Biomedical subjects

M Juusola

Publications and source records attributed to M Juusola.

26 records · Page 2Linked to original sources

PubMed1994

Sodium-dependent receptor current in a new mechanoreceptor preparation.

1. Intracellular microelectrodes recorded the receptor potential and receptor current in the neurons of spider slit sense organs during mechanical stimulation of the slits. 2. Mechanical stimulation produced two patterns of action potential discharge, corresponding to the two groups of neurons described previously by electrical stimulation. 3. Tetrodotoxin eliminated the action potentials and revealed a receptor potential with both static and adapting components. Voltage clamp gave an inward receptor current with a similar time course. 4. Replacement of sodium ions in the bath reversibly eliminated the receptor current, indicating that it is carried by sodium ions. However, this effect was comparatively slow, suggesting that the tips of the sensory dendrites lie in a chemically restricted environment.

Action Potentials↗

PubMed1993

Band-pass filtering by voltage-dependent membrane in an insect photoreceptor.

The membrane properties of short type blowfly photoreceptors (R1-6) were investigated in dark and light adaptation with single electrode current and voltage clamp techniques. The impedance of the cells was defined in frequency domain by using discontinuous current clamp and white-noise-modulated current injection. We found that the slow activation and relaxation of the voltage-dependent K+ conductance transform the photoreceptor membrane effectively into a band-pass filter. This behaviour could be observed under current clamp as voltage-dependent outward and inward rectification of the membrane. The voltage-dependent band-pass filtering is likely to be present in all neurons with graded potentials and voltage-dependent membrane conductances.

Adaptation, Physiological↗

PubMed1993

The dynamic nonlinear behavior of fly photoreceptors evoked by a wide range of light intensities.

Fly photoreceptor cells were stimulated with steps of light over a wide intensity range. First- and second-order Volterra kernels were then computed from sequences of combined step responses. Diagonal values of the second-order Volterra kernels were much greater than the off-diagonal values, and the diagonal values were roughly proportional to the corresponding first-order kernels, suggesting that the response could be approximated by a static nonlinearity followed by a dynamic linear component (Hammerstein model). The amplitudes of the second-order kernels were much smaller in light-adapted than in dark-adapted photoreceptors. Hammerstein models constructed from the step input/output measurements gave reasonable approximations to the actual photoreceptor responses, with light-adapted responses being relatively better fitted. However, Hammerstein models could not account for several features of the photoreceptor behavior, including the dependence of the step response shape on step amplitude. A model containing an additional static nonlinearity after the dynamic linear component gave significantly better fits to the data. These results indicate that blowfly photoreceptors have a strong early gain control nonlinearity acting before the processes that create the characteristic time course of the response, in addition to the nonlinearities caused by membrane conductances.

Animals↗

PubMed1992

Measurement of cell impedance in frequency domain using discontinuous current clamp and white-noise-modulated current injection.

A method is described for the determination of cellular input impedance of non-spiking neurones. The input impedance is important when cellular geometry and the effects of voltage-dependent channels are considered. Cells are impaled with a single glass microelectrod and current is injected using a time-sharing technique. The cell's impedance is measured by randomly modulating the injected current and calculating the impedance as a transfer function between current and recorded membrane voltage. Corresponding coherence functions can also be calculated for estimating the signal-to-noise ratio, and also linearity (i.e. possible activation of voltage-dependent conductances) of the membrane.

Animals↗

PubMedDate not supplied

Fast-acting compressive and facilitatory nonlinearities in light-adapted fly photoreceptors.

Light-adapted fly photoreceptor cells were stimulated with brief positive and negative contrast flashes (contrast = delta I/I, I = intensity). Membrane potential responses to a wide range of flash intensities were well-fitted by a static nonlinearity followed by a compartmental model represented by a gamma function. However, the agreement improved if one parameter of the gamma function, n, varied quadratically with input light intensity. Response amplitude and time to peak were estimated from the fitted parameters. Response amplitude varied approximately linearly with contrast but showed nonlinear compression with the largest negative flashes. Reducing the background light level by 3 decades or hyperpolarizing the cell electrically produced stronger nonlinear compression with both contrast polarities. This is probably due to fast voltage-activated K+ channels. Responses to double flashes with varying time separations were well-fitted by summed gamma functions, allowing separation of the individual flash responses. There was no detectable time-dependent interaction between paired positive flashes at all separations. However, the response to two negative flashes was greater than the linear prediction at short separations, and this facilitatory nonlinearity decayed with a time constant of about 1 msec. The facilitation is probably related to resonant behavior in light-adapted photoreceptors and may be due to an IP3-induced intracellular Ca2+ release.

Animals↗

PubMedDate not supplied

A method for determining photoreceptor signal-to-noise ratio in the time and frequency domains with a pseudorandom stimulus.

We have developed a method that utilizes repeated sequences of pseudorandomly modulated stimuli for calculation of the SNR either in the time or frequency domains. The method has the advantage that the distribution of SNR over relevant frequencies is readily observed. In addition, a SNR value, calculated as the ratio of the corresponding variances, is an estimate of the true SNR because it has been weighted by the cell's frequency response. The procedure offers significant advantages when studying signal transmission in nonspiking cells like photoreceptors.

Animals↗

PubMedDate not supplied

Principal dynamic mode analysis of nonlinear transduction in a spider mechanoreceptor.

The nonlinear dynamics of the mechanoelectrical transduction in an arthropod mechanoreceptor (cuticular slit sense organ of the spider Cupiennius salei) were studied using Volterra kernel measurements and the recently proposed method of principal dynamic modes. Since mechanoreceptors must operate with sufficient gain sensitivity to rapidly varying displacement stimuli over a broad bandwidth and for a wide range of amplitudes, the experimental data were generated by applying pseudorandom broadband mechanical displacements of various mean levels to the cuticular slits. The recorded response data were intracellular current and potential. The purpose of this paper is to demonstrate the use of the principal dynamic mode (PDM) methodology in elucidating the nonlinear dynamics of a spider mechanoreceptor. The results obtained demonstrate that two PDMs suffice to provide a complete nonlinear dynamic model of this insect mechanoreceptor. The first PDM resembles the first-order kernel and has a low pass characteristic (position dependent), while the second PDM has a high-pass characteristic (velocity-dependent) and resides entirely in the second-order kernel (nonlinear adaptation). This study may serve as an example of the proper use of this new methodology for the analysis of nonlinear physiological systems.

Animals↗

PubMedDate not supplied

Two methods for calculating the responses of photoreceptors to moving objects.

The responses of photoreceptor cells to moving stimuli are crucial to understanding motion detection in visual systems. However, these responses are not well characterized quantitatively because they result from a combination of spatial optical behavior in the lens systems with temporal behavior in the phototransduction mechanism. While both these processes can now be modeled quite well by relatively simple equations, their combination cannot be easily obtained in a closed form. Here, we present two approaches to this problem, based on well-established models for the lens and photoreceptors systems of the fly compound eye. The first approach leads to a recursive formula for predicting the photoreceptor response to a moving point object. The second method is approximate, but almost equally accurate and more rapid.

Adaptation, Ocular↗