Accommodation

What is accommodation?

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

What is the basic mechanism of accommodation?

1. Ciliary muscle contraction moves the apex of the ciliary body towards the axis of the eye and releases resting zonular tension around the lens equator

2. When zonular tension is released, the elastic lens capsule molds the young lens into a more spherical and accommodated form

How does the diameter change with accommodation?

Diameter decreases

How does the thickness of the lens change with accommodation?

Thickness increases (anterior-posterior length increases)

During accommodation the anterior lens surface moves blank and the posterior lens surface moves blank.

Anteriorly; posteriorly

How do the anterior and posterior lens surface curvatures change with accommodation?

Anterior and posterior lens surface curvatures increase

What does the increase in surface curvature of the anterior and posterior lens surfaces do? (two things)

1. Increases the optical power of the lens

2. Facilitates the ability of the eye to focus on near objects

What are the three categories of ciliary muscles?

Longitudinal fibers, radial fibers, circular fibers

Where do the longitudinal fibers of the ciliary muscles run?

Runs along inside of the sclera

Where do the radial fibers of the ciliary muscles run?

Fan out from chamber angle toward the ciliary processes

Where do the circular ciliary muscles run?

Runs parallel to Schlemm’s canal

What are the longitudinal/radial muscle fibers conceptionally broken down into?

Anterior and posterior portions

What do the anterior portions of longitudinal/radial muscle fibers do?

Move posteriorly (away from TM and towards pars plana) and applies force to the scleral spur and opens trabecular meshwork

What do the posterior portions of longitudinal/radial muscle fibers do?

More anteriorly (away from pars plana and towards TM) and applies force to pars plana moving it anteriorly

What happens to ciliary muscles thickness upon contraction?

1. Increase in thickness of the circular portions

2. Decrease in thickness of the radial and longitudinal portions

What does contraction of ciliary muscles do to the choroid and ciliary processes?

Contraction pulls the anterior choroid forward, moves the apex of ciliary processes towards the lens equator

What happens to ciliary muscles upon relaxation?

Ciliary body is returned to its unaccommodated configuration through the elasticity of the choroid and posterior ciliary zones

Which fibers are attached to the elastic choroid that helps allow the ciliary body to return to its unaccommodated configuration?

Radial fibers are attached to the elastic choroid

Which ANS division(s) innervates the ciliary muscle?

Parasympathetic and Sympathetic innervation

What does the parasympathetic innervation of the ciliary muscle do?

ACh binding to M3 receptors mediates contraction

What does the sympathetic innervation of the ciliary muscle do?

B2-adrenergic receptors mediate relaxation

The ciliary muscle is a/n blank muscle

An atypical smooth muscle

What does one stimulus cause in ciliary muscle?

One stimulus causes nearly simultaneous contraction of all muscle groups (smooth muscle-like)

Typical smooth and skeletal muscle is derived from what embryological lineage?

Mesodermal lineage

Ciliary body/muscle is embryologically derived from what lineage?

Ectodermal lineage

What is usually derived from ebryological ectodermal lineage?

Epithelial structures of the eye

What other muscles of the eye are from the embryological ectoderm lineage?

Iris dilator and sphincter muscles

What is the atypical smooth muscle-like development of the ciliary muscle most likely due to?

A distinct embryological origin

Ciliary muscle blank fibers are similar to blank twitch muscles.

Longitudinal; fast (very quick fast twitch muscles)

Fewer of what and more of what are present in longitudinal fibers than typically found in smooth muscle?

Fewer Mitochondria and more myofibrils

Fast twitch fiber proteins appear in muscles needed for what? Examples?

Muscles needed for fine movements like orbicularis oculi/LPS

(Larger/smaller) neuron size is found in ciliary muscle than is normally found in smooth muscle.

Larger

What does the larger neuron size than normally found in smooth muscle in ciliary muscle suggest?

Faster than typical smooth muscle

Radial/Circular portions of ciliary muscle have more of what than normally found in smooth muscle?

Mitochondria

What are the two functions of ciliary zonules?

1. Hold lens in place

2. Transmit tensile forces for accommodative shape change of the lens

What is important about the shape of ciliary zonules?

Form fibril structures that allow fluid to pass

How are ciliary zonule types organized?

Based on lens insertion

What are the three types of ciliary zonules/three areas zonule insertion is concentrated?

Anterior zonules, equatorial zonules, posterior zonules

What are the two main portions of the ciliary body?

Pars plicata and pars plana

How deep does zonule insertion go and into what layer of the lens?

Embedded into the superficial lens capsule (0.5-1 micron thick region)

When does zonule formation occur?

Occurs late in embryonic development

Do we synthesize zonules throughout life?

Only embryologically developed and cannot be remade

What are zonules synthesized by during embryonic development?

Synthesized by non-pigmented ciliary epithelial cells

What are zonule fibers primary made up of?

Glycoproteins

What are the specific glycoproteins that zonule fibers are made up of?

Fibrillin, Fibrillin-like proteins, MAGP-1

What can zonule structure be described as?

Beads on a string

Are the fibrillin and fibrillin-like proteins localized to the “beads”, the “string” or both?

Localized to beads and the string

What is MAGP-1?

Microfibril associated glycoprotein 1

Is MAGP-1 localized to the beads, the string, or both?

Localized to the “beads” only

What contributes to the elasticity of zonule fibers? What do they not have?

Fibrillin gives the elastic quality; Do not have collagen or elastin in their structure

What are fibrillins?

