Eye neurobiology Flashcards

Question

What is the third step of phototransduction

Answer 1

Channels that depolarize photoreceptors are cGMP gated, in the dark state when no light is coming into the cell, this channel is constantly open leading to depolarization Since cGMP is hydrolyzed to 5'GMP when light comes in, this closes the ion channel and causes hyperpolarization of the cell and reduced glutamate release (an excitatory neurotransmitter)

Answer 2

* Intracellular calcium is an important modulator in promoting the return of the photoreceptor to the dark state * Ca2+ controls at least 3 components in the transduction cascade * Low levels phosphorylation of Rhodopsin * Guanylate cyclase is accelerated to generate new cGMP * Increased affinity of cGMP to the Na2+ channels

Answer 3

Rods and cones do not produce action potentials ❚ They respond to light with graded fluctuations of membrane potentials ❚ Rods respond slowly so that the effects of all photons absorbed over a ~100 msec period are summed together.

Answer 4

This means that although rods can detect small amounts of light, they cannot resolve flickers faster than 12Hz ❚ Cones respond much faster and can resolve flickers up to at least 55Hz ❚ Generally speaking, we cannot separate flashes of light faster than about 16Hz the CRITICAL FUSION FREQUENCY. Conventional movies (24Hz), analogue TV (50-60Hz) and video monitors (>60Hz) or HDTV devices have high frame and/or refresh rates to avoid flickering

Answer 5

In the retina, only the ganglion cells (G) transmit information as a series of action potentials

Answer 6

Three specialized interneuron cell types connect the photoreceptors to the ganglion cells: bipolar cells (B), horizontal cells (H) and amacrine cells (A). These cells also use graded potentials. ❚ These interneurons do not simply transmit signals from the photoreceptor to the ganglion cells – they process signals from multiple photoreceptors (receptive fields) in response to different patterns of light stimulating the retina

Answer 7

ON-centre and OFF-centre bipolar cells establish parallel pathways for the signal from a single cone ❚ When the cone cell is activated by light it becomes hyperpolarized leading to a reduction of Glutamate release ❚ This causes the ON-centre bipolar cell to become excited (depolarized) and the OFF-centre bipolar cell to be inhibited (hyperpolarized). ❚ OFF centre RGC are hyperpolarized by OFF-centre bipolar neuron, whereas ON-centre RGC show a very typical neuronal response to depolarisation by firing APs.

Answer 8

The two different classes of bipolar cells respond differently to Glu because they have different post- synaptic receptors that gate different ion channels. The on bipolar cell is gated by metabotropic glutamate receptors while the off bipolar is gated by ionotropic receptors

Answer 9

In the fovea * Ganglion cells (both M and P types are found) have smaller receptive fields * Low convergence: Ratio of photoreceptors to ganglion cells is <10:1 * Cone photoreceptors only Peripheral retina * Higher ratio of rods to cones * High convergence: Higher ratio of photoreceptors to ganglion cells (>100:1) * More sensitive to light

Answer 10

Signals from cones in the periphery of the receptive field reach bipolars by way of horizontal cells, which are inhibitory. This creates the opposite effect to illumination at the receptive field centre.

Answer 11

Signals from the surround are mediated via horizontal cells ❚ Light falling on a photoreceptor in the surround of an ON-centre bipolar cell causes a hyperpolarization. ❚ This causes the horizontal cell to hyperpolarize. This reduces the amount of inhibitory neurotransmitter (GABA)* released by the horizontal cell onto postsynaptic cones in the receptive field centre. ❚ Thus, when the surround is illuminated the horizontal cells depolarize the photoreceptors in the field centre, causing the opposite effect to light falling directly on them (hyperpolarization). ❚ This causes an ON-centre bipolar cell to hyperpolarize When the surround area of the receptive field is illuminated, the horizontal cell becomes hyperpolarized by the surrounding photoreceptors, this leads to the horizontal cell inhibiting the central 'ON' bipolar cell

Answer 12

ON-centre cells depolarize in response to light falling on the centre of the field but hyperpolarize in response to light falling on the surround. ❚ OFF-centre cells hyperpolarize in response to light falling on the centre of the field but depolarize in response to light falling on the surround.

