Summary: Neurons in the midbrain receive strong limited synaptic input from retinal ganglion cells but only from small sensory neurons.

Source: Charity

For the first time, neuroscientists from the Charité – Universitätsmedizin Berlin and the Max Planck Institute for Biological Intelligence (in progress) have demonstrated a precise connection between sensory neurons in the retina and the superior colliculus in the midbrain.

Neuropixels probes are a relatively recent development that represent the next generation of electrodes. Packed with dense recording dots, neuropixels probes are used to record the activity of neurons, and these have recently facilitated insights into neuronal circuits.

Writing in Natural relationshipsThe researchers describe the basic principles common to the visual systems of mammals and birds.

Two brain structures are critical to the processing of visual stimuli: the visual cortex in the primary cerebral cortex and the superior colliculus, a structure in the midbrain. Vision and visual information processing involve very complex processes.

In simple terms, the visual cortex is responsible for general visual perception, while the evolutionarily mature midbrain structures are responsible for visually guided adaptive behaviors.

The mechanisms and principles involved in visual processing in the visual cortex are well known. The work carried out by a group of researchers led by Dr. Jens Kremkow has contributed to knowledge in this field and was completed in 2017 with the establishment of the Amy Notter Jr. Research Group at the Charité Neuroscience Research Center (NWFZ).

The main goal of the research group, which is supported by the German Research Foundation (DFG), is to further improve our understanding of the neurons involved in the visual system. Many unanswered questions remain, including the details of how visual information is processed in the superior colliculi of the midbrain.

Retinal ganglion cells, sensory cells located in the retina of the eye, respond to external visual stimuli and transmit the received information to the brain. Direct signaling pathways ensure that visual information received by retinal neurons also reaches the midbrain.

“Until now, little is known about the way neurons in the retina and neurons in the midbrain are functionally connected. The lack of knowledge about the way neurons in the superior colliculi perceive the processing of synaptic inputs is similarly pronounced,” said Dr. Krumkow, the leader of the study. .

“This information is very important to understand the mechanisms involved in the process of the midbrain.”

Until now, it has been impossible to measure the activity of synaptically connected retinal and midbrain neurons in living organisms. For their latest study, the research team developed a method based on measurements obtained with new high-density electrodes known as NeuroPixels probes.

Strictly speaking, NeuroPixels probes are tiny, straight electrodes that feature about a thousand recording sites on a narrow shank. Consisting of 384 electrodes to simultaneously record the electrical activity of neurons in the brain, these devices have become game-changers in the field of neuroscience.

Researchers working at Charité and the Max Planck Institute for Biological Intelligence have now used this new technology to define the corresponding midbrain structures in mice (superior colliculi) and birds (optic tectum).

Both brain structures have a common evolutionary origin and play an important role in visual processing of retinal input signals in both groups of animals.

Their work led the researchers to a surprising discovery: “Usually this type of electrophysiological recording measures electrical signals from the soma, the action potential that originates from the cell body of the neuron,” explained Dr. Kremkow.

“In our recording, however, we noticed signs that looked different from normal action potentials. We went on to investigate the cause of this phenomenon, and the input signals in the midbrain are caused by action potentials distributed in the ‘axonal arbors’ (branches) of the retinal ganglion cells. Our findings suggest that the new electron array technology from axons It is used to record the electrical signals generated by the projections of neurons that transmit neurons.This is a new discovery.

In an international first, Dr. Kremkow’s team was able to simultaneously capture the activity of neurons in the retina and the activity of synaptically connected target neurons in the midbrain.

Until now, the functional wiring between the eye and the midbrain has remained an unknown quantity. The researchers were able to show that, at the single-cell level, the spatial organization of inputs from retinal ganglion cells in the midbrain creates a highly accurate representation of the original retinal input.

This shows the iris
Retinal neuron signals travel through a mosaic of nerve tracts to be further processed by neurons in the midbrain. Represented by lines: electrical signals from axonal branches of retinal ganglion cells measured simultaneously by tiny electrodes of neuropixel probes in the midbrain. Credit: Charity | Jens Kremkow & Fotostudio Farbtonwerk I Bernhardt link

“Middle brain structures effectively provide a one-to-one replica of the structure of the retina,” Dr. Kremkow said.

“Another new discovery for us is that neurons in the midbrain receive stronger and more specific synaptic input from retinal ganglion cells, but only a small number of these sensory neurons,” he said. These neural pathways enable highly structured and functional communication between the retina of the eye and corresponding regions in the midbrain.

Among other things, this new understanding increases our understanding of a phenomenon known as blindsight, which can occur in individuals with damage to the visual cortex caused by trauma or tumors.

These individuals, who lack conscious awareness, retain the ability to process visual information, creating awareness of stimuli, shapes, movement, and even colors that appear to be connected to the midbrain.

To test whether the principles first observed in the mouse model could be applied to other vertebrates – and therefore generalizable in nature – Dr Kremkow and his team worked with a team from the Max Planck Institute for Biological Intelligence, Liz Meitner’s research group in birds, led by Dr Daniel Valentine. It focuses on the neural sectors responsible for coordinating precise movements.

“Using the same types of measurements, we were able to show that the spatial organization of neural tracts connecting the retina and midbrain in zebra finches follow the same principle,” Dr. Valentin said.

She added: “This discovery was surprising given that birds have superior eyesight and the evolutionary distance between birds and mammals is great.

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This shows the neurons in the postrema

The researchers’ observations showed that retinal ganglion cells in both the optic tectum and superior colliculi display similar spatial organization and functional wiring. Their findings led the researchers to conclude that the discovered principles must be critical for visual processing in the mammalian midbrain. These principles may be general in nature, applying to all vertebrate brains, including humans.

Regarding the researchers’ future plans, Dr. Kremkow said: “Now that we understand the functional and mosaic-like connections between retinal ganglion cells and neurons in the superior colliculi, we can investigate the way in which sensory information is processed in vision.” system, particularly in regions in the midbrain, and how they contribute to visually guided adaptive behavior.

The team also wants to see if the new method can be used in other structures and to measure axonal activity elsewhere in the brain. If this is possible, it opens up many new possibilities for investigating the underlying mechanisms of the brain.

So visual neuroscience research news

Author: Manuela Zingle
Source: Charity
Contact: Manuela Zingle – Charitable
Image: Image for Charité | Jens Kremkow & Fotostudio Farbtonwerk I Bernhardt link

Preliminary study: Open Access.
High-density electrode recordings reveal strong and specific connections between retinal ganglion cells and midbrain neurons” by Jens Kremkow et al. Natural relationships


High-density electrode recordings reveal strong and specific connections between retinal ganglion cells and midbrain neurons

The superior colliculus is a midbrain structure that plays important roles in visually guided behaviors in mammals. Neurons in the superior colliculus receive inputs from retinal ganglion cells, but how these inputs are integrated in vivo is not known.

Here, we found that high-density electrodes simultaneously capture the activity of retinal axons and postsynaptic target neurons in the superior colliculus, in vivo.

We show that retinal ganglion cell axons in the mouse provide input to the superior colliculus, a precise representation of a single retinal cell.

This isomorphic mapping builds the scaffold for proper retinotopic wiring and functionally specific connectivity. Our methods are broadly applicable, which we demonstrate by recording retinal inputs to the optic tectum in zebra finches.

In mice and zebra finches, we find common wiring patterns in retinal ganglion cells with neurons in retinoreceptor areas that provide an accurate representation of the visual world.

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