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Biomedical subjects

D A Baylor

Publications and source records attributed to D A Baylor.

At least 73 records · Page 4Linked to original sources
PubMed1979

The membrane current of single rod outer segments.

1. Outer segments of individual rods in the retina of the toad, Bufo marinus, were drawn into a glass pipette to record the membrane current. 2. Light flashes evoked transient outward currents. The peak response amplitude was related to flash intensity by a Michaelis equation with half-saturating intensity about 1 photon mum-2. 3. The saturating response amplitude ranged up to 27 pA and corresponded closely to complete suppression of the steady inward current present in darkness. 4. For a given cell the saturating response amplitude varied linearly with the length of outer segment within the pipette. This is consistent with a uniform density of light-sensitive channels and negligible gradient of membrane potential along the outer segment. 5. Responses to bright flashes never showed the relaxation from an initial peak seen previously in intracellular voltage recordings, suggesting that the conductance change responsible for the relaxation does not occur in the outer segment. 6. Responses to local illumination of only the recorded outer segment were very similar to those obtained with diffuse light at the same intensity, indicating that peripheral rods made little contribution to the responses. 7. The spectral sensitivity of 'red' rods was consistent with a retinal1-based pigment with lambda max = 498 +/- 2 nm. 8. The kinetics of the response were consistent with four stages of delay affecting action of the internal transmitter. Responses were faster at the basal end of the outer segment than at the distal tip. 9. Background light reduced the sensitivity to a superposed dim test flash and shortened the time course of the response, indicating that adapting light modifies the kinetics and gain of the transduction mechanism within the outer segment. 10. