The Visual System Flashcards
T or F. 1/2 of brain is used for vision processes.
T. We need the occipital cortex for perception of objects in space.
All the visual reflexes (see something and have a response to it)–provided by the brain stem and spinal cord.
Where are visual memories stored?
In the parietal and temporal lobes.
The visual system also provides information required for setting of the circadian rhythms, general metabolic rate, mood, etc. Where does this occur?
These parts of vision occur in the pineal gland (circadian rhythm) and diencephalon
For a clinician–the visual system is a critical system to analyze because it can provide a large amount of information on the patient. For example–the ____ is the only visible vascular system in the body.
retina. Hypertension will often be diagnosed through analyses of the retina before blood pressure changes are noted by the patient.
What part of the eye fine focuses incoming light?
the fovea
Describe the general way that light travels through the eye
As it enters the eye, light is:
refracted by the cornea and inverted through the pupil.
The image is then variably refracted by the lens (this is less effective with aging (bifocals), so image is not always completely clear unless glasses are worn)
Ultimately–the image (light) is projected onto the fovea (which is a yellow pigmented area in the optic area). The fovea is at the center of the macula of the eye.
Many animals do not have a macula or fovea–which is why their vision is not as crisp as ours.
A schematic of the eye. Notice how the arrow is upright outside (right) of the eye, but is smaller and inverted when it reaches into the fovea. Why is it smaller?
It is smaller due to refraction in the cornea and lens–but this also increases the clarity of the image.
The lens refraction abilities are controlled by ciliary muscles that are under sympathetic and parasympathetic nervous system control.
What is the role of the sclera?
It is just a protective layer for the retina and the choroid
What is the choroid?
The choroid is the vascular bed of the outer retina. It has higher blood flow of any organ in the body based on size.
The choroid provides blood flow and nutrients to what structures?
the photoreceptors and RPE cells.
The retinal blood flow is supplied by what?
the central retinal artery that enters through the optic nerves and supplies about 20% of the blood in the human retina. The rest is provided by the choroid.
T or F. The fovea is devoid of blood vessels.
T. If blood vessels grow there (as they do in Macular Degeneration)–it affects vision because the blood distorts the image.
Describe the layers of the retina
On the left is a schematic of the layers of the retina, while the right image is of a human retina.
The pigment cells are the retinal pigmented epithelial cells (RPE). What do they do?
They provide a barrier to the retina from the choroid. Photoreceptors (cones and rods–named based on their shape) in the photoreceptor layer come in contact with the RPE.
Where are the cell bodies of the photoreceptors located?
in the outer nuclear layer, while their projections lie in the outer plexiform layer.
The photoreceptors signal through the outer plexiform layer to where?
the bipolar cells, whose cell bodies are in the inner nuclear layer.
The final layer of neurons in the retina (deepest layer) are:
retinal ganglion cells (located in the ganglion cell layer). Once the visual object is transduced to the retinal ganglion cells, it will enter into the optic nerve and go to the brain.
What cells help in convergence of the image from the millions of photoreceptors to hundreds of bipolar cells to tens of ganglion cells?
Interneurons (horizontal cells and amacrine cells)
Ultimately–the image will be sent through the ganglion cells to the optic nerve (optic nerve entry into the retina is the blind spot)
In the fovea, the outer segments are present, but the rest of the retinal layers are displaced radially to allow for minimal distortion of the image.
Again, the retinal pigmented epithelium is the layer that seperates the choroid (vascular) from the neural retina. What does it contain?
the black pigment, melanin
How does macular degen affect the RPE?
Macular degeneration results when blood vessels damage the RPE and break through their barrier and grow into the macular region of the eye, causing visual distortion of the image.
What is another function of the RPE?
The 2nd main function of the RPE is to eat rod outer segments every 10 days. As we age–this becomes less effective. More proteins are left in the RPE, which can cause them to autofluoresce. This autofluorescence is called drusen and is a key factor in dry macular degeneration.
The melanin in the RPE help absorb some of the light that comes to the photoreceptors. When this absorption is reduced (blue-eyed people vs. brown eyed people), some think this may contribute to eye disease.
