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Granule cells in the hippocampus project axons to hippocampal CA3 pyramidal cells where they form large mossy fiber terminals. We have reported the presence of gap junction protein connexin36 (Cx36) specifically in the stratum lucidum of rat ventral hippocampus, thus creating morphologically mixed synapses that have the potential for dual chemical/electrical transmission capabilities. There is some electrophysiological evidence that these terminals have a gap junction-mediated electrical transmission component under some conditions. Here, we used various approaches to further characterize molecular and electrophysiological relationships between the Cx36-containing gap junctions at mossy fiber terminals and their postsynaptic elements. In the CA3b and CA3c hippocampal regions, the vast majority of these terminals, identified by their selective expression of vesicular zinc transporter-3 (ZnT3), displayed multiple, fine immunofluorescent Cx36-puncta representing gap junctions, which were absent at mossy fiber terminals in the dorsal hippocampus. These puncta were invariably found in close proximity to the protein constituents of adherens junctions (i.e., N-cadherin and nectin-1) that are a structural hallmark of mossy fiber terminals that contact dendritic shafts of CA3 pyramidal cells, thus indicating the loci of gap junctions at these contacts. Cx36-puncta were also associated with adherens junctions at mixed synapses in other regions of the CNS. Electrophysiologically induced long-term potentiation of field responses evoked by mossy fiber stimulation was greater in the ventral than dorsal hippocampus, and it remains to be determined whether the electrical component of transmission at mossy fiber terminals contributes to the enhancement of these responses.

Granule cells in the hippocampus project axons to hippocampal CA3 pyramidal cells where they form large mossy fiber terminals. We have reported the presence of gap junction protein connexin36 (Cx36) specifically in the stratum lucidum of rat ventral hippocampus, thus creating morphologically mixed synapses that have the potential for dual chemical/electrical transmission capabilities. There is some electrophysiological evidence that these terminals have a gap junction-mediated electrical transmission component under some conditions. Here, we used various approaches to further characterize molecular and electrophysiological relationships between the Cx36-containing gap junctions at mossy fiber terminals and their postsynaptic elements. In the CA3b and CA3c hippocampal regions, the vast majority of these terminals, identified by their selective expression of vesicular zinc transporter-3 (ZnT3), displayed multiple, fine immunofluorescent Cx36-puncta representing gap junctions, which were absent at mossy fiber terminals in the dorsal hippocampus. These puncta were invariably found in close proximity to the protein constituents of adherens junctions (i.e., N-cadherin and nectin-1) that are a structural hallmark of mossy fiber terminals that contact dendritic shafts of CA3 pyramidal cells, thus indicating the loci of gap junctions at these contacts. Cx36-puncta were also associated with adherens junctions at mixed synapses in other regions of the CNS. Electrophysiologically induced long-term potentiation of field responses evoked by mossy fiber stimulation was greater in the ventral than dorsal hippocampus, and it remains to be determined whether the electrical component of transmission at mossy fiber terminals contributes to the enhancement of these responses.

This paper studies synchronization of a network of hybrid quadratic integrate-and-fire spiking neurons communicating over a complete graph and interconnected by means of bidirectional electrical couplings. Synchronization of the network of identical neurons with a common and constant coupling strength is studied using a Lyapunov-based argument for sufficiently large coupling strength. In addition, a voltage-dependent coupling law is proposed. It is assumed that each neuron is coupled to each of its neighbors by a coupling law which depends on the voltage of its neighboring neuron. For the voltage-dependent case, a sufficient condition for synchronization of two interconnected neurons is presented. Moreover, a comparison between the two mechanisms is given. Simulation results are provided to verify the theoretical analysis., QC 20200702

