The Cortical States of Wakefulness

Sylvain Crochet, James Poulet
2019
Article

Résumé

Cortical neurons process information on a background of spontaneous, ongoing activity with distinct spatiotemporal profiles defining different cortical states. During wakefulness, cortical states alter constantly in relation to behavioral context, attentional level or general motor activity. In this review article, we will discuss our current understanding of cortical states in awake rodents, how they are controlled, their impact on sensory processing, and highlight areas for future research. A common observation in awake rodents is the rapid change in spontaneous cortical activity from high-amplitude, low-frequency (LF) fluctuations, when animals are quiet, to faster and smaller fluctuations when animals are active. This transition is typically thought of as a change in global brain state but recent work has shown variation in cortical states across regions, indicating the presence of a fine spatial scale control system. In sensory areas, the cortical state change is mediated by at least two convergent inputs, one from the thalamus and the other from cholinergic inputs in the basal forebrain. Cortical states have a major impact on the balance of activity between specific subtypes of neurons, on the synchronization between nearby neurons, as well as the functional coupling between distant cortical areas. This reorganization of the activity of cortical networks strongly affects sensory processing. Thus cortical states provide a dynamic control system for the moment-by-moment regulation of cortical processing.

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https://infoscience.epfl.ch/record/263467?ln=fr

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Sylvain Crochet, James Poulet
2019
Article

Résumé

Official source

https://infoscience.epfl.ch/record/263467?ln=fr

À propos de ce résultat

Concepts associés (11)

Concepts associés

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Publications associées (5)

Publications associées

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The neocortex is the most computationally advanced portion of the brain. It is currently assumed to be composed of a large number of "cortical columns" – intricate arrangements of cortical neurons approximately 300-500 µm in diameter and 2-5 mm in height in humans – that might serve as the elementary computational unit of the neocortex. Understanding the computation performed by this microcircuit is one of the keys to our comprehension of the brain. The so-called cortical column is not a static entity, however, and it evolves throughout a lifetime and continually adapts to the information from its cortical environment. Despite the differences between cortical columns across the cortex, a number of common features have been identified: a laminar structure, the dynamics of connections between identified neurons or the mechanisms for these connections to be modified (that give its specificity to each microcircuit). This thesis presents the description of the differential connectivity and synaptic dynamics across cell populations and the long term neuronal rewiring in a particular neuronal population within the cortical column. Somatic whole cell recordings have been performed to probe the connectivity, synaptic dynamics and plasticity of the connections in the rat neocortex. Two populations of layer V pyramidal neurons were studied in particular: cortico-callosally projecting pyramidal cells (CCPs) and thick tufted pyramidal cells (TTCs). The first major results from this work revealed the degree of connectivity and the linear dynamics of the CCPs population when compared to the TTCs. CCPs have nearly 4 times fewer interconnections and subsequent post-synaptic potentials were less decreasing in amplitude along a pre-synaptic series of action potentials. Long term configuration of TTC networks was explored. These experiments show for the first time the emergence of new functional synaptic connections between TTCs within hours. Activation of the slice by glutamate greatly increases their rate of emergence and this work demonstrated that metabotropic glutamate receptor 5 (mGluR5) activation and action potential firing are required for new connections to be formed in this experimental protocol. Newly formed connections respond in a more linear fashion and have weaker post-synaptic influence than already existing connections. Pre-existing connections are also modified after stimulation, requiring mGluR5, action potentials as well as α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors activation. The activation of group III metabotropic glutamate receptors (mGluRs) however results in a decrease in the strength of connections. Finally, the influence of inhibitory interneurons on the activity and connectivity between TTCs was also investigated. The results of this study show that firing of inhibitory interneurons can be triggered by the input of only one pyramidal cell. They further show that stimulation of a single TTC can result in an hyperpolarization of the post-synaptic TTC mediated by an interneurone. When the pre-synaptic neuron is also directly connected to the post-synaptic neuron with an excitatory synapse, the indirect inhibitory connection serves to curtail the excitatory response. This work has provided a new insight into the dynamic nature of the cortical microcircuitry, showing that it evolves rapidly and can adapt, reconfigure and rewire itself in remarkably short time-spans. It also describes the variety of dynamics exhibited by the different types of pyramidal cells, due to either the projecting site specificity or to the action of an intermediate interneuron.

EPFL2007

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Modified Trans-synaptic Tracing Viruses Reveal Novel Connectivity in the Rat Olfactory Bulb

Aurélie Pala

Olfaction and the olfactory pathway is a good system to study sensory processing in general, mainly because of the simplified organization of the cells constituting the olfactory bulb, the anatomical structure responsible for early odor encoding and processing. Olfaction is also a good model system, because the connections between olfactory neurons directly influence the functions performed by the network they are part of. In this thesis, we choose a particular kind of virus, the pseudorabies virus (PRV) as trans-synaptic tracer to understand the synaptic connections made within the olfactory bulb (OB) as well as between the OB and the olfactory cortex. By using PRV strains differing in their directions of propagation across the synapses and by genetically modifying them, we are able to highlight different anatomical and functional characteristics of the OB circuit. First, we confirm the columnar organization of the granule cells (GCs) present in the granule cell layer (GCL), previously demonstrated by our research group. Then, we show the extent in the GCL of the axon collateral network of the OB principal cells, mitral and tufted cells (MCs and TCs) and its targets: GCs columns but also neurons morphologically different than GCs known as short-axon cells, that we further try to characterize. Based on morphological information as well as electrical and protein expression profiles we conclude to a high level of heterogeneity amongst the short-axon population. Finally, we confirm the presence of projections from anterior piriform cortex (aPCX) pyramidal cells onto GCs columns as well as onto a short-axon cells population.

2008

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The neocortex makes up over 80% of the mammalian brain and is responsible for higher cognitive functions, processing of sensory inputs and orchestration of complex motor outputs. It is a 6-layered structure composed of billions of morphologically and electrically diverse neurons. Functionally, the basic unit is the neocortical column (NCC), a vertical structure of 0.5 mm wide, repeated millions of times across the neocortex and connected in an intricate but consistent way. In this thesis I investigated the role of morphologies (neurogeometry) in shaping the elaborate connectivity scheme within a column. First, I suggest a morphological basis for the higher than expected reciprocal connectivity reported experimentally between connected pairs, using a newly defined measure: the Reciprocity Index (RI), which arises from a purely mathematical concept and can be derived from the morphometric statistics of neurons. Second, I show that most experimentally reported synaptic patterns between different classes of neurons can be directly computed from the statistics of their morphologies, while some pathways would require additional functional mechanisms to refine an even more specific synaptic pattern. My thesis is done within the Blue Brain Project, the first comprehensive attempt to reverse-engineer the mammalian brain, starting with the somatosensory NCC of a P14 rat. I present in this thesis my contribution in constructing the framework for building biologically accurate circuitry. I explain how we start from 3D neuron morphologies obtained in vitro, repair them for slicing artefacts, build circuits and accurately detect potential synapses. I also explore how modelling of the experimental procedures can help us characterize biases and predict in vivo data from in vitro data. I finally present recent exploratory work on how to use the supercomputing power to design novel in silico protocols to investigate the emergent dynamics of the neocortical column.

EPFL2010

Chargement

EPFL2007