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56 Cards in this Set
- Front
- Back
- 3rd side (hint)
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Define "retina"
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-Located at the back of the eye
-Contains photoreceptors specialized to convert light energy to neural activity |
Converts __ to ___ .
Location. |
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The study of light rays and their interactions
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Optics
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Also a type of nerve in the eye.
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The bending of light that can occur when they travel from one transparent medium to another.
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Refraction
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How images are formed in the eye
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The opening that allows light to enter the eye and reach the retina.
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Pupil
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Black dot.
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Iris
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-Pigmentation provides eye's color
-Contains 2 muscles to contract/expand pupil. |
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This structure in the eye is continuous with the sclera and appears glassy/transparent.
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Cornea
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Sclera
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The white of the eye, continuous with cornea.
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Extraocular muscles
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-Located in the sclera
-Move the eyeball in the orbit -Normally not visible (behind conjunctiva) |
What do they move?
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The optic nerve
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Carries axons from the retina, exits the back of the eye (thru optics disks), passes through the orbit, and reaches the base of the brain near the pituitary gland.
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Starts by carrying axons from the retina, then what?
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Where do retinal vessels originate from?
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The optic disks
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Where the optic nerve exits the eye.
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Macula
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The part of the retina for central vision. Located in the middle of the retina.
-No large blood vessels. |
What type of vision?
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Fovea
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-Central retina
-Dark spot, 2mm in diamater -Retina thins here -Anatomical reference point |
Thickness of retina
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What are the two types of 'humors' in the eye?
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-Aqueous Humor: Fluid that nourishes the cornea
-Vitreous Humor: Viscous, jelly-like, lies between lens and retina. Pressure keeps eye spherical. |
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What is the lens?
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Located behind the iris.
-Suspended by ligaments called zonule fibers. -Changes in lens shape allow our eyes to adjust their focus to different viewing distances. |
Zonule fibers.
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Ciliary Muscles
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Attached to lens and sclera and form a ring inside the eye. Help to change shape of lens (viewing distances)
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Diopter
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-Refractive Power
-The reciprocal of the focal distance (in meters) is this unit of measurement. -Cornea = 42 diopters |
Unit of measurement. Measures what?
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Accomodation
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-Changing the shape of the lens to provide additional focusing power.
-Ciliary muscle contracts and swells in size., making the area outside the muscle smaller and decreasing the tension in the suspensory ligaments. -Lens = rounder/thicker -Increases refractive power. |
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What is the pupillary light reflex?
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-Connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils
-Pupils contract when light is great, expand in darkness. |
Size change
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Visual field
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How much we can see at one time
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The ability of the eye to distinguish two nearby points
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Visual acuity
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What is a photoreceptor?
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A specialized cell in the retina that transduces light energy into changes in membrane potentials
-Light-sensitive |
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Bipolar cells
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Connects photoreceptors to ganglion cells.
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Ganglion cells
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-Fire action potentials in response to light
-Only source of output from the retina |
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Horizontal cells
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-Receive input from the photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors.
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Similar to amacrine cells
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Amacrine cells
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-Receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells.
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Similar to horizontal cells
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Innermost layer of the retina containing cell bodies of the ganglion cells.
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Ganglion cell layer
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Second most inner layer of the retina containing cell bodies of bipolar cells, horizontal cells, and amacrine cells.
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The inner nuclear layer
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Second most outer layer of the retina containing the photoreceptors.
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Outer nuclear layer
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Outermost layer of the retina containing the light-sensitive elements of the retina. Embedded in epithelium.
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layer of photoreceptor outer segments.
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Outer plexiform layer
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Between outer and inner nuclear layers, where the photoreceptors make synaptic contact with the bipolar and horizontal cells.
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Rod receptors
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-Long, cylindrical outer segment, containing many disks.
-Under scotopic (nighttime) conditions, only these contribute to vision. |
Shape and when in use
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Cone receptors
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-Shorter than rods, tapering outer segment, fewer membranous disks
-Used primarily for photopic/daylight vision |
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Why is the retina called 'duplex'?
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Because it has a scotopic retina for nighttime vision and a photopic retina for daytime vision.
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How do rod photoreceptors transduce light?
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-Light stimulation of the photopigment activates g-proteins.
-G-proteins activate effector enzymes which changes the cytoplasmic concentration of a second messenger molecule. -This change causes a membrane ion channel to close, and the membrane potential is altered. |
5 general steps
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What is the dark current?