Electrical coupling between neurons is broadly present across brain areas and is typically assumed to synchronize network activity. However, intrinsic properties of the coupled cells can complicate this simple picture. Many cell types with electrical coupling show a diversity of post-spike subthreshold fluctuations, often linked to subthreshold resonance, which are transmitted through electrical synapses in addition to action potentials. Using the theory of weakly coupled oscillators, we explore the effect of both subthreshold and spike-mediated coupling on synchrony in small networks of electrically coupled resonate-and-fire neurons, a hybrid neuron model with damped subthreshold oscillations and a range of post-spike voltage dynamics. We calculate the phase response curve using an extension of the adjoint method that accounts for the discontinuous post-spike reset rule. We find that both spikes and subthreshold fluctuations can jointly promote synchronization. The subthreshold contribution is strongest when the voltage exhibits a significant post-spike elevation in voltage, or plateau potential. Additionally, we show that the geometry of trajectories approaching the spiking threshold causes a "reset-induced shear" effect that can oppose synchrony in the presence of network asymmetry, despite having no effect on the phase-locking of symmetrically coupled pairs.

A previously reported normally-off GaN double-channel (DC-) MOS-HEMT with a gate recess into the upper channel layer has achieved a remarkably low R-on. In this letter, we found that the double-channel structure itself does not guarantee a low Ron without careful consideration of the electrical coupling between the two channels. A strong channel-to-channel (C2C) coupling between the two channels is critical to reduce Ron in the DC-MOS-HEMT by exploiting both channels at the access region while allowing currents to converge into the heterojunction lower channel at the gate region. Otherwise, with a weak C2C coupling, the conduction through the upper channel is comprised due to the high-resistivity MOS-channel section at the gate region. The fabricated DC-MOS-HEMT with C2C distance (t(C2C)) of 7.5 nm exhibits an appreciably lower R-on than that with t(C2C) = 11.5 nm. Further analysis reveals the latter suffers from an increased sheet resistance at access region compared with the sheet resistance of the DC-heterostructure itself. A modified TLM characterization is proposed to analyze the C2C coupling in the DC-heterostructure. It is confirmed the DC-heterostructure with t(C2C) = 7.5 nm boasts a strong C2C coupling, while that with t(C2C) = 11.5 nm features relatively weak C2C coupling.

A previously reported normally-off GaN double-channel (DC-) MOS-HEMT with a gate recess into the upper channel layer has achieved a remarkably low R-on. In this letter, we found that the double-channel structure itself does not guarantee a low Ron without careful consideration of the electrical coupling between the two channels. A strong channel-to-channel (C2C) coupling between the two channels is critical to reduce Ron in the DC-MOS-HEMT by exploiting both channels at the access region while allowing currents to converge into the heterojunction lower channel at the gate region. Otherwise, with a weak C2C coupling, the conduction through the upper channel is comprised due to the high-resistivity MOS-channel section at the gate region. The fabricated DC-MOS-HEMT with C2C distance (t(C2C)) of 7.5 nm exhibits an appreciably lower R-on than that with t(C2C) = 11.5 nm. Further analysis reveals the latter suffers from an increased sheet resistance at access region compared with the sheet resistance of the DC-heterostructure itself. A modified TLM characterization is proposed to analyze the C2C coupling in the DC-heterostructure. It is confirmed the DC-heterostructure with t(C2C) = 7.5 nm boasts a strong C2C coupling, while that with t(C2C) = 11.5 nm features relatively weak C2C coupling.

A previously reported normally-off GaN double-channel (DC-) MOS-HEMT with a gate recess into the upper channel layer has achieved a remarkably low R-on. In this letter, we found that the double-channel structure itself does not guarantee a low Ron without careful consideration of the electrical coupling between the two channels. A strong channel-to-channel (C2C) coupling between the two channels is critical to reduce Ron in the DC-MOS-HEMT by exploiting both channels at the access region while allowing currents to converge into the heterojunction lower channel at the gate region. Otherwise, with a weak C2C coupling, the conduction through the upper channel is comprised due to the high-resistivity MOS-channel section at the gate region. The fabricated DC-MOS-HEMT with C2C distance (t(C2C)) of 7.5 nm exhibits an appreciably lower R-on than that with t(C2C) = 11.5 nm. Further analysis reveals the latter suffers from an increased sheet resistance at access region compared with the sheet resistance of the DC-heterostructure itself. A modified TLM characterization is proposed to analyze the C2C coupling in the DC-heterostructure. It is confirmed the DC-heterostructure with t(C2C) = 7.5 nm boasts a strong C2C coupling, while that with t(C2C) = 11.5 nm features relatively weak C2C coupling.

