Modular construction of nervous systems: a basic principle of design for invertebrates and vertebrates

As evidenced by the proliferation of papers in the last 30 years it is now well accepted that an iterative columnar or modular organization of the neocortex is characteristic of mammalian sensory, motor and frontal association areas. This does not imply that all mammalian neocortical areas are thus arranged; exceptions occur, particularly in the rodents

Cochlear injury and adaptive plasticity of the auditory cortex

Growing evidence suggests that cochlear stressors as noise exposure and aging can induce homeostatic/maladaptive changes in the central auditory system from the brainstem to the cortex. Studies centered on such changes have revealed several mechanisms that operate in the context of sensory disruption after insult (noise trauma, drug-, or age-related injury). The oxidative stress is central to current theories of induced sensory-neural hearing loss and aging, and interventions to attenuate the hearing loss are based on antioxidant agent. The present review addresses the recent literature on the alterations in hair cells and spiral ganglion neurons due to noise-induced oxidative stress in the cochlea, as well on the impact of cochlear damage on the auditory cortex neurons. The emerging image emphasizes that noise-induced deafferentation and upward spread of cochlear damage is associated with the altered dendritic architecture of auditory pyramidal neurons. The cortical modifications may be reversed by treatment with antioxidants counteracting the cochlear redox imbalance. These findings open new therapeutic approaches to treat the functional consequences of the cortical reorganization following cochlear damage

The barrel cortex—integrating molecular, cellular and systems physiology

A challenge for neurobiology is to integrate information across many levels of research, ranging from behaviour and neuronal networks to cells and molecules. The rodent whisker signalling pathway to the primary somatosensory neocortex with its remarkable somatotopic barrel map is emerging as a key system for such integrative studie

Properties and function of somatostatin-containing inhibitory interneurons in the somatosensory cortex of the mouse

GABAergic inhibitory interneurons play a pivotal role in balancing neuronal activity in the neocortex. They can be classified into different classes according to their variable morphological, electrophysiological, and neurochemical properties, including two major groups: parvalbumin-containing (PV+), fast-spiking (FS) cells and somatostatin-containing (SOM+) cells. Using transgenic mice, we identified two subgroups, distinct by all criteria, of SOM+ cells in the somatosensory (barrel) cortex of the mouse, one (called X94) in layer 4 and 5B, and the other one (X98) in deep layers (Ma et al., 2006). We found that X98 cells were calbindin-expressing (CB+), infragranular, layer 1--targeting Martinotti cells, and had a propensity to fire low-threshold calcium spikes, whereas X94 cells did not express CB, targeted mostly layer 4, discharged in stuttering pattern and with quasi fast-spiking properties. In the barrel cortex, it was previously shown that SOM+ cells mediate disynaptic inhibition in supragranular and infragranular layers. However, the roles of layer 4 SOM+ cells remain largely unknown. We used dual whole-cell recording to elucidate the synaptic circuits in layer 4 and the function of layer 4 SOM+ cells during cortical network activities. We found that layer 4 X94 SOM+ cells received strongly facilitating excitatory input and generated relatively slow rising inhibitory postsynaptic currents (IPSCs) compared to those evoked by FS cells. Strikingly, our data showed that SOM+ cells mediated strong synaptic inhibition of FS cells with connection probability greater than 90% in layer 4, but received very little reciprocal inhibition from FS cells, and no reciprocal inhibition from other SOM+ cells. Moreover, 100% of recorded SOM+-SOM+ cell pairs were electrically coupled with higher coupling ratio compared to that of electrically coupled FS cell pairs. In order to examine the functions of SOM+ cells, we applied 0 Mg2+ artificial cerebrospinal fluid (ACSF) to induce episodes of cortical network activity and observed that, during episodes of network activity, SOM+ cells fired robustly and synchronously, and produced strong inhibition of regular-spiking (RS) excitatory cells and inhibitory FS cells, especially the latter. Taken together, our data reveal that SOM+ cells in the barrel cortex can be sub-divided into different subtypes, and that layer 4 SOM+ cells exert a powerful inhibitory effect during high frequency network activity

From Neural Arbors to Daisies

Pyramidal neurons in layers 2 and 3 of the neocortex collectively form an horizontal lattice of long-range, periodic axonal projections, known as the superficial patch system. The precise pattern of projections varies between cortical areas, but the patch system has nevertheless been observed in every area of cortex in which it has been sought, in many higher mammals. Although the clustered axonal arbors of single pyramidal cells have been examined in detail, the precise rules by which these neurons collectively merge their arbors remain unknown. To discover these rules, we generated models of clustered axonal arbors following simple geometric patterns. We found that models assuming spatially aligned but independent formation of each axonal arbor do not produce patchy labeling patterns for large simulated injections into populations of generated axonal arbors. In contrast, a model that used information distributed across the cortical sheet to generate axonal projections reproduced every observed quality of cortical labeling patterns. We conclude that the patch system cannot be built during development using only information intrinsic to single neurons. Information shared across the population of patch-projecting neurons is required for the patch system to reach its adult state

Beyond the Cortical Column: Abundance and Physiology of Horizontal Connections Imply a Strong Role for Inputs from the Surround

