Exam 7 Vision Special Senses 73X Flashcards by Christopher Waller

In broad terms give the overview of Image formation?

1) Light refracted through refractive structures of eye
2) Receptor potential produced by retinal photoreceptors
3) Neural layer processes visual data, sends nerve impulses to optic nerve
4) Impulses conveyed to thalamus and onto the cerebral cortex for interpretation as images

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What is refraction and what causes it?

Light bending as it travels through different transparent substances of different densities

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What provides refraction of light rays entering the eye? Which structure provides the majority?

Cornea (Most)
Lens
Aqueous Humor (Minor Role)
Vitreous Body (Minor Role)

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An images formed on the retina has what orientation?

Upside Down

Left-to-right reversal

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The variable refraction of light rays reflected form objects further than about 20 feet from the viewer are essentially parallel, what does this mean regarding the ciliary muscle and lens?

Ciliary Muscle is relaxed

Lens is fairly flat

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The variable refraction of light rays reflected from objects closer than 20 feet from the viewer are not parallel, what does this mean regarding the ciliary muscle and lens?

Ciliary Muscle Contracts–> reducing tension on suspensory ligaments –>
Lens shape is more spherical (thicker)

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The increase in curvature of the lens for near vision is called what?

Accommodation

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What shape will the lens have if tension is reduced via the suspensory ligaments (zonular fibers)?

Convex shape

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If the ciliary muscles relax and tension is restored to the suspensory ligaments via the choroid (acts like a spring), what shape will the lens take?

Flat (less convex)

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How is tension released from the suspensory ligaments allowing the lens to thicken and take on a more convex shape?

Ciliary muscle contracts, stretching choroid and releasing tension

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What is the minimum distance from the eye an object can be clearly focused w/maximum accommodation?

Near Point of Vision

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What term is used to describe the loss of elasticity of the lens w/aging, and therefore cannot accommodate and focus the light?

Presbyopia

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The normal eye in which is able to refract light rays from an object 20 ft (6meters) away and form a clear image on the retina is called?

Emmetropic

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Refraction abnormalities that prevent a clear image to form on the retina are?

Myopia (Nearsightedness)\
Hypermetropia (Farsightedness)
Astigmatism (Irregular curvature of cornea)

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What occurs when the eyeball is too long relative to the focusing power of the cornea and lens?

Myopia

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Where does the image focus for a person suffering from Myopia?

In Front of the Retina

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Myopia may be caused by what and how is it corrected?

Elongated Eyeball or thickened Lens

Corrected: Use of Concave Lens

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What refraction abnormality is characterized by an eyeball that is too short relative to the focusing power of the cornea and lens (thin) and in which the image is focused behind the retina?

Hypermetropia

-Convex lens to correct

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What refractive abnormality is caused by irregular curvature of either the cornea or lens and results in parts of the image blurred or distorted, and correction is by a lens that rotates the axis of the light going into the eye?

Astigmatism

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What is the other part of the overall accommodation mechanism that occurs simultaneously w/changes in thickness of the lens and will allow the light rays to enter the eye in a more parallel orientation for better focus?

Pupil Constriction (circular muscles of the iris constrict)

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Since humans are only able to focus on one set of objects at a time this provides what type of vision?

Binocular Vision: Depth perception, three-dimensional perspective

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How is Binocular vision created in our human perception?

1) Two retinas see an object from slightly different angles

2) Brain fuses the two slightly different images creating stereoscopic vision (seeing solid)

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The eyes must move medially so that both are directed toward the object being viewed as we move closer and rotate medially so light rays strike the same point on both retinas, this is called?

Convergence

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The microscope view of rods and cones consists of what? Term given in the 1830’s due to their shape

Rods: Outer Segments cylindrical or rod-shaped
Cones: Tapered or cone-shaped

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Where are cones most highly concentrated and in which dramatically decrease in density moving away from this area?

Central Fovea (Cone Peak)

Where are rods most highly concentrated?

Ring around the foveal pit (18 degrees, 4.5mm)

Which photoreceptor tends to be more densely concentrated?

Rods more so than cones

What area of the retina of the eye is receptor free?

Optic Disc (blind Spot)

The outer segment of rods and cones are next to the pigmented layer and is responsible for what?

