Optical System II Flashcards
What is the function of CN IV?
Trochlear Nerve:
GSE = Innervates the Superior Oblique Muscle
Moves the eyes down and out
What is the function of CN III?
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
What is the function of CN VI?
Abducens Nerve:
GSE = Innervates the Lateral Rectus Muscle
Abducts the eyes
What is the function of levator palpebrae superioris?
Innervated by CN III (superior division), it opens the eyes
Has partial sympathetic innervation (reason for pseudoptosis of Horner’s syndrome)
What is the function of Superior Rectus?
Innervated by CN III (superior division), it elevates the eye with slight adduction
Tested by moving the eye up and out
What is the function of Inferior Rectus?
Innervated by CN III (inferior division), it depresses the eye with slight adduction
Tested by moving the eye down and out
What is the function of Medial rectus?
Innervated by CN III (inferior division), it adducts the eye
Tested by adducting the eye
What is the function of Inferior Oblique?
Innervated by CN III (inferior division), it moves the pupil up and out
Tested by moving the eye up and in
What is the function of Superior Oblique?
Innervated by CN IV, it moves the pupil down and out
Tested by moving the eye down and in
What is the function of lateral rectus?
Innervated by CN VI, it abducts the eye
Tested by abducting the eye
What will occur with a left CN III paralysis?
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
What occurs with a Left CN VI paralysis?
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
What is esotropia?
Deviation of visual axis of one eye towards that of the other
What is strabismus?
Deviation of one eye from parallelism with the other
What is diplopia?
double vision
What will occur with damage to CN IV?
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)
How do patients with Trochlear nerve palsy move their heads to reduce diplopia?
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
What are the three actions of the accomodation reflex?
- Convergence
- Accommodation (increased lens thickness)
- Constriction of pupil
What determines the amount of refraction of light?
- Difference between the velocities of light in the two media
- angle that the light hits the interface between the two media
What is a refractive index?
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
What is the definition of infinity for light viewed by a point source?
Infinity is reached when the light rays from a point source appear to be parallel
- 20 feet or farther from the eye is sufficient
What is the focal point of a lens?
The spot where parallel, axial rays from infinity are focused (or appear to be focused) at a common point
What is the focal distance?
Distance from the center of the lens to the focal point
What are properties of a converging lens?
- 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
What are properties of a diverging lens?
- 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
What are properties of a cylindrical lens?
- shaped like half of a cylinder
- lens bends light only in one plane (no vertical bending)
What is the refractive power of a lens?
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
What is the sign of the refractive power of a converging and diverging lens?
Converging refractive power >0
Diverging refractive power < 0
Where does the most refraction take place on the eyeball?
on the cornea
- this is the greatest change in velocity (between air and tissue)
Why does the lens change its refractive power?
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
How does accommodation work?
- 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
What is presbyopia?
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
What is emmetropia? What occurs when an object is at infinity?
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
What is myopia? What happens when an object is at infinity?
- 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)
What is used to treat myopia?
A negative (diverging) lens
What is hyperopia? What happens when an object is at infinity?
- 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)
What is used to treat hyperopia?
Positive (converging) lens
What is astigmatism?
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
What is used to treat astigmatism?
A cylindrical lens
–> lens is oriented to bend light in vertical direction only
