Anatomy of the Eye
What is the outermost tissue layer of the eye called?
Sclera
Shining a light onto a cone or rod will result in a ________.
Hyperpolarization!
Which type of photoreceptor provides color perception?
CONES!
What will happen to the firing rate of an on-center ganglion cell when both the center and surround is illuminated?
No change!
The primary visual cortex has how many layers? Which layer has multiple subdivisions?
The primary visual cortex has 6 layers. The 4th layer has several subdivisions.
Failure of adequate drainage of aqueous humor results in which disorder?
Glaucoma
Which type of channel(s) in photoreceptor cells is/are open in the dark, and what ions are flowing through them?
cGMP-gated cation channels are located in the outer segment, which allow Na and Ca to flow through. There are also K leak channels in the inner segment of photoreceptor cells, which allow K to leak out of the cell.
Which type of photoreceptor has lower acuity? WHY?
RODS, because they have larger receptor fields (larger degree of convergence).
Retinal ganglion cells respond best to what type of stimulus?
Retinal ganglion cells respond to EDGES! Ganglion cells whose activity will be most affected have receptive fields that lie along the light-dark border.
Is the input in the LGN monocular or binocular? In layer 4 of the striate cortex?
Information from both eyes remain segregated in the LGN and in layer 4 of the striate cortex. Most neurons in the visual cortex outside of layer 4 are binocular (signals from the 2 eyes converge).
80% of the volume of the eye is filled with a thick, gelatinous substance called...
Vitreous humor
What is Rhodopsin? What is it composed of?
Rhodopsin is composed of a GPCR, opsin, which lies in the disk membrane and retinal, the light absorbing chromophore which resides inside the opsin molecule.
Why do you have the greatest acuity in the foveola?
ONLY CONES, 1:1 convergence, and avascular (no blood vessels) therefore light is subjected to a minimum of scattering.
Where are simple cells primarily located? How do their visual fields differ from other cells?
In the visual cortex (layer 4)! They are larger, elongated, and respond best to bars of light that are in a particular orientation.
Where do M ganglion cells project in the LGN? And then in the striate cortex?
Layers 1 and 2 in the LGN, and 4C alpha in the striate cortex.
What are the five basic classes of neurons in the retina from the back of the retina to the front?
photoreceptors, bipolar cells / horizontal cells, amacrine cells, and ganglion cells.
What signalling cascade is activated when you shine light onto rhodopsin?
Light is absorbed by retinal, so it changes its conformation from 11-cis to all trans. This changes the shape of opsin and activates the G-protein transducin. The alpha subunit activates PDE, which hydrolyzes cGMP, reducing its concentration in the outer segement leading to a closure of cGMP-gated channels. Therefore, less Na and Ca enter the cell, while K continues to flow out, leading to a HYPERPOLARIZATION of the cell.
Why are men more likely to be colorblind?
The genes that encode the red, green, and blue pigments are located on the X chromosome and men only have one X chromosome.
If a cone has a DARK spot in the center of it, it will depolarize. Which type of bipolar cells will also show a depolarization and why?
Because the cone is depolarized it will release glutamate onto the bipolar cells. Glutamate will bind to AMPA and kainate receptors on the off-center bipolar cells, leading to a depolarization of these cells. The off-center bipolar cells will also release glutamate that will bind to AMPA, kaintaine, and NMDA receptors on the off-center ganglion cells and result in an increase in their firing rate.
The MT area in the extrastriate cortex responds to what type of visual information? What pathway is this region part of?
The MT (middle temoporal area) responds to the direction of movement. It is part of the dorsal (spatial vision) pathway and projects to the parietal lobe!
Why are ganglion cells the only cells within the retina that produce action potentials?
Most of the processing in the retina is through GRADED potentials because of the short distances involved therefore action potentials are not needed.
Why is light adaptation important?
This decreases the sensitivity of the photoreceptors as the levels of illumination increases, to prevent saturation of the photoreceptors. This greatly extends the range of light intensities over which they can operate!
Which type of photoreceptor exhibits action potentials?
NEITHER! Photoreceptors do NOT exhibit action potentials. They produce GRADED changes in membrane potentials that changes the amount of glutamate release onto the bipolar cells!
What is the function of horizontal cells?
Horizontal cells inhibit photoreceptor cells in the surrounding receptive fields by releasing GABA.
What was the "pinwheel" picture trying to show :)
Orientation preferences in the primary visual cortex! A small area contains neurons having the full range of orientation preferences.