Flashcards in Chapter 11 - Vision: The Eye Deck (87):
Mention as many eye structures as you can remember. Macro.
- Zonule Fibers
- Anterior chamber
- Aqueous humor
- Vitreous humor
- Posterior chamber
- Ciliary body
- Optic disc
- Optic nerve
(You need to mention >8 to get 2, and all to get 3 and above)
What are the zonule fibers?
A group of radially arranged connective tissue bands that are attached to the ciliary muscle. The zonule fibers keep the lens in place and places tension on the lens - flattening it.
What is the iris?
The iris is the most anterior portion of the uveal tract. It contains two sets of muscles with opposing actions, which allow the size of the pupil to be adjusted under neural control.
What is the uveal tract?
The uveal tract is the layer of tissue adjacent to the retina. This layer has three distinct, but continuous structures: the choroid, the ciliary body, and the iris.
What is the pupil?
The opening in the uveal tract, in the center of the iris.
What is the cornea?
The cornea is the transparent tissue that extends from the sclera.
What is the aqueous humor?
The aqueous humor is the clear, watery liquid that supplies nutrients to the cornea. It is produced by the ciliary processes in the posterior chamber, and is drained from the eye by cells lying at the junction of the iris and the cornea.
Insufficient drainage of the aqueous humor can be harmful. How, and what is the name of the disorder?
Failure of adequate drainage results in a disorder known as glaucoma, in which high levels of intraocular pressure can reduce the blood supply to the eye and eventually damage retinal neurons.
What is the vitreous humor?
The thick, gelatinous substance, which fills the space between the back of the lens and the surface of the retina. It contains phagocytic cells that remove debris and blood, and the thickness gives structure to the eye.
What are "floaters"?
Floaters are collections of debris in the vitreous chamber that are too large for phagocytic consumption. They remain to cast annoying shadows on the retina.
What is the posterior chamber?
The posterior chamber is the region between the lens and the iris. Cells here produce aqueous humor for the anterior chamber.
What is the anterior chamber?
The chamber just behind the cornea and in front of the lens.
What is the ciliary muscle?
The ciliary muscle is the muscular part of the ciliary body. The ciliary muscle is responsible for the accommodation of the eye.
What is the ciliary body?
The ring of tissue that encircles the lens and consists of two parts: the ciliary muscle and the ciliary processes.
What are the ciliary processes?
The ciliary processes are the vascular components of the ciliary body. They are responsible for producing the fluid that fills the front of the eye: the aqueous humor.
What is the choroid?
The choroid is the largest component of the uveal tract: the tissue middle tissue layer of the eye. The choroid is composed of a rich capillary bed as well as a high concentration of the light-absorbing pigment melanin.
What is the sclera?
The sclera forms the outermost tissue layer of the eye and is composed of a tough, white, fibrous tissue. At the front of the eye, this opaque outer layer is transformed into the cornea.
What is the retina?
The retina is the innermost layer of the eye and contains neurons that are sensitive to light and are capable of transmitting visual signals to central targets.
What is the fovea?
The fovea is the small depression or pit in the retina. It is located in the center of the macula lutea, and is the area of greatest acuity.
What is the optic disc?
The optic disk, or the optic papilla is the site in which the blood vessels enter the eye and the retinal axons leave the eye. This region contains no photoreceptors and produces the perceptual phenomena known as the blind spot.
What is the optic nerve?
The optic nerve is the nerve bundle that exits the eye and projects to central structures in the brain.
About half the cases of blindness in the world stem from one type of disorder. Which?
Opacitites in the lens known as cataracts.
Which structures are responsible for the refraction (bending) of light necessary for the formation of focused images on the photoreceptors of the retina?
The cornea and the lens.
What happens to the refraction of light when submerged in water?
Water has a refractive index close to that of the cornea; as a result, immersion in water virtually eliminates the refraction that normally occurs at the air/cornea interface.
What is accomodation?
Accommodation are dynamic changes in the refractive power of the lens.
The shape of the lens is determined by two opposing forces. Which?
The elasticity of the lens, which tends to keep it rounded up; and the tension exerted by the zonule fibers, which tends to flatten it.
How does the size of the pupil affect visual acuity?
Aberrations, either spherical or chromatical, are the greatest for light rays that pass farthest from the center of the lens. Narrowing the pupil reduces both types of aberration, because the light will pass only the center of the lens.
What is a spherical aberration?
Spherical aberration is an optical effect observed in an optical device (lens, mirror, etc.) that occurs due to the increased refraction of light rays when they strike a lens or a reflection of light rays when they strike a mirror near its edge, in comparison with those that strike nearer the centre.
What is a chromatic aberration?
In optics, chromatic aberration is an effect resulting from dispersion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have different refractive indices for different wavelengths of light.
