Flashcards in Lectures 5 & 6 - The Eye Deck (164):
What are 2 other names for the primary visual cortex?
Striate cortex, V1
What is vision?
Our perceptual experience of light: light is transduced by photoreceptor neurons and the signal is transmitted across many synapses in retina before reaching the optic nerve, which transmits signals to the brain
What are the 4 CNS structures for vision?
3. Primary visual cortex
4. Extrastriate cortex
What is another name for the extrastriate cortex?
Is visual system activity determined by absolute light levels?
NOPE, relative light levels
What is the dark current?
In the absence of light, photopigment proteins are inactive and therefore photoreceptors are depolarized and have current: cGMP opens cGMP gated cation channels (Na+ and Ca++ flow in)
What is the relative light level dependence of visual system activity due to?
Receptive field properties that begin in the retina
What are the 2 properties of light waves? What does each correspond to?
1. Amplitude: brightness
2. Wavelength: color
What is the wavelength related to?
Inverse of frequency
What is the range of visible light?
What is the visible light with the highest energy?
400 nm = blue light
What is the visible light with the lowest energy?
700 nm = red light
What are the 3 possible behaviors of light rays on different media?
When an object absorbs light, what light do we see?
The light that is NOT absorbed
What changes when light gets refracted?
What is velocity?
Direction + speed
If you have a red reflector strip on a bike, what behaviors of light are taking place when we look at it?
- Reflection of red light by strip
- Absorption of some light by strip
- Refraction within the eye
What are the 3 tissue layers of the eye?
1. Fibrous tunic
2. Vascular tunic
What are the 2 parts of the fibrous tunic of the eye? What does each constitute?
1. Sclera (posterior 2/3rds)
2. Cornea (anterior 1/3rd)
What are the 3 parts of the vascular tunic of the eye?
2. Ciliary body
List the 6 layers of the retina from superficial to deep.
1. Ganglion cell layer
2. Inner plexiform layer
3. Inner nuclear layer
4. Outer plexiform layer
5. Outer nuclear layer
6. Pigmented epithelium layer
What is the sclera of the eye?
- Opaque connective tissue
- Posterior 5/6th of fibrous tunic
- Part of the eye commonly known as the “white”
What is the insertion point for the extraocular eye muscles?
What is the cornea of the eye?
Transparent and convex anterior 1/3rd of the fibrous tunic
What is the iris of the eye?
Pigmented muscular curtain near the front of the eye, between the cornea and the lens, that is perforated by an opening called the pupil
What is the role of the ciliary body?
Secretes fluid into the eye
What is the choroid of the eye?
Posterior majority of vascular tunic made of vascularized connective tissue
What causes blood shot eyes?
Choroid becomes red
What branches off the ciliary body? Role?
Ciliary processes: secrete aqueous humor
What part of the eye is responsible for the pupillary light reflex?
What part of the eye is responsible for the accommodation reflex?
How many intraocular muscles in the iris? What are they? What is each innervated by? What do they do?
1. Sphincter (constrictor) muscle - Parasympathetic NS (CN III) -> constricts pupil
2. Dilator muscle -Sympathetic NS -> dilates pupil
==> Control pupil size
What kind of muscles are the intraocular iris muscles?
What are the zonula fibers of the eye?
Fibers that suspend the lens in position during accommodation
Describe the process of accommodation to near vision.
Near source => parasympathetic bilateral afferents in CN II => synapse at lateral geniculate nuclei of thalamus => synapse at visual cortex => synapse at visual association cortex => synapse at Edinger-Westphal nuclei of the midbrain => parasympathetic preganglionic neurons (CN III) => synapse at ciliary ganglia => postganglionic neurons => intraocular contraction of ciliary muscles (both eyes) => relaxation of zonula fibers => relaxation of lens => increases refractive power
What are the 2 effects of the pupillary reflex test? Describe each.
1. Direct: constriction of illuminated pupil
2. Consensual: constriction of contralateral pupil
What is a consensual response?
Any reflex observed on one side of the body when the other side has been stimulated
Describe the pathway of the pupillary light reflex.
Light => eyes => bilateral CN II parasympathetic afferents => synapse at both lobes of pretectal nucleus of midbrain => neurons from each PTN lobe to EW with crossing over from each PTN lobe => Edinger-Westphal nuclei of the midbrain => preganglionic parasympathetic neurons in the efferent axons in CN III going to both eyes => ciliary ganglia => postganglionic parasympathetic neurons => short fibers in ciliary nerve => sphincter muscles of iris
What is another name for the consensual effect?
What causes constriction of the contralateral pupil during the pupillary reflex test?
Crossing over of neurons from the pretectal nuclei to the Edinger-Westphal nuclei of the midbrain
Describe the innervation pathway of the dilation of the pupil.