Current concepts of cortical information processing and most cortical network models largely rest on the assumption that well-studied properties of local synaptic connectivity are sufficient to understand the generic properties of cortical networks. This view seems to be justified by the observation that the vertical connectivity within local volumes is strong, whereas horizontally, the connection probability between pairs of neurons drops sharply with distance. Recent neuroanatomical studies, however, have emphasized that a substantial fraction of synapses onto neocortical pyramidal neurons stems from cells outside the local volume. Here, we discuss recent findings on the signal integration from horizontal inputs, showing that they could serve as a substrate for reliable and temporally precise signal propagation. Quantification of connection probabilities and parameters of synaptic physiology as a function of lateral distance indicates that horizontal projections constitute a considerable fraction, if not the majority, of inputs from within the cortical network. Taking these non-local horizontal inputs into account may dramatically change our current view on cortical information processing

From Neural Arbors to Daisies

Pyramidal neurons in layers 2 and 3 of the neocortex collectively form an horizontal lattice of long-range, periodic axonal projections, known as the superficial patch system. The precise pattern of projections varies between cortical areas, but the patch system has nevertheless been observed in every area of cortex in which it has been sought, in many higher mammals. Although the clustered axonal arbors of single pyramidal cells have been examined in detail, the precise rules by which these neurons collectively merge their arbors remain unknown. To discover these rules, we generated models of clustered axonal arbors following simple geometric patterns. We found that models assuming spatially aligned but independent formation of each axonal arbor do not produce patchy labeling patterns for large simulated injections into populations of generated axonal arbors. In contrast, a model that used information distributed across the cortical sheet to generate axonal projections reproduced every observed quality of cortical labeling patterns. We conclude that the patch system cannot be built during development using only information intrinsic to single neurons. Information shared across the population of patch-projecting neurons is required for the patch system to reach its adult stat

Fast Rhythmic Bursting Cells: The Horizontal Fiber System in the Cat’s Primary Visual Cortex

One of the cellular mechanisms underlying the generation of gamma oscillations is a type of cortical pyramidal neuron named fast rhythmic bursting (FRB) cells. After 58 cells from 21 cats\u27 primary visual cortices were filled with Neurobiotin, the brains were cut, and the cells were photographed. From all cells, 1 non-pyramidal and 4 pyramidal cell (3 regular spiking (RS) cells & 1 FRB cell) were confocaled, reconstructed with Neurolucida, and analyzed with NeuroExplorer. All 5 cells showed a linear correlation (\u3e0.94) between length and number of intersections. Their polar histograms indicated that the FRB cell has triple dendritic length, twice the number of dendritic tree orders and mean length compared to the 4 other cells. We propose that FRB cells are key elements of the horizontal fiber system that links cell populations with similar feature selections throughout the primary visual cortex

Morpho-physiological analysis of interneuronal populations in the rat piriform cortex before and after kindling induced epilepsy

The piriform cortex (PC) is involved in olfactory sensory processing, associative learning tasks and is highly seizurogenic. Understanding how interneurons participate in these behaviours, especially their contribution in epileptogenic mechanisms, is hampered by an incomplete understanding of their functional and morphological diversity. The hypothesis in this work is that kindling-induced epilepsy alters the firing properties of PC interneuronal populations. Altered/impaired interneuronal firing could lead to abnormal processing in the PC and epileptogenesis. Therefore it was important to first identify and describe interneuronal morpho-functional properties in the unkindled brain and then to assess the electrophysiological parameters following kindling. Based on interneuronal calcium-binding protein content, immunohistochemical analysis of PC showed that the four distinct interneuronal populations (calretinin, calbindin, parvalbumin, and parvalbumin/calbindin containing interneurons) had distinct layer localizations, preferred dendritic arborization patterns and specific innervations onto interneurons and pyramidal cells. Whole cell patch-clamp recordings of PC interneuronal populations indicated a large heterogeneity of firing patterns that could be classified into five main patterns ranging from non-adapting very high frequency (NAvHF) to various degrees of spiking adaptation: adapting high frequency (AHF), adapting low frequency (ALF), strongly adapting low frequency (sALF), and weakly adapting low frequency (wALF). This high firing variability in the PC suggests that different interneuronal populations might have distinct functional means to control and regulate the olfactory network processing, memory coding and/or generation of oscillatory activities. However, after kindling, NAvHF and wALF firing patterns were absent. These changes were correlated to an increased K+ current in multipolar cells. This result was confirmed by quantitative real time polymerase chain reaction (QPCR) and immunohistochemistry studies indicating an increased expression of a voltage-gated potassium channel Kv1.6 after kindling. Thus, kindling-induced alteration of interneuronal firing properties, especially the absence of NAvHF firing behaviour, might reduce the efficacy of inhibition on the pyramidal cells leading to increased disinhibition and/or altered oscillatory activities in the PC. Overall, this work provides a morpho-functional analysis of PC interneuronal populations indicating a high complexity of innervation and firing behaviours. It also shows for the first time that kindling induces alterations of interneuronal firing patterns that might be responsible for epileptogenesis in this area