Optical System II

Question 1

What is the function of CN IV?

Answer 1

Trochlear Nerve:

GSE = Innervates the Superior Oblique Muscle

Moves the eyes down and out

Question 2

What is the function of CN III?

Answer 2

Oculomotor Nerve:

GSE = Innervates 5 of 7 extraocular muscles

GVE = Preganglionic parasympathetic axons synapse in ciliary ganglion, postganglionic axons innervate the sphincter pupillae muscle and cilliary muscle

Question 3

What is the function of CN VI?

Answer 3

Abducens Nerve:

GSE = Innervates the Lateral Rectus Muscle

Abducts the eyes

Question 4

What is the function of levator palpebrae superioris?

Answer 4

Innervated by CN III (superior division), it opens the eyes

Has partial sympathetic innervation (reason for pseudoptosis of Horner’s syndrome)

Question 5

What is the function of Superior Rectus?

Answer 5

Innervated by CN III (superior division), it elevates the eye with slight adduction

Tested by moving the eye up and out

Question 6

What is the function of Inferior Rectus?

Answer 6

Innervated by CN III (inferior division), it depresses the eye with slight adduction

Tested by moving the eye down and out

Question 7

What is the function of Medial rectus?

Answer 7

Innervated by CN III (inferior division), it adducts the eye

Tested by adducting the eye

Question 8

What is the function of Inferior Oblique?

Answer 8

Innervated by CN III (inferior division), it moves the pupil up and out

Tested by moving the eye up and in

Question 9

What is the function of Superior Oblique?

Answer 9

Innervated by CN IV, it moves the pupil down and out

Tested by moving the eye down and in

Question 10

What is the function of lateral rectus?

Answer 10

Innervated by CN VI, it abducts the eye

Tested by abducting the eye

Question 11

What will occur with a left CN III paralysis?

Answer 11

Ptosis
(Levator palpebrae superioris)

Abducted and downward eye position
(only lateral rectus and sup. oblique working)

Dilation and loss of accommodation
(parasympathetic nerve loss)

–> All while looking straight ahead

Question 12

What occurs with a Left CN VI paralysis?

Answer 12

Abducens Nerve: paralysis of left lateral rectus

Looking to the Right: Eyes are conjugate

Looking Straight ahead: Esotropia appears

Looking to the left: Esotropia maximum

Question 13

What is esotropia?

Answer 13

Deviation of visual axis of one eye towards that of the other

Question 14

What is strabismus?

Answer 14

Deviation of one eye from parallelism with the other

Question 15

What is diplopia?

Answer 15

double vision

Question 16

What will occur with damage to CN IV?

Answer 16

Trochlear nerve: Paralysis of Sup. Oblique Muscle

• Extorsion of affected eye
• Deficit produces an oblique diplopia
  maximum when in a downward gaze to the opposite side
  (eye cannot look downward when turned inward)

Question 17

How do patients with Trochlear nerve palsy move their heads to reduce diplopia?

Answer 17

tilt their heads to the unaffected side - causing the normal eye to intort and align with extorted eye

–> Superior oblique paralysis results in affected eye to become extorted, resulting in unalignment of the eyes and diplopia

Question 18

What are the three actions of the accomodation reflex?

Answer 18

1. Convergence
2. Accommodation (increased lens thickness)
3. Constriction of pupil

Question 19

What determines the amount of refraction of light?

Answer 19

• Difference between the velocities of light in the two media
• angle that the light hits the interface between the two media

Question 20

What is a refractive index?

Answer 20

It describes the velocity of light in a substance

Refractive Index = (Velocity of light in air)/(Velocity of light in substance)

–> Refractive index is always >1

Question 21

What is the definition of infinity for light viewed by a point source?

Answer 21

Infinity is reached when the light rays from a point source appear to be parallel

• 20 feet or farther from the eye is sufficient

Question 22

What is the focal point of a lens?

Answer 22

The spot where parallel, axial rays from infinity are focused (or appear to be focused) at a common point

Question 23

What is the focal distance?

Answer 23

Distance from the center of the lens to the focal point

Question 24

What are properties of a converging lens?

Answer 24

• Convex lens
• Thick in the center and thin at the edges
• focuses axial light from infinity to a focal point
• Type of lens in the eye

Question 25

What are properties of a diverging lens?

Answer 25

• Concave lens
• thin in the center and thick at the edges
• light from infinity is not focused at a point
• light passing through the lens diverges
• create an imaginary focal point on the side of the light source

Question 26

What are properties of a cylindrical lens?

Answer 26

• shaped like half of a cylinder
• lens bends light only in one plane (no vertical bending)

Question 27

What is the refractive power of a lens?

Answer 27

Power = 1/f

f = focal length

in D (diopters)

–> Strong lenses bend light more and have short focal lengths

–> Weak lenses bend light less and have longer focal lengths

Question 28

What is the sign of the refractive power of a converging and diverging lens?

Answer 28

Converging refractive power >0

Diverging refractive power < 0

Question 29

Where does the most refraction take place on the eyeball?

Answer 29

on the cornea

• this is the greatest change in velocity (between air and tissue)

Question 30

Why does the lens change its refractive power?

Answer 30

For objects far from the eye, the lens needs to be weak (~20D)

For objects close to the eyes the lens needs to be strong (~34D)

In order to focus the object on the retina

This is called accommodation

Question 31

How does accommodation work?

Answer 31

• Ciliary muscles contract; which revlieves tension on lens from zonal fibers
• -> lens becomes fatter, chubbier, optically stronger
• only the anterior portion of the lens moves

Question 32

What is presbyopia?

Answer 32

Recession of the near point and loss of ability to accommodate with age

• lens becomes more elastic and less able to expand–> ciliary muscle contracts, but lens doesn’t change shape

Question 33

What is emmetropia? What occurs when an object is at infinity?

Answer 33

Normal vision

• parallel light from infinity is sharply focused on retina
• When an object is at infinity:
  ciliary muscle is relaxed
  lens weak and flat
  Image is focused on fovea

Question 34

What is myopia? What happens when an object is at infinity?

Answer 34

• Nearsighted
• Parallel light from infinity is focused in front of the retina
• Eyeball is elongated compared to normal
• cornea is generally normal
• Object at infinity:
  Ciliary muscle relaxed
  Lens weak and flat
  Image focused in front of retina (fuzzy image)

Question 35

What is used to treat myopia?

Answer 35

A negative (diverging) lens

Question 36

What is hyperopia? What happens when an object is at infinity?

Answer 36

• Farsighted
• Light from infinity is focused behind retina
• eyeball is shortened compared to normal
• Cornea is normal
• Object at infinity:
  without accommodation
  ciliary muscle relaxed
  lens weak and flat
  image focused behind the retina (fuzzy)

with accommodation
ciliary muscle contracts
lens becomes chubby and optically stronger
image focused on retina (clear)

Question 37

What is used to treat hyperopia?

Answer 37

Positive (converging) lens

Question 38

What is astigmatism?

Answer 38

Results from asymmetrical curvature of cornea (although, can be from curvature of lens)

• No single point where all rays of light are focused
  image is blurred at any distance and accommodation won’t help

Question 39

What is used to treat astigmatism?

Answer 39

A cylindrical lens

–> lens is oriented to bend light in vertical direction only