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-Movement of positive charge across the membrane, which occurs in the dark.
-In rods |
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Cyclic guanosine monophosphate
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-cGMP
-Stimulates Na+ channels to open /gate -Intracellular 2nd Messenger -Continually produced in photoreceptor by guanylyl cyclase, keeping channels open -Light reduces cGMP, causing the channels to close = more negative membrane potential. |
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Do rod photoreceptors depolarize or hyperpolarize in response to light? Why?
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-Hyperpolarize
-Because light reduces the production of cGMP, causing sodium channels to close, leading to a more negative membrane potential in photoreceptors. |
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Rhodopsin
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-Receptor protein with a prebound chemical agonist.
-In rods, rhodopsin absorbs electromagnetic radiation in the membrane of the stacked disks in the rod outer segments. -First step in hyperpolarization in response to light. |
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What is amplification?
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-Gives our visual system the ability to detect as little as a single photon, the elementary unit of light energy.
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Bleaching
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-The absorption of light causes a change in the conformation of retinal so that it activates the opsin.
-Changes the wavelengths absorbed by the rhodopsin. -Stimulates a g-protein in the disk membrane which activates an effector enzyme. -This breaks down cGMP that is normally present in the cytoplasm of the rod in the dark. |
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How to cone photoreceptors transduce light?
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-Same as rods except:
The type of opsins in the mebranous disks of the outer segments. Cones contain three (blue, red, green). |
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Color detection and Thomas Young
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-Discovered that all the colors of the rainbow can be created using ratios of blue/red/green.
-At each point in retina is a cluster of 3 receptor types. Each type is sensitive to either blue/green/red. |
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Young-Helmholtz trichromacy theory
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-Hermann von Helmholtz and Thomas Young
-The brain assigns colors based on a comparison of readouts from the 3 cone types. -When all cone types are equally active, we perceive white. -Color blindness = missing cone receptors. |
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Dark adaptation
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-The transition from all cone daytime vision to all rod nighttime vision is not instantaneous.
-Takes about 20-25 minutes. -Getting used to the dark. |
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Light adaptation
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-Takes 5-10 minutes
-Reverses changes in retina from dark adaptation. |
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What is calcium's role in light adaptation?
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From dark to light:
-Cones are hyperpolarized as much as possible. -Gradual depolarization of the membrane back to -35mV -cGMP-gated sodium channels admit calcium. -When channels close, a process is initiated which gradually opens the channels even if light levels don't change. -Ensures that photoreceptors are always able to register relative changes in light level. |
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Which are the only cells involved in vision that fire action potentials?
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Ganglion cells.
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Not cones/rods, bipolar or horizontal/amacrine.
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Why do photoreceptors release fewer transmitter molecules in the light than in the dark?
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-Dark is the preferred stimulus for the photoreceptor
-Darkness = depolarization, release of neurotransmitter. -Somewhat counterintuitive. |
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OFF v. ON bipolar cells
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-OFF bipolar cells v. ON bipolar cells.
-OFF = glutamate-gated cation channels mediate a depolarizing EPSP from the influx of Na+. -ON = g-protein coupled receptors and respond to glutamate by hyperpolarizing. -OFF = more glutamate (light off) -ON = less glutamate (light on) |
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Receptive field
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The area of the retina that, when stimulated with light, changes the cell's membrane potential.
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Bipolar receptive field
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-Two parts:
-Circular area of retina providing direct photoreceptor input, called the receptive field center -Surrounding area of retina providing input via horizontal cells, called the receptive field suround. |
Receptive field center, receptive field surround
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Ganglion cell receptive fields
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-Dark in center = cell depolarizes
-Dark in surround = cell hyperpolarizes -Dark in both = effect reduced |
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M-type ganglion cells
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-Large
-M = magno -5% of ganglion cell population -Larger receptive fields, conduct action potentials more rapidly in optic nerve, more sensitive to low-level stimuli -Respond to stimulation with a transient burst of action potentials |
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P-type ganglion cells
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-Small
-P=parvo -90% of ganglion cells -Respond to stimulation of receptive fields with sustained discharge as long as the stimulus is on. |
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nonM-nonP ganglion cells
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-5% of ganglion cells
-less well characterized |
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Color-opponent cells
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-Color sensitive neurons
-Response to one wavelength in the receptive field center is canceled by showing another wavelength in the receptive field surround. -Color-canceling (e.g. red canceled by green) |
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