The Visual System Flashcards
What is light?
Electromagnetic energy that moves in waves
The color white is ______
A mix of wavelengths
A single wavelength is a ______
Color
The highest frequency color is ____
Blue
The lowest frequency color is ____
Red
What is the pupil?
The opening that allows light to enter
What is the iris?
Two muscles that control the diameter of the pupil
What is the cornea?
Transparent cover over the eye with a layer of fluid underneath
What is the sclera?
The white walls of the eye
What are the 3 parts of the retina?
1.) Optic Disc
2.) Macula
3.) Fovea
What is the optic disc?
Origin point of retinal blood vessels
What is the macula?
Region of retina for central vision
What is the fovea?
Thin central point on the retina
What is the light’s path through the eye?
Through the pupil onto the lens
How is the lens held in place?
By zonule fibers attached to the ciliary muscle
What causes a refraction of light?
Corneal fluid
What is the purpose of corneal fluid?
To focus light onto the retina at the back of the eye
What is responsible for light accommodation?
The lens
Why is the lens important?
To see things closer than ~9 meters away
What has a smaller refraction power? Lens or Cornea?
Lens
What are the three cell types that compromise the retina’s direct path for visual processing?
1.) Photoreceptors
2.) Bipolar cells
3.) Ganglion cells
What do ganglion cells do?
Output to form the optic nerve
What are the 3 types of retinal cells influenced by?
1.) Horizontal cells
2.) Amacrine cells
What do rods respond to?
All wavelengths in the visual spectrum; cannot distinguish color
What do cones respond to?
Specific and unique wavelengths in the visual spectrum
How many rods are in each eye?
90-120 million
How many cones are in each eye?
5-7 million
What are rods and cones comprised of?
Stacks of membranous discs
What is contained in discs?
Photopigments that are sensitive to light
Why can rods not distinguish colors?
All rods have the same photopigments
The fovea has a high density of ______
Cone cells
What is the fovea responsible for?
Precise visual acuity
Why does the fovea have precise visual acuity?
Light can strike the cones without passing through other layers
What is the resting voltage in rods and what is it due to?
-30mV due to dark current
What do the membranous discs in rods contain?
Rhodopsin
What is rhodopsin?
A photopigment that is a combination of opsin and retinal
What is Retinal derived from?
Vitamin A
What is bleaching?
Light striking rhodopsin changes retinal and activates the opsin protein
In bleaching, what color does rhodopsin change to?
From purple to yellow
What happens when opsin is triggered?
Intracellular signals close Na+ channels and hyperpolarize the rod cell
Rods _______ easily
Saturate
In daylight, vision is dependent on ______
Cones
Why does vision depend on cones in daylight?
Cone photopigments require more light to bleach
What colors do cones preferentially respond to?
Blue, green, or red light wavelengths
Light induces _______
Hyperpolarization
What happens when rods and cones are hyperpolarized?
It reduces the amount of glutamate NT released
What happens when there is a glutamate reduction?
Depolarization of bipolar cells, which then excite ganglion cells
Every ganglion neuron has a ________
Receptive field
What is a receptive field?
Area on the retina where light will induce an increase or decrease in ganglion neuron firing rate
What is dark-light adaptation?
Pupil dilation/constriction alters the amount of light striking the retinae.
This alters levels of unbleached rhodopsin, which creates changes in retinal circuitry
What is the Retinofugal pathway?
Pathway away from the retina to the brain
What does ‘fugal’ mean?
To flee from
What 3 things does the retinofugal pathway consist of?
1.) Optic Nerve
2.) Optic chiasm
3.) Optic tract
Where does the Optic nerve travel?
Begins at the ganglion cells, out the back of the eyes and enters through the skull
How does the optic chiasm work?
The nasal half of the ipsilateral eye combines with temporal half of the contralateral eye to create the visual field
Nerve axons from the _______ cross over at the chiasm
Temporal fields
What does decussation mean?
The crossing over of nerve fibers or pathways in the central nervous system to the opposite side
What is the binocular visual field?
The portion that is seen by both eyes
What are the temporal portions of the visual field?
The portion that is unique to each eye
How does temporal portions of the visual field travel?
It crosses over to the contralateral optic tracts at the optic chiasm. It is then processed in the contralateral hemisphere of the brain
What is the optic tract?
Pathways leaving the optic chiasm
What does the optic tract do?
Carries visual field information
What is the main location target of the optic tract?
Lateral Geniculate Nucleus of the thalamus
Where does the Lateral Geniculate Nucleus output to?
The visual cortex in the occipital lobe
What is the pathway of the LGN?
Optic radiation
Where else does the optic tract lead to in the brain?
Hypothalamus and midbrain
What is the role of the hypothalamus in the visual system?
Involved in waking/sleeping in sync with light/dark cycle
What is the role of the midbrain in the visual system?
Influences pupil dilation and some eye movements
Also houses the superior colliculus
What is the role of the superior colliculus in the visual system?
Involved in controlling saccadic eye movements
What are saccadic eye movements?
Quick movements of the eyes to center a novel object over the fovea
Which of Brodmann’s areas are involved in the visual cortex?
17 and 18
What is the secondary visual cortex?
Involved in visual sensory memory and the ability to relate what is being seen to things that have been seen before
What does farsightedness/hyperopia say about the eyeball?
The eyeball is too short
How do you correct hyperopia?
Convex corrective lens focuses light onto nearer retinae
What does nearsightedness/myopia say about the eyeball?
The eyeball is too long
How do you correct myopia?
Concave corrective lens focuses light onto more distant retinae
What is LASIK?
Surgical reshaping of the cornea to provide proper light refraction onto the retina.
The flap of the cornea is cut and cornea is reshaped from the inside by the laser
What does LASIK stand for?
Laser-Assisted In Situ Keratomileusis
What two injuries are often associated with optic nerve injuries?
1.) Multiple Sclerosis
2.) Diabetes Mellitus
