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Chapter 2

PSYB57H3 Chapter Notes - Chapter 2: Vitreous Body, Pupillary Light Reflex, Ciliary Muscle

Course Code
George Cree

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Chapter 2 The First Steps in Vision: From Light to
Neural Signals
A Little Light Physics
Two ways to conceptualize light:
o Wave
An oscillation that travels through a medium by transferring energy from
one particle or point to another without causing any permanent
displacement of the medium
Visible light waves have wavelengths between (violet) 400 and (red) 700
nanometres (nm; nm=10-9 metre)
o A stream of photons
Tiny particles that each consist of one quantum of energy
In empty space the electromagnetic radiation from a star travels in a straight line at the
speed of light (about 186,000 miles per second)
o Once it reaches the atmosphere:
Some light will be absorbed or scattered (by encounters with dust)
Most will make it through the atmosphere and will eventually hit the
surface of an object
If it hits something light-surfaced, most of the light is reflected
If it hits something dark-surfaced, most of the light is absorbed
If light is neither reflected or absorbed, it will be bent or refracted
Eyes That See Light
o The first tissue that light encounters when it reaches the eye
o Has a rich supply of transparent sensory nerve endings
There to force the eyes to close and tears if the cornea is scratched
Aqueous Humor
o A fluid derived from blood
o Fills the space immediately behind the cornea
o Supplies oxygen and nutrients to and removes waste from the cornea and the
crystalline lens
o Completely transparent
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o Shape of the lens is controlled by the ciliary muscle
o A hole in a muscular structure called the iris
o Controls the amount of light that reaches the retina (via the pupillary light reflex)
o Gives the eye its distinctive colour
o Iris automatically expands and contracts to allow more less light into the eye
o Photi seeze refle
Vitreous humor
o Light is refracted for the fourth and final time
o Comprises 80% of the internal volume of the eye
o Image is brought into focus here
o Only some of the light reaches the retina
Good deal of light is lost in the eyeball
o Detet light ad tell the rai about aspects of light that are related to objects
i the orld
Focusing Light onto the Retina
Cornea forms the most refractive surface in the eye
o Aqueous and vitreous humors also help refract light
o Lens can alter the refractive power by changing its shape
Accomplished through contraction of the ciliary muscle
Attahed to the iliar usles through zoules of )i ti fires
When the ciliary muscle is relaxed
o Zonules are stretched
o Lens is relatively flat
o Eye will be focused
o Eye will be focused on very distant objects
When the ciliary muscle contracts
o Reduced tension on zonules
o Enables the lens to bulge
o Allows us to focus on something closer
Accommodation enables the power of the lens to vary by as much as 15 diopters
o Ability declines with age
Starting from about 8 years old, we lose about 1 diopter of
accommodation every five years up to the age of 30
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Presbyopia loss of vision because of insufficient accommodation
Lens becomes sclerotic (harder) and the capsule that encircles the
lens loses its elasticity
o Opacities of the lens (composed of crystallins class of proteins packed very
Caused by irregularity of the crystallins
o Congenital cataracts (present at birth) are relatively rare
o Absorb and scatter more light than the normal lens does
o When the refractive power of the four optic components of the eye are perfectly
matched to the length of the eyeball
Myopia (nearsightedness)
o Occurs if the eyeball is too long
o The image will be focused in front of the retina
o Can be corrected with negative lenses
Hyperopia (farsightedness)
o Occurs if the eyeball is too short
o Image is focused behind the retina
o Can be corrected with positive lenses
Average adult human eye is 24 mm long
o Occurs when the cornea is not spherical
o Vertical lines must be focused slightly in front of the retina, while horizontal lines
are focused slightly behind it (or vice versa)
The Retina
The optics involved are similar to those in most cameras
o stop is aalogous to the iris i hua ees
The purpose of a camera is just to record an image, the purpose of the human visual
system is to interpret the image
Light is transduced into neural energy that can be interpreted in the brain
Ee dotors use a ophthalosope to look at the fudus ak surfae of their patiet’s
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