Subthreshold Resonance Research Articles

Intrinsically generated phase precession of grid cells in a network model

Event Abstract Back to Event Intrinsically generated phase precession of grid cells in a network model Kay Thurley1, 2*, Franziska Hellmundt1 and Christian Leibold1, 2 1 Ludwig-Maximilians-University, Munich, Department Biology II, Germany 2 Bernstein Center for Computational Neuroscience Munich, Germany Stellate cells in layer II of the medial entorhinal cortex exhibit place-specific firing in-vivo. Their firing fields are arranged on a spatial hexagonal lattice, generally referred to as grid. Grid-field activity is accompanied by oscillations of the local field potential (LFP) in the theta band (4-12 Hz). The theta phase of single spikes thereby decreases with the distance traveled in the field, a phenomenon called phase precession. Stellate cells have been characterized as type II oscillators with a subthreshold resonance in the theta range. As such they are considered to be pacemakers. It is unclear how spiking of such putative pacemaker neurons would be able to precess in phase relative to a self-generated oscillation. Based on a recent model of phase precession in the hippocampus (Geisler et al. 2010 Proc. Natl. Acad. Sci, U.S.A. 107: 7957), we developed a theory on how this paradox can be resolved. For the original hippocampus model, the core idea is that a population oscillation, i.e. LFP, is a little slower than the individual neuronal oscillators, when the latter have firing fields of limited spatial extent and are sequentially activated. The individual neurons hence phase precess with respect to the LFP. Extending the original model to grid fields is not straightforward since the periodicity of the grid fields generally destroys the phase coordination. Moreover, the original model strongly relies on a compression parameter that governs the transformation of the rate place code to spike timing. The compression parameter, however, has an entirely unclear mechanistic origin. Our simulations show that the type II property of stellate cells is instrumental in synchronizing small cell groups and thereby accounts for theta oscillations and phase precession. Direct excitatory coupling between the stellate cells, indirect inhibitory coupling via a gamma-oscillating network of interneurons, or both could mediate this phase coordination. The compression of the place code from rates to spike timing follows as a natural consequence of the self organization of firing phases. The model also explains phase precession for changes in the running speed of the animal and variable grid-spacing. Keywords: Entorhinal Cortex, stellate cells, Theta Rhythm Conference: Bernstein Conference 2012, Munich, Germany, 12 Sep - 14 Sep, 2012. Presentation Type: Poster Topic: Other Citation: Thurley K, Hellmundt F and Leibold C (2012). Intrinsically generated phase precession of grid cells in a network model. Front. Comput. Neurosci. Conference Abstract: Bernstein Conference 2012. doi: 10.3389/conf.fncom.2012.55.00161 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 18 Sep 2012; Published Online: 12 Sep 2012. * Correspondence: Dr. Kay Thurley, Ludwig-Maximilians-University, Munich, Department Biology II, Munich, Germany, thurley@bio.lmu.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Kay Thurley Franziska Hellmundt Christian Leibold Google Kay Thurley Franziska Hellmundt Christian Leibold Google Scholar Kay Thurley Franziska Hellmundt Christian Leibold PubMed Kay Thurley Franziska Hellmundt Christian Leibold Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Firing frequency response to current and conductance periodic inputs in a Ih/INap biophysical neuron model

Various neurons exhibit membrane potential (or subthreshold) resonance [1], a peak in the subthreshold voltage amplitude response to oscillatory current inputs at a certain preferred (resonant) input frequency. Previous theoretical work using linear models has shown that subthreshold resonance can be communicated to the supra-threshold regime [2,3]; i.e., the firing frequency (or signal gain) response to oscillatory input currents peaks at the subthreshold resonant frequency. Whether this property is maintained under more general conditions and for different types of inputs is still an open question. In this work, we investigate the firing frequency patterns generated in response to both current and conductance sinusoidal inputs in a biophysical (conductance-based) neuron model that includes two ionic currents: h- (Ih) and persistent sodium (INap). This model is based on measurements for stellate cells (SCs) in layer II of the medial entorhinal cortex [4] and captures various phenomena observed in SCs such as membrane potential oscillations and membrane potential (subthreshold) resonance at theta frequencies (4 - 10 Hz)[2]. The Ih/INap model describes the cell's subthreshold dynamics and the onset of spikes [5]. Spikes were generated artificially as in integrate/resonate-and-fire models (see [5]). We computed the firing rate and signal gain responses of the Ih/INap model to both current and conductance sinusoidal inputs for a wide range of frequencies and input amplitudes (Ain). The resulting patterns for current and conductance input are qualitatively different. For current inputs, the firing frequency (and signal gain) patterns show up to three prominent peaks with heights that differ only slightly. The number of peaks increases with increasing values of Ain. For small values of Ain, the peak input frequency coincides with the subthreshold resonant frequency. The voltage traces (voltage vs. time) corresponding different peaks for the same value of Ain have roughly the same number of spikes (and inter-spike intervals of similar size), and differ in the number of subthreshold oscillations interspersed in between two consecutive spikes. For conductance (excitatory) inputs, the firing rate patterns show multiple peaks with different heights. As Ain increases, the highest peak moves to the right. For input frequencies above 60 Hz, the firing rate and gain of the highest peak significantly increases relative to other peaks for the same value of Ain. This prominent peak reflects the fast time scale present in the model that has been shown to underlie hyper-excitable firing in SCs [6] observed in animal models of temporal lobe epilepsy. We use dynamical systems tools to explain the mechanism that govern the generation of the firing frequency patterns described above. We show that the nonlinearities present in the model and the time scale separation between voltage and the h-current gating variables play an important role in determining these patterns.