Bilateral input in low light => hypothalamus => neurons to cervical spinal cord => synapse with preganglionic sympathetic efferents in spinal nerve => inferior cervical ganglia => synapse in superior cervical ganglion => postganglionic sympathetic neurons => dilator muscles
What are the 3 important structures of the retina? Describe each.
1. Optic disc: 2mm hole in posterior retina = highly vascularized exit/entry point of optic nerve
2. Fovea: depression (1.5 mm diameter) of outer-most cellular neuronal layer of retina
3. Macula: 6 mm in diameter
What constitutes the blind spot of the eye? What does it mean?
The optic disc of the retina
No photoreceptors in this spot
What is an ophthalmoscope?
Tool used to view the retina through the pupil
Why does the fovea have improved acuity?
1. Because the depression allows for the light to directly reach the photoreceptors
2. Because its RGCs have a smaller receptive field size
What part of the eye has the majority of the refractive power? What other part helps?
The lens has additional and VARIABLE refractive power
What innervates refraction in the eye?
Parasympathetic reflexive control (accommodation)
What requires greater refraction?
Greater divergence of light rays
What does relaxation of the ciliary muscle mean for refraction?
What does contraction of the ciliary muscle mean for refraction?
Near vision: ciliary muscle contracted or relaxed?
Far vision: ciliary muscle contracted or relaxed?
What is the pathway of light through the eye? Use anatomical terms and reflection, refraction, and absorption
Light reflected from object => refraction in cornea => refraction in aqueous humor => pupil => refraction in lens => refraction in vitreous humor => absorption in retina by photoreceptors and signal transduction => bipolar cells => ganglion cells
Does a distant object require higher or lower refractive power? What does this mean for the shape of the lens?
Does a near object require higher or lower refractive power? What does this mean for the shape of the lens?
What is myopia? Too much or too little refraction? What is it caused by?
Too much refraction
Caused by steep curvature of the cornea
What is hyperopia? Too much or too little refraction? What is it caused by?
Too little refraction
Caused by flattening of the cornea
What is astigmatism?
Uneven curvature of the cornea causing uneven refraction
What are corrective lenses or laser surgery a treatment for?
Near and far sightedness
What leads to common visual impairments?
Errors in refractive power of cornea
Is visual acuity homogenous across the retina?
What is the purpose of the Snellen Eye Chart?
To test our ability to discriminate letters and numbers at 20 feet
What does the denominator correspond to in the Snellen Eye Chart?
Related to the size of the letters, which increases as you move up chart
What does the nominator correspond to in the Snellen Eye Chart?
Distance between the subject and the chart (always 20 ft)
Definition of visual acuity?
Ability to distinguish light from nearby points in visual field
Normal visual acuity?
1.74 mm @ 20 feet
What are the 5 cell types of the nuclear layer of the retina?
1. Bipolar cells
2. Horizontal cells
3. Amacrine cells
4. Photoreceptor cells
5. Ganglion cells
At what layer of the retina does pigmentation start?
Outer nuclear layer
Does transduction happen in the ganglion cell layer of the retina?
In what retina cell layer are photoreceptors found?
Outer nuclear layer
In what retina cell layer are amacrine cells found?
Inner plexiform layer
In what retina cell layer are bipolar cells found?
Inner nuclear later
In what retina cell layer are horizontal cells found?
Outer plexiform layer
By what cells is light transduced?
What is the direct pathway of light transduction? What NT does it use?
Photoreceptors => bipolar cells => ganglion cells
What are the only retinal cells capable of firing APs?
What is the lateral pathway of light transduction? What NT does it use?
Inhibitory information from amacrine and horizontal cells to dampen the responsiveness of adjacent cells in direct pathway
How many photoreceptors in each eye?
What are the 2 types of photoreceptor cells? % for each?
1. Rods: 90%
2. Cones: 10%
List the 3 portions of photoreceptors cells.
1. Synaptic terminals
2. Inner segments with cell bodies
3. Outer segments
How do cones and rods differ?
1. Number of disks: rods have many, cones have few
2. Sensitivity to light: high for rods, 1000x lower for cones
3. Types of photopigment protein (opsin part): rhodopsin for rods and photopsin 1-3 for cones
In dim light are we primarily using rods or cones?
How are the photoreceptors distributed across the retina?
How does the fovea's photoreceptor composition differ to that of the periphery? Explain each.
1. More cones: better color vision
2. Less rods: lower light sensitivity
3. Smaller ganglion cells receptive field in fovea: higher visual acuity
How does receptive field size relate to visual acuity?
Smaller ganglion cells receptive field = high visual acuity
What is the RGC receptive field size?
Small circular patch of photoreceptors that send signals to a single retinal ganglion cell
What are the 3 types of vision during the day? Is each rod or cone mediated? Describe each.