Answer 13

* An edge of a light stimulus moves into the receptive field surround of ON bipolar cell B. * This edge is also falling on the receptive field centre of ON bipolar cell C. * The light will cause bipolar cell C to depolarize because of the direct synapse with the photoreceptor. * The light will also cause bipolar cell B to hyperpolarize because of the indirect synapses through the horizontal cell. * The larger membrane potential difference between the cells B and C compared to without lateral inhibition. This leads to an enhancement in the perception between the dark and light side of the edge. * In parallel, this effect is enhanced through opposite modulation of OFF centre cell activity Works through lateral inhibiton of bipolar cell B Edge detection is pretty common sense, if the light falls on the edge of a receptive field, the photoreceptors nearest that edge of light will depolarize and the surrounding neurons will be inhibited

Answer 14

The structure of ganglion receptive fields allows ganglion cells to respond best to changes in light intensities (contrast) between the centre and surround of the receptive field

Answer 15

❚ Resolution of visual image is determined by density of sensory receptors and size of receptive fields in the retina. ❚ As the density of receptors increases and the size of the receptive field decreases spatial detail increases. ❚ Rods contribute to larger receptive fields and are relatively much more numerous than cones toward the periphery

Answer 16

Contrast is the difference in visual properties (brightness, colour) that makes an object distinguishable from other objects and the background.

Answer 17

Contrast detection results from the structure of ganglion cell receptive fields, which allow ganglion cells to respond best to changes in light intensities (contrast) between the centre and surround of the receptive field.

Answer 18

A moving object elicits stronger firing rate in the ganglion cell population near the edges of the objects image These are the only regions of spatial contrast and only regions where the light intensity changes over time The centre-surround antagonism is the basis for our ability to processes differences in illumination that falls onto photosensitive cells of the retina and are critical for spatiotemporal contrast sensitivity in addition to visual acuity

Answer 19

Colour is not essential for perception of detail. The majority of our ability to perceive detail results from our ability to discriminate variations in brightness within the visual image.

Answer 20

Note that the light falling on the apple is broad spectrum (white light). ❚ The red pigment in the apple appears red because the apple adsorbs other light frequencies but reflects predominately red light. Similarly the banana adsorbs most light frequencies but reflects predominantly yellow light.

Answer 21

According to the Young-Helmholtz trichromacy theory, originally proposed in the 19th century, the brain assigns colours based on an additive colour model, in which the three primary colours of red (R), green (G) and blue (B) light are added together to produce a broad array of different colours. . Today, we know the human eye contains 3 types of cones, which preferentially absorb blue (S), green (M) and red (L) light. This theory explains how each type of cone photoreceptor detects different wavelengths in light

Answer 22

❚ Each class of cones contains a different photopigment that gives it a distinct spectral sensitivity. ❚ S-cones are most susceptible to blue light (~420 nM peak), M-cones to green (~530 nM peak) and L-cones to red (~560 nM peak).

Answer 23

Light reflected from real surfaces in the natural world rarely contains a single wavelength. In fact most surfaces reflect light that represents a continuous distribution of wavelengths. ❚ A system that uses more than two types of cones would be able to differentiate more physically different surfaces that appear to be the similar colour.

Answer 24

Because the reflectance functions of the flowers shown here vary slowly across the spectrum, and the spectral sensitivities of the L and M cones are so similar across a broad spectral region, these two classes of cones will generate highly correlated signals when they absorb light from natural surfaces such as these ❚ This would create a considerable redundancy of information entering the visual pathway if each cone was connected independently to the visual pathway ❚ The solution is to transmit DIFFERENCES between signals, not the signals themselves.