Large cysteine-rich mutli-domain glycoproteins that polymerize in the extracellular space in a head-to-tail manner to form microfibrils

What is fibrillin a common constituent of?

Force-bearing structures such as blood vessels, lungs, and ligaments

What is fibrillin usually organized into?

10 to 12 nm diameter microfibrils that form a meshwork

What is weird about the structure of microfibrils of zonules?

They have periodic hollow cavities

What can lead to zonule failure?

Marfan syndrome

What tends to happen with zonule failure in those with Marfan syndrome?

Have dislocated lenses in one or both eyes (Ectopia Lentis)

What is the zonule failure in those with Marfan syndrome attributable to?

Mutation in Fibrillin gene

How many people with Marfan syndrome develop Ectopia Lentis?

About 6 in 10 people

How does the elasticity of the lens capsule compare to the lens?

2000X more elastic than the lens

What does the lens capsule do during accommodation?

Reshapes the lens upon zonule relaxation

What does the lens capsule change about the lens during accommodation?

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

What is the lens capsule?

A “thin” transparent elastic membrane

What is the lens capsule secreted by?

Lens and epithelial fiber cells

Where is the lens capsule the thickest?

Thickest just anterior and posterior to the equatorial region of the lens

Where is the lens capsule the thinnest?

Posteriorly

What is the lens capsule composed of?

Extracellular proteins consistent with basement membranes (it is a basement membrane)

What are the extracellular proteins the comprise the lens capsule?

Collagen type IV, Laminin, Heparin sulfate proteoglycans

Which specific heparin sulfate proteoglycans comprise the lens capsule?

Perlecan, Nidogen, Collagen type XVIII

How is the lens capsule attached to the lens?

Lens integrins associate with laminin network of ECM

What is the characterization (fast/slow) of the turnover rate of the lens capsule ECM proteins?

Turnover at a very slow rate compared to the basement membrane of other epithelia

What is the turnover rate of lens capsule? Other epithelia?

Lens capsule: months-years Other epithelia: Hours

What is the lens subject to because of the slow turnover rate and its thickness? When can it be exacerbated?

Subject to pathological changes that increase the lens capsule stiffness due to lysyl oxidase activity and crosslinking; Exacerbated if a lot of glucose present

What is presbyopia?

Gradual age related loss of accommodative amplitude

What is the rate of linear decline in presbyopia?

~2.5D/10 years

What is presbyopia due to generally?

Age-related changes in accommodative apparatus

How does the lens capsule thickness change from birth through young adulthood?

General increase in capsule thickness

How does the lens capsule thickness change in aging adults?

Occurs regionally

Where does the lens capsule increase in thickness in aging adults?

Thicker anteriorly (thinner posteriorly)

How does lens capsule elasticity change with age?

Lens capsule elasticity decreases

Why does lens capsule elasticity decrease with age? (two reasons)

1. Non-enzymatic glycation of collagen IV which increases stiffness

2. Extremely slow molecular turnover rates of lens capsule components

What can a prolonged increase in aqueous humor glucose induce in the lens capsule?

Can induce intra- and intermolecular cross-linking between lysine and amine groups (glycation events)

What specific component of the lens capsule is susceptible to glycation?

Collagen type IV

What exists in individuals with type 1 diabetes due to hyperglycemia?

Lowered amplitude of accommodation when compared with age-matched controls (capsule can no longer facilitate accommodation to a normal degree)

What contributes the most to presbyopia, stiffening of lens itself or the stiffening of the lens capsule?

Stiffening of the lens itself

How are zonules affected or unaffected by age? (two things)

1. No elasticity change

2. Zonular/Capsule insertion distance to the lens equator increases with age (more fanning out)

Why does the zonular/capsule inserstion distance to the lens equator increase with age?

Due to growth of underlying lens

How are ciliary muscles affected or unaffected by age?

1. Loss of muscle fibers and increase in connective tissue

2. Contractile force does NOT decrease with age

After accommodative loss, movement of ciliary body blank.

Still occurs

How do we know that after accommodative loss, movement of ciliary body still occurs?

Ciliary ring diameter decrease still occurs in older subjects

If the ciliary body contraction still occurs in older individuals, then why does presbyopia occur?

Because the lens does not change shape to allow accommodation to occur

How does the mass of the lens change with age?

Mass increases linearly with age

Why does the mass of the lens increase linearly with age?

Due to the increase in lens cell number without any cell loss

What aspects of the lens change due to the increase in mass with age? (two things)

Increase in axial thickness, increase in anterior and posterior curvature

What aspects of the lens do NOT change due to the increase in mass with age?

Equatorial lens diameter in the disaccommodated state

What is the lens paradox (related to refraction change due to aging)

Aged lens appears to be similar in shape to an accommodated lens, yet near vision is lost

What are the 2 reasons that explain the lens paradox?

1. Thickness differences

2. A decrease in refractive index near equatorial region of the lens with age (unsure why)

What are the thickness differences that explain the lens paradox? (two things)

1. With aging, thickness increases in the cortical layers only (not enough growth in thickness to compensate)

2. With accommodation, thickness of the entire lens (nucleus too) increases

What age related change is thought of as the primary reason for accommodative failure?

Changes in the lens stiffness

How does the pattern of lens stiffness increase due to age related changes?

Stiffness increases continue long after accommodative ability fails

Why does lens stiffness affect accommodation?

If the lens is not pliable it cannot be shaped by the lens capsule