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A F Kohn

Publications and source records attributed to A F Kohn.

13 recordsLinked to original sources
PubMed2006

Compartmental models of mammalian motoneurons of types S, FR and FF and their computer simulation.

Mathematical models of motoneurons (MNs) of types S, FR and FF were developed based on cat MN data. Each of the three models has an initial segment, a soma and a dendritic tree. The initial segment and the soma include models of several types of ionic currents, including a calcium-dependent slow potassium current. The dendritic tree is modeled as a series association of several electrically passive cylinders. Afterhyperpolarization parameters, current to frequency relation and the responses to input current steps, ramps and sinusoids were used for model validation. The effects of sinusoidally varying synaptic inputs at different levels of the dendritic tree were studied by computer simulation. The corresponding frequency response functions resulted of lowpass type with cutoff frequencies from 10 to 40 Hz, for synapses occurring more distally or more proximally, respectively. The nonlinear effects caused by two sinusoidally varying synaptic conductances (at 7 and 11 Hz), acting at different dendritic segments, were quantified by spectral analysis of the current reaching the soma. The simulations pointed to two main nonlinear effects: (i) harmonics of the two input frequencies (e.g., 14 Hz) and (ii) intermodulation terms (e.g., 4 Hz). When the two synaptic inputs occurred on more distal dendritic compartments the nonlinear effects were more pronounced.

Animals↗
PubMed2005

Spinal source for the synchronous fluctuations of bilateral monosynaptic reflexes in cats.

Successive stimuli of constant intensity applied to Ia afferents produce spinal monosynaptic reflexes (MSRs) of variable amplitude. We recorded simultaneous MSRs in the left and right L7 (or L6) ventral roots of anesthetized cats. We analyzed the cross-covariance (CCV) between the amplitudes of bilateral MSRs. Long-time series (5 to 8 h) of these bilateral MSRs exhibited transitory changes in their covariations (as measured by the zero-lag peak of their CCV), thus suggesting the existence of certain neural sources contributing to produce these changes. The aim of the present study was to show that spinal centers producing negative spontaneous cord dorsum potentials (nSCDPs) contribute to maintain correlations in the amplitude of bilateral MSRs. After spinal cord transection at the L1 segment, no significant changes were observed in the correlation between the amplitude of bilateral nSCDPs versus the amplitude of bilateral MSRs. However, this correlation, as well as the peak at zero lag in the CCV between bilateral MSRs and the CCV between bilateral nSCDPs, respectively, were abolished after a subsequent longitudinal bisection at the L1-S2 spinal segments. These results suggest that lumbar spinal neurons (bilaterally interconnected) contribute to maintain the synchronous fluctuations of bilateral MSRs.

Animals↗
PubMed1997

Presynaptic inhibition compared with homosynaptic depression as an explanation for soleus H-reflex depression in humans.

The H-reflex is depressed for seconds if elicited following a single H-reflex or train of H-reflexes. Presynaptic inhibition from flexor afferents (tibialis anterior) onto soleus Ia afferents elicited by either single or trains of stimuli had no effect on the soleus H-reflex on a time scale of seconds. Postsynaptic inhibition was also excluded by magnetic stimulation tests that showed that the excitability of the motoneuron pool was not changed at latencies within a range of seconds. Homosynaptic depression localized at the presynaptic terminal seems to be the mechanism behind the H-reflex depression in humans.

Adult↗
PubMed1997

Computer simulation of noise resulting from random synaptic activities.

Neuronal behavior is dependent on random inputs from a multitude of synaptic contacts on the soma and dendritic tree. Therefore, simulations of different types of noise are often required in the experimental and theoretical investigation of the properties of neurons and neuronal assemblies. The direct simulation of these noise sources by simple difference equations may therefore be quite useful and a general approach is presented in this paper. Initially, a first order model and its time-discretization are analyzed in detail, followed by a generalization to more complex models. The firing patterns of neurons are dependent on the random behaviors of their membrane potentials at the trigger zone. These depend on the propagation of the randomly occurring postsynaptic potentials from specific places on the dendritic tree or soma to the trigger zone. Different models may represent a variety of circumstances in which random membrane potentials arise at the trigger zone. Simulations of different types of noise are often required in the experimental and theoretical investigation of the properties of neurons and neuronal assemblies. The direct simulation of these noise sources by simple difference equations may therefore be quite useful and a general approach is presented in this paper. This paper presents a detailed analysis of the very useful first order model and its time discretization. The criterion used is that the autocovariance sequence of the discrete time model be a sample of the original autocovariance function. Several cases are presented which are of practical interest, including the case of constant output variance independent of the model's time constant. General models are time-discretized by the impulse response invariance method. Two applications are presented, one is related to the modeling of the synaptic currents by the alpha function instead of the delta function and the second deals with analog synaptic noise generation by D/A conversion of computer generated noise sequences.

