RESUMO
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.
Assuntos
Simulação por Computador , Modelos Neurológicos , Neurônios Motores/classificação , Neurônios Motores/fisiologia , Animais , Sinalização do Cálcio , Gatos , Dendritos/fisiologia , Eletrofisiologia , Sinapses/fisiologiaRESUMO
Numa proposta de pesquisa para estudar os correlatos eletrofisiológicos de processamento semântico de palavras apresentadas visualmente pretendia-se captar (e processar) quatro potenciais evocados cerebrais de algumas derivações num mesmo experimento, utilizando um equipamento comercial usado em centros hospitalares. Para viabilizar tal pesquisa, foi criado um protocolo especial, em que duas respostas visuais evocadas seqüencialmente, com uma latência de 4,15 s, eram adquiridas em uma mesma janela de aquisição. Como a taxa de amostragem resultante no equipamento foi adequada para a aplicação (100 Hz), foi possível realizar o experimento no próprio centro hospitalar, utilizando um equipamento com o qual os pesquisadores da área biomédica estavam bem familiarizados. Um problema adicional surgiu no tocante a artefatos por movimentação ocular. Este foi sanado graças à possibilidade de serem arquivadas todas as respostas individuais, e não somente a média coerente. Um algoritmo simples realizou a detecção de artefatos de forma eficiente. As soluções encontradas possibilitaram a realização da pesquisa proposta, sendo que essas mesmas soluções poderão permitir trabalhos em outros centos que se deparem com uma problemática semelhante à descrita.
Assuntos
Eletrofisiologia , Potenciais Evocados Visuais , Movimentos Oculares , Tempo de Reação , Cognição , Validação de Programas de ComputadorRESUMO
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.
Assuntos
Potenciais Somatossensoriais Evocados/fisiologia , Dedos/inervação , Células Receptoras Sensoriais/fisiologia , Aceleração , Adulto , Anestesia Local , Estimulação Elétrica , Eletromiografia , Feminino , Antebraço/inervação , Mãos/anatomia & histologia , Humanos , Masculino , Pessoa de Meia-Idade , Vias Neurais/fisiologia , Estimulação Física , Postura , Tempo de Reação/fisiologia , Couro Cabeludo/inervação , Processamento de Sinais Assistido por ComputadorRESUMO
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.
Assuntos
Modelos Neurológicos , Neurônios Motores/fisiologiaRESUMO
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.
Assuntos
Modelos Neurológicos , Neurônios/fisiologia , Sinapses/fisiologia , Potenciais de AçãoRESUMO
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.
Assuntos
Simulação por Computador , Modelos Neurológicos , Sinapses/fisiologia , Potenciais de Ação/fisiologia , Algoritmos , Dendritos/fisiologia , Condutividade Elétrica , Impedância Elétrica , Humanos , Potenciais da Membrana/fisiologia , Neurônios/fisiologia , Transmissão Sináptica/fisiologia , Fatores de TempoRESUMO
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.