Biomedical subjects

F Awiszus

Publications and source records attributed to F Awiszus.

At least 37 records · Page 2Linked to original sources

PubMed1993

Subdivision of primary afferents from passive cat muscle spindles based on a single slow-adaptation parameter.

38 primary afferents originating from de-efferented cat tibialis anterior muscle spindles were investigated. Ramp-and-hold stretches of the host muscle were performed with a varying amount of muscle pre-stretch while recording the primary afferent discharges. From the discharge responses an interspike interval function was estimated. This revealed a slow adaptation during the hold phase of stretch which could be approximated quite well by a power function. The slow-adaptation power function exponent (SAE) was found to be rather independent of the amount of pre-stretch applied to the host muscle and grouped around a value characteristic for each afferent. These 'characteristic SAEs' showed a clearly bimodal distribution within the population of primaries studied. Moreover, the distribution around both modes was narrow enough to justify the subdivision of the primary afferents according to their characteristic SAE as either high-SAE (10 afferents; 26%) or low-SAE (28 afferents; 74%) afferents. The most likely explanation for this bimodality of slow-adaptation behavior in primary afferents is given by the assumption that the afferent discharge of the passive spindle is mainly provided from a branch innervating either the bag1 (for high-SAE units) or the bag2 and chain (for low-SAE units) intrafusal fibers.

Action Potentials↗

PubMed1993

Sensitivity of different stimulus-timing strategies for the detection of small excitations in noisy spike train data.

There are several different strategies to control the timing of a stimulus with respect to the ongoing discharge during the recording of neuronal stimulus-response characteristics. One possible strategy consists of delivering stimuli in such a way that a constant pre-stimulus spike density is reached. Another strategy enforces spike application with a constant stimulus latency after a spontaneous discharge. In this paper the sensitivity of these different strategies for statistical verification of small excitatory response components was investigated. It was found that the difference between observed post-stimulus spike distribution and expected spike distribution under the null hypothesis of no stimulus effect was larger using a constant-stimulus-latency (CSL) strategy with an appropriate value for the stimulus latency. Thus, the statistical verification of neuronal response components is clearly facilitated if a CSL strategy is used. This superiority of the CSL strategy is marked, especially for small excitations at neurons discharging slowly with low discharge variability.

Action Potentials↗

PubMed1993

Quantification and statistical verification of neuronal stimulus responses from noisy spike train data.

Usually neuronal responses to short-lasting stimuli are displayed as peri-stimulus time histogram. The function estimated by such a histogram allows to obtain informations about stimulus-induced postsynaptic events as long as the interpretation is restricted to the first response component after the stimulus. The interpretation of secondary response components is much more difficult, as they may be either due to stimulus effects or represent an "echo" of the primary response. In the present paper two output functions are developed that do not show such an echoing of responses. The first one, the interspike interval change function, represents an ideal way to quantify a neuronal stimulus response as its amplitude was found to be almost independent of the stimulation strategy used during acquisition of the spike train data. The other function, the displaced impulses function, allows to verify the statistical significance of an observed response component. Both functions may be estimated from stimulus-correlated spike train data, even if the neuron under investigation shows considerable interspike-interval variability in the absence of stimulation. The concepts underlying these neuronal output functions are developed on simulated responses of a Hodgkin-Huxley-type model for a mammalian neuron at body temperature that is exposed to a transient excitatory conductance increase. Additionally, estimation of these output functions is also demonstrated on responses of human soleus motoneurons that were exposed to electrical stimuli of the tibial nerve in the popliteal fossa.

Action Potentials↗

PubMed1993

The relationship between estimates of Ia-EPSP amplitude and conduction velocity in human soleus motoneurons.

