Biomedical subjects

R R Poznanski

Publications and source records attributed to R R Poznanski.

9 recordsLinked to original sources

PubMed2000

A dendritic cable model for the amplification of synaptic potentials by an ensemble average of persistent sodium channels.

The persistent sodium current density (I(NaP)) at the soma measured with the 'whole-cell' patch-clamp recording method is linearized about the resting state and used as a current source along the dendritic cable (depicting the spatial distribution of voltage-dependent persistent sodium ionic channels). This procedure allows time-dependent analytical solutions to be obtained for the membrane depolarization. Computer simulated response to a dendritic current injection in the form of synaptically-induced voltage change located at a distance from the recording site in a cable with unequally distributed persistent sodium ion channel densities per unit length of cable (the so-called 'hot-spots') is used to obtain conclusions on the density and distribution of persistent sodium ion channels. It is shown that the excitatory postsynaptic potentials (EPSPs) are amplified if hot-spots of persistent sodium ion channels are spatially distributed along the dendritic cable, with the local density of I(NaP) with respect to the recording site shown to specifically increase the peak amplitude of the EPSP for a proximally placed synaptic input, while the spatial distribution of I(NaP) serves to broaden the time course of the amplified EPSP. However, in the case of a distally positioned synaptic input, both local and nonlocal densities yield an approximately identical enhancement of EPSPs in contradiction to the computer simulations performed by Lipowsky et al. [J. Neurophysiol. 76 (1996) 2181]. The results indicate that persistent sodium channels produce EPSP amplification even when their distribution is relatively sparse (i.e. , approximately 1-2% of the transient sodium channels are found in dendrites of CA1 hippocampal pyramidal neurons). This gives a strong impetus for the use of the theory as a novel approach in the investigation of synaptic integration of signals in active dendrites represented as ionic cables.

Animals↗

PubMed2000

Theoretical analysis of the amplification of synaptic potentials by small clusters of persistent sodium channels in dendrites.

We extend on the work developed by R.R. Poznanski and J. Bell from a linearized somatic persistent sodium current source to a non-linear representation of the dendritic Na(+)P current source associated with a small number of persistent sodium channels. The main objective is to investigate the modulation in the amplification of excitatory postsynaptic potentials (EPSPs) in dendrites studded with persistent sodium channels. The relation between membrane potential (V) and persistent sodium current density (I(NaP)) is approximated heuristically with a sigmoidal function and the resultant cable equation is solved analytically using a regular perturbation expansion and Green's function techniques. The transient simulated (non-evoked) response is found as a result of current injection in the form of synaptically induced voltage change located at a distance from the recording site in a cable with a uniform distribution of ion channel densities per unit length of cable (the so-called 'hot-spots') and with the conductance of each hot-spot (i.e., number of channels per hot-spot) assumed to be a constant. The results show an amplification in the observed EPSPs to be compatible with the experimentally derived estimates, and in addition a saturation in the amplification is observed indicating an optimum number of ionic channels.

Animals↗

PubMed1997

Syncytial integration by a network of coupled bipolar cells in the retina.

A model system for syncytial integration is the outer vertebrate retina, where graded signals or electrotonic potentials interact laterally via gap junctions to form an integrated response that is relayed by chemical synapses to the next layer of interconnected cells. Morphological and physiological experiments confirm that bipolar cells form quasisyncytial lattices, and so this review will aim to address two important issues: the function of coupling in visual information processing and the construction of a robust mathematical model that can adequately simulate signal spread in the bipolar cell syncytium. It is shown that the role of coupling in bipolar cells differs from that associated in the presynaptic networks, namely, loss in spatial resolution in order to increase the signal-to-noise ratio. The intrinsic membrane properties of bipolar cells which give rise to voltage-dependent currents are inactive over the normal in vivo operating range of membrane potential and may be shunted as a direct result of electrotonic coupling, suppressing any possibility of action potential propagation in the bipolar cell syncytium. It is therefore speculated that the mechanisms underlying processing of information in bipolar networks are dependent on the structure of bipolar cells and in particular, on the presence of gap junctions. It is proposed that a three-dimensional model which incorporates the spatial properties of each bipolar cell in the network in the form of a leaky cable is the most likely model to simulate signal spread in the bipolar cell syncytium in vivo. This is because discrete network models represent each bipolar cell in the syncytium as isopotential units without any spatial structure, and thus are unable to reproduce the temporal characteristics of electrotonic potential spread within the central receptive field of bipolar cells.

Animals↗

PubMed1996

Transient response in a tapering cable model with somatic shunt.

Transient voltage responses (charging transients) to current stimulation are presented for a non-linearly tapering cable model of a neurone with a shunt at the soma. Exact expressions are given for the time constants of exponential decay expressed in terms of the passive membrane time constant and the taper rate constant. The effect of a somatic shunt on the membrane potential transients at the soma is shown to result in a more rapid final decay of the membrane potential than simple passive decay with membrane time constant. The theoretical analysis should be of use to electrophysiologists wishing to interpret their experimentally observed charging transients recorded at the some in terms of cable properties of neurones in vivo.