First proton-transfer study of18F+presonances relevant for novae

The ${}^{18}\mathrm{F}(p,\ensuremath{\alpha}){}^{15}\mathrm{O}$ reaction is the predominant destruction mechanism in novae of the radionuclide $^{18}\mathrm{F}$, a target of $\ensuremath{\gamma}$-ray observatories. Thus, its rate is important for understanding $^{18}\mathrm{F}$ production in novae. We have studied resonances in the ${}^{18}\mathrm{F}+p$ system by making a measurement of a proton-transfer reaction ${}^{18}\mathrm{F}(d,n)$. We have observed 15 $^{19}\mathrm{Ne}$ levels, 5 of which are below the proton threshold, including a subthreshold state, which has significant ${l}_{p}=0$ strength. Our data provide a direct determination of the spectroscopic strength of these states and new constraints on their spins and parities, thereby resolving a controversy, which involves the 8- and 38-keV resonances. The ${}^{18}\mathrm{F}(p,\ensuremath{\alpha}){}^{15}\mathrm{O}$ reaction rate is reevaluated, which takes the subthreshold resonance and other new information determined in this experiment into account.

Theoretical interpretation of anomalous tritium and neutron productions during Pd/D co-deposition experiments

The recent experimental observations of triple tracks in solid-state nuclear track detectors, CR-39, during Pd/D co-deposition experiments indicate that the triple tracks are due to ∼14 MeV neutrons, which appear to originate from hot fusion reaction D(t,n) 4 He. Nuclear theory interpretation of the origin of ∼14 MeV neutrons is presented in terms of a sub-threshold resonance reaction involving (T + p) resonance state of 4 He ∗ (J π =0 + ) at 20.21 MeV, which produces 1.01 MeV T. An upper limit of the branching ratio, R(n)/R(T), between neutron production rate and tritium production rate is calculated to be R(n)/R(T) < 10 −4 . Experimental tests of the proposed theoretical interpretation are proposed.

GAD67-GFP+ Neurons in the Nucleus of Roller. II. Subthreshold and Firing Resonance Properties

In the companion paper we show that GAD67-GFP+ (GFP+) inhibitory neurons located in the Nucleus of Roller of the mouse brain stem can be classified into two main groups (tonic and phasic) based on their firing patterns in responses to injected depolarizing current steps. In this study we examined the responses of GFP+ cells to fluctuating sinusoidal ("chirp") current stimuli. Membrane impedance profiles in response to chirp stimulation showed that nearly all phasic cells exhibited subthreshold resonance, whereas the majority of tonic GFP+ cells were nonresonant. In general, subthreshold resonance was associated with a relatively fast passive membrane time constant and low input resistance. In response to suprathreshold chirp current stimulation at a holding potential just below spike threshold the majority of tonic GFP+ cells fired multiple action potentials per cycle at low input frequencies (<5 Hz) and either stopped firing or were not entrained by the chirp at higher input frequencies (= tonic low-pass cells). A smaller group of phasic GFP+ cells did not fire at low input frequency but were able to phase-lock 1:1 at intermediate chirp frequencies (= band-pass cells). Spike timing reliability was tested with repeated chirp stimuli and our results show that phasic cells were able to reliably fire when they phase-locked 1:1 over a relatively broad range of input frequencies. Most tonic low-pass cells showed low reliability and poor phase-locking ability. Computer modeling suggested that these different firing resonance properties among GFP+ cells are due to differences in passive and active membrane properties and spiking mechanisms. This heterogeneity of resonance properties might serve to selectively activate subgroups of interneurons.