There are several parameters associated with motoneuron size, among which are the conduction velocity of the axon as well as the size of the excitatory postsynaptic potential (EPSP) induced by stimulation of Ia afferents in the corresponding muscle nerve. In particular, it has been established in animal experiments that small motoneurons with a low conduction velocity exhibit large Ia EPSPs, whereas large motoneurons with a high conduction velocity show small Ia EPSPs. Thus small motoneurons are recruited earlier than large ones. In this study, we investigated whether such a relationship between motoaxon conduction velocity and size of the Ia EPSPs could also be found for human soleus motoneurons. In total, 36 motor units from six healthy volunteers were activated by a slight voluntary contraction and exposed to 200 stimuli of the tibial nerve in the popliteal fossa. Stimuli were delivered using a special stimulus protocol ensuring a constant pre-stimulus spike density along with a constant rate of discharge of the investigated unit. From the stimulus-correlated spike train data a measure of Ia-EPSP amplitude was obtained, along with the single-unit H-reflex latency. Additionally, for each unit, the so-called surface macro EMG was recorded, which measures the complete electrical activity attributable to the unit investigated. From the macro EMB, the intramuscular delay from arrival of each action potential at the soleus muscle and the detection of the muscle-fiber action potential picked up by the recording needle electrode were measured. All single-unit H-reflex latencies were corrected for the corresponding intramuscular delays. From the corrected latencies, single-unit conduction velocities were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Abnormal EPSPs evoked by magnetic brain stimulation in hand muscle motoneurons of patients with amyotrophic lateral sclerosis.

Using cross-correlation between magnetic brain stimulation and discharges of a motoneuron made active by a slight voluntary contraction, an indirect estimate of the EPSP evoked by magnetic brain stimulation in single hand muscle motoneurons was obtained in patients with amyotrophic lateral sclerosis (ALS) and normal controls. In total, 60 motoneurons of 3 normal subjects and 70 motoneurons of 7 patients with ALS were investigated. All motoneurons of normal subjects responded to the magnetic brain stimulus with a short-latency EPSP with a rise time between 1 and 5 msec. In contrast, only 67% of the motoneurons from ALS patients responded with an EPSP while the remaining 33% exhibited a clear short-latency inhibition in response to the brain stimulus. For those units of ALS patients showing an EPSP, both latency and EPSP amplitude were indistinguishable from those of normal subjects. The EPSP rise time, however, was massively prolonged in some units (up to 18 msec). These results provide a physiological basis for the interpretation of surface EMG responses in patients with ALS.

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PubMed1992

Reduction of a Hodgkin-Huxley-type model for a mammalian neuron at body temperature.

Hodgkin-Huxley-type models mimic the electrical behavior of excitable membranes quite realistically. However, inclusion of many different ionic channels into such a model yields a highly complex set of differential equations. In this paper a reduction of a "full" Hodgkin-Huxley-type model based on voltage-clamp data from small rat neurons in the supraoptic nucleus area is introduced. It was found that two of the ionic channel gating variables of the full model preserved a rather close relationship during simulations. This allowed to express one of these gating variables in terms of the other one thus reducing the number of differential equations the model is based on. The behavior of the reduced model was very similar to that of the full model. In particular, important physiological features as spike shape and constant-input-to-interspike-interval relationship were (almost) identical in the full and the reduced model.

Animals↗

PubMed1992

Analytical reconstruction of the neuronal input current from spike train data.

The time course of the current driving action potential generation at a neuron investigated experimentally is in general not measurable directly. In this paper an indirect method is introduced that allows estimation of this unknown current time course using only spike train data. Assuming the leaky integrator model as valid for the action potential encoding site of the investigated neuron, the unknown input current is obtained by determining (analytically) a current time course that upon injection into the leaky integrator model evokes action potential sequences identical to those observed experimentally. Applications of this current-reconstruction procedure to neuronal output data obtained from a leaky integrator model showed that the procedure allows a good estimation of the underlying input current even if the membrane time constant of the investigated neuron is not known exactly. Additionally, an application of current reconstruction to experimental data obtained from a cat muscle spindle primary afferent subject to repeated gamma-stimuli is demonstrated.

Action Potentials↗

PubMed1992

The relationship between a neuronal cross-correlogram and the underlying postsynaptic current.

