Lecture 7: vision I Flashcards
Refraction?
bending of light waves at an angulated surface of transient material
What can alter the degree of refraction?
- ratio of two reflective indices
- degree of angle between interface and entering wave light
What is refractive index?
- ratio of velocity of light in air: velocity of light traveling in substance
What is refractive Power?
- measure of how much a lens bends light waves. measure in diopters
What is focal Point?
- point where all parallel rays converge after passing through the lens
What is focal length?
- distance from center of lens to the focal point
Describe how the depth of focus of the lens changes?
The depth of focus is inversely related to the size of the pupil diameter
What is Emmetropia?
- eye has normal depth of focus
- when ciliary muscles relax distant object light rays sharply focus on the retina
What is hyperopia?
- farsightedness
- distant objects seen clearly
- caused by eyeball that is too short
What is Myopia?
- nearsightedness
What causes light rays to converge in from of the retina?
- myopia
- caused by eyeball that is too long
What is the maximum visual acuity for two-point sources?
1.5 to 2 mm
What is visual acuity?
measures the resolving power of eye
- eye chart test
Where does the aqueous humor come from?
ciliary processes
In order to pull the aqueous humor into the anterior chamber, what is the environment like?
it would be high osmolarity. High concentration of ions to draw fluid through via osmosis
What is glaucoma?
Increased intraocular pressure that will lead to damage of the optic nerve and potentially vision loss, if not corrected
What is the photosenstivie pigment of rods?
rhodopsin
What is the photosensitive pigment in cones?
contain three different pigments
What two components join together to form rhodopsin?
scotopsin and 11-cis retinal
What happens when rhodopsin is exposed to light waves?
the cis is converted to trans and does not have the proper orientation to bind with scotopsin
- forms bathorhodopsin
What is activated rhodopsin and what does it do?
- metarhodopsin II
- excites electrical charges in rods that signal to optic nerve potential
Degradation of metarhodopsin II produces what compounds?
- all trans retinal and scotopsin
- isomerize with isomerase to the cis form
How can increased levels of Vitamin A be helpful with vision?
Vitamin A is isomerized to 11-cis retinol and converted to 11-cis retinal
What is the resulting receptor potential when the rods are exposed to light?
increasing negativity. hyperpolarizing
Why does activation of rhodopsin cause hyperpolarization?
- the decompensation of rhodopsin decreases the rod membrane conductance for Na ions, in the outer segment of the rod.
- and the rod is continually pumping Na ions out of the membrane
Explain how a -40 mV potential is generated in the rod in a normal dark condition?
- cGMP gated channels are open and allow leaking of Na ions back inside the rod membrane
In what conditions are you likely to find a hyperpolarized rod, and a less polarized rod?
- hyperpolarized: in light conditions
- non polarized: in dark conditions
What is light adaptation?
- results in the reduction of rhodopsin and photosensitive chemicals
- reduces the sensitivity of the eye to light
What is dark adaptation?
- results in low light and the accumulation of rhodopsin and other light sensitive pigments
- increases the sensitivity of the eye to light
How do we perceive color?
- based on the mixing ratio of blue, green , red stimulation of the cones
Green and red cones are required to see what colors?
- green
- yellow
- orange
- red
Blue, green and red cones are required to see what colors?
- violet
- blue
- green
What is protanope?
loss of red cones
What is deuteranope?
- loss of green cones
What is loss of blue cones?
- blue weakness
What are the different neuronal cell types of the retina from superficial to deep?
- rods and cones
- horizontal cells
- bipolar cells
- amacrine cells
- ganglion cells
Where do rods and cones synapse?
- bipolar and horizontal cells
What is the function of horizontal cells?
- transmit signal from outer plexiform layer to horizontally from rods and cones to the bipolar cells
What is the function of bipolar cells?
- transmit signal from rods, cones, horizontal cells to inner plexiform layer
- synapse with ganglion and amacrine cells
What is the function of amacrine cells?
- transmit signal from bipolar to ganglion cells
- send signal across inner plexiform layer
What is the function of the ganglion cells?
- transmit signal from retina to the optic nerve
What is the interplexiform cell?
- transmits retrograde signal from inner to outer plexiform layer
- inhibitory and control lateral spread of vision
What neurons are in the foveal region direct pathway (aka: fast cone system)?
- cones
- bipolar cells
- ganglion cells
What four neurons purely make up rod vision?
- rods
- bipolar cells
- amacrine cells
- ganglion cells
What neurotransmitter is released at the synapse of rods/cones and bipolar cells?
glutatmate
Amacrine cells secrete what neurotransmitters that are all inhibitory?
- GABA
- glycine
- dopamine
- acetylcholine
- indolamine
Which neurons are the only ones that will transmit their visual signals as an action potential?
- ganglion cells
What is the importance of electrotonic conduction within the eyes?