We examine the effects of dendritic filtering on the existence, stability, and robustness of phase-locked states to heterogeneity and noise in a pair of electrically coupled ball-and-stick neurons with passive dendrites. We use the theory of weakly coupled oscillators and analytically derived filtering properties of the dendritic coupling to systematically explore how the electrotonic length and diameter of dendrites can alter phase-locking. In the case of a fixed value of the coupling conductance (gc) taken from the literature, we find that repeated exchanges in stability between the synchronous and anti-phase states can occur as the electrical coupling becomes more distally located on the dendrites. However, the robustness of the phase-locked states in this case decreases rapidly towards zero as the distance between the electrical coupling and the somata increases. Published estimates of gc are calculated from the experimentally measured coupling coefficient (CC) based on a single-compartment description of a neuron, and therefore may be severe underestimates of gc. With this in mind, we re-examine the stability and robustness of phase-locking using a fixed value of CC, which imposes a limit on the maximum distance the electrical coupling can be located away from the somata. In this case, although the phase-locked states remain robust over the entire range of possible coupling locations, no exchanges in stability with changing coupling position are observed except for a single exchange that occurs in the case of a high somatic firing frequency and a large dendritic radius. Thus, our analysis suggests that multiple exchanges in stability with changing coupling location are unlikely to be observed in real neural systems.

We examine the effects of dendritic filtering on the existence, stability, and robustness of phase-locked states to heterogeneity and noise in a pair of electrically coupled ball-and-stick neurons with passive dendrites. We use the theory of weakly coupled oscillators and analytically derived filtering properties of the dendritic coupling to systematically explore how the electrotonic length and diameter of dendrites can alter phase-locking. In the case of a fixed value of the coupling conductance (gc) taken from the literature, we find that repeated exchanges in stability between the synchronous and anti-phase states can occur as the electrical coupling becomes more distally located on the dendrites. However, the robustness of the phase-locked states in this case decreases rapidly towards zero as the distance between the electrical coupling and the somata increases. Published estimates of gc are calculated from the experimentally measured coupling coefficient (CC) based on a single-compartment description of a neuron, and therefore may be severe underestimates of gc. With this in mind, we re-examine the stability and robustness of phase-locking using a fixed value of CC, which imposes a limit on the maximum distance the electrical coupling can be located away from the somata. In this case, although the phase-locked states remain robust over the entire range of possible coupling locations, no exchanges in stability with changing coupling position are observed except for a single exchange that occurs in the case of a high somatic firing frequency and a large dendritic radius. Thus, our analysis suggests that multiple exchanges in stability with changing coupling location are unlikely to be observed in real neural systems.

This paper combines the infrared lock-in thermography (IR-LIT) and heterodyne excitation techniques to detect high-frequency capacitive currents due to intradie electrical coupling between microelectronic devices or more complex systems. Modulating the excitation with the heterodyne approach, we drive devices or complex systems with high frequency electrical signals in such a way that they behave as low frequency heat sources, modulating their temperature field at a frequency detectable by an IR-LIT system. This approach is analytically studied and extended to a bi-dimensional scenario, showing that the thermal information at low frequency depends on the electrical characteristics of the sample at high frequency., Postprint (published version)

Fast-spiking (FS) cells in layer IV of the somatosensory cortex receive direct thalamocortical (TC) input and provide feed-forward inhibition onto layer IV excitatory cells. The level of synchronous firing of FS cells will affect the shape of this feed-forward output. Two factors that contribute to the synchrony are correlated TC input and electrical coupling between FS cells. Using a cell-pair model, we show that these two factors act synergistically to increase synchrony, and we examine the underlying mechanism. © 2006 Elsevier B.V. All rights reserved.