Differences in subthreshold resonance of hippocampal pyramidal cells and interneurons: the role of h-current and passive membrane characteristics.

The intrinsic properties of distinct types of neuron play important roles in cortical network dynamics. One crucial determinant of neuronal behaviour is the cell's response to rhythmic subthreshold input, characterised by the input impedance, which can be determined by measuring the amplitude and phase of the membrane potential response to sinusoidal currents as a function of input frequency. In this study, we determined the impedance profiles of anatomically identified neurons in the CA1 region of the rat hippocampus (pyramidal cells as well as interneurons located in the stratum oriens, including OLM cells, fast-spiking perisomatic region-targeting interneurons and cells with axonal arbour in strata oriens and radiatum). The basic features of the impedance profiles, as well as the passive membrane characteristics and the properties of the sag in the voltage response to negative current steps, were cell-type specific. With the exception of fast-spiking interneurons, all cell types showed subthreshold resonance, albeit with distinct features. The HCN channel blocker ZD7288 (10 μm) eliminated the resonance and changed the shape of the impedance curves, indicating the involvement of the hyperpolarisation-activated cation current Ih. Whole-cell voltage-clamp recordings uncovered differences in the voltage-dependent activation and kinetics of Ih between different cell types. Biophysical modelling demonstrated that the cell-type specificity of the impedance profiles can be largely explained by the properties of Ih in combination with the passive membrane characteristics. We conclude that differences in Ih and passive membrane properties result in a cell-type-specific response to inputs at given frequencies, and may explain, at least in part, the differential involvement of distinct types of neuron in various network oscillations.

Bound, virtual, and resonanceS-matrix poles from the Schrödinger equation

A general method, which we call the potential $S$-matrix pole method, is developed for obtaining the $S$-matrix pole parameters for bound, virtual and resonant states based on numerical solutions of the Schr\"odinger equation. This method is well-known for bound states. In this work we generalize it for resonant and virtual states, although the corresponding solutions increase exponentially when $r\to\infty$. Concrete calculations are performed for the $1^+$ ground and the $0^+$ first excited states of $^{14}\rm{N}$, the resonance $^{15}\rm{F}$ states ($1/2^+$, $5/2^+$), low-lying states of $^{11}\rm{Be}$ and $^{11}\rm{N}$, and the subthreshold resonances in the proton-proton system. We also demonstrate that in the case the broad resonances their energy and width can be found from the fitting of the experimental phase shifts using the analytical expression for the elastic scattering $S$-matrix. We compare the $S$-matrix pole and the $R$-matrix for broad $s_{1/2}$ resonance in ${}^{15}{\rm F}$

Cholinergic modulation of the resonance properties of stellate cells in layer II of medial entorhinal cortex.

In vitro whole cell patch-clamp recordings of stellate cells in layer II of medial entorhinal cortex show a subthreshold membrane potential resonance in response to a sinusoidal current injection of varying frequency. Physiological recordings from awake behaving animals show that neurons in layer II medial entorhinal cortex, termed "grid cells," fire in a spatially selective manner such that each cell's multiple firing fields form a hexagonal grid. Both the spatial periodicity of the grid fields and the resonance frequency change systematically in neurons along the dorsal to ventral axis of medial entorhinal cortex. Previous work has also shown that grid field spacing and acetylcholine levels change as a function of the novelty to a particular environment. Using in vitro whole cell patch-clamp recordings, our study shows that both resonance frequency and resonance strength vary as a function of cholinergic modulation. Furthermore, our data suggest that these changes in resonance properties are mediated through modulation of h-current and m-current.

Characteristics and physiological role of hyperpolarization activated currents in mouse cold thermoreceptors.

