Searching the neuronal network

Grafik: MW

Think globally – act locally; this principle is also crucial to the brain. Single nerves can only contribute to complex behavior if they first find partners at the level of local networks.

Scientific support: Prof. Dr. Hans-Christian Pape

Published: 12.10.2015

At a glance
  • Networks constitute an important organizational principle in the brain. They permit the development of complex patterns through the coordinated activity of nerve cells.
  • Functional ensembles consisting of small "neighborhood" organizations of groups of nerves play a particular role. Through concerted spatiotemporal action, researchers believe, they form a decisive bridge between the cellular level and the systematic representation of brain content.
  • Functional ensembles can encode mental maps or sharpen contrasts in visual tasks. They also play a role in the reward system and in higher cortical functions such as planning  behavior.
  • In functional ensembles, participating cells become active with clearly defined roles in diverse situations, while non-participating neighbors remain silent. They can react flexibly to changes in the environment by adapting their activity pattern or recruiting extra partners.
Networks, networks everywhere

Research into networks has been profitable to scientists in areas well beyond brain research. Similar principles govern every system in which many players interact in complex patterns. The spectrum ranges from biochemical systems and computer networks to social interactions and the Internet.

The question of what types of mechanisms and rules form and determine such networks is the subject of interdisciplinary network research. They are particularly helpful in difficult mathematical, theoretical and graphical contexts, as seen in theoretical physics and other fields.

Of particular interest is research into complex networks that exhibit so-called not-trivial topological characteristics. This means that the connection between elements are neither completely rule-governed nor random, but rather assume a range of patterns of distribution whose particular rules follow their own logic. These include specific hierarchies or community structures.

The human nervous system comprises about 86 billion nerve cells – over ten times the number of humans on Earth. In contrast to the approximate average number of friends a person has on Facebook – 342 – each neuron in the brain forms around 1000 connections in its social network. How thoughts, behavior, and feelings arise from these billions of contact points in the brain remains a central question of neuroscience.

Scientists have been able to observe the behavior of the brain through functional imaging, with the help of ever more powerful and refined types of microscopes and derivitive techniques, peering into the inner workings of neurons and puzzling together gigantic sets of molecular data to generate profiles of the gene activity of neurons. But how various levels of organization in the brain create and exchange complex information has remained unresolved.
 

Network concepts can explain a lot

Local networks play a key role in the search for an explanation for the way genes, molecules, and electrical impulses produce specific information and generate a behavioral response. In principle, information can be encoded at many levels – in the gene activity of a cell, in dosages of messenger molecules such as neurotransmitters, in the frequencies of electrical signals, in the number of connections between neurons, or in the activity of neuronal networks within and between regions of the brain. Sometimes the activity of a single cell suffices to retain complex information such as the face of a familiar actress Star-​Wars-​Neurone und Jenifer-​Aniston-​Zellen.

But in most cases it is the main characteristic of nerve cells – their communicative nature – that encodes information. The interplay of neurons in local networks appears to support the higher capacities of the brain. The functional connections between neurons are coordinated in space and time to provide bridges between the organisational levels of the brain. This is, in any case, the principle underlying networks in dances of genes and the environment Netzwerke im Tanz von Genen und Umwelt.

Most researchers agree that local communities are the determining link between nerve cells and complex brain functions. In Germany a Special Research Area (SFB 1134) has been established on the basis of this concept of "Functional Ensembles." With this term, researchers have established the concept of groups of cells that become active as a unit within a specific frame of time. The choreography of the ensemble in a pattern of activity is somewhat like the poses assumed by a team of cheerleaders to deliver specific information – such as a face, or a familiar place.

Mental maps based on location cells

The idea of functional ensembles has attracted scientists for quite some time. Evidence of the communal activity of small groups of nerves has regularly presented itself – they act as "neuronal cliques" which contribute to a memory, performing as a group a so-called "cortical song" Von neuronalen Cliquen und cortikalen Liedern. Researchers of SFB 1134 regard the term ensemble as the best designation for nerve cells which collaborate in an activity. Currently they are investigating whether these groups of neurons manage to create the fundamental units of complex brain functions in spatiotemporal patterns.

One topic of intense study has been the so-called "orientation cells" and the networks they form to generate a part of the brain's "navigation system." The discovery of these cells led to the award of the Nobel Prize for Physiology or Medicine in 2014 for John O'Keefe and May-Britt and Eduard Moser  Gesucht und gefunden: Orientierungszellen. They found that primarily the hippocampus – but also other specific regions of the brain – contain location and grid cells that create functional ensembles. The location cells of the hippocampus are highly specialized. They frequently fire when an experimental animal occupies a particular position in a space, and single neurons react to different locations: meaning that neuronal activity creates a sort of landmark system. An ensemble of diverse orientation cells creates a mental map of the complete space in which an animal is moving, and encodes this map through a type of activity pattern as a spatial memory.