Fast-spiking (FS) cells in layer IV of the somatosensory cortex receive direct thalamocortical (TC) input and provide feed-forward inhibition onto layer IV excitatory cells. The level of synchronous firing of FS cells will affect the shape of this feed-forward output. Two factors that contribute to the synchrony are correlated TC input and electrical coupling between FS cells. Using a cell-pair model, we show that these two factors act synergistically to increase synchrony, and we examine the underlying mechanism. © 2006 Elsevier B.V. All rights reserved.

We investigated gap-junctional coupling of rods and cones in macaque retina. Cone voltage responses evoked by light absorption in neighboring rods were briefer and smaller than responses recorded in the rods themselves. Rod detection thresholds, calculated from noise and response amplitude histograms, closely matched the threshold for an ideal detector limited by quantal fluctuations in the stimulus. Surprisingly, cone thresholds were only approximately two times higher. Amplitude fluctuations in cones could be explained by a Poisson distribution of photoisomerizations within a pool of seven or more coupled rods. Neurobiotin coupling between rods and cones was consistent with our electrical recordings, with approximately six rods labeled per injected cone. The spatial distribution of tracer-coupled rods matched the light-evoked cone receptive field. The gap junction inhibitor carbenoxolone abolished both electrical and tracer coupling. Amplitude fluctuations in most rods were accounted for by the expected rate of light absorption in their outer segments. The fluctuations in some rods, however, were consistent with a summation pool of up to six rods. When single rods were injected with Neurobiotin, up to 10 rods were labeled. Rod-rod and rod-cone electrical coupling is expected to extend the range of scotopic vision by circumventing saturation at the rod to rod-bipolar cell synapse; however, because coupling also renders the rod synapse less effective at separating out photon signals from dark noise, coupling is expected to elevate the absolute threshold of dark-adapted observers.

We investigated gap-junctional coupling of rods and cones in macaque retina. Cone voltage responses evoked by light absorption in neighboring rods were briefer and smaller than responses recorded in the rods themselves. Rod detection thresholds, calculated from noise and response amplitude histograms, closely matched the threshold for an ideal detector limited by quantal fluctuations in the stimulus. Surprisingly, cone thresholds were only approximately two times higher. Amplitude fluctuations in cones could be explained by a Poisson distribution of photoisomerizations within a pool of seven or more coupled rods. Neurobiotin coupling between rods and cones was consistent with our electrical recordings, with approximately six rods labeled per injected cone. The spatial distribution of tracer-coupled rods matched the light-evoked cone receptive field. The gap junction inhibitor carbenoxolone abolished both electrical and tracer coupling. Amplitude fluctuations in most rods were accounted for by the expected rate of light absorption in their outer segments. The fluctuations in some rods, however, were consistent with a summation pool of up to six rods. When single rods were injected with Neurobiotin, up to 10 rods were labeled. Rod-rod and rod-cone electrical coupling is expected to extend the range of scotopic vision by circumventing saturation at the rod to rod-bipolar cell synapse; however, because coupling also renders the rod synapse less effective at separating out photon signals from dark noise, coupling is expected to elevate the absolute threshold of dark-adapted observers.

We will briefly outline a computational theory of the first stages of human vision according to which (a) the retinal image is filtered by a set of centre-surround receptive fields (of about 5 different spatial sizes) which are approximately bandpass in spatial frequency and (b) zero-crossings are detected independently in the output of each of these channels. Zero-crossings in each channel are then a set of discrete symbols which may be used for later processing such as contour extraction and stereopsis. A formulation of Logan's zero-crossing results is proved for the case of Fourier polynomials and an extension of Logan's theorem to 2-dimentsional functions is also approved. Within this framework, we shall describe an experimental and theoretical approach (developed by one of us with M. Fahle) to the problem of visual acuity and hyperacuity of human vision. The positional accuracy achieved, for instance, in reading a vernier is astonishingly high, corresponding to a fraction of the spacing between adjacent photoreceptors in the fovea. Stroboscopic presentation of a moving object can be interpolated by our visual system into the perception of continuous motion; and this "spatio-temporal" interpolation also can be very accurate. It is suggested that the known spatiotemporal properties of the channels envisaged by the theory of visual processing outlined above implement an interpolation scheme which can explain human vernier acuity for moving targets.