Hyperpolarization-activated currents (I(h)) are mediated by the expression of combinations of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits (HCN1-4). These cation currents are key regulators of cellular excitability in the heart and many neurons in the nervous system. Subunit composition determines the gating properties and cAMP sensitivity of native I(h) currents. We investigated the functional properties of I(h) in adult mouse cold thermoreceptor neurons from the trigeminal ganglion, identified by their high sensitivity to moderate cooling and responsiveness to menthol. All cultured cold-sensitive (CS) neurons expressed a fast activating I(h), which was fully blocked by extracellular Cs(+) or ZD7288 and had biophysical properties consistent with those of heteromeric HCN1-HCN2 channels. In CS neurons from HCN1(-/-) animals, I(h) was greatly reduced but not abolished. We find that I(h) activity is not essential for the transduction of cold stimuli in CS neurons. Nevertheless, I(h) has the potential to shape the excitability of CS neurons. First, I(h) blockade caused a membrane hyperpolarization in CS neurons of about 5 mV. Furthermore, impedance power analysis showed that all CS neurons had a prominent subthreshold membrane resonance in the 5-7 Hz range, completely abolished upon blockade of I(h) and absent in HCN1 null mice. This frequency range matches the spontaneous firing frequency of cold thermoreceptor terminals in vivo. Behavioural responses to cooling were reduced in HCN1 null mice and after peripheral pharmacological blockade of I(h) with ZD7288, suggesting that I(h) plays an important role in peripheral sensitivity to cold.

Role of theN*(1535)inη'production

We study the near-threshold ${\ensuremath{\eta}}^{'}$ production mechanism in nucleon-nucleon and $\ensuremath{\pi}N$ collisions under the assumption that subthreshold resonance ${N}^{*}(1535)$ is predominant. In an effective Lagrangian approach that gives a reasonable description to the $\mathit{pN}\ensuremath{\rightarrow}\mathit{pN}\ensuremath{\eta}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}n\ensuremath{\eta}$ reactions, we find that the excitation of ${N}^{*}(1535)$ resonance from the t-channel \ensuremath{\pi} exchange makes the dominate contribution to the $\mathit{pN}\ensuremath{\rightarrow}\mathit{pN}{\ensuremath{\eta}}^{'}$ process, and a value of 6.5 for the ratio of $\ensuremath{\sigma}(\mathit{pn}\ensuremath{\rightarrow}\mathit{pn}{\ensuremath{\eta}}^{'})$ to $\ensuremath{\sigma}(\mathit{pp}\ensuremath{\rightarrow}\mathit{pp}{\ensuremath{\eta}}^{'})$ is predicted. A strongcoupling strength of ${N}^{*}(1535)$ to ${\ensuremath{\eta}}^{'}N$ (${g}_{{\ensuremath{\eta}}^{'}{\mathit{NN}}^{*}}^{2}/4\ensuremath{\pi}=1.1$) is extracted from a combined analysis to $\mathit{pp}\ensuremath{\rightarrow}\mathit{pp}{\ensuremath{\eta}}^{'}$ and $\ensuremath{\pi}N\ensuremath{\rightarrow}N{\ensuremath{\eta}}^{'}$, and the possible implication to the intrinsic component of ${N}^{*}(1535)$ is explored.

Trojan horse method applied to9Be(p,α)6Liat astrophysical energies

The low-energy bare-nucleus cross section for ${}^{9}\mathrm{Be}(p,\ensuremath{\alpha}){}^{6}\mathrm{Li}$ has been extracted by means of the Trojan horse method (THM) applied to the ${}^{2}\mathrm{H}({}^{9}\mathrm{Be}$, $\ensuremath{\alpha},{}^{6}\mathrm{Li})n$ reaction at a beam energy of $^{9}\mathrm{Be}$ of 22.35 MeV. For the first time, we assume an intermediate process, ${}^{9}\mathrm{Be}+{}^{2}\mathrm{H}\ensuremath{\rightarrow}{}^{9}\mathrm{Be}+p+n$, and considered this process as one criterion of the quasifree condition. Accordingly, sequential decay processes were eliminated. The derived astrophysical $S(E)$ factor for the two-body process ${}^{9}\mathrm{Be}(p,\ensuremath{\alpha}){}^{6}\mathrm{Li}$ is compared with that obtained from direct experiments. We have found good agreement between the two results, leading to an improved determination of the $S(E)$ with $S(0)=21.0\ifmmode\pm\else\textpm\fi{}0.8$ MeV b. Furthermore, the electron screening potential energy ${U}_{e}=676\ifmmode\pm\else\textpm\fi{}86$ eV has also been extracted in a model-independent way by comparing the direct and THM data. The value is significantly higher than that predicted by current theoretical models, whereas it is lower than $\mathit{Ue}\ensuremath{\simeq}830$ eV, which was extracted from direct measurements with inclusion of the ${E}_{\mathrm{c}.\mathrm{m}.}=\ensuremath{-}23$ keV subthreshold resonance.

Subthreshold membrane-potential resonances shape spike-train patterns in the entorhinal cortex.

