Chapter 9: Vision

What is a receptive field?

A portion of the retina that when a small portion of light shines on it the firing rate changes. Light anywhere else on the retina outside this receptive field would have no effect on firing rate.

What is accommodation

Accommodation is changing the shape of the lens which is used to change focusing power.

Why does light bend when going through two different mediums?

Bending occurs because of the difference between the speed of light through the two medias. Light travels faster through air than it does through water. Bending is known as refraction

What are the maximal activation wavelengths of blue, red, and green cones?

Blue- 430 nm Green- 530 nm Red- 560 nm

What are the two pathways from photoreceptors to bipolar cells?

Direct path- involved no horizontal cell interference, Light shined on to cons will depolarise turn On the bipolar cell, making it an ON bipolar cell Indirect- horizontal cell hyperpolarize with the OFF bipolar cell,

Only ______ cells fire.

Ganglion cells fire action potentials, all other cells in the retina respond to stimulation with graded changes in membrane potential.

What is the neurotransmitter that bipolar cells respond to?

Glutamate is the NT that bipolar cells respond too

What is the basic system of retinal information processing?

Information about light flows from the photoreceptors to bipolar cells, to ganglion cells which project axons out of the eye into the optic nerve. Horizontal and amacrine cells modify the response of the bipolar and ganglion cells by lateral connection.

List the layers of the retina in order that light passes through and what each layer contains.

Innermost closest to the light ganglion cell layer- cell Bodies (Inner plexiform layer)- synaptic connections between bipolar cells amacrine cells and ganglion cells Inner nuclear layer- cell bodies of the bipolar cells, horizontal cells and amacrine cells (Outer plexiform layer)- where photoreceptors make synaptic contact between bipolar and horizontal cells. Outer nuclear layer- cell bodies of photoreceptors Layer of photoreceptor outer segments - contains the light sensitive elements of the retina Pigmented epithelium Outermost/ furthest from the light

What types of information does the ganglion cell population send (Hint: there are 3 types)?

Light vs dark Red vs green Blue vs yellow

Which type of ganglion cell have no color opponency?

M ganglion cells lack color opponency, their responses are non-color specific.

What stimulates the sodium channels to open up?

Na + channels are stimulated by intracellular second messenger proteins cGMP

What are the differences between ON and OFF bipolar cells?

On bipolar cell depolarize in the presence of light and off bipolar cell hyperpolarize in the presense of light.

Compare and contrast M and P-type ganglion cells. What percent of ganglions do these cells make up?

P cells constitute 90% of the ganglion cell population. M cells constitute 5% of the ganglion cell population. And nonM-nonP ganglion cells types make up the last 5% M ganglion cells Larger receptive fields Conduct action potentials more rapidly in the optic nerve More sensitive to low contrast stimuli lack color opponency with their responses are non color specific P type ganglion cells Color opponent cells, reflecting the fact that the response to color in the receptive field center is canceled by showing another color in the receptive field surround. are color sensensitive neurons

What photopigment does light activate? How is retinal related to this process?

Rhodopsin, a receptor proteins found in the rod outer segment. Opsin is the name of the receptor protein, the prebound agonist is called retinal. The absorption of light causes a change in the conformation of retinal so that it activates the opsin. The process is called bleaching, because it changes the wavelengths absorbed by rhodopsin.

how do Rods and cones photoreceptors differ functionally and structurally?

Rod photoreceptors - Have long cylindrical outer segments contatining many disks great number of disks and higher photopigment concentration, making them more sensitive to light - Makes vision possible in low light Cone photoreceptors - shorter tapering outer segments containing few membranous disks - In day time lighting, cones are used most - three type of cones each containing different pigments

Make sure you understand the mechanisms of phototransduction in rods and cones. How is it different between rods and cones?

Rods - Phototransduction is done by a photopigment called rhodopsin(opsin is the receptor protein and retinal is the agonist) - The G-protein called transducin is stimulated - Transducin activates the effector enzyme PDE - PDE activity reduces the cGMP level - As a result, Na+ close and the cell membrane hyperpolarizes Cones - Same form of transduction except for the opsins in the membranous disks of the outer segments. - First set of cones are activated by short wave length, blue cones. The second set of cones are activated by medium wavelength green cones. The third set of cones are activated by long wave light red cones. Each cone type is activated by a broad range of wavelengths within the range, and there is overlap between groups of cones. The cone types are known as short medium and long.

