Accomodation

Question 1

Describe the unaccomodated eye in relation to anatomical structures?

Answer 1

Ciliary muscle relaxed
Zonules under tension
Induce centrifugal force on lens capsule
Stretches the lens, flattening ant and post surfaces (less convex)

Question 2

Describe the accomodated eye in relation to its anatomical structures

Answer 2

Ciliary muscle contracts
Tension on zonules relaxes
Lens becomes more spherical-> increasing its optic power

Question 3

What regulates contraction of the ciliary muscle?

Answer 3

Muscuranic receptors M3 via parasympathetic stimulation

Question 4

Where does the ciliary muscle anchor itself and how does this relate to its movement?

Answer 4

Anteriorly onto scleral spur
Posteriorly not attached to sclera, only choroid
Therefore contraction moves ciliary body anteriorly

Question 5

What are the three layers of the ciliary muscle?

Answer 5

Longitudinal: anterior movement, run parallel to sclera
Radial/oblique : constriction and anterior movement
Circular: ring like constriction

Question 6

What are zonules?

Answer 6

Meshwork of fibres of mucopolysaccahrides, glycoprotein and elastin
Attach like velcro onto outer surface of lens capsule
Secreted by ciliary epithelium
Insert onto 2 rings on lens capsule, some insert equatorially
Aqueous can flow through them into the vitreous

Question 7

Which region of the lens capsule is thickest?

Answer 7

Mid peripheral anterior
next anterior
Equator
Then posterior-thinnest (dangerous in cataract surgery)

All made up of type IV collagen

Question 8

How does the lens change in accomodation?

Answer 8

Bulges anteriorly and posteriorly
Diameter decreases
Anterior radius of curvature increases
Peripheral anterior and posterior lens surfaces don't change
Anterior chamber shallows
Lens shrinks slightly due to gravity

Question 9

What is neural control of accommodation?

Answer 9

Parasympathetic control via CNIII fibres to ciliary muscle
Begins at Edinger Westphal nucleus and travel with occulatormotor nerve and synapse at ciliary ganglion, entering the eye as either the long or short ciliary nerves

Question 10

How do antimuscarinics effect accommodation?

Answer 10

Act as parasympathetic antagonists
Cause cyloplegia, blocking accomodation
Eg. Cyclopentolate and atropine

Question 11

Which nerves control miosis?

Answer 11

Parasympathetic control via CNIII fibres to ciliary muscle
Begins at Edinger Westphal nucleus and travel with occulatormotor nerve and synapse at ciliary ganglion, entering the eye as either the long or short ciliary nerves

Question 12

Which nerves control convergence?

Answer 12

Somatic efferent motor fibres
Originate from 3rd nerve motor nucleus (next to Edinger Westphal)
Fibres travel in 3rd nerve to supply both medial recti

Question 13

What role do sympathetic fibres play in accomodation?

Answer 13

Sympathetic stimulation decreases ciliary muscle tension and reduces amplitude of accomodation

Question 14

What is presbyopia?

Answer 14

Age related reduction in accomodation

Question 15

What are the causes of presbyopia?

Answer 15

Capsule loses elastic properties
Lens increases in diameter, less potential change in zonular tension on ciliary muscle contraction
Loss of smooth muscle cells in ciliary muscle

Question 16

What is the main theory of accommodation?

Answer 16

Helmholtz theory

Question 17

What is the ora serrata?

Answer 17

Where pars planar terminates and transitions to choroid

Question 18

How does the ciliary body action on the scleral spur affect the trabecular meshwork?

Answer 18

When ciliary body contracts, pulls sclera spur posteriorly, increasing the pore size of the trabecular meshwork
Encouraging aqueous outflow into canal of schlemm

Question 19

How do muscarinic agonists work in glaucoma?

Answer 19

Eg. pilocarpine

improve ciliary muscle contraction, opening trabecular meshwork and encouraging aqueous outflow through canal of schlemm