Biomedical subjects

O Bernander

Publications and source records attributed to O Bernander.

3 recordsLinked to original sources

PubMed1995

Do neurons have a voltage or a current threshold for action potential initiation?

The majority of neural network models consider the output of single neurons to be a continuous, positive, and saturating firing rate f (t), while a minority treat neuronal output as a series of delta pulses sigma delta (t-ti). We here argue that the issue of the proper output representation relates to the biophysics of the cells in question and, in particular, to whether initiation of somatic action potentials occurs when a certain threshold voltage or a threshold current is exceeded. We approach this issue using numerical simulations of the electrical behavior of a layer 5 pyramidal cell from cat visual cortex. The dendritic tree is passive while the cell body includes eight voltage- and calcium-dependent membrane conductances. We compute both the steady-state (Istatic(infinity)(Vm)) and the instantaneous (I0(Vm)) I-V relationships and argue that the amplitude of the local maximum in Istatic(infinity)(Vm) corresponds to the current threshold Ith for sustained inputs, while the location of the middle zero-crossing of I0 corresponds to a fixed voltage threshold Vth for rapid inputs. We confirm this using numerical simulations: for "rapid" synaptic inputs, spikes are initiated if the somatic potential exceeds Vth, while for slowly varying input Ith must be exceeded. Due to the presence of the large dendritic tree, no charge threshold Qth exists for physiological input. Introducing the temporal average of the somatic membrane potential while the cell is spiking repetitively, allows us to define a dynamic I-V relationship Idynamic(infinity)( ). We find an exponential relationship between and the net current sunk by the somatic membrane during spiking (diode-like behavior). The slope of Idynamic(infinity)( ) allows us to define a dynamic input conductance and a time constant that characterizes how rapidly the cell changes its output firing frequency in response to a change in its input.

Action Potentials↗

PubMed1994

Amplification and linearization of distal synaptic input to cortical pyramidal cells.

1. Computer simulations were used to study the effect of voltage-dependent calcium and potassium conductances in the apical dendritic tree of a pyramidal cell on the synaptic efficacy of apical synaptic input. The apical tuft in layers 1 and 2 is the target of feedback projections from other cortical areas. 2. The current, Isoma, flowing into the soma in response to synaptic input was used to assess synaptic efficacy. This measure takes full account of all the relevant nonlinearities in the dendrities and can be used during spiking activity. Isoma emphasizes current flowing in response to synaptic input rather than synaptically induced voltage change. This measure also permits explicit characterization of the input-output relationship of the entire neuron by computing the relationship between presynaptic input and postsynaptic output frequency. 3. Simulations were based on two models. The first was a biophysically detailed 400-compartment model of a morphologically characterized layer 5 pyramidal cell from striate cortex of an adult cat. In this model eight voltage-dependent conductances were incorporated into the somatic membrane to provide the observed firing behavior of a regular spiking cell. The second model was a highly simplified three-compartment equivalent electrical circuit. 4. If the dendritic tree is entirely passive, excitatory synaptic input of the non-N-methyl-D-aspartate (non-NMDA) type to layers 1, 2, and 3 saturate at very moderate input rates, because of the high input impedance of the apical tuft. Layers 1 and 2 together can deliver only 0.25 nA current to the soma. This modest effect is surprising in view of the important afferents that synapse on the apical tuft and is inconsistent with experimental data indicating a more powerful effect. 5. We introduced in a controlled manner a voltage-dependent potassium conductance in the apical tuft, gK, to prevent saturation of the synaptic response. This conductance was designed to linearize the relationship between presynaptic input frequency and the somatic current. We also introduced a voltage-dependent calcium conductance along the apical trunk, gCa, to amplify the apical signal, i.e., the synaptic current reaching the soma. 6. To arrive at a specific relationship between the presynaptic input rate and the somatic current delivered by the synaptic input, we derived the activation curves of gK and gCa either analytically or numerically. The resultant voltage-dependent behavior of both conductances was similar to experimentally measured activation curves.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Synaptic background activity influences spatiotemporal integration in single pyramidal cells.

The standard one-dimensional Rall cable model assumes that the electrotonic structure of neurons does not change in response to synaptic input. This model is used in a great number of both theoretical and anatomical-physiological structure-function studies. In particular, the membrane time constant, tau m, the somatic input resistance, Rin, and the electrotonic length are used to characterize single cells. However, these studies do not take into account that neurons are embedded in a network of spontaneously active cells. Synapses from these cells will contribute significantly to the membrane conductance, especially if recent evidence of very high specific membrane resistance, Rm = 100 k omega.cm2, is taken into account. We numerically simulated the electrical behavior of an anatomically reconstructed layer V cortical pyramidal cell receiving input from 4000 excitatory and 1000 inhibitory cells firing spontaneously at 0-7 Hz. We found that, over this range of synaptic background activity, tau m and Rin change by a factor of 10 (80-7 msec, 110-14 M omega) and the electrotonic length of the cell changes by a factor of 3. We show that this significantly changes the response of the cell to temporal desynchronized versus temporal synchronized synaptic input distributed throughout the neuron. Thus, the global activity of the network can control how individual cells perform spatial and temporal integration.

Action Potentials↗