Many neurons exhibit subthreshold membrane-potential resonances, such that the largest voltage responses occur at preferred stimulation frequencies. Because subthreshold resonances are known to influence the rhythmic activity at the network level, it is vital to understand how they affect spike generation on the single-cell level. We therefore investigated both resonant and nonresonant neurons of rat entorhinal cortex. A minimal resonate-and-fire type model based on measured physiological parameters captures fundamental properties of neuronal firing statistics surprisingly well and helps to shed light on the mechanisms that shape spike patterns: 1) subthreshold resonance together with a spike-induced reset of subthreshold oscillations leads to spike clustering and 2) spike-induced dynamics influence the fine structure of interspike interval (ISI) distributions and are responsible for ISI correlations appearing at higher firing rates (> or =3 Hz). Both mechanisms are likely to account for the specific discharge characteristics of various cell types.

Proton capture by 14N at astrophysical energies

Calculations are reported for low energy, proton capture by 14N with the recoil corrected continuum shell model. An interaction from a fit to Cohen and Kurath (6–16) p-shell matrix elements and Reid soft-core g-matrix elements is employed. The prediction for 14N(p, γ)15O, based on this model and available data, is that S(0) equals 1.676 and S(30) is 1.666 keV b. Good agreement with available cross section data support this result. No evidence is found for significant contributions from the subthreshold resonance.

Strange resonances measured in Al+Al collisions at GeV with the FOPI detector

The FOPI Collaboration performed a high statistic experiment to study the strangeness production in Al+Al collisions at a beam kinetic energy of 1.9A GeV. The collected huge data sample allows for the first time the reconstruction of sub-threshold resonances production such as the Σ(1385) and the K(892). These measurements constrain the parameters of statistical model and provide information on in-medium cross sections for the different processes involved in the strangeness production close to or below their thresholds.

Strangeness Production Process pp → nK+Σ+ within Resonance Model

We explore production mechanism and final state interaction in the pp → nK+Σ+ channel based on the inconsistent experimental data published respectively by COSY-11 and COSY-ANKE. The scattering parameter a > 0 for nΣ+ interaction is favoured by large near-threshold cross section within a nonrelativistic parametrization investigation, and a strong nΣ+ interaction comparable to pp interaction is also indicated. Based on this analysis we calculate the contribution from resonance Δ* (1920) through π+ exchange within resonance model, and the numerical result suggests a rather small near-threshold total cross section, which is consistent with the COSY-ANKE data. With an additional sub-threshold resonance Δ* (1620), the model gives a much better description to the rather large near-threshold total cross section published by COSY-11.

Plasticity of Intrinsic Excitability during Long-Term Depression Is Mediated through mGluR-Dependent Changes inIhin Hippocampal CA1 Pyramidal Neurons

Bidirectional changes in synaptic strength are the proposed cellular correlate for information storage in the brain. Plasticity of intrinsic excitability, however, may also be critical for regulating the firing of neurons during mnemonic tasks. We demonstrated previously that the induction long-term potentiation was accompanied by a persistent decrease in CA1 pyramidal neuron excitability (Fan et al., 2005). We show here that induction of long-term depression (LTD) by 3 Hz pairing of back-propagating action potentials with Schaffer collateral EPSPs was accompanied by an overall increase in CA1 neuronal excitability. This increase was observed as an increase in the number of action potentials elicited by somatic current injection and was caused by an increase in neuronal input resistance. After LTD, voltage sag during hyperpolarizing current injections and subthreshold resonance frequency were decreased. All changes were blocked by ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride), suggesting that a physiological loss of dendritic h-channels was responsible for the increase in excitability. Furthermore, block of group 1 metabotropic glutamate receptors (mGluRs) or protein kinase C prevented the increase in excitability, whereas the group 1 mGluR agonist DHPG [(RS)-3,5-dihydroxyphenylglycine] mimicked the effects. We conclude that 3 Hz synaptic stimulation downregulates I(h) via activation of group 1 mGluRs and subsequent stimulation of protein kinase C. We propose these changes as part of a homeostatic and bidirectional control mechanism for intrinsic excitability during learning.