Describe rods
- long outer segment
- big synaptic terminal
- high sensitivity (over 1000 closed disks, more photopigment, scotopic night vision) and amplification (one photon stops the entry of about 107 Na+ ions)
- saturates in daylight
- low temporal resolution (slow response and long integration time)
- sensitive to scattered light (poor spatial resolution)
Describe cones
- short outer segment
- HUGE synaptic terminal
- Low sensitivity (many fewer open disks, less photopigment, less per disk, and photopic day vision) and amplification
- saturates only in intense light
- High temporal resolution (fast response ans short integration time)
- sensitive to direct axial light (good spatial resolution)
Describe the rod system
low acuity (rode absent in central fovea, and highly convergent pathways)
Achromatic (one type of rhodopsin)
Describe the cone system
High acuity (cones concentrated in fovea, slight divergence in pathways)
Chromatic (color vision) (three types of cones each with different photopigment)
Rod photoreceptors are named for the shape of the outer segment, which reaches and lays against the RPE cells What type of vision are they responsible for?
for black and white (night) vision (aka scotopic info).
They are much simpler than cones, since there are multiple cones for the different colors.
More about rods
Light transduction occurs in the outer segment.
The outermost outer segment is phagocytosed by the RPE cell layer every 10 days. A new outer segment extends from the inner segment and joins up with the other segments.
Cones are also named for the shape of their outer segment–it is much smaller and shorter than rod outer segments. What are they responsible for?
Cones are responsible for color vision. There are 3 types of cones for the different wavelengths of light
What are the types of cones?
L-cones -long wavelengths (red)
M-cones-medium wavelength (green)
S-cones-short wavelength (blue)
Each of these responds to a different color of light
Every color is represented by a unique combination of L-,M-, and S-cones
What chromosome carries the M and L cones?
M-cone and L-cones are carried on the X-chromosome–which is why men are much more likely to be color-blind in the red-green, since they only have 1 X-chromosome and women have 2.
•If one type of cone is absent because of genetic defects-person will be missing the corresponding opsin=color-blind.
What is the function of photoreceptors?
To absorb quanta of light (photons) and convert to an electrical signal
•Both cones and rods undergo HYPERPOLARIZATION in response to light (this is the only sensory system in which hyperpolarization is the response to the stimulus)
What is the first step of phototransduction in rods (same in cones, but they have 3 opsins, instead of one rhodopsin)?
Step 1–light comes in and converts 11-cis-retinal to all-trans retinal–which activates the rod opsin–rhodopsin
What can active opsin/rhodopsin do?
Active rhodopsin can activate Transducin by replacting GDP with GTP, which then causes transducin to dissociate into TBy and Ta-GTP, which activates cGMP phosphodiesterase (PDE) by removing its inhibitory subunit
What does Active PDE (no inhibitory subunit) do?
lowers cGMP levels (converts it to 5’-GMP) enough to hyperpolarize the membrane and close influxing sodium and calcium channels. This signal is the visual response and is passed to the CNS
What happens once enough sodium and calcium channels are shut off in the membrane?
Calcium begins to be pumped out through the sodium/calcium exchanger. As calcium is reduced in the membrane, cGMP levels rise again and re-polarize the membrane to pre-stimulus levels–which re-opens the channels
Slowly–rhodopsin is de-phosphorylated and all-trans-retinal is converted back to 11-cis-retinal.
Now–rhodopsin kinase comes back to re-phosphorylate rhodopsin, which is bound by arrestin to render it inactive. Once light stimulus occurs again–arrestin will be removed by the conversion of 11-cis-retinal to all-trans-retinal.
The process begins again and again.
This is a schematic of the processing of the retina from photoreceptors to the bipolar cells. The signal from the photoreceptors—>bipolar cells can be modified and convergence occurs by the horizontal cells.
Once photoreceptors have processed the light–the signal is passed alone to the bipolar cells. Rod photoreceptors transduce their signal to rod bipolar cells (periphery) and cone photoreceptors to cone bipolar cells (central retina).
At this point, Horizontal cells help do what?
converge the signals from multiple rod photoreceptors to fewer rod bipolar cells. (high degree of convergence here- 100 million receptor cells synapsing onto 1 million ganglion cells)
At the bipolar-ganglion cell synapse, _____ cells detect major changes in activity levels
amacrine
Ganglion cells are more complicated that photoreceptors or bipolar cells. There are multiple types of ganglion cells that will activate based upon the image presented. They are categorized by physiological role and size. What ganglion cells dominate in the peripheral retine (most input from rods)?
a-ganglion cells
Describe a-ganglion cells
- extensive dendritic trees
- large axons
- participate little in color perception
Where do a-ganglion cells project to?