1. Scotopic vision: rod mediated so poor acuity and high sensitivity
2. Mesopic vision: rod and cone mediated
3. Photopic vision: cone mediated so poor sensitivity and high acuity and color vision
When is scotopic vision used?
Overcast, no moon
When is mesopic vision used?
Moolight to early twilight
When is photopic vision used?
Early twilight to daytime (inside and outside)
Why is the photopic vision cone mediated?
Because the rods are saturated
1. Progressive degeneration of photoreceptors in the macula
2. Loss of vision in the central portion of the visual field
3. None other than retinal transplant
What is the cause of macular degeneration?
Unknown: genetics, CVD, oxidative stress
What is the initial symptom of macular degeneration?
Definition of phototransduction?
Process by which the outer segments of photoreceptors transduce light to an electrical signal = hyperpolarization
What are the 2 parts of the photopigment proteins? Describe each and its role.
1. Retinal = aldehyde of VA: light-absorbing molecule
2. Opsin = GPCR
Describe the phototransduction pathway in rods.
Light hits retinal portion of rhodopsin => rhodopsin is now activated and facilitates the exchange of GDP for GTP on transducin => transducin activates phosphodiesterase to convert cGMP to GMP => cGMP gated cation channels close => hyperpolarization of photoreceptor cell => IPSP => inhibition of glutamate release
Where is rhodopsin found in rods?
What is transducin?
G-protein that activates phosphodiesterase upon activation
How does light affect retinal on rhodopsin exactly? Explain.
It bleaches it: photo absorption breaks the double bond between C11 and C12 which permits rotation and conversion from 11-cis-retinal to all-trans-retinal
What is the initiating step in vision?
Can retinal absorb more light after bleaching? What does this mean?
NOPE so regeneration of retinal is necessary
What does retinal regeneration require?
Enzymes in the pigment epithelium
How do the photopsin varieties differ?
What is the trichromacy theory of color?
Net activity across 3 cone types is analyzed by brain to produce all color varieties
Describe the 3 types of photopsin.
1. Long wavelength: red
2. Medium wavelength: green
3. Short wavelength: blue
Does the wavelength sensitivity of cones overlap?
YUP, mainly between green and red ones
How does the sensitivity of photoreceptors vary with light variations?
1. Dark adaptation = sensitization
2. Light adaptation = desensitization
What are the 2 mechanisms of dark adaptation?
1. Dilation of pupils (minor)
2. Regeneration of unbleached retinal (major)
Does dark adaptation go faster in rods or cones? Why?
Cones because they have a smaller outer segment and same size pigmented epithelium to be able to regenerate a smaller amount of bleached retinal
What are the 2 mechanisms of light adaptation?
1. Bleaching retinal (immediate)
2. Decrease in intracellular Ca++
Describe the mechanism of decrease in intracellular Ca++ during light adaptation?
Light => decreases Ca++ conductance => less Ca++ in cells to inhibit the GC => more cGMP => cGMP gated channels open => opposes effect of light
Describe the receptive field characteristics of bipolar and retinal ganglion cells.
Special center-surround receptive field characteristics:
- Their response to light in the center receptive field is either excitatory (ON) or inhibitory (OFF)
- Their response to light in the surround receptive field opposes the effect of light in the center receptive field which is further inhibited by GABAergic horizontal and amacrine cells
Are the center-surround receptive field characteristics of non-transducing cells in the direct pathway inherited by cells in target structures?
What do the center-surround receptive field characteristics of non-transducing cells in the direct pathway allow us to do?
Describe how an ON bipolar cell works (dark and light).
DEPOLARIZES w/ light in CENTER receptive field:
- Dark => glutamate released by photoreceptors => mGluR6 activated => K+ efflux => hyperpolarization
- Light => no glutamate released by photoreceptors => mGluR6 inactive => no K+ efflux => depolarization
Describe how an OFF bipolar cell works (dark and light).
HYPERPOLARIZES w/ light in CENTER receptive field:
- Dark => glutamate released by photoreceptors => AMPA active => Na+ influx => depolarization
- Light => no glutamate released by photoreceptors => AMPA not activated => NO Na+ influx => hyperpolarization
What receptors do ON bipolar cells express?
What receptors do OFF bipolar cells express?
What NT do horizontal cells release?
What NT do photoreceptors release?
Describe the transduction pathway from photoreceptors to ON bipolar cells in both the center and surround receptive fields.
- Light in center receptive field => photoreceptor hyperpolarization => no glutamate release => bipolar cell depolarized
- Light in surround receptive field => center photoreceptors release glutamate and surround ones do not => horizontal cells inhibit center photoreceptors with GABA => hyperpolarization of bipolar cells
What forms the optic nerve?
Axons of retinal ganglion cells
What are the 2 basic types of RGC receptive fields?
When is the maximal response in an ON-center/OFF-surround ganglion cell achieved?