Name the two most common refractive errors?
Myopia (nearsightedness) and Hyperopia (farsightedness).
What is the cause of myopia?
Myopia can be caused by a corneal surface that is too curved or by an eyeball that is too long.
What is the cause of hyperopia?
Hyperopia can be caused by an eyeball that is too short or a refracting system that is too weak.
Where is the focus point of people suffering from myopia?
The focus point is in front of, rather than on, the retina.
What is the macula lutea?
The macula lutea is an oval spot containing yellow pigment (xanthophyll) that is located near the center of the retina. The macula is the region of the retina that supports high visual acuity.
There are five basic classes of neurons in the retina. Which?
- Bipolar cells
- Ganglion cells
- Horizontal cells
- Amacrine cells.
What cell layers do we find in the retina?
- Pigment epithelium
- Photoreceptor outer segments
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve fiber layer
What is the three-neuron chain in the retina?
The three-neuron chain is the most direct pathway of information flow from photoreceptors to the optic nerve. It consists of photoreceptor cells - bipolar cells - ganglion cells.
Horizontal cells and amacrine cells have their cell bodies in which layer(s)?
The inner nuclear layer.
The horizontal cells and amacrine cells have their processes in which layer(s)?
Horizontal: outer plexiform layer.
Amacrine: Inner plexiform layer.
The pigment epithelium plays two roles that are critical to the function of retinal photoreceptors. Which?
1. Membranous disks in the outer segment have a relatively limited life span (12 days) and are shed at the tip of the outer segment. This is phagocytosed by the epithelium.
2. The epithelium regenerates the photopigment molecules after they have been exposed to light.
From where does the photoreceptors gain most of their nourishment?
From the pigment epithelium.
What is phototransduction?
The process of making nerve impulses out of photon stimulation.
What is the resting membrane potential of a photoreceptor.
How is the membrane potential of a photoreceptor affected by increased illumination?
The potential becomes more polarized, until the saturation of about -65mV.
Describe the inward current in a photoreceptor during dark conditions.
In the cark, cations (both Na+ and Ca2+) flow into the outer segment through membrane channels that are gated by cGMP .
Describe the outward current in a photoreceptor during dark conditions.
K+ leaks out of the cell due to potassium-selective channels.
What is the reason for the open cation channels during darkness in a photoreceptor?
In the dark, the photoreceptors has a large concentration of cGMP, which opens channels.
... so cGMP stands for?
Cyclic guanosine monophosphate.
What part of the photoreceptor reacts to a photon?
The light-absorbing retinal, which is coupled with a class of protein called opsins.
What happens to retinal when hit by a photon?
When retinal absorbs a photon of light, one of the double bonds between the carbon atoms in the retinal molecule breaks, and its configuration changes from the 11-cis isomer to the all-trans retinal.
What happens when the retinal chromophore breaks into all-trans retinal?
It triggers a series of alterations in the opsin component of the molecule.
What happens when opsin in changed by all-trans retinal?
It leads to the activation of an intracellular messenger called transducin.
What happens in a photoreceptor when transducin is activated?
Transducin activates a phosphodiesterase that hydrolyzes cGMP.
How does the hydrolisys of cGMP affect the photoreceptor?
It lowers the concentration of cGMP throughout the outer segment of the photoreceptor, which reduces the number of cGMP molecules available to bind to the channels in the surface of the outer segment membrane and leading in turn to channel closure.
How do you write available?
Shortly mention the structures or molecules involved when light hits a photoreceptor.
Retinal - Opsin -Transducin - Phosphodiesterase and cGMP hydrolization.
What mechanism is in place to keep activated rhodopsin from signaling for a long time?
It is phosphorylated by rhodopsin kinase.
How does the phosphorylation of rhodopsin happen?
Rhodopsin kinase permits the protein arrestin to bind to rhodopsin. Bound arrestin blocks the ability of activated rhodopsin to activate transducin, thus effectively truncating the phototransduction cascade.
What is the retinoid cycle?
The retinoid cycle is a complex process which restores retinal to its 11-cis form.
Describe the retinoid cycle.
The all-trans retinal dissociates from opsin and diffuses into the cytosol of the outer segment; there it is converted to all-trans retinol and transported into the pigment epithelium via a chaperone protein called interphotoreceptor retinoid binding protein (IRBP). In the pigment epithelium it is converted into 11-cis retinal, and transported by IRBP back.
Mention how Ca2+ has regulatory effects on the phototransduction cascade. 3 effects known.
1. Decreases in Ca2+ increases the activity of guanylate cyclase, the cGMP synthesizing enzyme.
2. Decreases in Ca2+ increases the activity of rhodopsin kinase, which permits more arrestin to bind to rhodopsin.
3. Decreases in Ca2+ increases the affinity of the cGMP-gated channels for cGMP.
What is scotopic vision?
