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C N Christakos

Publications and source records attributed to C N Christakos.

27 records · Page 2Linked to original sources
PubMed1984

Note on the estimation of the correlation function of neural spike trains.

The use of time-bins in the estimation of the correlation function of neural spike trains has a filtering effect on the estimate and results in distortion and aliasing. Prior low-pass filtering of the spike trains, on the other hand, and computation of the correlation function of the emerging waveforms in the standard way result in an estimate that is also a filtered version of the original function but distortion- and alias -free. In addition, the correlation function so computed can be normalized. An analogous definition of the correlation coefficient for the first technique enables the comparison of these various correlation estimates and clarifies their properties.

Animals↗
PubMed1984

The use of frequency domain techniques in the study of signal transmission in skeletal muscle.

Spectral analysis provides a description of the moments of random signals and enables the characterization of the behaviour of systems in terms of input-output relations. The merits of such an approach in the study of signal transmission in skeletal muscle are described in this paper. The representation of neural spike trains as impulse sequences and the subsequent treatment appropriate for this kind of analysis are discussed together with some practical problems. Spectral analysis of muscle afferent signals is applied to data obtained from cat experiments, and the use of the related frequency-domain techniques is demonstrated on a subsystem of the stretch reflex.

Action Potentials↗
PubMed1983

The 'M2' electromyographic response to random perturbations of arm movements is missing in long-trained monkeys.

Adult monkeys who have been under training over a long period of time show a loss of the M2 component in their biceps electromyographic response to brief, random perturbations of an alternating arm movement, the M1 component being apparently enhanced. These changes of the stretch-induced responses indicate a long-term plasticity of the sensorimotor system of monkeys. This result also provides hints for the origin and possible functional significance of the short- and long-latency components of the electromyographic response in subjects with less experience.

Animals↗
PubMed1982

A linear stochastic model of the single motor unit.

The production of force and of the electrical signal by an active motor unit is theoretically described. Neural spikes are modelled using the Dirac delta function. Mechanisms for the generation of random impulse trains and the properties of the corresponding stochastic processes are discussed; the "renewal" model is proposed as the most appropriate. The possibility of using a linear model for the systems that produce force and electrical signal in the unit is examined. It is concluded that the linear assumption is justifiable during steady, constant-strength contractions of muscle. This linear stochastic model of the motor unit is used in two subsequent papers to study the muscle force and the electromyogram.

Animals↗
PubMed1982

A study of the muscle force waveform using a population stochastic model of skeletal muscle.

A population stochastic model based on the differing properties and the independent activation of motor units is used to describe the production of force in the contracting skeletal muscle. Detailed force predictions of the model concerning a hand muscle are obtained by computer simulation. General features of the force signal are established analytically on the basis of the general properties of the neuromuscular system which the population model takes into account. The results show that the asynchronous activity of motor units and the distribution of their filtering and firing properties at various levels of muscle contraction are responsible, at least partially, for the main features of the muscle force waveform, including tremor.

Animals↗
PubMed1982

A study of the electromyogram using a population stochastic model of skeletal muscle.

The features of the electromyogram (EMG) are studied using a population model of skeletal muscle based on the differing properties and the independent activation of motor units (MUs). It is shown, both analytically and by computer simulation, that: (a) The power spectrum of the EMG is determined by the distribution of filtering and firing properties of the active MUs. (b) A tendency towards a rhythmical grouping of action potentials is to be expected from a set of asynchronous MUs firing semiregularly at similar rates; the grouped electrical activity has a phase-lead over the force output of the set of about 180 degrees. A unified explanation of the properties of the muscle force waveform and the electromyogram, in terms of asynchronous activity of MUs, is proposed. The explanation covers the relationship and the differences between the two signals.

Animals↗
PubMed1980

Lumped and population stochastic models of skeletal muscle: implications and predictions.

Lumped models of skeletal muscle have been assumed a) in the design of experiments and the interpretation of experimental findings, b) in theoretical studies. In this paper, a population model that takes into account the differing properties and separate (independent) activation of motor units is presented as the most appropriate for muscle. A realistic (for muscle) transformation, population leads to lumped model, resulting in the lumping of motor unit neural signals or system responses, is proposed. On this basis, the possibility of modelling muscle as a single system is examined; and the consequences of treating muscle as a lumped system, in experiments or theoretical studies, are discussed. Also, the advantages of lumping, in models of muscle, are reviewed. Predictions of a computer population model, together with actual recordings from a hand muscle, are used to confirm the results of the analysis.

Axons↗
PubMed1979

Force predictions relating to tremor and other consequences of a 'distributed' model of skeletal muscle.

In this brief report, a 'distributed' computer model of skeletal muscle is described, and its predictions regarding muscle force and EMG are presented. The force and myo-electric signals are produced as the sum of the contributions of independently activated motor units possessing a distribution of properties. The results of the simulation as well as theoretical study show that: (a) asynchronous activity of motor units can be responsible for the fairly regular oscillations observed in muscle tremor; (b) the use, in the case of muscle, of the limit theorem for the superposition of independent point processes leads to erroneous conclusions with respect to the frequency characteristics of the muscle force waveform, and possibly, with regard to those of the EMG.

Electromyography↗