Cross-correlations between stimuli and neuronal discharges yield information about synaptic events at the investigated neuron. In this paper it is shown that the time course estimated by a cross-correlogram, the cross-correlation function (ccf), represents the input current that upon injection into the perfect integrator model evokes spike sequences that are (almost) identical to those used for estimation of the ccf. Thus, the shape of a ccf may be regarded as an estimate of the underlying postsynaptic current, if the neuron investigated behaves, at least to a first approximation, like a perfect integrator model.

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PubMed1991

The influence of an unmyelinated terminal on repetitive firing of a mammalian receptor afferent fiber.

The distal end of a myelinated receptor afferent fiber consists of an unmyelinated terminal membrane which is assumed to be the site of sensory transduction, whereas the action potential encoding appears at a distal node of Ranvier. In the present paper a model of a mammalian myelinated nerve fiber was augmented by an unmyelinated terminal segment into which stimulating current was injected thus modelling the situation at a myelinated receptor afferent fiber. It was found that the introduction of the unmyelinated terminal reduces the repetitive firing rate shown by the model. However, also the amplitude of the spikes at the site of action potential generation diminishes through the large electrical load which the unmyelinated terminal imposes onto the active parts of the nerve fiber model. This "loss" of spike amplitude can abolish the ability of the model to show repetitive activity, if the unmyelinated terminal increases in size. On the other hand, the incorporation of sodium channels into the terminal membrane compensates the spike amplitude reduction introduced by the electrical load of that membrane. This allows repetitive firing at a lower frequency than would be possible for a model with an equivalent sodium-channel-free terminal. The results show that the unmyelinated terminal present at the distal end of myelinated receptor afferent fibers has not only the ability to provide sensory transduction but evokes also a reduction in the discharge rate of the encoding membrane.

Action Potentials↗

PubMed1991

On a method to detect long-latency excitations and inhibitions of single hand muscle motoneurons in man.

The peri-stimulus-time histogram (PSTH) analysis of stimulus-related neuronal spike train data is usually regarded as a method to detect stimulus-induced excitations or inhibitions. However, for a fairly regularly discharging neuron such as the human alpha-motoneuron, long-latency modulations of a PSTH are difficult to interpret as PSTH modulations can also occur as a consequence of a modulated neuronal autocorrelation. The experiments reported here were made (i) to investigate the extent to which a PSTH of a human hand-muscle motoneuron may be contaminated by features of the autocorrelation and (ii) to develop methods that display the motoneuronal excitations and inhibitions without such contamination. Responses of 29 single motor units to electrical ulnar nerve stimulation below motor threshold were investigated in the first dorsal interosseous muscle of three healthy volunteers using an experimental protocol capable of demonstrating the presence of autocorrelative modulations in the neuronal response. It was found for all units that the PSTH as well as the cumulative sum (CUSUM) derived from these responses were severely affected by the presence of autocorrelative features. On the other hand, calculating the CUSUM in a slightly modified form yielded--for all units investigated--a neuronal output feature sensitive only to motoneuronal excitations and inhibitions induced by the afferent volley. The price that has to be paid to arrive at such a modified CUSUM (mCUSUM) was a high computational effort prohibiting the on-line availability of this output feature during the experiment. It was found, however, that an interspike interval superposition plot (IISP)--easily obtainable during the experiment--is also free of autocorrelative features.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1990

The singularly perturbed Hodgkin-Huxley equations as a tool for the analysis of repetitive nerve activity.

A qualitative analysis of the Hodgkin-Huxley model (Hodgkin and Huxley 1952), which closely mimics the ionic processes at a real nerve membrane, is performed by means of a singular perturbation theory. This was achieved by introducing a perturbation parameter that, if decreased, "speeds up" the fast variables of the Hodgkin-Huxley equations (membrane potential and sodium activation), whereas it does not affect the slow variables (sodium inactivation and potassium activation). In the most extreme case, if the perturbation parameter is set to zero, the original four-dimensional system "degenerates" to a system with only two differential equations. This degenerate system is easier to analyze and much more intuitive than the original Hodgkin-Huxley equations. It shows, like the original model, an infinite train of action potentials if stimulated by an input current in a suitable range. Additionally, explanations for the increased sensitivity to depolarizing current steps that precedes an action potential can be found by analysis of the degenerate system. Using the theory of Mishchenko and Rozov (1980) it is shown that the degenerate system does not only represent a simplification of the original Hodgkin-Huxley equations but also gives a valid approximation of the original model at least for stimulating currents that are constant within a suitable range.