Scotopic vision is vision at the lowest levels of illumination, where only the rods are activated.
What is photopic vision?
Photopic vision is the range of illumination where the rods are saturated (all channels are already closed), so vision is primarily guided by cones.
What is mesopic vision?
Mesopic vision occurs in levels of light at which both rods and cones contribute.
What is the difference in convergence between rod and cone circuitry?
Each rod bipolar cell is contacted by a number of rods, and many rod bipolar cells contact a given amacrine cell. In contrast, the cone system is much less convergent. Thus, each of the retinal ganglion cells that dominate central vision receives input from only one cone bipolar cell, which, in turn, is contacted by a single cone.
What can the differences in convergence between rod and cone systems tell us about their function?
Convergence makes the rod system a better detector of light, because small signals from many rods are pooled to generate a large response in the bipolar cell. At the same time, convergence reduces the spatial resolution of the rod system, since the source of a signal in a rod bipolar cell or retinal ganglion cell could have come from anywhere within a relatively large area of the retinal surface.
Are there rods in the fovea?
Yes, but, reversibly to the rest of the retina, the fovea has more cones than rods. The foveola, which is the very center of the fovea has no rods.
What area of the retina does not receive its blood from the retinal blood vessel?
The fovea. It is dependent entirely on the underlying choroid and pigment epithelium.
We normally distinguish between three types of cones. Which?
L, M and S. The letters refer to their length: long,medium and short-wavelength cones.
What is found to be the biological causes of dichromacy?
Human dichromats lack one of the three cone pigments, either because the corresponding gene is missing or because it exists as a hybrid of the red and green pigment genes.
What are on-center ganglion cells?
On-center ganglion cells produce bursts of APs when stimulus (light) is applied to the center of their receptive fields, hardly any APs when a dark spot is placed in that center, and a moderate amount of APs when stimulus is applied to the entire receptive field (center + surround).
What are off-center ganglion cells?
Off-center ganglion cells produce bursts of APs when a dark spot is applied to the center of their receptive fields, hardly any APs when a light spot is placed in that center, and a moderate amount of APs when no light is shining on the entire receptive field.
Bipolar cells have APs or Graded potentials?
Ganglion cells have APs or Graded potentials?
How can you explain the selective response of on- and off-center bipolar cells?
They express different types of glutamate receptor.
What kind of neurotransmitter is released by the photoreceptors?
Off-center bipolar cells react in what way to glutamate?
Off-center bipolar cells have ionotropic receptors (AMPA and kainate) that cause the cells to depolarize in response to glutamate released from photoreceptor terminals.
On-center bipolar cells react in what way to glutamate?
On-center bipolar cells express a G-protein-coupled metabotropic glutamate receptor (mGluR6). When bound to glutamate, these receptors activate and intracellular cascade that closes cGMP-gate Na+-channels, reducing inward current and hyperpolarizing the cell.
Photoreceptor synapses with off-center bipolar cells are described as "sign-conserving". Why?
They are described as sign-conserving because the sign of the change in the membrane potential of the bipolar cells (depolarization or hyperpolarization) is the same as that in the photoreceptor.
Photoreceptor synapses with on-center bipolar cells are called "sign-inverting". Why?
They are called sign-inverting because the change in the membrane potential of the bipolar cell is the opposite of that in the photoreceptor.
Kuffler's work called attention to the fact that retinal ganglion cells do not act as simple photodetectors. Indeed, most ganglion cells are relatively poor at signaling differences the level of diffuse illumination.. So what are they good at?
They are very good at sensing the differences between the level of illumination that falls on the receptive field center and the level of illumination that falls on the surround - that is, to luminance contrast.
Name the two most important functions that are enabled by the center-surround mechanisms of the retinal neurons.
1. Luminance contrast sensitivity.
2. Light adaptation.
How does background illumination influence ganglion firing rates?
The intensity of spot illumination required to evoke a given discharge rate is dependent on the background level of illumination. Increases in background level of illumination are accompanied by adaptive shifts in the cell's operating range such that greater stimulus intensities are required to achiever the same discharge rate.
What is meant by "firing rate is not an absolute measure of light intensity"?
Firing rate of retinal ganglion cells signals not the absolute measure of light intensity. This is because it is related to the background level of illumination. It does, however, signal the difference between the two.
Why is light adaptation useful?
It leads to a downplaying of the background level of illumination. This enables to us focus on the differences in illumination instead. This arrangement presumably explains why the relative brightness of objects remains much the same over a wide range of lighting-conditions.
Although the details of horizontal cell functions are not entirely known, they are thought to...
exert their influence via the release of neurotransmitter directly onto photoreceptor terminals, regulating the amount of transmitter that the photoreceptors release onto bipolar cell dendrites.