Action Potentials↗
PubMed1997

H-reflexes of different sizes exhibit differential sensitivity to low frequency depression.

The amplitude of the H-reflex declines when activated repetitively. The magnitude of decline is greater when the amplitude of the H-reflex is small. To explore whether pre- or postsynaptic factors contribute to the differences observed in H-reflexes of different sizes, changes in the amplitude of H-reflexes of different sizes were measured during a train of stimulation in 10 normal subjects. Amplitudes of different sizes were obtained using differing stimulus intensities or during superimposed contraction, two manipulations which differently affect the number of active afferents and the excitation of the motoneuron pool. Small amplitude H-reflexes depressed to a lower plateau than larger H-reflexes and superimposed contraction did not alleviate the depression during each train. Nearly all the decline in larger amplitude H-reflexes occurred in a component that was in common with smaller amplitude H-reflexes. This suggests that the depressibility of the earliest activated units is greater than later activated units in H-reflexes and that the magnitude of decline is affected by prior activity as well as size.

Adult↗
PubMed1989

A versatile waveform generator for testing neuroelectric signal processors.

A multi-channel waveform generator was designed for testing neuroelectric signal processors. Smooth transient signals that resemble action potentials or evoked potentials are generated by a second order switched capacitor filter excited by brief rectangular pulses. The choice of an integrated circuit switched capacitor filter simplified the design by circumventing some of the disadvantages of conventional active filters. The waveform generator is versatile, with several signal parameters being independently adjustable from front panel controls: duration, waveshape, latency, amplitude and signal-to-noise ratio. The generator has been used for testing evoked potential acquisition and processing systems, for evaluating the effects of analog filters on evoked potentials and for testing systems designed to detect and classify trains of multi-unit action potentials.

Action Potentials↗
PubMed1989

Dendritic transformations on random synaptic inputs as measured from a neuron's spike train--modeling and simulation.

Extracellular spike trains recorded from central nervous system neurons reflect the random activations from a multitude of presynaptic cells making contacts mainly on the extensive dendritic trees. The dendritic potential variations are propagated towards the trigger zone where action potentials are generated. In this paper, two dendritic propagation modes are modeled: passive and quasi-active. Synaptic bombardments are modeled as being applied apically, somatically, or distributed over the dendritic tree. The resulting simulated neuronal spike trains are analyzed by point process techniques. Dendritic inputs resulted in a tendency for random bursting, interspike interval histograms with a long tail and coefficients of variation larger than one. The autocorrelation histograms reflected dynamics of the dendritic tree and they were able to discriminate between a passive or a quasi-active propagation mode and between dendritic and somatic synaptic inputs.

Action Potentials↗
PubMed1983

Pervasive locking, saturation, asymmetric rate sensitivity and double-valuedness in crayfish stretch receptors.

The correspondence between afferent discharges and sinusoidal length modulations (0.2--10 cps, under 10% of the natural length variations) was studied in isolated fast-adapting stretch receptor organs (FAO) of crayfish, largely using average displays of rate vs. length (or derivatives) along the cycle. Rate modulations were greatest during early cycles and then stabilized, an initial adjustment remindful of mechanical preconditioning. Responses to stimulation in the FAO, as in the slowly-adapting organs (SAO) and possibly other receptors, exhibit the following features, all striking because of their magnitude and ubiquity. i) A zig-zag overall afferent rate vs. stimulus frequency graph with positively and negatively sloped segments. This precludes the straightforward use of Bode plots. ii) Marked non-linearities as an obvious stimulus-response locking in the positively sloped segments, a double-valuedness with one rate while stretching and another while shortening, a lower-limit saturation with the receptor silent for more than half a cycle, and an asymmetric rate sensitivity. iii) Clear-cut discharge leads relative to the stimulus at low frequencies and lags at high ones. The FAO responds worse than the SAO to low frequencies, and better to high ones; it is locked 1-to-1 in a much broader range (e.g., 3--100 vs. 1--3 cps). All features were strongly frequency-dependent. With higher frequencies: i) the number of impulses per cycle fell from several to just one and finally to one every several cycles at higher values; ii) the two values of each length approached one another usually but not always; iii) the silent proportion of the cycle increased; and iv) the rate sensitivity changed. Each feature can arise in principle at any of the transduction stages from length to discharge: the mechanical transduction from length to dendritic deformation, an the encoder one from generator potentials to discharges are particularly likely candidates.