Long-Term Potentiation in Rat Hippocampal Neurons Is Accompanied by Spatially Widespread Changes in Intrinsic Oscillatory Dynamics and Excitability

Oscillations in neural activity are a prominent feature of many brain states. Individual hippocampal neurons exhibit intrinsic membrane potential oscillations and intrinsic resonance in the theta frequency range. We found that the subthreshold resonance frequency of CA1 pyramidal neurons was location dependent, varying more than 3-fold between the soma and the distal dendrites. Furthermore, activity- and NMDA-receptor-dependent long-term plasticity increased this resonance frequency through changes in h channel properties. The increase in resonance frequency and an associated reduction in excitability were nearly identical in the soma and the first 300 mum of the apical dendrites. These spatially widespread changes accompanying long-term synaptic potentiation also reduced the neuron's ability to elicit spikes evoked through a nonpotentiated synaptic pathway. Our results suggest that the frequency response of these neurons depends on the dendritic location of their inputs and that activity can regulate their response dynamics within an oscillating neural network.

Cell-Type–Specific Modulation of Intrinsic Firing Properties and Subthreshold Membrane Oscillations by the M(Kv7)-Current in Neurons of the Entorhinal Cortex

The M-current (current through Kv7 channels) is a low-threshold noninactivating potassium current that is suppressed by muscarinic agonists. Recent studies have shown its role in spike burst generation and intrinsic subthreshold theta resonance, both of which are important for memory function. However, little is known about its role in principal cells of the entorhinal cortex (EC). In this study, using whole cell patch recording techniques in a rat EC slice preparation, we have examined the effects of the M-current blockers linopirdine and XE991 on the membrane dynamics of principal cells in the EC. When the M-current was blocked, layer II nonstellate cells (non-SCs) and layer III cells switched from tonic discharge to intermittent firing mode, during which layer II non-SCs showed high-frequency short-duration spike bursts due to increased fast spike afterdepolarization (ADP). When three spikes were elicited at 50 Hz, these two types of cells reacted with a slow ADP that drove delayed firing. In contrast, layer II stellate cells (SCs) and layer V cells never displayed intermittent firing, bursting behavior, or delayed firing. Under the M-current block, intrinsic excitability increased significantly in layer III and layer V cells but not in layer II SCs and non-SCs. The M-current block also had contrasting effects on the subthreshold excitability, greatly suppressing the subthreshold membrane potential oscillations in layer V cells but not in layer II SCs. Modulation of the M-current thus shifts the firing behavior, intrinsic excitability, and subthreshold membrane potential oscillations of EC principal cells in a cell-type-dependent manner.

Differential Intrinsic Response Dynamics Determine Synaptic Signal Processing in Frog Vestibular Neurons

Central vestibular neurons process head movement-related sensory signals over a wide dynamic range. In the isolated frog whole brain, second-order vestibular neurons were identified by monosynaptic responses after electrical stimulation of individual semicircular canal nerve branches. Neurons were classified as tonic or phasic vestibular neurons based on their different discharge patterns in response to positive current steps. With increasing frequency of sinusoidally modulated current injections, up to 100 Hz, there was a concomitant decrease in the impedance of tonic vestibular neurons. Subthreshold responses as well as spike discharge showed classical low-pass filter-like characteristics with corner frequencies ranging from 5 to 20 Hz. In contrast, the impedance of phasic vestibular neurons was relatively constant over a wider range of frequencies or showed a resonance at approximately 40 Hz. Above spike threshold, single spikes of phasic neurons were synchronized with the sinusoidal stimulation between approximately 20 and 50 Hz, thus showing characteristic bandpass filter-like properties. Both the subthreshold resonance and bandpass filter-like discharge pattern depend on the activation of an I(D) potassium conductance. External current or synaptic stimulation that produced impedance increases (i.e., depolarization in tonic or hyperpolarization in phasic neurons) had opposite and complementary effects on the responses of the two types of neurons. Thus, membrane depolarization by current steps or repetitive synaptic excitation amplified synaptic inputs in tonic vestibular neurons and reduced them in phasic neurons. These differential, opposite membrane response properties render the two neuronal types particularly suitable for either integration (tonic neurons) or signal detection (phasic neurons), respectively, and dampens variations of the resting membrane potential in the latter.

Direct reactions with exotic nuclei, nuclear structure and astrophysics

Intermediate energy Coulomb excitation and dissociation is a useful tool for nuclear structure and astrophysics studies. Low-lying strength in nuclei far from stability was discovered by this method. The effective range theory for low-lying strength in one-neutron halo nuclei is summarized and extended to two-neutron halo nuclei. This is of special interest in view of recent rather accurate experimental results on the low-lying electric dipole strength in 11Li. Another indirect approach to nuclear astrophysics is the Trojan-horse method. It is pointed out that it is a suitable tool to investigate subthreshold resonances.