Retinal Information Processing Flashcards
Molecules called visual pigments are made in the _____ and stored in the _______ where they are incorporated into the membrane
inner segment ; outer segment
Chromophore
Captures light photons and is the part of a molecule responsible for its color.
Bipolar Cells
The intermediary between the photoreceptors and the ganglion cells
How do photoreceptors pass their information onto bipolar cells?
Via graded potentials which vary in size instead of an all or nothing action potential
The relationship between glutamate and cell Synapse
Amount of glutamate present in the photoreceptor- bipolar cell synapse at any one time by inversely proportional to the number of photons being absorbed by the photoreceptor
Information Pathway of a Photon
1) A photon is captured by one of the photoreceptors and is absorbed by a visual pigment
2) The energy of the photon is transferred to the chromophore portion of the molecule in a process known as photoactivation
3) Hyperpolarization triggers the reduction of glutamate molecules released in the synapse 4) Finally, this change alerts the bipolar cells that a photon has been captured
5) Since the trigger is a reduction in glutamate concentration, the magnitude of reduction is used as an indicator of how many photons are absorbed
Lateral Inhibition
The phenomenon in which a neuron’s response to a stimulus is inhibited by the excitation of a neighboring neuron
Role of horizonal cells
Play a role in lateral inhibition and send their information down the pathway to the ganglion cells
Where do amacrine cells receive info?
Amacrine cells do not receive information from the photoreceptors, instead communicating with bipolar cells and other amacrine cells
Bipolar to Ganglion Cell Tranmission
The bipolar cells transmit the signals from the photoreceptors or the horizontal cells, and pass it on to the ganglion cells directly or indirectly (via amacrine cells)
Where do bipolar cells receive input?
Either from rods or cones but not both
Difference between diffuse bipolar cells and midget cells
Diffuse BC: Cells that get information from multiple different photoreceptors
Midget BC: Cells that get information form a single cone
Lateral Geniculate Nucleus (LGN)
A structure in the thalamus that receives input from the retinal ganglion cells and has input and output connections to the visual cortex
How many layers does the LGN have?
6 layers
top four are parvocellular -two layers from each eye
bottom two are magnocellular - one layer from each eye
Where do layers 1 & 2 (magnocellular layers) receive input from?
Large neurons receive input from m ganglion cells in the retina which means they receive input from diffuse bipolar cells
Where do layers 3-6 (parvocellular layers) receive input from?
smaller cells that receive input from retina’s P ganglion cells which means they recieve input from midget ganglion cells
Receptive Field
Region of retina in which visual stimuli influence the neuron’s firing rate
ON Centre Ganglion Cell Firing Rate
Increases its firing rate when a light is turned on in the center of its receptive field and decreases its firing rate when light is turned on in the surround
OFF Centre Ganglion Cell Firing Rate
Decreases their firing rates when a light is turned on in a spot in the center of its receptive field and increases when light is turned on in a spot in the surround
What is the size of the receptive field determined by?
The size of the receptive field is determined by how many photoreceptors are converging into the ganglion cell
- More photoreceptors = larger receptive field
How do retinal ganglion cells act as a filter?
Retinal ganglion cells act as a filter by responding best to stimuli that are just the right size and less to stimuli that are larger or smaller
What are ganglion cells most and less sensitive to?
Sensitive to differences in the intensity of light in the centre and in the surround, and they are less affected by the average intensity of light
How do the receptive fields of P and M cells differ?
1) Receptive fields of P cells is always smaller than M cells at all eccentricities
2) M cells are more sensitive and better able to detect visual stimuli than P cells under low light conditions, however the smaller receptive fields on P cells result in greater acuity than M cells
The magnocellular pathway carries information about?
Large, fast things (low spatial frequency, high temporal frequency) and is colorblind
The parvocellular pathway carries information about?
Information about small, slow, colorful things (high spatial frequency, low temporal frequency
Difference in firing rate between P and M Ganglion Cells
1) P cells have a sustained firing rate over the course of stimulus presentation
2) M cells have a transient firing rate, it gives a brief burst and then returns to
spontaneous firing even when the stimulus is present
What signals do P and M cells signal different information to the brain?
- P cells provide information about mainly the contrast in the retinal image
- M cells signal information about how the image changes over time
Intrinsically photosensitive Retinal Ganglion Cells (ipRGCs)
Refer to ganglion cells that do not receive info from photoreceptors
What are the first photoreceptors to mature in the retina and therefore are the first to send light driven signals to the brain?
Intrinsically photosensitive Retinal Ganglion Cells (ipRGCs)
