NEUR 0010 - Chapter 9 Flashcards
What does the retina actually process, rather than a faithful representation of light intensity?
Differences in light intensity on different parts of it
What is the LGN?
First synaptic relay in the visual perception pathway; in thalamus; receives info from the optic nerves and sends it to the cerebral cortex
Why does the pupil appear dark?
Because of the light-absorbing retina that’s behind it
The cornea is continuous with what part of the eye?
The sclera
What are extraocular muscles?
Three pairs of muscles in the sclera
What is the sclera?
The white of the eye; tough outer wall of the eyeball
What is the conjunctiva?
The membrane that folds back from the eyelid inside and joins with the sclera
What is the macula?
The part of the retina used for central vision; distinguished by yellow color and lack of large blood vessels
What is the fovea?
A dark spot in the macula where the retina is thinner; marks the center of the retina, and so delineates nasal vs temporal retina
What are two specializations of the part of the retina dedicated to central vision?
Lack of large blood vessels, fovea is thinner
What is interesting about the nutrient system of the cornea?
Has no blood vessels, so receives nutrients from the aqueous humor behind it
What is the lens? How is its size controlled?
Behind the iris: suspended by zonule fibers that are attached to ciliary muscles
How is focal distance related to curvature of the lens?
The greater the curvature, the smaller the focal distance
What is the focal distance?
The distance from the refractive surface to the point at which refracted light rays converge
How does lens accommodation work?
The ciliary muscles contract and swell, loosening the zonule fibers and allowing the lens to thicken; or the ciliary muscles relax and the diminish, tightening the zonule fibers and forcing the lens to thin
What does it mean that the pupillary light reflex is consensual?
If you trigger a response in one eye, it happens in both eyes: one pupil won’t dilate without the other, etc.
Lack of consensual response for pupillary light reflex signifies a neurological disorder of what brain region?
Brainstem
What effect does pupil constriction have?
Increasing depth of focus (reduces blurriness of far objects)
What are two factors that affect visual acuity?
Spacing of photoreceptors in the retina; precision of the eye’s refraction
What is the most direct pathway for visual information to exit the eye?
From photoreceptors to bipolar cells to ganglion cells
Which retina cells fire action potentials?
Just the ganglion cells
What are horizontal cells?
Receive input from the photoreceptors, project neurites laterally to influence surrounding bipolar cells and photoreceptors
What are amacrine cells?
Receive input from bipolar cells, project neurites laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells
What is the laminar organization of the retina from inner to outer?
Ganglion cell layer, inner plexiform layer (amacrine-ganglion/bipolar/amacrine synapses); inner nuclear layer (bipolar cells); outer plexiform layer (horizontal-bipolar/photoreceptor synapses); outer nuclear layer (photoreceptor cell bodies); layer of photoreceptor outer segments; pigmented epithelium
Why is it a good thing that the laminar organization is seemingly backwards?
It’s good to have the pigmented epithelium at the bottom: helps maintain photoreceptors, absorbs any extra light and minimizes reflection
What is the significance of the greater number of photopigmented disks in rods?
Higher photopigment concentration, so MUCH more sensitive to light
What are the characteristics of the peripheral retina pertaining to rods/cones and photoreceptors/ganglion cells?
More rods than cones; higher photoreceptor/ganglion ratio
What is more sensitive to light levels, peripheral retina or fovea?
Peripheral retina: more rods
What is the significance of the pit-like structure of the fovea?
Clears the other layers out of the way to allow for better acuity
What is the basic mechanism for signal transduction in the visual system?
G-proteins
What is the Vm for a rod outer segment in complete darkness?
-30 mV
Why is the Vm of a rod outer segment in darkness higher than usual?
Dark current from Na+ leak in from cGMP channels
Where does the cGMP come from for the dark current?
Made by guanylyl cyclase in the cytoplasm
What is the pigment in rods that responds to light?
Rhodopsin
What does it mean that rhodopsin can be thought of as “a receptor protein with a pre-bound chemical agonist?”
Receptor protein is opsin like a normal GPCR; pre-bound agonist is retinal (derivative of vitamin A)
What happens when light hits rhodopsin?
Changes from purpose to yellow: stimulates GP called transducin in the disk membrane; activates PDE to break down cGMP; no dark current; hyperpolarization
Why does daylight vision depend entirely on cones?
Because the rods hit a point of saturation where they can’t be hyperpolarized anymore
What is the peak sensitive of rods? What color is this?
500 nm, blue-green
What contributes to dark adaptation?
Pupil dilation; and when that maxes out: regeneration of unbleached rhodopsin and adjustment of functional circuitry of retina (make info from more rods available to each ganglion cell)
What is Ca++’s role in light adaptation?
If immediate light, then hyperpolarized as much as possible to Ek; gradually depolarizes back to -35 mV because lack of Ca++ entry to inhibit cGMP (so more cGMP is made and the Na+/Ca++ channels open back up and allow depolarization
What is Ca++’s effect on guanylyl cyclase?
Inhibits: prevents cGMP production
When does a photoreceptor release neurotransmitter, light or dark?
Dark: preferred stimulus
What does it mean to be an OFF bipolar cell or an ON bipolar cell?
Different responses ot glutamate released by photoreceptors
What is an OFF bipolar cell?
Glutamate-gated cation channels mediate classic depolarizing EPSP from influx of Na+
What is an ON bipolar cell?
GPCRs respond to glutamate by hyperpolarizing
Why are bipolar cells called off or on? What does that refer to?
When they depolarize: OFF bipolar cells depolarize when light is off (more glutamate allows for influx of Na+, causing depolarization); ON bipolar cells depolarize when light is on (less glutamate released, since glutamate causes hyperpolarization)
What is the receptive field of a bipolar cell?
The area of the retina that, when stimulated with light, changes the cell’s membrane potential
What is the relationship between response of bipolar center vs surround?
Opposite effects
What does it mean to say that ganglion cells are ON-center or OFF-center?
ON-center ganglion cells receive input from an ON-center bipolar cell, and the opposite for OFF
What kind of visual information do ganglion cells respond to?
Differences in light intensity
What are the two main types of ganglion cell?
M and P
How do M cells differ from P cells?
Less numerous; larger receptive fields; conduct APs more rapidly; more sensitive to low-contrast stimuli
What is the difference in AP firing between M and P cells?
M is transient burst, P is sustained discharge
What are color-opponent cells?
Color-sensitive ganglion cells that can detect difference in wavelength; can be P or nonPnonM
What are the two types of color-opponent cells?
Red-green and Blue-yellow
Why aren’t M cells color-opponent?
Receive input from multiple types of cones
Why are P and nonPnonM cells color-opponent?
The center of the receptive field is fed mainly by one type of cone