Action Potentials↗

PubMed1990

Effects of a slow potassium permeability on repetitive activity of the frog node of Ranvier.

Adding a potassium permeability with slow kinetics to the Frankenhaeuser-Huxley equations describing action potential generation at a frog node of Ranvier has a twofold effect on the maintained repetitive firing the model can show. If the contribution of the slow to the total potassium permeability is increased, the maintained discharge frequency for a given stimulating current experiences a decrease. On the other hand, addition of the slow channel narrows the range of currents for which the model can generate repetitive activity. If as little as 6.2% of the total potassium permeability are provided by the slow channels, the Frankenhaeuser-Huxley equations completely lose the ability to show maintained firing. The introduction of the slow potassium current abolishes especially repetitive activity at low values of stimulating current. This effect is so marked that the minimal discharge frequency the model can maintain increases with increasing contribution of the slow channel. Therefore, an important purpose of the slow potassium channel present at the frog nodal membrane could consist of preventing the node of Ranvier from generating consistent firing on its own.

Animals↗

PubMed1990

Effects of paranodal potassium permeability on repetitive activity of mammalian myelinated nerve fiber models.

Almost all potassium channels within mammalian myelinated nerve fibers are covered by the myelin sheath and their majority is concentrated in a small paranodal region. In order to investigate effects of this paranodal potassium permeability on nerve fiber behavior via a simulation approach, a myelinated fiber model is required that treats myelin sheath and internodal axolemma as separate entities. Such a fiber description was developed by Blight (1985) and his model was used to investigate the effects paranodal potassium channels have on the ability of maintaining repetitive firing in response to a constant current injected into the fiber. It was found that increasing the potassium channel density at the paranode from low to moderate values widened the range of injected currents with a repetitive response. This promotion of repetitive activity by the introduction of additional potassium channels occurred up to an "optimal" value beyond which a further increase in paranodal potassium permeability narrowed the range of currents with a repetitive response. Finally, if a certain limit in paranodal potassium channel density was exceeded, repetitive activity was abolished completely. These results were obtained regardless of the assumptions about the electrical resistance of the myelin sheath. On the other hand, in the absence of potassium channels repetitive firing could be observed only when a high resistance myelin sheath was assumed, whereas a nerve fiber model with electrical properties inferred from intracellular recordings needed at least some potassium channels within the paranodal region for repetitive firing in response to an injected current.

Animals↗

PubMed1989

Re-afferent effects of individual static and dynamic gamma-stimuli during maintained fusimotor stimulation.

The ability of maintained dynamic and static fusimotor stimulation to modulate the primary afferent response of the muscle spindle in the rhythm of gamma-stimulation was investigated using a highly sensitive method for modulation detection. The effect of 41 gamma-fibers (13 dynamic; 28 static) on 38 primary afferents obtained from the tibialis anterior muscle of the cat was studied. It was found that maintained stimulation of 10 out of the 13 dynamic (77%) and of 25 out of the 28 static (89%) gamma-fibers could evoke significant modulations of the primary afferent response in the rhythm of fusimotor stimulation at a minimum of one stimulation rate. Moreover, both static and dynamic gamma-stimulations could evoke significant primary afferent modulations almost over the entire range of stimulation rates studied (30-300 stimuli per second). These results show that both gamma-systems can modulate the primary afferent response in the rhythm of fusimotor stimulation over a wide range of stimulation rates; thus the central nervous system may be provided with re-afferent information about the effect of each individual gamma-motoneuron discharge. Some hypotheses for the internal spindle mechanism responsible for the afferent modulations are discussed.

Action Potentials↗

PubMed1989

Continuous functions for the analysis of sensory transduction.