Transducing light energy into receptor potential

How do Rods and cones increase the surface area available for trapping light?

Outer Segments are layered

How do the layers of the outer segment of rods and cones differ?

Rods: Stack Discs w/fold near base membrane Cones: Folds (long pleated fabric)

What destroys the outer segments of rods and cones and what occurs to the destroyed portions?

Light Destroys the outer segments of rods and cones | -Sloughed off and phagocytized by pigment epithelial cells

How/When does the Phagocytic activity of Rod and Cone tips occur?

Circadian Rhythm: -Rod tips: morning after first exposure to light -Cone tips: triggered by darkness (occurs at night) (Turnover of outer segment is fairly rapid)

what is contained in the inner segment of rods and cones?

Cell Nucleus Golgi Apparatus Mitochondria (Many)

The proximal end of rods and cones expands into what?

Synaptic terminal containing numerous synaptic vesicles

What are the colored, integral proteins in the plasma membrane of the outer segment of rods and cones?

Photopigments

Photopigments undergo structural changes when they absorb light and are contain two parts, what are the two parts?

Retinal: Derivative of Vit. A formed from Carotene Opsin: Glycoprotein

The photopigment in rods is called what and absorbs what light most effectively?

Rhodopsin (Retinal + Opsin = Rhodopsin_) | Blue to Green Light

Cones have three different photopigments due to the three types of cones, what are they called and what light do they absorb?

``` Photopsin I (Yellowish Absorption Maximum) Photopsin II (Green Absorption Maximum) Photopsin III (Blue-Violet Absorption Maximum) ```

Which portion of the Photopigment is essential for transducing light energy into electrical signals and which portion is different in each?

- Retinal: Same in all photopigments and essential in transducing light energy to electrical signal - Opsin: Different in each

In darkness what shape does retinal have and what is it called? This allows for what regarding Opsin?

Cis-Retinal (Bent Shape) | Opsin and Retinal fit together nicely

What occurs when the Cis-retinal absorbs a photon of light?

Straightens to trans-retinal and Opsin and retinal no longer fit, they dissociate

The conversion from cis-retinal to trans-retinal is called?

Isomerization

In the dark what ion flows into the photoreceptor outer segment through what channels?

Sodium | Via Ligand-gated Sodium Channels

The Ligand-gated Sodium Channels are held open by what? What effect does this have?

Ligand Cyclic GMP (cGMP) | -Partially depolarizes photoreceptor (Sodium-Potassium Pump expels Sodium ions out of the cell)

What does the Partial Depolarization during darkness trigger?

Continual Release of Glutamate (Neurotransmitter) at Synaptic Terminal

What is significant about Glutamate at the synaptic terminal?

Inhibitory of bipolar cell (usually it is excitatory)

Glutamate triggers inhibitory postsynaptic potentials that have what effect?

Hyperpolarize the Bipolar cells, preventing them from sending signals to ganglion cells when it is dark

What occurs when light strikes the retina?

Isomerization of cis-retinal activates enzymes that break down cGMP

Lack of cGMP causes what to happen?

Ligand-gated sodium channels close

What is the enzyme that breaks down cGMP?

Phosphodiesterase

The closing of cGMP channels has what effect?

Sodium ion inflow decreases Membrane potential becomes more negative (resulting in hyperpolarizing receptor potential decreasing release of glutamate)

The decrease in the inhibitory glutamate neurotransmitter allows for what?

Graded Potential produced by activated Bipolar Cells

Rhodopsin has a reddish color, in the presence of light cis-retinal is converted to trans-retinal and the Opsin minus the retinal have what color? What is this termed?

Colorless | Bleaching of Photopigment

What enzyme converts trans-retinal back to cis-retinal which allows it to rebind to Opsin reforming a functional photopigment? what is this called?

Enzyme: Retinal Isomerase Called: Regeneration

What photopigment contributes little to daylight vision and why?

Rods: regeneration is to slow | Cones regenerate rapidly

What is dark adaptation and how long does it take?

- Photopigments able to regenerate in the dark without being bleached by light - 30-40 minutes for bleached rhodopsin to fully regenerate

What is an inability to distinguish between certain colors due to an absence or deficiency of one of the three cone photopigments? What is the most common?

Color Blindness Common: Red-Green Color Blindness Generally inherited trait

What is another term for night blindness in which refers to the inability to see well at low light levels?

Nyctalopia

What is a typical cause of Nyctalopia?

Prolonged Vit. A deficiency and consequent below normal amount of rhodopsin, rod degeneration -Supplement will restore as long as before degenerative changes

The reina processing of images at synapses causes inputs to be enhanced and others discarded, why is the pathway typically one of convergence rather than divergence?

Photoreceptors vastly outnumber ganglion cells

A Single bipolar cell will synapse with how many Rods? what is the significance of this?

6-600 rods per bipolar cell | Convergence increase light sensitivity but decreases acuity of image

A single bipolar cell will typically synapse with how many cones?

One, less sensitive but higher acuity

What cells attenuate the differences between parts of the retina that are strongly stimulated and parts that are less stimulated, as well as assist in the differentiation of colors?

Horizontal cells | Amacrine Cells

What is the visual pathway from Optic Nerve to Occipital Lobe?

