Mechanism of Accommodation

Question 1

List the structures involved in accommodation (5)

Answer 1

1. Ciliary body (muscle)
2. Choroid
3. Iris
4. Anterior and Posterior Zones
5. Crystalline Lens (capsule)

Question 2

What are the 3 parts of the ciliary muscle?

Answer 2

1. Longitudinal (Brucke’s)
2. Radial
3. Circular/Annular (Circumferential)

Question 3

What type of muscle is the ciliary muscle?

Answer 3

Smooth muscle

Question 4

What innervates the ciliary muscle?

Answer 4

Both parasympathetic (dominant) and sympathetic

Question 5

What is parasympathetic innervation responsible for in the ciliary muscle?

Answer 5

Contraction of the ciliary muscle fibers

Question 6

What is sympathetic innervation responsible for in the ciliary muscle?

Answer 6

Induces relaxation

Question 7

Why are both PNS and SNS needed for ciliary muscle?

Answer 7

Allows for smooth accommodative tracking.

Ciliary muscle acts as its own antagonist.

Question 8

Where do the longitudinal muscle fibers originate from?

Answer 8

“Epichoroidal Stars” that attach the muscle to the outer surface of the choroid and continue forward to the sclera spur

Question 9

Where do the tendons of the longitudinal ciliary muscle fibers continue to after scleral spur attachment?

Answer 9

Trabecular meshwork

Question 10

What happens to the ciliary muscles during accommodation?

Answer 10

All 3 contract - there is a gradual rearrangement of the fibers (longitudinal and radial) to transition into circular/annular fibers

Question 11

Which fibers (long., radial, or circular) have the greatest contribution to the changing lens shape during accommodation?

Answer 11

Circular/Annular

Question 12

What happens to the anterior choroid during accommodation?

Answer 12

Contraction of the entire ciliary muscle pulls the anterior choroid forward

Question 13

What is the result of the anterior choroid being pulled forward during accommodation?

Answer 13

Ciliary muscle releases resting tension on the zonular fibers

Question 14

At rest, are the zonular fibers relaxed or tense?

Answer 14

Tense

Question 15

What happens to the scleral spur during accommodation?

Answer 15

Moves posteriorly and enhances flow across the entire trabecular meshwork by opening up flow channels, increasing effective filtration area

Question 16

What happens to the scleral spur during relaxed state?

Answer 16

Returns to anatomical position and decreases flow across trabecular meshwork by closing flow channels, decreasing effective filtration area

Question 17

When is the effective filtration area increased?

Answer 17

Accommodation

Question 18

When is the effective filtration area decreased?

Answer 18

Relaxed state

Question 19

What is responsible for an increased effective filtration area?

Answer 19

Posterior movement of scleral spur during accommodation and opening flow channels in trabecular meshwork

Question 20

What path do zonular fibers channel to the lens capsule?

Answer 20

Through the valleys between adjacent processes in the pars plicata

Question 21

What type of fibers are zonules?

Answer 21

Elastin-based fibers

Question 22

Where do zonular fibers come from?

Answer 22

Secreted by non-pigmented ciliary epithelium in valleys between ciliary processes

Question 23

What is the primary function of zonules?

Answer 23

Stabilize the lens

Question 24

What is secreted from the non-pigmented ciliary epithelium at the tips of ciliary processes?

Answer 24

Aqueous humor

Question 25

What are the divisions of the zonules?

Answer 25

Anterior Zonules (Anterior, Equatorial, and Posterior) and Posterior Zonules

Question 26

What is the function of the posterior zonules?

Answer 26

Aid in pulling the ciliary muscle back to its unaccommodated state position after accommodation has ceased

Question 27

Where do posterior zonules originate?

Answer 27

Non-pigmented ciliary epithelium near the ora serrata

Question 28

Where do anterior and posterior zonules attach?

Answer 28

Anterior: Superficial Anterior, Equator, and Posterior capsule
Posterior: non-pigmented ciliary epithelium in valleys or walls of ciliary processes

Question 29

Where is tension located when eyes are relaxed?

Answer 29

Anterior zonular fibers - pulling lens outwardly and keeping it flat

Question 30

Where is tension located when eyes are accommodated?

Answer 30

Posterior zonular fibers - contraction of ciliary body pulls ciliary body forward and releases tension on anterior zonular fibers

Question 31

What is the shape of the lens like during accommodation? What molds the lens into this shape?

Answer 31

Spherical shape; molded by capsule of lens

Question 32

What does the spherical shape of the lens provide?

Answer 32

More + power

Question 33

Explain what increases/decreases during accommodation:
Lens diameter
Lens thickness
Anterior curvature
Anterior chamber depth

Answer 33

Lens diameter decreases
Lens thickness increases
Anterior curvature increases
Anterior chamber depth increases

Question 34

What is responsible for pulling the ciliary body posteriorly upon cessation of accommodation?

Answer 34

Elasticity of the ciliary muscle to the choroid and tension of the posterior zonules

Question 35

What supplies the major force to accommodate the lens?

Answer 35

Lens capsule

Question 36

What are the 2 functions of the lens content

Answer 36

1. Reduce the rate of capsule movement

2. Retard thee extent of change in lens configuration

Question 37

List 3 physiological responses to accommodative processes. What are these called all together?

Answer 37

1. Pupillary constriction
2. Convergence of the eyes
3. Accommodation
   “Accommodative Triad” or “Near Reflex”

Question 38

What structure sends out preganglionic parasympathetic fibers for the accommodative triad?

Answer 38

Edinger-Westphal Nucleus

Question 39

What is presbyopia? (simple terms)

Answer 39

Age-related loss of accommodation

Question 40

What causes presbyopia?

Answer 40

Deterioration of the accommodative apparatus; multifactorial

Question 41

How do you correct presbyopia?

Answer 41

Bifocals