Afferent Pathways↗
PubMedDate not supplied

The amplitude and phase responses of the firing rates of some motoneuron models.

A vertebrate motoneuron receives an enormous amount of synaptic activity from descending pathways, from spinal cord interneurons and directly from mechanoreceptor afferents. The intrinsic characteristics of the motoneuron will determine how its output spike train will encode the activities of all its inputs. Therefore, the essence of the intrinsic motoneuron characteristics should be well studied and modelled if the roles of the motoneuron as a processing or encoding element are to be well understood. Mathematical models of motoneurons have been described in the literature and tested mostly under static conditions. To increase the reality of the validation of such models, the objective of the present work is to test a few selected models described in the literature using sinusoidal injected current of different frequencies. The resulting frequency responses are compared with data available in the literature from cat type F motoneurons. Discrepancies between some of the models' responses and real motoneuron data suggest that improvements are needed in the modelling of the afterhyperpolarization mechanism.

Models, Neurological↗
PubMedDate not supplied

Effects of synaptic noise on a neuronal pool model with strong excitatory drive and recurrent inhibition.

A model originally proposed by Akazawa and Kato (1990) for the spinal cord was adopted as prototypical of a neuronal pool with strong excitatory drive and strong recurrent inhibition. Our simulations of the model have shown that a strong synchronization occurs between the spike trains in the neuronal pool. This happens because the proposed model has a single and strong excitatory drive on the neuronal pool. However, usually a multitude of other randomly occurring synaptic inputs impinge on the neuronal pool and therefore a new investigation was carried out to study the effects of synaptic noise on the network behavior. The synaptic noise decreased the degree of synchronization of the neuronal spike trains but on the other hand caused an unexpected decrease in the mean firing rate of the neuronal pool. A detailed analysis indicated that this phenomenon is due to a combination of two mechanisms: a saturation of the feedback inhibition and a decrease of the synchronization in the neuronal pool with synaptic noise. The synaptic noise caused a more frequent activation of the saturated recurrent inhibitory feedback loop along time, thereby increasing the inhibitory effect on the neuronal pool.

Action Potentials↗
PubMedDate not supplied

Proximal receptors and the mechanical stimulation of the fingers: a somatosensory evoked potential study.

OBJECTIVE: To characterize scalp responses to mechanical stimulation of the fingers and evaluate the contribution of different receptors. DESIGN AND METHODS: Somatosensory evoked responses to mechanical stimulation of the right third finger were recorded from a P3-P4 montage (n = 15) and from a F3-P4 montage (n = 9) as well as after electrical stimulation (n = 9). Responses after mechanical stimulation of the distal region of the finger, with the hand at different positions, were also recorded (n = 8). Complementary experiments in a small number of individuals included EMG and accelerometer recordings as well as anaesthesia of the finger. RESULTS: Scalp responses characterized by an initial sequence of waves, here called NI-PI-NII, were recorded from the P3-P4 montage. Mean peak latencies were 20, 23 and 26 ms, respectively; electrical stimulation of the same region evoked an initial negativity (mean peak latency 23 ms). EMG recordings suggested the involvement of different receptors in response to electrical and mechanical stimulation. Accelerometer recordings showed the spread of a sizable mechanical wave at the forearm. Anaesthesia did not change the responses to mechanical stimulation. CONCLUSIONS: Relatively small mechanical stimuli applied at distal phalanxes may activate proximal receptors which generate scalp recorded responses that may completely occlude the contribution of the distal receptors.

Acceleration↗