When the entire center of the receptive field is illuminated
When is the minimal response in an ON-center/OFF-surround ganglion cell achieved?
When the entire surround of the receptive field is illuminated (ring of light)
Which effect is stronger on the RGC: center or surround?
What happens when you illuminate the entire receptive field of an ON-center/OFF surround RGC? Why?
Response will be just above baseline because the center effects are slightly stronger than surround
When is the maximal response in an OFF-center/ON-surround ganglion cell achieved?
When the entire surround of the receptive field is illuminated (ring of light)
When is the minimal response in an OFF-center/ON-surround ganglion cell achieved?
When the entire center of the receptive field is illuminated
Describe the transduction pathway from photoreceptors to OFF bipolar cells in both the center and surround receptive fields.
- Light in center receptive field => photoreceptor hyperpolarization => no glutamate release => bipolar cell is hyperpolarized
- Light in surround receptive field => center photoreceptors release glutamate and surround ones do not => horizontal cells do not inhibit center photoreceptors with GABA => bipolar cell is depolarized
What are the 2 basic types of RGCs?
ON or OFF cells
What kind of receptors do RGCs have?
What is the color-opponent processing in RGCs?
The receptive fields of ganglion cells innervated by cones have a center-surround organization also and the center and surround are also color-opponent, meaning the ganglion cell is excited by one color in the center and inhibited by the "opposite" color in the surrounding area
What are the opposite colors of the color-opponent processing in RGCs?
What does total color blindness lead to?
What does partial colorblindness lead to? What are the 2 types?
Dichromacy = loss of one cone type
1. Red-green color blindness: lack green or red cones
2. Yellow-blue color blindness: lack blue cones
On what gene is the red-green color blindness? What does this mean?
Men are more prone to it
On what gene is the yellow-blue color blindness?
What are the 3 ways of classifying RGCs?
1. Appearange: large (Magno) or small (Parvo) or other (non-M non-P)
2. Connectivity: to rods or cones
3. Electrophysiology: sensitivity to light, adaptation rate, and conduction velocity
Describe the 3 characteristics of M-type RGCs. What % of the total RGC population do they constitute?
1. Large with a large receptive field
2. Connected to rods via bipolar cells
3. Highly sensitive, fast adaptation (shorter AP), and rapid conduction
What is another way of saying an RGC shows fast adaptation?
It has a transient response
Can each type of RGC be ON or OFF type?
Describe the 3 characteristics of P-type RGCs. What % of the total RGC population do they constitute?
1. Small with a small receptive field
2. Connected to cones via bipolar cells
3. Low sensitivity to light and high sensitivity to color, slow adaptation (longer AP), and slow conduction
Describe the 1 characteristicsof non-M non-P-type RGCs. What % of the total RGC population do they constitute?
Connected to cones (little else known)
What does tunnel vision mean?
Loss of peripheral vision
Give an example of rods being more sensitive to light than cones?
1 photo can change the membrane potential in rods, but not in cones
What kind of layers are the inner and outer plexiform layers?
How does the macula's photoreceptor composition differ to that of the periphery?
Smaller ganglion cells receptive field in macula
What is the visual field?
The environmental space viewed by one eye or both eyes
How is visual field different from receptive field?
Visual field is the space around us and the receptive field corresponds to the number of photoreceptors sending signals to each bipolar cell/RGC
How do acuity and sensitivity relate to each other?
Inverse relationship: the higher the acuity, the lower the sensitivity
Do the photoreceptors absorb all of the light that reaches the retina?
NOPE, they absorb the light that the pigmented epithelium did not absorb
How do horizontal cells work? What is their role?
Horizontal cells are activated by glutamate secreted by either the center or surround photoreceptors and secrete GABA on GABAa receptors on either the center or surround photoreceptors to further inhibit them and strengthen the contrast of the center/surround receptive field
What happens when you illuminate the entire receptive field of an OFF-center/ON surround RGC? Why?
Response will be just below baseline because the center effects are slightly stronger than surround
What is a diopter?
Unit of refractive power that is equal to the reciprocal of the focal length (in meters) of a given lens
What is the focal length?
Distance between the cornea and retina
What is a fundus exam of the eye?
Physician looks through opthalmoscope to look at fundus and uses drops to dilate pupil to have a better view
What is the fundus of the eye?
The interior lining of the eyeball, including the retina, optic disc, and the macula
What is papilledema? Usually bilateral or unilateral?
Bulging of the optic disc due to swelling that is caused by increased intracranial pressure
What other non-ocular structure can a papilledema affect?
Put pressure on the olfactory bulb
What is nystagmus?
Involuntary movement of the eye
Role of sudomotor nerve? Where does it come from?
Innervate face to control vasodilation and sweating
and smooth muscles of eyelid
Same pathway as sympathetic innervation of the pupil but branches off at the external carotid artery