Sensory transduction at a primary receptor neuron yields a current that drives the generation of action potentials. Due to the inaccessibility of that current for direct measurements the analysis of sensory transduction requires the use of neuronal output functions that give an indirect measure for the "input" current, i.e. the current at the impulse initiating site. Three continuous neuronal output functions are investigated with respect to their ability to reconstruct the input current (i) the membrane potential recorded under sodium channel block referred to as "receptor potential", (ii) the interspike-interval function (Awiszus 1988a) and (iii) the phase lag function which is introduced in this paper. The behaviour of these three functions for constant and dynamically varying input is studied at the Hodgkin-Huxley model (Hodgkin and Huxley 1952) because for this model neuron it is possible to compare the input current estimates obtained from the output functions with the true input current. It was found that for constant and for sufficiently slow varying input all three functions allow a valid reconstruction of the input current time course. On the other hand, if the input current changes rapidly all three estimated input current time courses show considerable deviations from the true time course. The largest maximal deviation is shown by the current estimate obtained from the receptor potential whereas the phase lag function yields the smallest input current misjudgement. An experimental example to illustrate the procedure to obtain the phase lag function for a muscle spindle primary afferent is given.

Action Potentials↗

PubMed1989

On the description of neuronal output properties using spike train data.

Neuronal output properties for input stimuli that evoke a deterministic response can be efficiently described by the interspike-interval function (Awiszus 1988a). It is shown in this paper that there are stimuli for which both the Hodgkin-Huxley (HH-) model of an action potential encoding membrane (Hodgkin and Huxley 1952) and a muscle spindle primary afferent generate responses which violate the conditions for a deterministic one. Instead of being stochastic these responses follow systematic rules, namely those for a semi-deterministic response, a class of neuronal responses established in this paper that includes the deterministic one. Instead of being stochastic these output properties are best described by the interspike-interval curve. A phase plane analysis of the internal properties of the HH-model underlying such responses shows that it is reasonable to assume that responses of an HH-model and consequently, all neurons for which an HH-model is a valid description of the action potential encoding process, always fall into the class of semi-deterministic responses, regardless of the input current density time course as long as it is large enough to maintain spike activity. Consequences of this assumption for the analysis of neuronal output properties are discussed with respect to output measures and efficient input stimuli.

Action Potentials↗

PubMed1988

The adaptation ability of neuronal models subject to a current step stimulus.

Three neuronal models of the spike initiating process were investigated with respect to their ability to show adaptation to a current step: (i) the perfect integrator model (PIM), (ii) the leaky integrator model (LIM), and (iii) the Hodgkin-Huxley (HH)-model. It was found that although each neuronal model will generate different response spike trains to a given stimulus, all responses fulfilled the criteria of a deterministic neural response (Awiszus 1988). The results show that both PIM and LIM are unable to show adaptation regardless of the choice of model parameters whereas the HH-model shows a clear rate of discharge adaptation. The reason for this adaptation lies in the fact that there are conditions for the HH-model where a step stimulus is highly effective. These conditions are investigated by means of a phase plane analysis. Consequences of these results for the explanation of neuronal adaptation and the validity of the neuronal models investigated are discussed.

Action Potentials↗

PubMed1988

Continuous functions determined by spike trains of a neuron subject to stimulation.

Several ways of estimating a continuous function from the spike train output of a neuron subjected to repeated stimuli are compared: (i) the probability of firing function estimated by a PST-histogram (ii) the rate of discharge function estimated by a "frequencygram" (Bessou et al. 1968) and (iii) the interspike-interval function which is introduced in this paper. For a special class of neuronal responses, called deterministic, these functions may be expressed in terms of each other. It is shown that the current clamped Hodgkin-Huxley model of an action potential encoding membrane (Hodgkin and Huxley 1952) is able to generate such deterministic responses. As an experimental example, a deterministic response of a primary muscle spindle afferent is used to demonstrate the estimation of the functions. Interpretability and numerical estimatability of these spike train describing functions are discussed for deterministic neuronal responses.

Action Potentials↗