Answer 25

Ewald Hering (1892) noticed that the hues red, yellow, green and blue have special properties. ❚ They are fundamental because all other hues (spectral colours) can be described as mixtures of them, and yet they seem to be related as mutually exclusive pairs. ❚ These led Hering to propose that vision depended on three opponent mechanisms ❚ All of the colours of the spectrum could be derived in terms of variation in two opposing colour pairs: red-green and blue-yellow. He proposed that a third mechanism would capture the light-dark variation

Answer 26

The Hering theory can be used to accurately model natural surface reflections ❚ The natural surface reflection curve can be realistically modeled by combining three separate component functions: one comprising the brightness of the image, one comprising the red- green variation and the other yellow-blue variation. ❚ These three component functions can account for >99% of the variance in surface reflective functions of natural objects.

Answer 27

Hering’s theory basically describes a way to transform the visual information provided by the three cone types in the Young-Helmholtz theory into a more compact form. This is called “second-stage transformation”. ❚ He proposed that the ideal transformation is one that yields, for the kinds of visual stimuli the eye normally receives, the smallest correlation among signals in the three pathways. These are: ❚ The sum of all three cone signals: ❙ L + M + S ❚ The difference between the L and M cones: ❙ L – M ❚ The difference between the S cones and some combination of L and M cones: ❙ S - LM

Answer 28

❚ Dichromats have only two classes of receptors, instead of three. ❚ Two kinds of dichromacy are most common: loss of either L cones or M cones (protanopia or deuteranopia), with each having about 1% prevalence. ❚ Since either will principally affect the L-M channel, they are collectively known as “red-green colour-blindness”

Answer 29

1st stage - light is transduced by three types of cone photoreceptors, L (long), M (middle), and S (short) -wavelength sensitive. 2nd stage - cone signals are integrated through four cone-opponent mechanisms (L+M−, M+L−, S+(L+M)−, and (L+M)+S−) instantiated by retinal circuits connecting specific cone types to retinal ganglion cell types. Hypothesized 3rd stage - cone-opponent signals are predicted to combine in four specific combinations to generate colour-opponent mechanisms, as well as neurons whose activities underlie the perception of four “unique colours”

Answer 30

P cells are more numerous and have small receptive fields. They respond selectively to specific wavelengths and are believed to be involved in the perception of form and colour. ❚ P cells fall into two subtypes: ❙ 1. Neurons that receive opposed signals from L and M cones. ❙ 2. Neurons that receive signals from S cones opposed to some combined signal from L and M cones. ❚ Therefore these two subtypes of P cells appear to have the necessary characteristics to provide the red-green and yellow-blue channels postulated by Hering

Answer 31

Red-green subtype of P-cells receives inputs from only L and M cones, yellow-blue subtype receives inputs from all three classes L, S and M. ❚ S cones are much less frequent (<10%). And because it is much harder to focus short wavelength light, they are largely absent from the centre of the fovea. ❚ Therefore it seems that only the inputs from L and M cones are significantly used to compute the brightness signal.

Answer 32

Both types of information must be conveyed through the discharge of action potentials by the P cells and ultimately this must be resolved by the visual cortex.

Answer 33

P-cells respond well to brightness variations in the fine structure of the image, but also respond well to colour variation in the coarse structure of the image The spatial organization of the P-cell’s receptive field allows the cell to convey BOTH brightness and colour information. When light falling on a P cell’s receptive field covers both centre and surround, the P-cell will respond well to variations in colour between centre and surround – being excited by some hues and inhibited by others

Answer 34

The four ventral layers are known as parvocellular layers and receive inputs mainly from P cells ❚ Both the magnocellular and parvocellular layers contain on-centre and off-centre cells, just as in the retina ❚ P-cells in the lateral geniculate nucleus are also tightly clustered into ‘red-green” and “yellow- blue” groups

Answer 35

❚ The functions of the M and P pathways can be investigated using selective lesioning techniques because the two pathways are separated into different layers of the lateral geniculate nucleus. ❚ In this case the lesions were induced using an excitotoxin, ibotenic acid.

Eye neurobiology Flashcards

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