- this process allows for rods and cones to respond to varying levels of lighting
- eliminates the all or none action potential
With the diverse amount of amacrine cells, what if the most likely function common to them?
- interneurons that analyze visual signals before they leave the retina
If an amacrine cell is directionally sensitive, what does that mean?
- responds to movement of a spot across the retina
What is the effect of fewer rods and cones converging on an optic nerve at the fovea?
- increased visual acuity
- the central fovea only contains slender cones (increases visual acuity)
The absence of rods at the central fovea reduces what aspect of sight, at this area?
- sensitivity to weak light.
Why is the peripheral retina more sensitive to weak light?
- multiple rods converge on an optic nerve
Which type of retinal ganglion cells are important for crude rod vision in dark conditions?
- type W
What types of signals are sent from the type W ganglion cells?
- slow velocity
- excitation from rods
- transmit by bipolar and amacrine cells
Color vision most likely comes from what retinal ganglion cells?
- X
What type of signals can be represented by type X ganglion cells?
- fine detailed images on discrete retinal locations
- due to fact that cells receive input from one or more cones
What is characteristic about the Y ganglion cells?
- responds to rapid changes in vision
- most likely to apprise the CNS of new visual activity with low accuracy
What portions of the optic nerves cross at the optic chiasm?
nasal halves of the retina
What visual fields compose the left optic tract and where does it synapse at?
- the left lateral field and right nasal field
- synapse in the dorsal lateral geniculate nucleus in the thalamus
What visual fields compose the right optic tract and where will it synapse?
- the right lateral field and the left nasal field
- synapse in the dorsal lateral geniculate nucleus of the thalamus
From the dorsal lateral geniculate nucleus where do optic fibers pass to provide vision?
- geniculocalacrine fibers move too the primary visual cortex via optic radiation
Where is the primary visual cortex located?
- the calcarine fissure of the medial occipital lobe
What are the two primary functions of the dorsal lateral geniculate nucleus?
- relay visual information from optic tract to visual cortex via optic radiation
- regulates amount of signals that pass to the cortex.
The dorsal lateral geniculate nucleus receives signals from the lateral half of the ipsilateral retina on what layers?
- layers II, III, V
What visual fields do layers I, IV, and VI of the dorsal lateral geniculate nucleus receive?
- medial field of the contralateral eye
What fields of view do layers II, III, V of the DLGN receive?
- lateral ipsilateral views
What is contained in layers I and II of the DLGN?
- magnocellular layers
- input from Y ganglion cells
- rapid conduction of visual cortex
- black and white only; poor point to point transmission
What are layers III and IV of the DLGN, and what do they respond to?
- parvocellular layers
- input from X ganglion
- transmit color
- accurate point to point transmission
What is the source of DLGN transmission gating?
- corticofugal fibers from primary visual cortex
- reticular area of mesencephalon
- both are inhibitory and highlight visual information that is allowed to pass
The primary cortex contains an region where the macular area of sight terminates. Where is that?
- near the occipital pole
Where do signals from the peripheral retina terminate?
- concentric half anterior to pole, still within the calcarine fissure
Where do geniculocalcarine fibers terminate in the primary visual cortex?
- layer IV
Type Y ganglion fibers terminate in what layer of the primary visual cortex?
- layer IVc alpha
Type X ganglion fibers terminate in what layer of the primary visual cortex?
- layer IVc beta and IVa
After passing into the primary cortex where do signals next encounter?
- brodmanns’ area 18
- here the signals are analyzed for visual meaning
In layer IV of the cortical area, images are analyzed to help adjust what?
- directional gaze of each eye
- used in stereopsis
“Complex cells” are able to detect what?
- detect line orientation when a line is displaced laterally or vertically in the visual field
“Simple” cells are able to detect what and are located where?
- able to detect orientation of lines, and different orientations stimulate different sets of cells
- these cells are found in the layer IV of primary visual cortex
What is the autonomic nerve pathway of the eye starting with the synapse from the pretectal region?
- preganglionic fibers arise in the Edinger-Westphal nucleus and pass with CN III to synapse in ciliary ganglion
- ciliary ganglion synapse converts to postganlgionic parasympathetic fibers that move through ciliary nerves to the eyeball
What are the actions of the ciliary postganglionic ciliary nerves of the eye?
- excite the ciliary muscle to control accommodation
- control the sphincter that constricts the pupil
When the lens in spherical what are the ciliary muscles doing?
- contracted
- long distance objects
When the ciliary muscles contract, what type of shape does the lens take on?
- spherical
- objects at short distance
What two fibers are under control of ciliary nerves (CN III)?
- meridional fibers that contract and release lens tension, forming a sphere
- circular fibers that also decrease lens tension
What is presbyopia and when does it occur?
- loss of the ability to accommodate
- occurs by 45-70 y/o
- due to lens thickening and becoming larger
What is the effect of the larger and thicker lens in elderly individuals
- presbyopia
- loss of accommodation to significant degree