The Magnocellular layer of lateral geniculate nucleus (their main job is to help locate the object in space)
What ganglion cells dominate in the central retina?
B-ganglion cells (for fine colors and texture)
Describe B ganglion cells
- small receptive fields
- small dendritic arbors
- responsive to color stimuli (cones)
Where do B ganglion cells project to?
The parvocellular region in lateral geniculate nucleus, to define color and texture of object
While all cell types (photoreceptors, bipolar and ganglion cells) are in the retina in all regions–the fovea has the cones (outer segment) and the rest of their signaling pathway (bipolar cells, ganglion cells) project radially away from the fovea. This is to minimize distortion of the image.
T or F. Unlike the rest of the retina–in the fovea–convergence does NOT occur.
T.
Below, you see that the one cone is connected to one cone bipolar cell and 1 ganglion cell. There are not horizontal or amacrine cell interactions.
This design is unique and is the reason why humans have a crisp image representation
This is the general representation of the eye with the temporal, nasal and inferior, superior presented. Most times, this view will be what is noted in the retina through the ophthalmoscope and will help you have orientation.
Note how the blood vessels radiate from the optic disk.
Describe central visual pathway
Once the image is transduced from the ganglion cells, they will enter into the central pathways to be processed by the appropriate regions of the brain. The ganglion cell axons in the optic nerve partially decussate at the optic chiasm (only nasal fibers cross), and each optic tract projects to the lateral genicular.
From there, optic radiations carry geniculate fibers to the primary visual cortex in the opposite occipital lobe
Note the plane on the left and the pelican on the right. This image shows what the eyes “see” and where they end up in the occipital lobe.
Note that the nasal side (solid blue line and dashed black line) cross at the optic chiasm. The temporal image do not cross.
How does the visual cortex handle input?
- Cells that respond preferentially to input from one eye (ocular dominant) are segregated into layers in the lateral geniculate and the visual cortex
- The M pathway originates from magnocellular ganglion cells, projects to lateral geniculate layers 1 and 2, to layer 4C-alpha in the cortex - space information
- The P pathway originates from parvocellular ganglion cells, projects to lateral geniculate layers 3-6, to layer 4C-beta in the cortex - form information
This is what a patient with advanced diabetic retinopathy will see
What is this showing?
This is an image of the retina from a patient with proliferative DIABETIC retinopathy. Note the light spots at the bottom of the image–these are likely areas without blood flow (due to damage).
Note in the inferior region below the optic disk–the blood vessel has been damaged and there is edema occurring.
What causes diabetic retinopathy?
We do not exactly know what causes retinopathy. The high glucose causes damage to retinal endothelial cells and pericytes–eventually causing hypoxia and activating growth factors.
High glucose also damages the neurons.
What a patient with macular degeneration sees
What is this?
A retinal image of a patient with advanced macular degeneration. Note all the hyperfluorescence and areas of none vascularity
Compare and contrast of macular degeneration vs. retinopathy. Since macular degeneration is the leading cause of blindness in older people and retinopathy is the leading cause of blindness in working age adults.
Macular degeneration involves damage to the choroidal vasculature that breaks through the RPE cells and damages the macula. Central vision is lost due to the damage of the macula
There are 2 types of macular degeneration, namely:
Wet and Dry AMD
Normally all people have a little drusen. When a patient has abnormal amounts of drusen and begins to lose vision–often in 1 eye–it is termed ______________
dry macular degeneration or atrophic macular degeneration. This represents about 90% of the cases of macular degeneration. With visual aids, these people can continue to function
In the wet form (10% of cases)–the blood vessels grow into the macula and cause significant vision loss.
What factors contribute to the formation of AMD?
The biggest cause of AMD is age. Smoking is also a big contributor. Some new research has shown some genetic predisposition due to altered inflammatory responses.
Also changes in angiogenic and angiostatit factors
In the last 5 years–patients with AMD have hope. Using intravitreal injections (about monthly)–they can get a drug that blocks the vascular growth. It does not restore vision in most patients, but it often helps them maintain what vision they have. Eventually they will still lose vision, but the drugs are much better than previous options.
