Turtle Cones Research Articles

Cone response latency and log sensitivity: Proportional changes with light adaptation

Field adaptation causes proportional changes in cone response latency and log sensitivity for (1)psychophysical reaction times to threshold stimuli, (2)intracellular recordings of turtle cone responses, (3)local electroretinogram recordings of monkey cone responses and (4)theoretical impulse response functions derived from psychophysical flicker thresholds. To a good approximation, this relationship holds for all levels of cone adaptation except those that cause significant photopigment bleaching. For L and M cones, the proportionality constant between latency and log sensitivity appears to be cone-specific and largely independent of spatiotemporal stimulus parameters. For S cones, the proportionality constant depends to some extent on influences from other cone types.

The incremental sensitivity curve of turtle cone photoreceptors

The incremental sensitivity curve of turtle cone photoreceptors

Hardware cone cell model; operational characteristics

An electronic model of a salamander cone cell is derived to simulate the membrane potential changes recorded intracellulary in the salamander retina from flashes or steps of light. The electronic model is based on the theoretical work of Bayloret al. on turtle cone cells and employs hyperpolarising and depolarising conductances to generate the simulated membrane potential changes. These conductance changes and other internal parameters are monitored under the control of a minicomputer simulation system.

Feed-back modulation of cone synapses by L-horizontal cells of turtle retina.

Light stimulation of the periphery of the receptive field of turtle cones can evoke both transient and sustained increases of the cone Ca2+ conductance, which may become regenerative. Such increase in the cone Ca2+ conductance evoked by peripheral illumination results from the activation of a polysynaptic pathway involving a feed-back connexion from the L-horizontal cells (L-HC) to the cones. Thus the hyperpolarization of a L-HC by inward current injection can evoke a Ca2+ conductance increase in neighbouring cones. The cone Ca2+ channels thus activated are likely located at its synaptic endings and probably intervene in the cone transmitter release. Therefore the feed-back connexion between L-HC and cones by modifying the Ca2+ conductance of cones could actually modulate the transmitter release from cone synapses. Such feed-back modulation of cone synapses plays a role in the organization of the colour-coded responses of the chromaticity type-horizontal cells and probably of other second order neurones, post-synaptic to the cones. The mechanisms operating the feed-back connexion from L-HC to cones are discussed.

Photoreceptor signals at visual threshold.

Electrical responses of cone photoreceptors in the retina of the freshwater turtle have been characterised for flashes and steps of light in darkness and in the presence of background light. These intracellular measurements have been combined with the behavioural increment threshold curve to yield an estimate of 5-10 muV for the signal developed in a cone when the turtle can just detect an increment flash. The signal developed when the cones under the stimulus image are dark-adapted is of interest, for its measurement would help to explain how known physiological processes subserve visual detection for a variety of photic conditions. The effective quantal absorption of dark-adapted, red-sensitive cones of Pseudemys scripta elegans for a stimulus that the turtle can just detect is reported here. By combining this result with previous electrical measurements on red-sensitive cones of this species, an estimate of 35-70 muV is obtained for the signal developed in a dark-adapted cone at behavioural threshold. This larger signal required for detection of a flash in darkness is of particular interest in view of the recent observation that the intrinsic noise of turtle cones in darkness is larger than that of illuminated cones.

Characteristics and ionic processes involved in feedback spikes of turtle cones.

In about 20% of the cones of untreated retinas of turtles, bright flash illumination of the periphery of their receptive field evokes a spike through the feedback mechanism from the L-horizontal cell. Such feedback spikes, never observed with central stimulation, are labile, but after they have disappeared they can be regained by depolarizing the cone. Feedback spikes are actual regenerative responses, since they show a critical threshold potential, are facilitated by cone depolarization and are blocked by hyperpolarization. They are associated with a membrane resistance decrease; tetrodotoxin (10(-5) M) does not block them. High Ca2+ media facilitate their appearance, but their effect is transient because of the cone hyperpolarization and the light response block that Ca2+ ions induce. Sr2+ ions (4-10 mM) facilitate the discharge of feedback spikes in response to peripheral illumination in every cone, whether or not it has previously shown feedback effects. In Sr2+ media, feedback spikes are stable and can be evoked by dim lights. Ba2+ (2-6 mM) also facilitates and stabilizes the discharge of feedback spikes. Co2+ and D-600 block the feedback spikes. Pharmacological agents that depolarize the L-horizontal cells, such as GABA, glutamate or nicotine, also block the feedback spikes. Both Sr2+ and Ba2+ also induce the appearance of spontaneous and off spikes, which are also blocked by Co2+, but these are not related to the feedback mechanism. These results strongly suggest that every turtle cone receives a feedback input from the L-horizontal cells, which would be able to induce an increase of the cone Ca2+ conductance, which may become regenerative.

