capsule and hyperglycemia

increase in AH glucose can induce intra- and intermolecular cross-linking between lysine and amine groups

which collagen is susceptible to glycation in the lens capsule?

collagen type IV

what happens to accommodation in type 1 diabetics?

lowered amplitude of accommodation

what happens to zonular/capsule insertion distance as you age?

increases with age due to growth of underlying lens

ciliary muscle changes with age

loss of muscle fibers and increase in CT, contractile force does not decrease

what still happens even after accommodative loss?

movement of ciliary body, ciliary ring diameter still decreases

how does lens mass change with age?

increases linearly with age due to increase in lens cell number without any cells loss, increase in axial thickness and anterior/posterior lens curvatures

significantly reduced light stimulus (awake)

1. CNS/brainstem originating inhibitory input

2. parasympathetic nerve to sphincter muscle not stimualted

3. inhibitory signal comes from sympathetic nerve

4. mydriasis

some light stimulus (awake)

1. some competition to inhibitory signal

2. some stimulation along parasympathetic nerve to iris sphincter

3. some miosis occurs

more light stimulus (awake)

1. more competition to inhibitory signal

2. more stimulation along parasympathetic nerve to iris sphincter

3. more miosis occurs

when can lack of inhibition occur?

sleep, anesthesia, sympathetic inhibition suppressed, baseline level of positive signal from light-induced response causes miosis

secondary mechanism of pupil size regulation

contraction facilitates a super charge of mydriasis, not a requirement for pupil dilation, enhances speed and maximal pupil diameter