Special Senses Vision Flashcards
Describe light rays and the eye for objects nearby
Light rays are still diverging
It takes more refraction (bending) to make diverging rays focus on the retina
Describe the cornea and refraction
The cornea is the first site in the eye where refraction occurs
It bends the light the most of any structure in the eye and accounts for 2/3 of the ability of the eye to bend light
Amount of bending is fixed
What does accommodation require?
Ciliary muscle
Suspensory ligaments
Lens
What does near vision require?
Requires contraction of the ciliary muscle
As the ciliary muscle contracts, the tension on the suspensory ligaments is released
The suspensory ligaments become slack
The natural elasticity of the lens causes the lens to become rounder
The increased curvature of the lens increases the refractive power of the lens
Describe contraction of the ciliary muscle and diopters during life
In young kids, this adds 20 diopters
In young adults, loss of elasticity results in strength of 10 diopters
By the 50’s, loss of elasticity results in 1 diopter
What does increased curvature of the lens do to light rays?
Allows the light rays from a nearby object to be bent sufficiently to bring them into focus on the retina
What does far vision require?
Requires relaxation of the ciliary muscle
As the ciliary muscle relaxes, the tension on the suspensory ligaments is increased
Suspensory ligaments become taut, which pulls the lens into a flat position
The decreased curvature of the lens decreases the refractive power of the lens
Besides contraction of the ciliary muscles, what else does near vision require?
Convergence of the eyes to point of focus
Constriction of the pupil
How does constriction of the pupil help near vision?
By constricting the pupil, the opening for light to enter is reduced
This eliminates some of the diverging light rays and allows better focus
What is the vertical pathway of the retina composed of?
Photoreceptor
Bipolar cell
Ganglion cell
Describe photoreceptors and release of glutamate
When it is dark (no stimulation), the release of glutamate is high
When a photon activates the photoreceptor, the release of glutamate decreases
Light hyperpolarizes rods and cones
Describe vertical pathway for cones
Photoreceptor is struck by a photon of light
This decreases glutamate release onto a bipolar cell
Describe on-center bipolar cells
Activation of a photoreceptor in the center of the bipolar cell’s receptive field causes depolarization of the bipolar cell
Activation of a photoreceptor in the surrounding region causes hyperpolarization of the bipolar cell
Describe off-center bipolar cells
Activation of a photoreceptor in the center of the bipolar’s receptive field causes hyperpolarization of the bipolar cell
Activation of a photoreceptor in the surrounding region causes depolarization of the bipolar cell
What happens if a photoreceptor activates an on-center bipolar cell?
A metabotropic receptor is activated
Neurotransmitter release causes a decrease in cation influx
In the dark, the on-center cell is hyperpolarized
With light, release of glutamate from the photoreceptor decreases, so the cation current increases, and the on-center cell is depolarized
What happens when the photoreceptor activates an off-center bipolar cell?
An AMPA receptor is activated
Neurotransmitter release causes an increase in cation influx
In the dark, the off-center cell is depolarized
With light, glutamate release from the photoreceptor decreases, the AMPA receptor is not activated, and the off-center cell is hyperpolarized
Describe on-center and off-center ganglion cells
Cells are what the connecting bipolar cells is
The bipolar cell releases glutamate to excite the ganglion cell
Ganglion cell axons become fibers on the optic nerve
Cause action potentials
Relies on glutamate as neurotransmitter
Describe the vertical pathway for rods
Composed of many rods
Converge on one on-center bipolar cell
Which then synapses on an A11 amacrine cell
Which synapses on a cone on-center bipolar cell
Which then activates on a ganglion cell
Rods do well in low-light situations
What is the role of on- and off-center cells?
On-center cell tells us where something is
Off-center cell tells us where it ends
What do amacrine and horizontal cells do?
Provide inhibitory signals that modify the activity of neighboring photoreceptors, bipolar cells, or ganglion cells
The retina from each eye sees ____ visual fields
Two different
Describe activation of temporal retina and nasal retina
The left temporal retina is activated by light coming from the right visual field as is the right nasal retina
The right temporal retina is activated by light coming from the left visual field as is the left nasal retina
Describe the axons in the optic chiasm
The axons from the nasal retina on each side will cross and join the axons from the temporal retina
All the information from the right visual field is sent to the same part of the brain (in the case of the right visual field, the left side of the brain)
What are the functions of the lateral geniculate body?
Control the motions of the eyes, so they converge on the desired point of interest (convergence/divergence)
Control focusing so that the image we are interested in is in best focus possible
Identify major elements within our visual image and send that identification to the cortex
Identify motion within our visual image
Describe the 6 layers of the primary visual cortex
Layers 1, 2, and 3 allow networking between primary visual cortex and other parts of the cortex
Layer 4 receives inputs from the lateral geniculate nucleus
Layers 5 and 6 send information back to LGN
Describe the columns in the cortex
Cortex is composed of columns that start from surface of cortex and extend through all 6 layers
Each column does a different job
Neighboring columns have related jobs
If one column detects horizontal edges, neighboring column detects edges oriented at 30 degrees
What is the major job of the primary visual cortex V1?
Identify the edges/contours of components in our visual image
What is the major job of V2?
Identify disparities in the visual images presented by the two eyes
Disparities between the two eyes are used for depth perception
What are blobs?
Collections of neurons within the columns
Enable color detection
What is the major role of V4?
Complete processing of color inputs
What does the dorsal pathway that leaves the occipital cortex do?
Relays visual information to the motor cortices and enables us to complete motor acts based on visual input
What does the ventral pathway do?
Relays visual information to areas of the brain involved in higher processing of sensory inputs
Integration of visual input with other sensory inputs allows us to use that visual image to accomplish higher order functions, including naming the object and copying the object
Describe the light rays and the eye for objects far away
Light rays have diverged far enough that only parallel light rays enter the eye
Therefore, it doesn’t take much refraction to make parallel rays focus on retina
