Week 10 - Vision - finished Flashcards
What are the 3 layers of the eye?
Outermost sclera
Choroid
Innermost retina
Where does the light enter the eye and through what structures?
Anteriorly through the cornea and lens to be focussed on the retina where the visual perception begins
What is the function of the cornea and the lens together?
A simple converging lens meaning the image reaches the retina both reversed and inverted.
How much does the lens contribute to the refractive power of the eyes? Where does the rest of the refractive power come from?
15%
The majority of the eyes refractive power comes from the cornea, the lens further aids this process by changing it’s shape, this process is known as accommodation
Are the cells of the retina PNS or CNS cells?
The CNS, because of their development from the optic vesicle of the diencephalon.
What are the 5 basic cell types within the retina? List them from innermost to outermost
Ganglion cells Amacrine cells Horizontal cells Bipolar cells Photoreceptors
Where does visual processing begin?
At the photo receptors at the retina
What are the cells from innermost to outermost that light has to pass through before it gets to the photoreceptors?
Ganglion cells
Amacrine cells
Horizontal cells
Bipolar cells
What are photoreceptors responsive to?
Light
What is the basic pathway through the retina in terms of impulse?
Photoreceptors to the bipolar cells to the ganglion cells which are the cells of the optic nerve
What are the ganglion cells basically cells of?
The optic nerve
What are the 4 cells layers in the retina other than photoreceptors sensitive to?
The chemical change in the photo receptors when they are exposed to light. Before the light reaches the photoreceptors, these cells are unresponsive
What gives a higher degree of differentiation in vision?
Placing greater emphasis on differences in intensity rather than absolute intensities.
What are the 2 key classes of photoreceptors?
Rods and cones
How mich to rods outnumber cones?
20-1
What are rods sensitive to?
Light
When do rods function best?
In dim light or low light conditions (our night vision)
Do rods function much during the day?
No
What kind of information do rods give us?
Information about gross shape and form
Where are rods most highly concentrated?
Around the periphery of the retina
Does the fovea contain rods or cones or both?
Cones only
What kind of information do cones give us?
Detailed nformation about vision and colour
What is the fovea?
The area of highest visual acquity
How much of the information sent to the primary visual cortex comes from the fovea?
Almost half
What factors make the fovea the area of highest visual acquity?
Increased by the displacement of all other retinal cells and their axons. Blood vessels are also displaced at the fovea where the cells receive nutrients via diffusion
What kind of output do rods and cones have? When does this output get inhibited?
They have tonic or basal glutamate output. It gets inhibited my exposure to light.
What are technically the second order neurons in the input of visual information?
The bipolar cells
What makes bipolar cells fire?
They fire in response to decreased glutamate output from the rods and cones
What bipolar cells receive information from rods?
Is this input large or small (do they receive input from a few or many rods)? What does this mean for the visual information that is processed here?
Rod bipolar cells
Rod bipolar cells receive input from several rods giving them a large receptive field, this decreases the clarity of the visual input but increases their sensitivity to light.
What bipolar cells receive information from the cones?
Is this input large or small (do they receive input from a few or many cones)? What does this mean for the visual information that is processed here?
Cone bipolar cells.
Cone bipolar cells typically receive inputs from fewer cone cells, usually around one or two; this helps to maintain high visual acuity
What are the 3rd order neurons of the visual system?
The ganglion cells
Where do the axons of the ganglion cells leave the retina?
Through the optic disc and form the optic nerve
Where do the axons of the ganglion cells become myelinated?
Once they exit the retina and become the optic nerve
Describe the visual pathway:
Hemidecussation of visual information via the optic chiasm.
Only information from the nasal half of each retina decussates.
Axons of the ganglion cells terminate at the lateral geniculate nucleus.
From the lateral geniculate nucleus information travels back to the calcarine cortex in the optic radiations.
Each radiation is divided into a superior and inferior loop which travel separate courses back to the cortex.
This is protective measure to reduce the impact of a lesion in the pathway
Describe the direct and indirect pathways of the visual system
Information from the photoreceptors can reach these output cells via two pathways the direct/vertical pathway or the indirect/horizontal pathway.
The direct pathway is that outlined above; photoreceptors -> bipolar cells -> ganglion cells.
The indirect pathway involves greater processing and includes the horizontal and amacrine cells, in this pathway processing still begins with the photoreceptors. Photoreceptors -> bipolar cells -> horizontal cells -> bipolar cells -> amacrine cells -> ganglion cells.
These two pathways are typically functioning simultaneously.
