Signal transmission through the dark-adapted retina of the toad (Bufo marinus). Gain, convergence, and signal/noise.

Responses to light were recorded from rods, horizontal cells, and ganglion cells in dark-adapted toad eyecups. Sensitivity was defined as response amplitude per isomerization per rod for dim flashes covering the excitatory receptive field centers. Both sensitivity and spatial summation were found to increase by one order of magnitude between rods and horizontal cells, and by two orders of magnitude between rods and ganglion cells. Recordings from two hyperpolarizing bipolar cells showed a 20 times response increase between rods and bipolars. At absolute threshold for ganglion cells (Copenhagen, D.R., K. Donner, and T. Reuter. 1987. J. Physiol. 393:667-680) the dim flashes produce 10-50-microV responses in the rods. The cumulative gain exhibited at each subsequent synaptic transfer from the rods to the ganglion cells serves to boost these small amplitude signals to the level required for initiation of action potentials in the ganglion cells. The convergence of rod signals through increasing spatial summation serves to decrease the variation of responses to dim flashes, thereby increasing the signal-to-noise ratio. Thus, at absolute threshold for ganglion cells, the convergence typically increases the maximal signal-to-noise ratio from 0.6 in rods to 4.6 in ganglion cells

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

1. 1. The rod photocurrent was studied by recording the transretinal voltage from the aspartatetreated isolated frog retina before and after perfusion with 2 mM adenosine, which inhibited 60–80% of the light-induced rhodopsin phosphorylation. 2. 2. Adenosine did not afifect the time courses of the flash photoresponses or the OFF responses after a steady light. 3. 3. 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. 4. 4. The results indicate that rhodopsin phosphorylation does not determine the time course of the decay of excitation

Secretory S complex of Bacillus subtilis: sequence analysis and identity to pyruvate dehydrogenase

Peer reviewe

Transient sensitivity reduction and biphasic photoresponses observed when retinal rods are oxidized

1. 1. Rod photoresponses and the effects of oxidation have been studied by recording either the transretinal voltage in aspartate-treated retinas or the outer segment current of single rods. 2. 2. Oxidizing conditions transiently decreased, reducing conditions increased sensitivity. 3. 3. Biphasic photoresponses were seen when the level of oxidation was rising and also in some other sensitivity-depressing conditions. 4. 4. A model is proposed which explains the biphasic responses in terms of sensitivity differences between the tip and the base of the rod outer segment

On the relation between ERG waves and retinal function : Inverted Rod photoresponses from the frog retina

In rod mass receptor photoresponses recorded across the isolated frog retina, a paradoxical cornea-positive wave may precede the response of normal polarity. We present a model which shows that the light-induced decrease in rod current can give rise to inverted or biphasic ERG signals if the distal part (tip) of the rod outer segment responds more slowly and/or less sensitively than the proximal part (base). The condition is that current entering at the tip is represented with greater weight in the ERG. The model reproduces recorded ERG waveforms well. It further predicts that if there is a light-in sensitive conductance in the tip membrane, ERG photoresponses may be non-recordable although current photoresponses are only slightly reduced. The model reveals a type of complexity in the relation between mass potentials and underlying physiological processes which has not previously received attention

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 = 0.5NS 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 = 0.2NS (NS = 13.7 rods and NN = 2.8 rods). In time, the relation is tN [approximate] 0.7tS 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* = NS 2/NN and summation time t* = tS2/tN

Effects of sulfhydryl binding reagents on the photoresponses of amphibian retinal rods

1. 1. We have studied the effects of four sulfhydryl binding reagents (NEM, PHMB, PCMP and IAA) and the disulfide reducing agent, DTT, on the photocurrent of vertebrate rods by recording the ERG across the aspartate-treated retina of the frog, and by suction pipette recording from isolated rods of the tiger salamander. 2. 2. SH-reagents brought about three types of effects on rods: (1) a fast transitory increase in light sensitivity and photocurrent; (2) a “leakage” current that could not be turned off by light; and (3) a slower irreversible loss of sensitivity. 3. 3. The fast effects, including the leakage current, are attributable in part to direct action on the sodium channels in the plasma membrane. 4. 4. NEM, PHMB and PCMP were able to affect the transduction machinery inside the rod, which first contributed to the growth of photoresponses, but gradually depressed light sensitivity irreversibly. 5. 5. Typically, the reagents also induced large ERG transients of non-receptor (glial) origin. 6. 6. The fast effects of DTT on isolated rods were similar to those of the SH-reagents. This drug) however, had no clear effect on ERG photoresponses, suggesting that the oxidation of sodium-channel SH-groups is modest in the intact retina as compared with isolated rods

Signal transmission through the dark-adapted retina of the toad (Bufo marinus). Gain, convergence, and signal/noise.

ABSTRACT Responses to light were recorded from rods, horizontal cells, and ganglion cells in dark-adapted toad eyecups. Sensitivity was defined as response amplitude per isomerization per rod for dim flashes covering the excitatory receptive field centers. Both sensitivity and spatial summation were found to increase by one order of magnitude between rods and horizontal cells, and by two orders of magnitude between rods and ganglion cells. Recordings from two hyperpolarizing bipolar cells showed a 20 times response increase between rods and bipolars. At absolute threshold for ganglion cells (Copenhagen, D.R., K. Donner, and T. Reuter. 1987. J. Physiol. 393:667-680) the dim flashes produce 10-50-#V responses in the rods. The cumulative gain exhibited at each subsequent synaptic transfer from the rods to the ganglion cells serves to boost these small amplitude signals to the level required for initiation of action potentials in the ganglion cells. The convergence of rod signals through increasing spatial summation serves to decrease the variation of responses to dim flashes, thereby increasing the signalto-noise ratio. Thus, at absolute threshold for ganglion cells, the convergence typically increases the maximal signal-to-noise ratio from 0.6 in rods to 4.6 in ganglion cells