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Kyle Eaton

Mechanics, Aging and Neurological Control of Accommodation

Near Response or Near Triad : necessary for shifting one’s gaze from a distant target to a near target

3 parts: Accommodation.

Convergence: aligning eyes to see a single object

Pupil Constriction: helps in accommodation/ helps to focus

Accommodation: ability to focus images of objects at various distances in space onto the retina. i.e. various objects can be made conjugate to the retina.

Conjugacy: refers to objects and images being interchangeable as far as optics is concerned.

Gaussian equation: describes optically objects and images which are conjugate

1/Obj + 1/Im = 1/f = F

Obj: object distance from cornea

Im: axial length of eye, image distance from cornea

f: focal length

F: optical power of eye (lens and cornea), referred to cornea location

Multiple Mechanisms of Accommodation: ways we accommodate

    1. Variable axial length: (normal adult 24mm, normal neonate 17mm) we change the length of our eye as we grow…roughly 20yrs to accomplish this growth of 7mm.

      2. 1mm axial change = 5 Diopters

      In neonates, cornea is more powerful then adults thus babies are hyperopic but as we grow 2 things occur: 1. Cornea becomes flatter-losing power = process of

      2. Eye grows longer emmetropization

      Emmetropization: process by which we match the optics of the eye to the axial length. The ocular growth resulting in axial length increase is stimulated by blur during the first two decades of life.

      Some animals have different mechanisms to change axial length more quickly. For example, eels have a muscle that moves the entire lens forward and back

    2. Corneal Power: 2 ways
      1. Dynamic Acc.: by squinting we increase the corneal power, thus myopes can push the image back onto the retina.
      2. Static Acc.: Astigmatism- in essence astigmatism acts as a bifocal capable for imaging objects at two different distances
    3. Lenticular Power:

Translation: moving the lens forward to increase power or back to decrease power (used

by some animals such as cats, raccoons, most fish). Humans

Lens pinching: changes the curvature of the lens. The front surface of lens is pinched by a

muscle to increase the power (mostly amphibians, some fish, some water fowl as the

cornea underwater has no power)

Lens pulling: changes the curvature of the lens. Flattening and thickening lens changes

power (human mechanism)

Lens Astigmatism: minor contribution

D. Isoindical Surfaces: different indicies within the lens produce gradient within the lens.

-indicies greatest at the core, weakest at the cortex

-gradient decreases with age: less and gradient and power as we age which is fortunate because as we age the lens thickens causing and increase in surface power. The decrease in gradient power offsets the increase in surface power. This is called the lens paradox

and is a form of accommodation which occurs very slowly.
    1. Pupil Size: with a tiny pupil or pinhole, we are able to see great depth; however, it’s not a

great mechanism in low light levels as it cuts out a lot of light. Therefore we

have an adjusting, dynamic mechanism rather than a static one such as a tiny

pupil.

However, we use this principle as we age. After age 20, pupil constricts when

Accommodating as the lens starts losing power.

Anatomy

Lens: an elastic bag filled with very viscous lens fibers

Zonules: support lens in place, attach to equator of lens with some posterior and some anterior

-run to ciliary body passing thru the ciliary processes and anchors near the ora serrata

Ciliary Body: muscle which constricts causing forward movement of choroid and ora serrata and

thus relaxes the tension on the zonules

 

Relaxation Theory of Accommodation-accounts for majority of accommodative ability in humans

To accommodate -ciliary muscle contracts pulling the choroid forward*

-tension on the zonules is released

-lens thickens via pressure from the lens capsule (lens capsule molds the lens)

-lens capsule able to mold the lens because of changes in it’s own thickness

(lens if thinnest and flattest at it’s anterior pole so this is where most of the shape change occurs)

-lens molding only capable when elastic force of the capsule is greater than the

elastic resistence of the lens (presbyopia: when resistance of lens is greater than elastic force of capsule)

*ciliary muscle is anchored to the scleral spur

To unaccommodate -relax the ciliary muscle

-choroid is allowed to pull back via elastic properties

-restores tension on zonules causing lens to flatten

How do we know if anterior pole is changing the most? By observing Purkinje Images

Purkinje Images: special catoptric images which are images reflected from

Optical surfaces

4 images off eye: 2 off cornea which are so close they look like a single image

2 off lens (the 3rd and 4th images) – it’s this third image, off the

front of the lens which changes location during

accommodation, thus we know that the anterior pole changes

curvature the most

Schiener pupil – a double pinhole held close to the eye shows power of eye changes

Neural Control of Accommodation: the autonomic nervous system

-ciliary body is stimulated by parasympathetic innervation from the Edinger-Westphal

nucleus to stimulate increase constriction of ciliary body

-sympathetic innervation via the super cervical ganglion and long posterior ciliary

processes

-these have antagonistic functions: only see sympathetic activity during parasympathetic

activity – sympathetic inhibits accommodation only when accommodation has already

been activated

Presbyopia:

Absolute Presbyopia: the near point equals the far point. Amplitude of accommodation = Zero

Functional Presbyopia: -the near point recedes out beyond the near working distance

-the near working distance requires more than a full amplitude of

accommodation – can accommodate, but not enough to focus at one’s

near working distance.