Biomedical subjects

I Segev

Publications and source records attributed to I Segev.

39 records · Page 3Linked to original sources

PubMedDate not supplied

Subthreshold voltage noise due to channel fluctuations in active neuronal membranes.

Voltage-gated ion channels in neuronal membranes fluctuate randomly between different conformational states due to thermal agitation. Fluctuations between conducting and nonconducting states give rise to noisy membrane currents and subthreshold voltage fluctuations and may contribute to variability in spike timing. Here we study subthreshold voltage fluctuations due to active voltage-gated Na+ and K+ channels as predicted by two commonly used kinetic schemes: the Mainen et al. (1995) (MJHS) kinetic scheme, which has been used to model dendritic channels in cortical neurons, and the classical Hodgkin-Huxley (1952) (HH) kinetic scheme for the squid giant axon. We compute the magnitudes, amplitude distributions, and power spectral densities of the voltage noise in isopotential membrane patches predicted by these kinetic schemes. For both schemes, noise magnitudes increase rapidly with depolarization from rest. Noise is larger for smaller patch areas but is smaller for increased model temperatures. We contrast the results from Monte Carlo simulations of the stochastic nonlinear kinetic schemes with analytical, closed-form expressions derived using passive and quasi-active linear approximations to the kinetic schemes. For all subthreshold voltage ranges, the quasi-active linearized approximation is accurate within 8% and may thus be used in large-scale simulations of realistic neuronal geometries.

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PubMedDate not supplied

A brief history of time (constants).

That the cerebral cortex processes information at prodigious speeds cannot be doubted. Yet the passive time constant, tau(m), of neurons, often thought of as a measure of the neuron's "response time' to synaptic input, is relatively long. In the 1950s, tau(m) was estimated to be only a few milliseconds for mammalian central neurons; with improvement in recording techniques, its estimated value grew over the years and it now stands near 20-100 msec. However, as we will argue here, the functional meaning of tau(m) is ambiguous. On the basis of a newly introduced definition of local delay, we show that the time window for synaptic integration in passive dendritic trees can be much smaller than the time constant. We argue that the voltage response to very brief synaptic inputs is essentially independent of tau(m). We discuss how tau(m) can change dynamically with the global activity of the network, as well as the difficulties of defining a time constant in structures with voltage-dependent elements. We conclude that the classically defined tau(m) only provides a very rough estimate, typically an overestimate, of the response time of neurons and that alternative measures are required to capture the dependency of the time course of the membrane potential on ligand-gated and/or voltage-dependent membrane conductances.

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PubMedDate not supplied

Factors that control the efficacy of group Ia synapses in alpha-motoneurons.

1.) This review considers factors that produce systematic variations in the amplitude of monosynaptic group Ia EPSPs in triceps surae alpha-motoneurons belonging to different motor unit types. 2.) Anatomical studies using horseradish peroxidase to label functionally-identified group Ia afferents and motoneurons postsynaptic to them, and combined anatomicalelectro-physiological studies of type-identified alpha-motoneurons, have constrained some of the factors that produce variations in peak Ia EPSP amplitude in different cells. 3.) Computer modeling studies based on these experimental data, together with other evidence in the literature, suggest that the major factor that produces systematic variation in Ia EPSP amplitudes in type FF, FR, and S motoneurons is a corresponding variation in the density of active group Ia synapses. 4.) Although EPSP amplitudes are also affected by the relative conductance of the somatic membrane, as reflected in the dendritic-to-somatic conductance ratio, it is possible that at least some of this influence is an artifact produced by microelectrode penetration.

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