Lateral inhibition creates contrasts

Lateral inhibition is a classic textbook example of ensemble performance that helps us perceive the environment as a set of sharp contrasts. Whether through vision  (Ball oder Backstein), touch or another sense, the principle is always the same: contours such as the transition from light to dark are emphasized in a local neuronal network through a special switching system that involves the inhibition of neighboring cells that are presented with the same stimulus. "Light" cells surrounded by other illuminated cells are more suppressed than those which are surrounded by "dark" cells, and vice versa. Because cells in transition areas are inhibited to a lesser degree, this ensures that they send particularly strong signals regarding the darkness or lightness of a stimulus. The result is an improved, sharpened contrast.

Despite these differences, in both examples the pattern in the environment is transmitted by local communities of nerves into the network through a coordinated stimulation of activity over a specific period of time. This establishes a particular pattern, representing the information to be retained, at the neural level.

Common features of ensembles

Despite their many differences, ensembles are thought to share further features: the shut-down of "background noise" caused by neurons that are not currently part of an active ensemble; the activation of a particular ensemble in diverse situations – such as learning and calling up a particular memory; plasticity – or the network's ability to adapt by reacting to changes in the environment or the inner state of the brain through motivation, attention, or other states – Von neuronalen Cliquen und cortikalen Liedern.

An important question regards whether the translation of external information causes coordinated waves of activity among communities of cells and thus creates a "neuronal code" – It’s the rhythm.Of particular interest to scientists in SFB 1134 are questions about how networks form, and how single cells are integrated or shut out – and how the networks respond to changes in the environment or behavior – Bildhauern mit dem Erfahrungsmeißel.

SFB 1134 compares the organization of ensembles in four diverse brain systems: the network that builds memories in the hippocampus and enthorinal cortex; the network of sensory perception in the olafactory centers and somatosensory cortex; the motivational network in the reward system of the midbrain; and in networks in the prefontal cortex, where "higher" cognitive processes such as reflection and behavioral planning are located  – Netzwerke unter der Lupe. By systematically comparing the organization and functions of these networks, the scientists hope to gain hints as to the extent of the cooperativity and adaptability of the various networks. At the same time, their efforts should help in the attempt to decode the molecular mechanisms that underly specific network functions.

Alongside these basic aspects of networks and the systematic and theoretical work that will be necessary to expose the principle features of the networks, the scientists are hoping to answer some questions that at first might seem unusual: do glial cells participate in ensembles? Can neurons take part in several networks simultaneously? What role do energy requirements play in the functions of networks?

Clearly there remains much to understand about many aspects of networks – including systems outside the brain (see Info-Box). The challenge is to find patterns, weave bridges and understand the enormous range of intersections. The structural precision of a spider's web fascinates us, and the infectivity of viral hypes through social networks alarm us. But in comparison to the events in our brain, the complex mechanisms underlying these natural and artificial network activities seem almost banal. Surely neural networks have many fascinating surprises in store for us.

zum Weiterlesen:

  • Buzsáki G, Draguhn A: Neuronal oscillations in cortical networks. Science. 2004 (304): 1926 – 1929 (zum Abstract)
  • O’Keefe J, Recce ML: Phase relationship between hippocampal place units and the EEG theta rhythm. Hippocampus. 1993 (3):317 – 330 (zum Text)

EEG

Elektroencephalogramm/-/electroencephalography

Bei dem Elektroencephalogramm, kurz EEG handelt es sich um eine Aufzeichnung der elektrischen Aktivität des Gehirns (Hirnströme). Die Hirnströme werden an der Kopfoberfläche oder mittels implantierter Elektroden im Gehirn selbst gemessen. Die Zeitauflösung liegt im Millisekundenbereich, die räumliche Auflösung ist hingegen sehr schlecht. Entdecker der elektrischen Hirnwellen bzw. des EEG ist der Neurologe Hans Berger (1873−1941) aus Jena.

Hippocampus

Hippocampus/Hippocampus/hippocampual formatio

Der Hippocampus ist der größte Teil des Archicortex und ein Areal im Temporallappen. Er ist zudem ein wichtiger Teil des limbischen Systems. Funktional ist er an Gedächtnisprozessen, aber auch an räumlicher Orientierung beteiligt. Er umfasst das Subiculum, den Gyrus dentatus und das Ammonshorn mit seinen vier Feldern CA1-​CA4.

Veränderungen in der Struktur des Hippocampus durch Stress werden mit Schmerzchronifizierung in Zusammenhang gebracht. Der Hippocampus spielt auch eine wichtige Rolle bei der Verstärkung von Schmerz durch Angst.

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