``` Optic Nerve Optic Chiasm Optic Tract Lateral Geniculate Nucleus of Thalamus Optic Radiations Primary Visual Area of Cortex (area 17) in Occipital Lobe ```

What provides output from the retina to the brain and via what structure is this accomplished?

Axons of Retinal Ganglion cells via Optic Nerve

Axons of the Optic Nerve pass through what and at this location what occurs?

Pass through: Optic Chiasm | Some cross to opposite side, some do not

What are the axons considered once they pass through the Optic Chiasm?

Optic Tract

The Optic tract terminates where?

Lateral Geniculate Nucleus of the Thalamus

Lateral Geniculate Nucleus of the Thalamus synapse w/neurons having axons that form what and in which those have projections that travel where?

Optic Radiations which project to the primary visual area (17) in the occipital lobes of the cerebral cortex

The Visual Field refers to what?

Everything that can be seen by that eye

The visual fields of our eyes overlap and provide binocular vision, each eye has two regions of their visual field?

Nasal (Central) Half | Temporal (Peripheral) Half

The light rays from objects in the nasal half of the visual field fall on what portion of the retina?

Temporal half of the retina

The light from objects in the Temporal Half of the visual field fall on what portion of the retina?

Nasal Half of the retina

What visual information is conveyed to the left side of the brain due to the crossing/not crossing over pattern at the optic chiasma?

Right half of each visual field | Left eye: Nasal Half, Right eye: Temporal Half

What visual information is conveyed to the right side of the brain due to the crossing/not crossing over at the optic chiasma?

Left half of each visual field | Left eye: Temporal half, Right eye: Nasal Half

What axons do not cross at the Optic Chiasma but continue directly to the lateral geniculate nucleus of the thalamus on the same side?

Temporal half of each retina (which field of vision is Nasal half)

Axons of collaterals of retinal ganglion cells project to the midbrain and are involved in what?

Circuits that constrict pupils and coordinate head and eye movement

Collaterals will also travel to what portion of the brain in which establishes circadian rhythms based in part on impulses of light perception from the eyes?

Suprachiasmatic nucleus

What are the projection of axons from the thalamus to the primary visual area of the cortex on the same side?

Optic Radiations

Loss of vision in an otherwise normal eye, caused by either no transmission or poor transmission of visual stimulation through the optic nerve to the brain during early childhood or for sustained period due to strabismus, anisometropia cataract, trauma, is called?

Amblyopia

What is the name of the fissure slightly above the occipital pole and travels along the middle cerebrum between the cuneus (cuneate) and lingual (linguate) gyrus?

Calcarine Sulcus

What is concentrated on the banks of the calcarine Sulcus?

Primary Visual Cortex

Central Visual Field is located on what portion of the Calcarine Sulcus?

Posterior Portion

Peripheral Visual Field is located on what portion of the Calcarine Sulcus?

Anterior Portion

The upper bank of the calcarine sulcus responds strongly to which portion of the visual field?

Lower half (below center)

The lower bank of the calcarine sulcus responds strongly to which portion of the visual field?

Upper Half (Above Center)

Processing of visual signals involves a number of separate systems within the cerebral cortex and to operate in concert to interpret what different aspects of visual images?

1) Process information related to shape 2) Process information related to color 3) Process information related to movement, location, and spatial organizaiton