Where are cones and rods located?

Rods and cones are towards the back of the retina in the second outermost section behind the pigmented epithelium

What is the role of calcium in light adaptation

The constriction of the pupil helps in reducing light entering the eye but the most important change is a gradual depolariation of the membrane back to about -35 mV. This happens because of the fact that cGMP gated channels, also admit calcium ions. In dark Ca2+ enters the cones and has an inhibitory affect on the enzyme that makes cGMP, guanylyl cyclase. When the cGMP gated channels close from light, the calcium flow stops along with the sodium flow, inturn increasing the synthesis of cGMP. Which thereby allowing the cGMP gated channels to open again. In short: Ca2+ allows us to see changes in different amounts of light.

How does the curve of the cornea impact focal distances

The distance from the refractice surface to the point where parallel light rays converge is called the focal distance. Focal distance depends on the curvature of the cornea-the tighter the curve, the shorter the focal distance

What is the function of the fovea?

The fovea is a thinning of the retina at the center of the macula. The lateral displacement of the cells above the photoreceptor allows light to strike the photoreceptor without passing through the other retinal cell layers. This structural specialization maximizeses visual acuity at the fovea by pushing aside other cells that might scatter light and blur the image.

What is the membrane potential in complete darkness? What happens when light is applied?

The membrane potential in complete darkness is -30 mV in rods. This constant depolarization is caused with a steady influx of Na+ through special channels in the outer segment membrane. This flow of positive charge across the membrane, occurring in the dark is known as dark current. When light is applied an enzyme activates that destroys the second messenger molecule cGMP responsible for keeping the sodium channels open, thereby shutting off the Na+ current which makes the membrane potential more negative, inturn hyperpolarizing the cell

What is dark current?

The movement of positive charge across the membrane, which occurs in the dark, is called the dark current.

What is pupillary light reflex?

The pupillary light reflex involves connections of neurons between the retina and the brain stem that control muscles in the pupil. Which constricts the pupil in response o light, thereby adjusting the amount of light that reaches the retina.

What are antagonistic center-surround receptive fields?

The response of a bipolars cells receptive field center is opposite to that of the receptor field surround.

How are rods/cones distributed in relation to the fovea? What the implications for vision?

The rods are found mainly in the peripheral retina and there are no rods in the central fovea. Cones are found primarily in the central retina within 10 degree of the fovea

What is the Young-Helmholtz Trichromacy Theory?

The theory that the brain assigns colors based on a comparative read out of the three cone types

Which range of wavelength is visible to human eye?

Visible light ranges wavelengths from 400-700 nm

What is the visual field

Visual field is the range of area you can see without moving your head or your eyes.

What are 3 ways one characterizes/describes light waves?

Wavelength- the distances between successive waves and peaks Frequency- the number of waves per second Amplitude- the difference between peak and trough

When light travels and hits a surface, what are the three thongs that light can do? Which of these processes is the method vision occurs?

When light his the surface, it can reflect, absorb, and refract. Vision uses refraction

What are ipRGCs?

ipRGCs are a small percentage of retinal ganglion cells that actually transudce light. ipRGCs depolarize to light, they have large dendritic fields meaning the cells sum light input is over muchlarger areas of the retina than rods and cones.

How does the cornea and lens focus light to one spot of the retina

light that enters the eye perpendicular to the corneal surface passes straight to the retina, but light rays that strike the curved surface of the cornea at angles other than perpendicular are bent such that they converge on the back of the retina.

Visual acuity

the ability to distinguish two-point near each other is known as visual acuity

What is one advantage to inside out organization?

the photoreceptors play a critical role in the maintenance of the photoreceptors and photopigments. The pigmented epithelium also absorbs any light that passes entirely through the retina, thus minimizing the scattering of light within the eye that would blur the image.