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S Hemilä

Publications and source records attributed to S Hemilä.

31 records · Page 2Linked to original sources
PubMed1985

Rhodopsin phosphorylation inhibited by adenosine in frog rods: lack of effects on excitation.

The rod photocurrent was studied by recording the transretinal voltage from the aspartate-treated isolated frog retina before and after perfusion with 2 mM adenosine, which inhibited 60-80% of the light-induced rhodopsin phosphorylation. Adenosine did not affect the time courses of the flash photoresponses or the OFF responses after a steady light. The introduction of adenosine while the retina was illuminated by a steady background did not enhance the effect of light. Instead, the opposite change, due to PDE inhibition, was observed. The results indicate that rhodopsin phosphorylation does not determine the time course of the decay of excitation.

1-Methyl-3-isobutylxanthine↗
PubMed1981

Longitudinal spread of adaptation in the rods of the frog's retina.

1. The stimulus-response function of the red rods in the retina of the common frog (Rana temporaria) was determined in different adaptational states by measuring aspartate-isolated receptor responses. 2. Flash stimuli, background adaptations and bleaches were delivered through the same optical channel forming an oblique light-beam striking the receptor side of the isolated and flat-mounted retina at an angle of 10 degrees. 3. When the light was blue-green and optimally polarized the absorbance of the receptor layer was about 2, from which follows that 70-80% of the light was absorbed in the distal third of the rod outer segments, i.e. the exposure was local. Homogeneous exposures of the whole rod outer segments were obtained with orange and red lights. 4. Combinations of homogeneous and local stimuli with homogeneous and local adaptations were used to investigate the longitudinal spread of background, intermediate and opsin adaptation, i.e. the sensitivity-reducing effect of a background light, and the transient and permanent sensitivity losses following a bleach isomerizing 3.5-26% (usually 10%) of the rhodopsin in the retina. 5. The results obtained were related to predictions based both on the assumption that the adaptation effects spread longitudinally within the rod outer segments and the assumption that they are strictly confined to the disks absorbing the adapting lights. 6. These comparisons reveal that all three types of adaptation spread longitudinally. It is for instance clear that the sensitivity loss observed with homogeneous stimuli and local adaptation (as compared to homogeneous adaptation) is larger than that predicted by the non-spreading hypothesis. 7. The longitudinal spread of background adaptation is largely finished within 10 sec after turning on the background light, while an efficient spread of the intermediate adaptation effect may require minutes. 8. A background light decreasing the sensitivity by about one log unit decreases the time from flash to response maximum from 5 to 1 sec (small responses). Corresponding opsin adaptation effects are accompanied by less dramatic changes in response kinetics. 9. Independent of adaptation type - homogeneous or local, background, intermediate or opsin - it was found that local stimuli are less efficient that homogeneous stimuli in light-adapted retinae. This effect can be explained assuming that the sensitivity-reducing effects are pronounced in the distal than in the proximal parts of the rod outer segments. 10. The opsin adaptation effect following 10% local bleaches decreases the sensitivity to both homogeneous and local stimuli 2-3 times more than corresponding homogeneous bleaches. This means that the strength of the opsin effect is not related to the average percentage bleached but to the fraction bleached in the distal part of the rod, or generally to the fraction bleached in the most affected region. 11...

Adaptation, Ocular↗
PubMed1978

An analysis of rod outer segment adaptation based on a simple equivalent circuit.

This model of rod outer segment adaptation is based on the hypothesis that transmitter substance released by bleached rhodopsin closes sodium channels in the outer segment plasma membrane, leading to hyperpolarization of the receptor. The outer segment adaptation processes of the model are associated with the transmitter release, the transmitter background concentration and the plasma membrane leakage. Changes in the three model parameters correspond to the three types of outer segment adaptation process. According to the model the stimulus-response function is in every adaptational state U/Umax = I/(I + IH). The model predicts how each adaptation process affects IH and Umax. Specifically, when the number of liberated transmitter molecules per isomerizing quantum decreases, the amplitude Umax remains constant but IH increases. A short description of this model has been published in a paper reporting experimental results on background adaptation (Hemilä, 1977).

Adaptation, Ocular↗
PubMed1978

Directional selectivity and colour coding in the frog retina.

The impulse discharge of ganglion cells was recorded with extracellular micro-electrodes in the excised and opened eye of the common frog, Rana temporaria. The responses of different ganglion cell types to a standard moving spot with various spot-background contrasts are described. Information about such stimulus parameters as the size and contrast as a moving object is given by different classes of ganglion cells with preferences for different stimulus features. Of 171 sustained cells with small receptive fields 29 were found directionally selective, i.e. they responded well to movements only in some directions. Experiments with double stimulus fields suggest that this selectivity is due to an amacrine cell-mediated lateral inhibition nonsymmetrically arranged around the centre of the receptive field. The dichromatic colour vision of the frog is based on partly opponent signals from yellow-sensitive cones and blue-sensitive green rods. These opponent inputs make the ganglion cells respond to blue spots moving against a yellow-green background, irrespective of the relative intensities of the two colours. When the green rods are stimulated with blue light the ganglion cells produce long "on"-responses with significantly lower impulse frequencies than the short cone-mediated responses.

Action Potentials↗
PubMed1978

Excitation and adaptation in the vertebrate rod photoreceptor.