Sustained feedback effects of L-horizontal cells on turtle cones.

Prolonged stimulation of the periphery of their receptive field can evoke in turtle cones sustained complex depolarizations or sustained membrane oscillations. In cones in which such effects of prolonged peripheral stimulation are not apparent, the injection of short depolarizing pulses can reveal a sustained increase of electrical excitability in response to prolonged peripheral illumination. The sustained effects of prolonged peripheral illumination have characteristics similar to those of the feedback depolarizations evoked by flash peripheral stimulation: they are labile in untreated retinas, can be blocked by either hyperpolarization, Co2+ or agents that depolarize the L-horizontal cells. They are associated with a decrease in the membrane input resistance. In retinas bathed in Sr2+- or Ba2+-containing media, prolonged peripheral illumination evokes a sustained repetitive discharge of spikes. These experiments demonstrate that the feedback effects of the L-horizontal cells on the cones are not only transient but also sustained and that the sustained effects of peripheral stimulations are associated with an increase in membrane Ca2+ conductance. The possible nature of the feedback connection between L-horizontal cells and the cones is discussed.

Artificial cone responses: A computer-driven hardware model

A hardware model of a cone cell has been designed and fabricated according to the Baylor, Hodgkin and Lamb theoretical model ( J. Physiol. 242, 685–727, 759–791). The cone model is interfaced to a minicomputer which organizes the simulation of light stimuli and analyzes the responses. The fit of the simulated electrical responses with those recorded intracellularly from turtle cones is very good for 10 msec flashes and 670 msec steps of light, whose intensity ranged over 2.5 log units with the value of the unattenuated light of 1.5 × 10 17 photons sec −1 cm −2. The model parameters can be adjusted to give the closest reproduction of the actual photoresponses for different animals; in particular we obtained a good fit for the cone responses of the tiger salamander retina.

Endogenous levels of neurotransmitter candidates in photoreceptor cells of the turtle retina.

Abstract—The neurotransmitter used by vertebrate photoreceptors has not been identified, although aspartic acid, glutamic acid and acetylcholine have all been suggested as possible candidates. In the present study, we have measured the endogenous levels of these substances in isolated photoreceptors of the turtle retina. Fractions containing over 80% of identified photoreceptors were obtained by enzymatic dissociation of the retina followed by velocity sedimentation at unit gravity. The endogenous levels of free amino acids in these fractions were measured by amino acid analysis and the ACh content was measured by a radiochemical assay. In addition, identified photoreceptors were drawn into a micropipet individually under visual observation and their ACh content was determined. Our results show that while the concentrations of free aspartic acid and glutamic acid in isolated photoreceptors are similar to those found in the retina, the concentration of ACh in cone photoreceptors (0.1–0.2 mM) is 2‐ to 3‐fold higher than that in the turtle retina. This result combined with the findings of others suggests that turtle cone photoreceptors may be cholinergic

Origin of non-linearity of voltage-current relationships of turtle cones

The voltage-current relations of the cone membrane were measured by means of pulses of current injected into single cones of turtle retina. In darkness the voltage-current curve was almost linear. During illumination with a small (dia. 115 μm) spot the steepness of the curve increased but its linearity was preserved. However, during illumination with a big (dia. 2 mm) spot the positive pulses of current evoked considerably higher potential drops than negative pulses of the same intensity. Accordingly, the depolarizing portion of the voltage-current curve was steeper than the hyperpolarizing one. This non-linear behaviour of cone membrane displayed only with a large-area illumination can be explained as a combined effect of non-linearity of presynaptic membrane of the cone and of the mechanism of feedback between the horizontal cells and cones. The equivalent circuit of synapse between the photoreceptor and the horizontal cell, realized as a physical model, is described. The model reproduces the effect of area illuminated on the shape of the voltage-current curves of the cone, as well as some other experimental manifestations of the feedback mechanism in cones.

Activation of a regenerative calcium conductance in turtle cones by peripheral stimulation.

Light stimulation of the periphery of the receptive field of some turtle cones evokes spikes which are tetrodotoxin-insensitive. In all cones, Sr 2+ or Ba 2+ ions (4–2 mM) induce the appearance of spikes in response to pheripheral illumination. These spikes are blocked by Co 2+ ions and by agents which depolarize the L-horizontal cells and block their responses. It is concluded that peripheral illumination evokes Ca 2+ spikes in turtle cones through a feed-back synaptic mechanism from the L-horizontal cells.

Analysis of electrical noise in turtle cones.

1. Properties of the light-sensitive voltage noise in cones in the retina of the turtle, Pseudemys scripta elegans, have been studied by intracellular recording.2. Suppression of the noise by light was a function of the hyperpolarizing response of a cone but not of the size or pattern of illumination.3. Power density spectra of the noise were fitted in many cones by the product of two Lorentzians with characteristic time constants tau(1) and tau(2) averaging 40 and 7 msec respectively. The spectra of some cells were peaked and could be fitted by a resonance curve.4. Spectra in dim light exhibited decreased low frequency power. They could often be fitted by a product of two Lorentzians using the same value of tau(2) as used in darkness but decreasing tau(1) and the zero frequency asymptote. An e-fold reduction in tau(1) occurred with lights which hyperpolarized by 4-7 mV.5. Injection of hyperpolarizing currents of about 0.1-0.2 nA into weakly coupled cones reduced the noise, and also reduced the sensitivity to dim flashes.6. The variance-voltage relation during steady illumination of different intensities differed from cone to cone. Dim lights increased the noise in some cells and decreased it in others, but moderately bright lights which gave steady responses of more than about one third maximal reduced the noise in all cells.7. When the cell was transiently depolarized during the differentiated component following steady illumination, the noise was less than it was after prolonged darkness.8. In the after-effect of bright light, the time course of recovery of noise was the same as that of flash sensitivity and voltage. The noise was reduced e-fold for hyperpolarizations averaging 3 mV while for sensitivity this reduction occurred for 1.3 mV. For a given hyperpolarization the noise was lower during the after-effect than during steady dim illumination.9. When a series of dim flashes was delivered to a cone, no significant increase in variance over the dark noise was detected during the photo-response. This implies that each photoisomerization evokes no more than about 1.5 muV at the peak of the response in a coupled cone, corresponding to about 50 muV in an isolated cone.10. The elementary shot events underlying the noise are about 100 muV in amplitude in an isolated cone, have a characteristic time constant of 16-60 msec and reflect unit conductance fluctuations of about 16 pS (S, Siemen identical with Omega(-1)).11. It is concluded that the noise source is internal to the cones. We postulate that the noise arises from opening and closing of the light-sensitive ionic channels in the outer segment, and that in darkness there is a residual concentration of the blocking substance which on average closes up to about one third of the channels. It seems likely that the unit event involves a considerable number of blocking molecules and ionic channels.

Internal recording of the early receptor potential in turtle cones.

1. Early receptor potentials (E.R.P.s) were recorded with internal electrodes in turtle cones by applying brief flashes from a xenon tube with a maximum photon density equivalent to 2-3 x 10(8) photons micronm-2 at the optimum wave-length. 2. The E.R.P. was separated from the late receptor potential (L.R.P.) by superposing in flash on a step of light which was strong enough to saturate the L.R.P. 3. In red-sensitive cones the E.R.P. consisted of a brief depolarizing phase (R1) followed by a hyperpolarizing phase (R2) of maximum amplitude 10 mV and duration 30-40 msec. R1 was small or absent in green-sensitive cones. 4. With flashes of increasing intensity the E.R.P. approached its maximum exponentially with an exponential constant Q of about 10(8) photons micronm-2 which is of the same order as the reciprocal of the photosensitivity of porphyropsin; the implication of this result, which is considered in the theoretical section, is the the E.R.P. is proportional to the number of photoisomerizations. 5. When tested with a constant xenon flash at varying times after the beginning of a bleaching light the E.R.P. declined exponentially with a similar value of Q. 6. After prolonged bleaches the E.R.P. recovered with a time constant of about 100 sec but much quicker recoveries were observed after relatively brief bleaches. 7. The form and size of the E.R.P. are consistent with the accepted view that it arises from a redistribution of charge in the cone pigment molecule. 8. The effect of a single photoisomerization in an isolated cone was estimated as about 10(-10) V or one electronic charge through about 10% of the membrane.

