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Pyramid cells cluster, vibrate and signal our brain

Our cerebral cortex acts like a vast switchboard, responding to countless lines of information about our changing environment — gathered from our sensory organs. The flow of this data is influenced by individual pyramid cells clustered like bells on a chord, ringing for attention.

Scientists at the Max Planck Institute of Neurobiology in Martinsried [Munich], Germany, show for the first time that contact points between specific neurons cluster along a target neuron — much like bells along a dendrite cord.

The cells of the cerebral cortex receive information according to which area of the brain is sending a signal. For example, signals from retinal cells arrive in the visual cortex, which gives priority to objects in motion — most likely differentiating signals by the frequency of their vibrations.

As there are thousands of synapses receiving impulses in any given moment, pyramid cells regulate transmissions by volume of signal received, frequency received, and from which location. Very possibly signals are grouped together making them command more attention — just like a set of bells.

Each cell receives information from several thousand lines of pyramid cells. Then, pyramid cells relay those signals to the cerebral cortex — or not. As information is electrical, neurobiologists measure signals at various contact points along a neuron, first by their frequency, then by their final destination.

The research was published in the July 19, issue of elife.

Combining several techniques, neurobiologists in Tobias Bonhoeffer's lab analysed the spatial arrangement between synapses. Using optogenetics, they were able to selectively activate pyramid cells in brain slices of mice. And with "calcium imaging", were able to observe and record the activity of individual synapses using a two-photon microscope. Therefore, they succeeded in mapping how synapses are arranged in respect to one another.

These maps made clear that pyramidal cells cluster in groups of 4 to 14 synapses, arranged in an area less than 30 micrometres or 0.0011811 inches, along a dendrite.

"Signals a cell receives from ten simultaneously active synapses — can be greater than the sum of signals from ten individual synapses. Until now, it was unclear how to interprete the arrangement of synapses on pyramid cells."

Volker Scheuss PhD, Department Synapses - Circuits - Plasticity, Max Planck Institute of Neurobiology, Martinsried, Germany.

"The existence of these clusters suggests synapses interact with each other to control [magnify] the strength of their combined signal," explains Onur Gökçe, author of the study. This is the first anatomical explanation for the disproportionate strength of clustered synapse signals in comparison to individual signals.

The observation in layer 5 pyramid cells was of particular interest to the scientists as those cells oscillate in synchrony.

"This rhythmic activity, which probably influences the processing of visual information, could synchronously activate synapse clusters — boosting the overall signal received."

Volker Scheuss PhD

The spatial organization of synaptic inputs on the dendritic tree of cortical neurons plays a major role for dendritic integration and neural computations, yet, remarkably little is known about it. We mapped the spatial organization of glutamatergic synapses between layer 5 pyramidal cells by combining optogenetics and 2-photon calcium imaging in mouse neocortical slices. To mathematically characterize the organization of inputs we developed an approach based on combinatorial analysis of the likelihoods of specific synapse arrangements. We found that the synapses of intralaminar inputs form clusters on the basal dendrites of layer 5 pyramidal cells. These clusters contain 4 to 14 synapses within ≤30 µm of dendrite. According to the spatiotemporal characteristics of synaptic summation, these numbers suggest that there will be non-linear dendritic integration of synaptic inputs during synchronous activation.

Onur Gökçe, Tobias Bonhoeffer, Volker Scheuss
Clusters of synaptic inputs on dendrites of layer 5 pyramidal cells in mouse visual cortex. eLife; 19 July, 2016

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Synapses of pyramid cells in the cerebral cortex form functional groups — like bells on a string.
Some of the related synapses are shown in green in this computer reconstruction.
Image Credit:MPI of Neurobiology / Scheuss



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