Neural Circuits

Question 1

What are the three main areas involved in the “standard” visual pathway?

Answer 1

Retina, lateral geniculate neuron, ventral stream

Question 2

Rostrally to caudally, order the following retinal structures:

Inner nuclear layer
Ganglion cell layer
Outer nuclear layer

And associate the following components to each layer: photoreceptors, interneurons, retinal ganglion cells

Answer 2

Outer nuclear layer - photoreceptors
Inner nuclear layer - interneurons
Ganglion cell layer - retinal ganglion cells

Question 3

Why do all eyes see in the 400 - 700 nm range?

Answer 3

Sea water transmits best this visible spectrum, and eyes (and their opsins) have likely evolved under water

Question 4

What is an opsin?

Answer 4

A light-sensitive G-protein-coupled receptor for which the signalling cased is activated upon the conformational change of its ligand 11-cis retinal to all-trans retinal, which occurs upon absorption of a photon

Question 5

How come can different opsins have different spectral differences?

Answer 5

Different aa. opsin compositions.

Question 6

Broadly, what are the respective functions of the photoreceptors, interneurons (amacrine cells, bipolar cells, horizontal cells), and the retinal ganglion cells in the retina?

Answer 6

Photoreceptors: light detection, initiating signal
Interneurons: processing and refining information
Retinal ganglion cells: relaying information upstream in the visual pathway

Question 7

What are the respective roles of amacrine cells, bipolar cells, and horizontal cells?

Answer 7

Amacrine cells: provide lateral inhibition to bipolar and ganglion cells ->responsible for direction selection + center-surround receptive field
Bipolar cells: relay photoreceptor signal through excitation to RGCs
Horizontal cells: provide lateral inhibition to photoreceptor, refining inputs

Question 8

In which structure of the photoreceptor are contained opsins?

Answer 8

In its “discs” like organelles

Question 9

Describe the phototransduction cascade happening in the disc organelles of photoreceptors.

Answer 9

Light creates a conformational change in rhodopsin -> activating the coupled G-protein ->subunit activating cGMP phosphodiesterase ->cGMP converted to 5’-GMP -> cGMP-gated channel is closed -> Na+ cannot enter the photoreceptor anymore ->hyperpolarization

Question 10

What is a synaptic ribbon?

Answer 10

Structure of the photoreceptor along which SVs “fall”, along a sustained release in the dark condition

Question 11

What sort of glutamate receptors do ON and OFF bipolar cells express? How does that affect the transduction of the photoreceptor signals?

Answer 11

OFF bipolar cell -> kainate receptors, driving depolarization of the cell upon glutamate binding = same activity than photoreceptors

ON bipolar cell ->MGLUR6 receptors, driving hyperpolarization of the cell upon glutamate binding through inactivation of non-selective cation channels = inverse activity of photoreceptors

Question 12

How many photoreceptors do bipolar cells contact?

Answer 12

About 10

Question 13

What is the functional advantage of separating light intensity into ON and OFF bipolar cells?

Answer 13

Helps the comparison between neighbouring light intensities

Question 14

What is a receptive field?

Answer 14

Region of the world that affects the activity of the neuron

Question 15

What is retinotopy?

Answer 15

Specific spatial organization of retinal ganglion cells that results in the clustering of RGCs that respond to similar locations in the visual space

Question 16

In which region of the brain are maps of visual, auditory, somatosensory, and motor space are aligned and stacked on top of each other?

Answer 16

Superior colliculus

Question 17

In the frog, how does ephrin signalling pattern the retinotopy of the tectum?

Answer 17

Rostral RGCs in the retina express low amount of EphA receptors (repulsive) -> they project caudally in the high concentration region of the EphrinA gradient (ligand)

Caudal RGCs in the retina express high amount of EphA receptors (repulsive) -> they project rostrally in the low concentration region of the EphrinA gradient (ligand)

Dorsal RGCs in the retina express low amount of EphB receptors (attractive) -> they project laterally in the low concentration region of the EphrinB gradient (ligand)

Ventral RGCs in the retina express high amount of EphB receptors (attractive) -> they project medially in the low concentration region of the EphrinB gradient (ligand)

Question 18

What is the role of neural activity in the patterning of the superior colliculus (or optic tectum)?

Answer 18

Synaptic pruning, allowing functional outputs to cluster -> refining of the map

Question 19

From what part of the brain did Wiesel and Hubel record in their famous experiment?

Answer 19

Recorded from V1

Question 20

What is spike-triggered average?

Answer 20

STA is a stimulus-response assay where random sequences of black and white squares are shown on a screen and the corresponding visual responses recorded, allowing to generate receptive fields for the neurons recorded

Question 21

What is the proportion of synapses onto the LGN that come from the retina?

Answer 21

Only 7% -> rest probably feedback mechanisms from upstream regions in the visual pathway

Question 22

How did Wiesel and Hubel think that orientation selectivity arised in V1?

Answer 22

A single V1 neuron would take many inputs coming from LGN neurons with the centres of their receptive fields lined up in a specific way = bar-like arrangement

Question 23

In terms of orientation selectivity, how are V1 cells organized?

Answer 23

Systematically in pinwheels.

Question 24

What is the current view of the transmission of functional visual information from LGN to V1?

Answer 24

Mix of relay and emergent properties, yielding an enhanced orientation bias in V1

Question 25

How are the cortical layers generated?

Answer 25

Inside-out fashion from the ventricular zone containing the precursor cells.

Question 26

True or false: interneurons do not originate from the ventricular zone.

Answer 26

True, they originate elsewhere in the brain and migrate throughout the cortical layers.

Question 27

What is the handshake hypothesis regarding the connections between V1 cortical neurons and thalamic neurons?

Answer 27

During dev., thalamic neurons project to the subplate of the developing cortex, where cortical neurons of layer VI are also projecting to (downstream), leading to meeting, or the “handshake” of the two sets of neurons

Question 28

If retrograde dyes were to be injected in V1, what would be observed in dLGN?

Answer 28

The segragation of the dyes would be preserved -> suggesting that geniculate projections form retinotopic maps (organized with signals like Eph-ephrin)

Question 29

What is connectomics? What does it allow in the understanding of the patterning of LGN axons to the cortical layers of V1?

Answer 29

Connectomics: taking thin sections through cortex of mice, reconstructing them using EM, every single neuron within the reconstructed structure is traced out

Question 30

In vitro it was found that neighbouring V1 cells share similar orientation tuning, what does that suggest for the patterning of LGN axons to the cortical layers of V1

Answer 30

LGN axons (some) carry orientation biases, and the biases are strengthened in V1

Question 31

What results lead to suggest that orientation selectivity might be emergent to V1 neurons themselves?

Answer 31

Different orientation responses across individual V1 neurons -> different responses in the dendrites and the axons = preference of the cell might be emergent to itself

Question 32

What are the two main schools of thought on the functional information transmission between LGN and V1?

Answer 32

Bias amplifier at V1 occurs through circuitry

Bias amplifier occurs at the level of single V1 neurons