Unit 9: Visual System 1

Amount of hyperpolarization

- If we record directly from photoreceptor, at resting potential of -35mV, then with the stimulus of light, the photoreceptor hyperpolarizes. - The amount of hyperpolarization is directly related to the intensity of light. - This is similar to how the increase in action potential firing in the odorant receptor cell is directly proportional to the amount or intensity of the odorent. - With dim light; little hyperpolarization; little reduction of glutamate release - With bright light, a lot of hyperpolarization; a great reduction in glutamate release.

Rods and Cones and distance from Fovea

- If you are in the center of the eye, then cones used for detection - Only cones in the fovea - Out in the periphery, only rods used for detection - Nothing in the optic disc because axons from ganglion cells leave to the brain through the optic disc (no photoreceptors here - can't detect light) - With constant movement of eye, we can

Convergence

- Low convergence of cones; 1 cone synapse onto 1 bipolar cell onto 1 retinal ganglion cell; so you get one neuron in the brain that is only processing that one cone - Can be thought of as "pixels"; more pixels for cones - High convergence for rods - You have 10 rods converging onto 4 bipolar cells, onto 1 retinal ganglion cell with the help of amacrine cell. - So one neuron in the brain is representing an average data collected from10 rods. - So low levels of pixels that is representing this averaged out information. - Data in the fovea, center of vision is very clear; periphery with rods is not as clear in our vision - Cones detect color; rods can only detect black and white -- you have really good color in the center of your vision

The inhibitory response of rods to light

- Normally in the dark there is cyclic GMP and Na+ channels kept open and Na+ ions flow in and there is depolarization and release of glutamate. - When light hits, rhodopsin (cone opsin in cones) become bleached, there is breakdown of cyclic GMP AKA Na+ close and Na+ ions don't enter the cell. - The rods hyperpolarize instead, no action potential, and no release of glutamate/ excitatory NTM.

How can different bipolar cells give opposite responses to direct cone input?

- OFF bipolar cells have iono- tropic glutamate receptors, and these glutamate-gated channels mediate a classical depolarizing excitatory postsynaptic potential from the influx of Na+. Hyperpolarization of the cone causes less neurotransmitter to be released, resulting in a more hyperpolarized bipolar cell. - On the other hand, ON bipolar cells have G-protein-coupled (metabotropic) receptors and respond to glutamate by hyperpolarizing. AKA less glutamate release means less hyperpolarization and more activation of bipolar cells.

How can the same photoreceptor excite some bipolar cells, while inhibiting others?

- Photoreceptors are hyperpolarized by light and this results in a decreased release of glutamate onto bipolar cells. - Key: Neurons at different levels of the visual system have very different receptive fields - If you reduce glutamate onto an Inhibitory Metabotropic receptor, then it will cause ON-CENTER bipolar cell to be depolarized and ON-CENTER ganglion cell to increase firing. - If you have an Excitatory Ionotropic receptor and you reduce glutamate, then you hyperpolarize an OFF-CENTER bipolar cell and decrease firing rate onto an OFF-CENTER ganglion cell. (usually true with light stimulus as glutamate is excitatory)

Design an experiment to find the receptive field of a ganglion cell

- Present light and depending on where the location is, then this can record where the receptive cells are - Hubel and Weisel recorded from LGN and Cortical cells.

Compare the similarities and differences between cones and rods

- Rods - (scotopic system) and have sensitivity and detection to low levels of light - Cones - (photopic system) not activated until you have high levels of light - At high levels of light, all rods would be activated and you will not be able to tell the difference. So you would only use your cones to tell the difference.

Phototransduction

- Similar for rods and cones but each have different proteins. - Photoreceptors are constitutively active in the dark. Na+ channels are kept open in the dark. They are depolarizing and are releasing NTM glutamate (these are excitatory) - When light hits, the rods become hyperpolarized as sodium channels close

How could you use your experiment to determine the center and surround of the receptive field of a ganglion cell?

- Spot of light in center: Bipolar cell depolarization and increase in action potential firing in ganglion cell - Spot of light in the surround: hyperpolarization in the bipolar cell and decrease in firing action potentials in the ganglion cell. - With light shined on entire on-center-off-surround cell: the center-surround organization of the receptive fields leads to a neural response that emphasizes the contrast at light-dark edges. **Look at diagram on ppw -- Opp. for off-center/ on- surround cell

Determining the receptive field of a ganglion cell

- The receptive field of a ganglion cell is located by recording from the neuron's axon in the optic nerve. A small spot of light is projected onto various parts of the retina; the receptive field consists of the locations that increase or decrease the ganglion cell's firing rate. - A receptive field on the retina corresponds to light coming from a particular location in the visual field

Neurons at Different Levels of the Visual System have very different Receptive Fields

- The receptive fields of retinal ganglion cells are concentric - aka a circular central area with a ring around it, the surround - Both bipolar and ganglion cells have 2 types: - On-center/off-surround - Off-center/on-surround - The center and the its surround are always antagonistic

Rods and Cones differences

-Cones: At phototopic levels (daylight), the most obvious is that we have much greater spatial sensitivity on our central retina. - Cones make the perception of color possible (only activated at high levels of light); aka the reason for why we can't perceive color differences at night with minima light that only activates rods - Rods: we are much poorer at discriminating colors on our peripheral retina because of smaller number of cones - We are much more sensitive to low levels of light on our peripheral retina. - Rods respond better to low levels of light and rods are abundant in the periphery

3 important points to remember:

1. With one exception, the only light-sensitive cells in the retina are the rod and cone photoreceptors. 2. The ganglion cells are the only source of output from the retina (through the optic nerve) 3. Ganglion cells are the only retinal neurons that fire action potentials and this is essential for transmitting information outside the eye.

Describe the cells in the retina. How are they connected?

Ganglion cells --> Bipolar cells --> Photoreceptors (receiving photons - rods and cones) - Then when photoreceptors receive stimuli (light), they either increase or decrease firing of bipolar cells, then synapse onto ganglion cells --> then they increase or decrease the firing rate of the ganglion cells. - Amacrine in between bipolar and ganglion cells help with the convergence of the synapse onto the ganglion cells.

Transduction of light by rods summary:

Naturally aka without light, Na+ channels are kept open by the secondary messengers cGMP: 1. Light activates (bleaches) rhodopsin 2. Transducin, the G-protein is stimulated 3. Phosphodiesterase (PDE), the effecter enzyme is activated 4. PDE activity reduces cGMP level 5. Na+ channels close, and the cell membrane hyperpolarizes.

Identify and describe the cells of the retina, and discuss the pattern of their interconnections

The most direct pathway for visual information to exit the eye is from photoreceptors to bipolar cells to ganglion cells. - Besides the cells in this direct path from photoreceptor to brain, retinal processing is influenced by two additional cell types: - Horizontal cells receive input from the photoreceptors and project neurites laterally to influence surrounding bipolar cells. - A wide variety of types of amacrine cells generally receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells.

Light has hit the surround of these cell's receptive fields, what kind of cells are they?

Use action potential firing to determine answer