Electrophysiology↗

PubMed1995

Electrotonic coupling between two CA3 hippocampal pyramidal neurons: a distributed cable model with somatic gap-junction.

A model of a pair of electrotonically coupled CA3 hippocampal pyramidal neurons is presented. Each neuron is represented by a tapered equivalent cable attached to an isopotential soma. The synaptic potential in a neuron soma is determined as a consequence of electrical coupling to another soma that receives a synaptic input on its dendritic tree. Estimates of the coupling resistances, soma input resistances and soma-to-dendritic tree conductance ratio show that a substantial current may arise in a neuron as a consequence of synaptic activity in a neuron coupled to it. The small increase in decay time due to coupling in the model indicates that actual coupling is between more than just pairs of neurons.

Animals↗

PubMed1993

Extracellular current flow and potential during quantal transmission from varicosities in a smooth muscle syncytium.

A discrete model has been developed that describes the extracellular current that flows in a smooth muscle syncytium upon the secretion of a quantum of transmitter onto a smooth muscle cell in the syncytium. This allows a description to be given of the current (called the excitatory junctional current (EJC)) recorded by an electrode of given diameter placed on the surface of the muscle, during synaptic transmission from a varicosity situated anywhere in the muscle. The EJC is of maximum negative amplitude when the varicosity is at the surface of the muscle near the inside rim of the electrode and decreases as the varicosity moves to the centre of the electrode. It is of maximum positive amplitude when the varicosity is at the surface near the outside rim of the electrode and declines rapidly in amplitude as the varicosity is removed further from the outside rim. Smaller diameter electrodes give larger EJCs than larger diameter electrodes for most positions of the varicosity on the surface of the muscle. The EJC amplitude declines for varicosities beneath the electrode that are not on the surface of the muscle, but deep in the tissue. The rate of this decline is greater the smaller the diameter of the electrode. The time-course of the EJC is largely invariant under changes in the position of the varicosity with respect to the recording electrode. Changes in the polarity of the current flow during a single EJC can occur, however, if two varicosities secrete transmitter simultaneously, one inside the electrode and one outside, and the time-course of the currents due to the individual varicosities is either the same or slightly different. This theoretical work has been used to interpret a number of recent experimental studies of extracellular current flow during autonomic neuromuscular transmission.

Animals↗

PubMed1992

Modelling the electrotonic structure of starburst amacrine cells in the rabbit retina: a functional interpretation of dendritic morphology.

A detailed morphometric analysis of a Lucifer yellow-filled Cb amacrine cell was undertaken to provide raw data for the construction of a neuronal cable model. The cable model was employed to determine whether distal input-output regions of dendrites were electrically isolated from the soma and each other. Calculations of steady state electrotonic current spread suggested reasonable electrical communication between cell body and dendrites. In particular, the centripetal voltage attenuation revealed that a synaptic signal introduced at the distal end of the equivalent dendrite could spread passively along the dendrite and reach the soma with little loss in amplitude. A functional interpretation of this result could favour a postsynaptic rather than a presynaptic scheme for the operation of directional selectivity in the rabbit retina. On the other hand, dendrites of starburst amacrine cells process information electrotonically with a bias towards the centrifugal direction and for a restricted range of membrane resistance values the voltage attenuation in the centripetal direction suggests that the action of these dendrites can be confined locally. A functional interpretation of this result favours a presynaptic version of Vaney's cotransmission model which attempts to explain how the neural network of starburst amacrine cells might account for directionally selective responses observed in the rabbit retina.

Animals↗

PubMed1991

A generalized tapering equivalent cable model for dendritic neurons.

A mathematical model has been developed which collapses a dendritic neuron of complex geometry into a single electrotonically tapering equivalent cable. The modified cable equation governing the transient distribution of subthreshold membrane potential in a branching tree is transformed, becoming amenable to analytic solution. This transformation results in a Riccati differential equation whose six solutions (expressed in terms of elementary functions) control the amount and degree of taper found in the equivalent cable model. To illustrate the theory, an analytic solution (in series form) of the modified cable equation is obtained for a voltage-clamp present at the soma of a quadratically tapering equivalent cable whose distal end is sealed.

Dendrites↗

PubMed1987

Transient response in a somatic shunt cable model for synaptic input activated at the terminal.

Rall's neuron model is extended by including a non-uniform time constant together with synaptic input modeled as a square step of conductance. An analytic solution (in series form) for the electrotonic potential is obtained. The major conclusion reached is that a lower somatic time constant attenuates the amplitude of the potential at the soma, brought about by the activation of a synapse located at the distal end of the dendritic cable in an initially polarized neuron.

Dendrites↗