Electrical responses of rods in the retina of Bufo marinus

1. Intracellular responses to flashes and steps of light have been recorded from the outer segment and the cell body of rods in the retina of the Bufo marinus. The identification of the origin of recorded responses has been confirmed by intracellular marking.2. Responses to flashes delivered in darkness or superimposed on a background were analysed. Responses recorded from outer segments conform to the principle of ;spectral univariance'. The shape of the response is not affected by enlarging the spot diameter from 150 to 1000 mum.3. The membrane potential measured in darkness at the outer segments varied from -15 to -25 mV. Injection of steady hyperpolarizing currents increases the size of the response to light; depolarizing currents reduce the response. The mean value of the input resistance is 97 +/- 30 MOmega in darkness and increases by 20-30% during illumination.4. The responses obtained from the cell body of rods have the same shape, time course and spectral sensitivity of those recorded at the outer segment. Injection of steady current at the cell body produces different effects than at the outer segment: hyperpolarizing currents reduce the amplitude of the response to light; depolarizing currents increase the response.5. The experimental data are fitted according to a model similar to that used to describe the responses of turtle cones (Baylor & Hodgkin, 1974; Baylor, Hodgkin & Lamb, 1974a, b).6. The model reproduces the electrical responses of the rod outer segment to a variety of stimuli: (a) brief flashes and steps of light in dark adapted conditions; (b) bright flashes superimposed on background illuminations; (c) pairs of flashes delivered at different time intervals. Responses to hyperpolarizing steps of current are also reproduced by the model.

Effects of applied currents on turtle cones in darkness and during the photoresponse.

1. The voltage changes, produced by pulses of current of various intensities and polarities into turtle cones, were examined both in darkness and during the photoresponse. 2. Depolarizing pulses of current produced voltage deflexions which, during the photoresponse, were increased to a greater extent than those produced by pulses of current of the same intensity and opposite polarity. 3. Voltage-current relationships were measured both in darkness and during illumination. During illumination they display a non-linear behaviour, the curve being steeper for positive than for negative currents. 4. These non-linearities are greatly attenuated following exposure of the retina to a solution containing 5 mM-CoCl2 which is known to block synaptic transmission. The effects of CoCl2 are readily reversible when the retina is returned to normal conditions. 5. By comparing the effects produced by pulses of current injected into individual cones with those observed in horizontal cells following stimulation with a radial current, it is observed that the non-linear behaviour of the cone membrane during the photoresponse can be, at least in part, accounted for by events occurring in post-synaptic elements (i.e. horizontal cells).

Closure of ionic channels in turtle cones.

Recent observations from turtle cones on the kinetics of the light response and the variance and power spectrum of spontaneous noise are discussed in terms of the behaviour of membrane ionic channels.

Ultrastructural analysis of functional changes in the synaptic endings of turtle cone cells.

The idea that upon synaptic activity, the limiting membrane of synaptic vesicles is transiently incorporated into the cell membrane, subsequently internalized by coated vesicles, and finally returned to the vesicular compartment is based on a morphometric and ultrastructural tracer analysis of the events following repetitive electrical stimulation of the neuromuscular junction (Heuser and Reese 1973). The controversy on the various phases of this synaptic vesicle membrane recycling hypothesis, however, has not yet been satisfactorily settled. Although a number of reports indicate that upon intense stimulation, the surface of the presynaptic membrane increases in both the neuromuscular junction (Clark et al. 1972; Ceccarelli et al. 1972; Heuser and Reese 1973) and the superior cervical ganglion (Pysh and Wiley 1974), it has been recently denied that synaptic vesicles do indeed fuse with the cell membrane (Birks 1974). Conflicting opinions also exist on the mechanism of membrane interiorization and the fate of the...

Light path and photon capture in turtle photoreceptors.

