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Lecture 2

BIO SCI N110 Lecture Notes - Lecture 2: Melanin, Sclera, Choroid

Biological Sciences
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Fri 4.26.19 | The Eye: Structure and Function
The Structure of the Eye
Resolution vs Sensitivity
Resolution (acuity): The ability to detect fine details
Sensitivity: The ability to detect faint light
An ideal eye would have both, but in reality optimizing one comes at the expense of the other
We are diurnal creatures -- we have higher resolution and low sensitivity
Layers and "Humors"
Clear = want light to go through it
White part, tough, maintains shape
Blood vessels = gets blood through
cells of the eye bc cells need oxygen
Melanin = increases resolution,
decreases sensitivity
Skin pigmentation, also in eye
Melanin is dark = absorbs light
→ light will be absorbed
Good night vision = more
Light sensing part, brain! back of
the eye
Nourishes cornea = liquid in front of
eye, like water
Jelly like helps hold shape, pressure
Controls amount of light entering eye
Eye contracts or dilates → do this in
ER to test neurological reflex
Can also happen if you're excited,
nervous, scared … evolve as a social
signal, focus vision under

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Accommodation (focus)
Focuses light that is coming in
from all directions into the
back of the eye with all the
photoreceptors and can be
Achieved by ciliary muscle
Ppl wear glasses bc
ciliary muscle has
issues focusing the
The eye accommodates for close vision by tightening the ciliary muscles, allowing the lens to become
more rounded
Distance viewing → light rays from distant objects are nearly parallel and don't need as much refraction
to bring them to focus
Lens at minimum strength
Zonule fibers taut
Ciliary muscles relaxed = unaccommodated
Near viewing light rays from close objects diverge and require more refraction for focusing
Lens rounds to greater strength
Fibers slack
Ciliary muscles contracted = accomodated
Clinical Problems: Refractive Errors
Changes in the ability of the lens to round up (accommodate) with age

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Clinical Problems: Cataracts
clouding of the lens in the eye
The Retina
Transduction and Processing of Visual Info
The retina is part of the central nervous system
It transduces patterns of light (photons) into the language of the nervous system
It performs the first steps in the processing of visual info
The Image of the World is Upside Down on the Retina
The Photoreceptors
Two Types: Rods and Cones
Transduction occurs within the photopigment that resides inside the disks at the outer segment of
rods and cones
After being activated, the photoreceptors will be inactivated for >100ms
Rod: very sensitive to light, poor acuity (sharpness of vision), no color good for sensitivity
Cone: less sensitive to light, high acuity, color vision good for resolution
Respond by Hyperpolarization
Intracellular recording from a single cone stimulated by different amounts of light
Flash of light will change the membrane potential depending on the amount of light
Light is actually an inhibitory signal → hyperpolarizing effect
No axons or action potentials in the retina, only subthreshold synaptic potentials (except ganglion
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