PHYSIOLOGY - Accomodation Flashcards
What is the main refractive power of cornea and lens?
Cornea - +45D
Lens - +15D
Total = + 60D
What are the differences in light rays towards the eye from a distant object vs a near object?
distant object - parallel rays from infinity towards eye
near object - diverging rays from object towards eye therefore requires more converging power to focus on retina.
What is the definition of accomodation?
The necessary increase in the conergence power of the eyeball by increasing the refractive power of the crystalline lens, by increasing the curvature of its anterior surface –> accomodation.
How much does the anterior curvature radius change when in accomodation?
anterior surface radius curvature changes from 10mm to 6mm
posterior surface radius curvature remains the same at 10mm
What structure connects the ciliary body to the equator of the lens?
the suspensory ligament of lens
What is the mechanism of accomodation?
- M3 muscarinic receptors from parasympathetic supply from short ciliary nerves from ciliary ganglion cause ciliary muscle contraction.
- ciliary muscle contraction causes to pull itself forward and releases tension on zonular fibres/suspensory ligaments causing them to relax
- lens becomes free to move (lens capsule is elastic) and so moves forward so increases anterior convexity of lens so lens becomes globular, allowing for lens to focus diverging rays onto retina.
What is the near point and far point?
near point - punctum proximum
far point - punctum remotum
near point is at which small objects can be accurately focused on the eye
far point is the farthest a person can see.
What is the near point? What is it affected by?
What is the far point? What is it affected by?
Near point - small objects can be accurately focused. Affected by age.
Far point - the farthest a person can see. Affected by refractive status - emmetropia is infinity.
What is the difference between far point and near point called? How is it measured by?
range.
Measured by RAF rule
What is the amplitude of accomodation?
Difference of REFRACTIVE power of the eye when accomodated (recpirocal near point) minus rest (reciprocal far point)
What is the amplitude of accomodation at 5 years old vs 75 years old.
see table.
What is the near triad?
accomodation
pupillary constriction
convergence
What happens to the axial width of the lens during accomodation? What happens to the equatorial diameter of the lens during accomodation?
axial width increases
equatorial diameter decreases
What is the effect of muscarinic agonists on accomodation? What effect does it have on pressure?
Pilocarpine produces accomodation by acting on the ciliary muscle.
It also by traction on the trabecular meshwork fibres via the scleral spur to aid aqueous outflow to reduce pressure
What is the AC/A ratio?
What ratio is normal?
the number of prism diopters of convergence induced by each diopter of accomodation
Normal range is 3:1 to 5:1