We will briefly outline a computational theory of the first stages of human vision according to which (a) the retinal image is filtered by a set of centre-surround receptive fields (of about 5 different spatial sizes) which are approximately bandpass in spatial frequency and (b) zero-crossings are detected independently in the output of each of these channels. Zero-crossings in each channel are then a set of discrete symbols which may be used for later processing such as contour extraction and stereopsis. A formulation of Logan's zero-crossing results is proved for the case of Fourier polynomials and an extension of Logan's theorem to 2-dimentsional functions is also approved. Within this framework, we shall describe an experimental and theoretical approach (developed by one of us with M. Fahle) to the problem of visual acuity and hyperacuity of human vision. The positional accuracy achieved, for instance, in reading a vernier is astonishingly high, corresponding to a fraction of the spacing between adjacent photoreceptors in the fovea. Stroboscopic presentation of a moving object can be interpolated by our visual system into the perception of continuous motion; and this "spatio-temporal" interpolation also can be very accurate. It is suggested that the known spatiotemporal properties of the channels envisaged by the theory of visual processing outlined above implement an interpolation scheme which can explain human vernier acuity for moving targets.

Breathing pattern is postulated to be generated by brainstem neurons. However, determination of the underlying cellular mechanisms, and in particular the synaptic interactions between respiratory neurons, has been difficult. Here we used dual recordings from two distinct populations of brainstem respiratory neurons, hypoglossal (XII) motoneurons, and rhythmogenic (type-1) neurons in the preBötzinger complex (pre-BötC), the hypothesized site for respiratory rhythm generation, to determine whether electrical and chemical transmission is present. Using an in vitro brainstem slice preparation from newborn mice, we found that intracellularly recorded pairs of XII motoneurons and pairs of preBötC inspiratory type-1 neurons showed bidirectional electrical coupling. Coupling strength was low (<0.10), and the current that passed between two neurons was heavily filtered (corner frequency, <10 Hz). Dual recordings also demonstrated unidirectional excitatory chemical transmission (EPSPs of ~3 mV) between type-1 neurons. These data indicate that respiratory motor output from the brainstem involves gap junction-mediated current transfer between motoneurons. Furthermore, bidirectional electrical coupling and unidirectional excitatory chemical transmission are present between type-1 neurons in the preBötC and may be important for generation or modulation of breathing rhythm.

Breathing pattern is postulated to be generated by brainstem neurons. However, determination of the underlying cellular mechanisms, and in particular the synaptic interactions between respiratory neurons, has been difficult. Here we used dual recordings from two distinct populations of brainstem respiratory neurons, hypoglossal (XII) motoneurons, and rhythmogenic (type-1) neurons in the preBötzinger complex (pre-BötC), the hypothesized site for respiratory rhythm generation, to determine whether electrical and chemical transmission is present. Using an in vitro brainstem slice preparation from newborn mice, we found that intracellularly recorded pairs of XII motoneurons and pairs of preBötC inspiratory type-1 neurons showed bidirectional electrical coupling. Coupling strength was low (<0.10), and the current that passed between two neurons was heavily filtered (corner frequency, <10 Hz). Dual recordings also demonstrated unidirectional excitatory chemical transmission (EPSPs of ~3 mV) between type-1 neurons. These data indicate that respiratory motor output from the brainstem involves gap junction-mediated current transfer between motoneurons. Furthermore, bidirectional electrical coupling and unidirectional excitatory chemical transmission are present between type-1 neurons in the preBötC and may be important for generation or modulation of breathing rhythm.