Physiology of Vision Flashcards
So there are lots of coordinated layers of activity in the various sections of the visual system, from the retina, to the lateral glaticulate nucleus, to the primary visual cortex. What layers particularly go all the way to PVC?
Layers II and III.
To what nervous system do neural cells of retina belong?
They are actually part of the central nervous system! No periphery here!
The photoreceptors of the retina have to do a special activity to make light “see-able” to the brain. What do these cells do?
They TRANSDUCE photons into electrical signals. The photons cause hyperpolarizing potentials (instead of the regular depolarizing action potentials).
What type of potentials are generated and sent down axons when rods and cones are exposed to light?
Hyperpolarization potentials. Different than normal. In the dark, Na+/Ca++ inward channels and K+ outward channels are always open, causing constant depolarization (regulated by internal Gproteins). In light, the influx of Na+/Ca++ is shut off, and the net efflux of charge (because K+ still going out) causes hyperpolarization and a potential.
Describe the process of transduction.
GPCR-mediated system. Opsins are the GCPR, with light-sensitive 11-cis retinal (VitA derivative) and the G-protein transducin. Light (photon) activation causes retinal to change to transformation, causing phosphorylation and release of transducin. Transducin-P moves cues phosphodiesterase to hydrolyze the cGMPs that are keeping the Na+/Ca++ inward channels open on the membrane, and so they move away from the transmembrane protein and close the channel. This stops inward movement of Na+ and Ca++, but not eflux of K+, so the membrane becomes hyperpolarized.
What is luminance?
Measure of amount of light reflected by a surface, proportional to how many photons availble.
What are the various ranges of luminance that the eyes can pick up?
- Scotopic, only the rods are sensitive enough to pick up light, but they don’t feed into the form and acuity neural pathways.
- Mesopic, both rods and cones are active.
- Photopic, Rods become saturated (anything more than indoor light), and eventually cones become saturated.
What is graded neural signaling, and why is it important?
Graded neural signaling means that there are more or less neurotransmitters being sent, rather than an all-or-nothing ON/OFF switch. Need this for:
Topography
Luminance
CONTRAST (edges)
Distance
MOvement - there is a distinct mechanism for motion sensitivity
What is visual topography?
Array of overlapping visual receptive fields across retina. There are regions of the visual field that maximally excite one particular retinal cell (much like regions of sensation in somatosensory system). These regions all overlay to create pictures, like pixelation.
* Not all receptive fields are the same size. Smaller in the focal region of the visual field, which concentrates on the fovea, therefore more detailed.
If rods and cones don’t actually create action potential, how does the electrical message get sent to the brain?
Rods and cones speak in chemical language, changing the amount of neurotransmitter secreted. It is at the RETINAL GANGLIONS that the chemical message is translated into action potentials.
What is a spatial receptive field?
As visual topography ties together areas of the visual field with individual maximally excited rods/cones, the Spatial Receptive Field ties together regions of the visual field with individual retinal ganglions and cause an action potential. Therefore, this helps our brain understand WHERE in a visual field to place an object, because there are specific nerves that are tied to that particular space in the visual field.
What translates the chemical message from the photoreptor cells to an electrical message through the retinal ganglion cells?
BIPOLAR CELLS. Some sense a reduction in neurotransmitter secretion (i.e., the photoreceptor cell caught a photon of light) as needing to emit MORE neurotransmitter directly to retinal ganglion cell to start an action potential, and other bipolar cells sense the same message and emit LESS neurotransmitter. The combination allows for fine control, and there are different pathways for when things get brighter vs. darker.
Where are rods? Where are cones?
Rods are most dense around the fovea and then gradually lose density moving away from the fovea. There are VERY FEW rods in the fovea.
There are basically all cones in the fovea, and basically no cones anywhere else.
How does the brain accomodate the need to process information more acutely in the focal range (the macular range) in the field of vision?
The brain actually gives MORE SPACE to the macula nerve cells than to areas of the visual field that excite fewer neurons. Though they are bigger portions of the visual field, the brain devotes a lot less space to them.
Luminance vs. Contrast
Luminance is quantifiable physical entity.
Contrast is comparative, and depend son perceptual quality and neural circuit ability to compare luminance levels WITHOUT absolute fidelity to physical quantification of luminance.
