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  • 1
    Electronic Resource
    Electronic Resource
    Axons, but not cell bodies, are activated by electrical stimulation in cortical gray matter I. Evidence from chronaxie measurements (1998)
    Springer
    Experimental brain research 118 (1998), S. 477-488 
    Details
    ISSN: 1432-1106
    Keywords: Key words Visual cortex ; Brain slice ; Strength-duration relations ; Conduction velocity ; Rat
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Extracellular electrical stimulation of the gray matter is often used to determine the function of a given cortical area or pathway. However, when it is used to elicit postsynaptic effects, the presynaptic neuronal elements activated by electrical stimulation have never been clearly identified: it could be the excitable dendrites, the cell body, the axon initial segment, or the axonal branches. To identify these elements, we performed two series of experiments on slices of rat visual cortex maintained in vitro. The first series of experiments, reported in this paper, was aimed at determining the chronaxie, a temporal parameter related to the membrane properties of the neuronal elements. In order to identify the presynaptic elements that were activated by extracellular electrical stimulation, chronaxies corresponding to postsynaptic responses were measured and compared with those corresponding to the activation of axons (antidromic activation) and those corresponding to the activation of cell bodies (intracellular current injection in intracellularly recorded neurons). The chronaxie for orthodromic activation was similar to that for axonal activation, but was 40 times smaller than the chronaxie for direct cell body activation. This suggests that, whenever a postsynaptic response is elicited after electrical stimulation of the cortical gray matter, axons (either axonal branches or axon initial segments), but not cell bodies, are the neuronal elements activated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002210050304
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  • 2
    Electronic Resource
    Electronic Resource
    Axons, but not cell bodies, are activated by electrical stimulation in cortical gray matter II. Evidence from selective inactivation of cell bodies and axon initial segments (1998)
    Springer
    Experimental brain research 118 (1998), S. 489-500 
    Details
    ISSN: 1432-1106
    Keywords: Key words Visual cortex ; Brain slice ; Intracortical microstimulation ; NMDA ; Rat
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  The results presented in the companion paper showed that extracellular electrical stimulation of the gray matter directly activates axons, but not cell bodies. The second set of experiments presented here was designed to separate the contribution of the axon initial segments and cell bodies from that of the axonal branches to the pool of presynaptic neuronal elements activated by electrical stimulation. For that purpose, N-methyl-d-aspartate (NMDA) iontophoresis was used to induce a selective inactivation of the cell body and of the adjoining portion of the axon by depolarization block, without affecting axonal branches that lack NMDA receptors. After NMDA iontophoresis, the neurons located near the iontophoresis electrode became unable to generate action potentials in an irreversible manner. When the NMDA-induced depolarization block was performed at the site of electrical stimulation, an unexpected increase in the amplitude of the orthodromic responses was observed. Several control experiments suggested that the field potential increase was due to changes of the local environment in the vicinity of the iontophoresis pipette, which led to an increased excitability of the axons. After the period of superexcitability, the orthodromic responses displayed an amplitude that was 15—20% lower than that observed before the NMDA-induced depolarization block, even though cell bodies and axon initial segment at the site of stimulation could not be activated by electrical stimulation. This result shows a low contribution for axon initial segments to the pool of neuronal elements activated by the electrical stimulation. Altogether, these experiments demonstrate that the postsynaptic responses obtained after electrical stimulation of the cortical gray matter result almost exclusively from the activation of axonal branches. Since the neocortex is organised as a network of local and long-range reciprocal connections, great attention must be paid to the interpretation of data obtained with electrical stimualtion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002210050305
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Corticocortical connections between visual areas 17 and 18a of the rat studied in vitro: spatial and temporal organisation of functional synaptic responses (1997)
    Springer
    Experimental brain research 117 (1997), S. 219-241 
    Details
    ISSN: 1432-1106
    Keywords: Key words Visual cortex ; Slice ; Field potentials ; Current source density ; Latency
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Much is known about the anatomy of corticocortical connections, yet little is known concerning their physiology. In order to have access to the synaptic and temporal aspects of the activity elicited through corticocortical connections, we developed an in vitro approach on slices of rat visual cortex. We used extracellular recordings of field potentials combined with electrical stimulation to localise regions of areas 17 and 18a that are connected. We found that corticocortical connections between areas 17 and 18a can be preserved in 500 μm thick slices, with a focus of activity separated from the stimulating electrode by 1.5 mm to more than 3 mm. The potentials elicited in one area after stimulation of its neighbour displayed fast events, corresponding to action potentials, and slow events, corresponding to synaptic potentials. Intracellular recordings showed that the earliest synaptic responses consisted of monosynaptic excitatory potentials. Measurement of response latency showed that axons involved in both feedforward and feedback corticocortical connections are slowly conducting (0.3–0.8 m/s). Conduction velocity for antidromically activated cells was not significantly different for the two sets of connections. In an attempt to establish the spatial organisation of functional synaptic inputs, field potential recordings were performed in the different cortical layers and used to establish current source density (CSD) graphs along the depth axis. The CSD maps obtained were found to be somewhat variable from one case to another. It is suggested that this variability results from the use of electrical stimulation, which activates axons that are both afferent and efferent to a given cortical area. The field potentials are therefore likely to contain responses that correspond to the activity mediated by the intrinsic collaterals mixed in variable amount with responses produced by corticocortical synapses. With this restriction in mind, it is suggested that, after stimulation of the supragranular layers, the functional synaptic inputs of feedforward connections are concentrated in layer 4 and the bottom of layer 3, while those of feedback axons involve mainly the upper part of the supragranular layers. The intrinsic collaterals of the neurones participating in corticocortical connections seem also to provide the bulk of their inputs to the upper part of the supragranular layers. The laminar pattern of activity obtained after infragranular layer stimulation was comparable to that obtained after supragranular layer stimulation, except for the addition of a supplementary region of activated synapses in the infragranular layers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002210050218
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    Paper (German National Licenses)
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