1. The directional selectivity of individual cones was examined by intracellular recording in the eye of the turtle. Sensitivites were determined from linear responses to dim flashes of monochromatic light incident on a cell over a range of angles to its long axis. 2. With light near the optimum wave-length, some red- and green-sensitive cones showed a high sensitivity for light entering axially and lower sensitivities for light entering obliquely. In contrast, other cells had lower peak sensitivities and less pronounced directional selectivities. The highest axial sensitivities observed in red receptors were about 320 muV photon(-1) mu2; in these cells, the sensitivity declined to half for rays 6-9 degrees off the axis as measured in the retina. Green receptors had lower axial sensitivities and broader angular profiles. 3. On the assumption that rays at all angles contribute independently to the over-all sensitivity, the sensitivity of a cell to large cones of rays was successfully predicted from the angular selectivity determined with a narrow pencil of rays. The shape of small responses to dim stimuli delivered on and off the axis of the cell was invariant, implying that a cone signals the number of photons absorbed but not their angle of incidence. 4. Short wave-lengths have previously been shown to be filtered out by the oil droplets present in turtle cones. At short wave-lengths, the angular profiles showed a depression in axial sensitivity consistent with this filtering action. 5. Diameters of inner segments, oil droplets, and outer segments were measured in red-, green-, and blue-sensitive cones, since these dimensions are expected to influence the cones' angular acceptances and ability to collect light. The diameters of the structure were in approximately the same proportions for each type of receptor, but the absolute values of the diameters were found to be scaled in relation to the wave-length of maximum sensitivity. 6. Optical determinations of the efficiency with which axial rays are concentrated by red receptors gave a mean value of 55%. 7. Receptors in histological sections of the whole eye were found to be oriented with their long axes directed approximately toward the pupil. 8. The observed directional selectivities and collecting efficiencies agree well with the behaviour of a model retinal cone developed by Winston & Enoch (1971) on a geometrical optical treatment. 9. Effective collecting areas are derived for red-, green- and blue-sensitive cones; these permit conversion of observed flash sensitivities into the mean peak hyperpolarization produced by isomerization of a visual pigment molecule. The figure obtained is about 25 muV for red-sensitive cones and 21muV for green-sensitive cones.

Properties of centre-hyperpolarizing, red-sensitive bipolar cells in the turtle retina.

1. Responses of centre-hyperpolarizing, red-sensitive bipolar cells were studied by intracellular recording in the retina of the turtle, Pseudemys scripta elegans. The identity of these cells was confirmed by Procion Yellow marking. 2. Circles of light produced hyperpolarizing waves that were graded with intensity and could exceed -30mV in amplitude. The operating intensity range was similar to that of turtle cones. 3. Flashes in the form of an annulus evoked graded depolarizations which could be greater than 10 mV in the dark-adapted state or about 30mV when applied over central backgrounds. 4. Responses proportional to intensity were produced by dim circular stimuli. For radii less than about 200 mum these responses reached peak in approximately 120 msec and were invariant with respect to wave-length or area of illumination. Absolute flash sensitivity varied greatly from cell to cell but in the most sensitive cell encountered was about 460 muV photon(-1) um2. 5. Sensitivity of both bipolar cells and red-sensitive cones was enhanced progressively for enlargements of a circular flash up to 150-200 mum in radius. 6. Increasing the radius of a circle from 200 to 1250 mum caused a decrease of about 75% in bipolar cell sensitivity. This decrease was associated with a marked shortening of the response for all colours. The same enlargement decreased sensitivity of red-sensitive cones by approximately 20% and did not appreciably alter the time course of their response. These effects are attributed to impingement from type I red-sensitive horizontal cells because they have the requisite spatial and spectral properties. 7. Responses of a few bipolar cells were already shortened for 200 mum flashes; this property suggests impingement from type II horizontal cells. 8. For small circles the spectral sensitivity of the bipolar cells considered resembled closely that of red-sensitive cones or horizontal cells. Red backgrounds enhanced the relative sensitivity to green flashes suggesting that these bipolar cells receive input from red-sensitive members of double cones as well as single red-sensitive cones. 9. Steady depolarizing currents injected into bipolar cells decreased the response to either central or annular illumination; hyperpolarizing currents decreased the response to a central flash and increased the response to an annulus.

Reconstruction of the electrical responses of turtle cones to flashes and steps of light.

1. Theoretical equations which predict the electrical response of turtle cones to a wide range of light stimuli are developed from the experiments described in previous papers.2. The central points in the theory are that (a) light starts a chain of reactions leading to the production of a substance which blocks ionic channels in the outer segment, (b) an equilibrium between blocking molecules and open channels is rapidly established, (c) the blocking molecules are removed or inactivated by a chain of reactions, the first of which is autocatalytic, (d) in addition to the conductance which decreases with light there is also a conductance which increases with a delay when the cone is hyperpolarized.3. Parameters in the theory were deduced by approximate equations from the experiments described in the previous papers.4. There was good agreement between the properties of real and model cones in the following cases: (a) the response to 10 msec flashes and 0.7 sec steps of light calculated to give between 20 and 5 x 10(7) photoisomerizations per cone at times extending to about 2 sec; (b) the complicated changes in the response to a test flash that occur when it is superposed on background lights of increasing intensity; (c) the after-hyperpolarization and period of reduced sensitivity following a strong flash.5. The main defect of the theory is that the effect of background light in shortening the time to maximum of the response to a flash was more pronounced in a real cone than in the model.