The excitatory processes and the reactions of the rhodopsin photoproducts in vertebrate rods are summarized, and an adaptation model of the rod is outlined. The transmitter factor Q describes the ability of the discs to release a transmitter carrying the signal from the site of light adaptation to the plasma membrane of the receptor. If Q decreases appropriately when the background light intensity increases, the response range of the rod is preserved and the rod obeys Weber's law. This kind of receptor adaptation is experimentally observed in several species. A photoisomerization in a disc apparently affects the release of a transmitter also in neighbouring discs. The background light also affects the neuronal network of the retina, preserving the response range of the ganglion cell. After an exposure bleaching an appreciable fraction of rhodopsin the sensitivity of the rod returns in two phases: a fast intermediate adaptation and a slow opsin adaptation. After a rather small bleach the photoresponse is fairly well preserved (mainly Q-adaptation), but large bleaches cause a strong reduction in the response range. Different experimental methods utilized when correlating rhodopsin photoproducts and intermediate adaptation are discussed.

Adaptation, Ocular↗
PubMed1977

Background adaptation in the rods of the frog's retina.

1. Aspartate-isolated photoresponses of the red rods to flashes and steps of light have been recorded, both in the presence of and without background lights of varying strength. 2. The results are interpreted in terms of a model of rod outer segment adaptation, where the three model parameters correspond to the adaptation processes associated with the transmitter release, the transmitter background concentration and the plasma membrane leakage, respectively. 3. The stimulus-response function deviated somewhat from the Michaelis equation U/Umax=I/(I + IH). During light-adaptation the operating curve, the stimulus-response function plotted in a log-log diagram, retained approximately its shape while moving strongly to the right along the log intensity axis and to a lesser degree downwards (Umax-decrease). 4. The movement of the operating curve was such that the rods approximately obeyed Weber's law. In the cases of flash and step of light stimuli the movement of the operating curve was about the same. 5. When a moderate background light was turned on a large decrease of sensitivity was first observed. During a period 0-5-1 min the sensitivity increased towards the stationary value. After extinguishing the background light the dark sensitivity returned in 0-5-1 min and then a period of hypersensitivity lasting typically 1 min was observed. 6. The experimental results, as interpreted according to the model, indicate that light-adaptation decreases q, the number of transmitter molecules released by one bleached rhodopsin molecule. 7. There is probably an adaptation process also in the rod inner segment, which increases the sensitivty of the rod to transient stimuli.

Action Potentials↗
PubMedDate not supplied

Modelling the spatio-temporal modulation response of ganglion cells with difference-of-Gaussians receptive fields: relation to photoreceptor response kinetics.

Difference-of-Gaussians (DOG) models for the receptive fields of retinal ganglion cells accurately predict linear responses to both periodic stimuli (typically moving sinusoidal gratings) and aperiodic stimuli (typically circular fields presented as square-wave pulses). While the relation of spatial organization to retinal anatomy has received considerable attention, temporal characteristics have been only loosely connected to retinal physiology. Here we integrate realistic photoreceptor response waveforms into the DOG model to clarify how far a single set of physiological parameters predict temporal aspects of linear responses to both periodic and aperiodic stimuli. Traditional filter-cascade models provide a useful first-order approximation of the single-photon response in photoreceptors. The absolute time scale of these, plus a time for retinal transmission, here construed as a fixed delay, are obtained from flash/step data. Using these values, we find that the DOG model predicts the main features of both the amplitude and phase response of linear cat ganglion cells to sinusoidal flicker. Where the simplest model formulation fails, it serves to reveal additional mechanisms. Unforeseen facts are the attenuation of low temporal frequencies even in pure center-type responses, and the phase advance of the response relative to the stimulus at low frequencies. Neither can be explained by any experimentally documented cone response waveform, but both would be explained by signal differentiation, e.g. in the retinal transmission pathway, as demonstrated at least in turtle retina.

Animals↗
PubMedDate not supplied

Noise-equivalent and signal-equivalent visual summation of quantal events in space and time.

Noise recorded in visual neurons, or variability in psychophysical experiments, may be quantified in terms of quantal fluctuations from an "equivalent" steady illumination. The conversion requires assumptions concerning how photon signals are pooled in space and time, i.e. how to pass from light fluxes to numbers of photon events relevant to the Poisson statistics describing signal/noise. It is usual to approximate real weighting profiles for the integration of photon events in space and time (the sensitivity distribution of the receptive field [RF] and the waveform of the impulse response [IR]) by sharp-bordered apertures of "complete," equal-weight summation of events. Apertures based on signal-equivalence cannot provide noise-equivalence, however, because greater numbers of events summed with smaller weights (as in reality) have lower variances than smaller numbers summed with full weight. Thus sharp-bordered apertures are necessarily smaller if defined for noise- than for signal-equivalence. We here consider the difference for some commonly encountered RF and IR profiles. Summation areas, expressed as numbers of photoreceptors (cones or rods) contributing with equal weight, are denoted NS for signal and NN for noise; sharply delimited summation times are correspondingly denoted tS and tN. We show that the relation in space is NN = 0.5 NS for the Gaussian distribution (e.g. for the RF center mechanism of retinal ganglion cells). For a photoreceptor in an electrically coupled network the difference is even larger, e.g., for rods in the toad retina NN = 0.2 NS (NS = 13.7 rods and NN = 2.8 rods). In time, the relation is tN approximately 0.7 tS for realistic quantal response waveforms of photoreceptors. The surround input in a difference-of-Gaussians RF may either decrease or increase total noise, depending on the degree of correlation of center and surround noise. We introduce a third useful definition of sharp-bordered summation apertures: one that provides the same signal-to-noise ratio (SNR) for large-long stimuli as the real integration profiles. The SNR-equivalent summation area is N* = N2S/NN and summation time t* = t2S/tN.

Animals↗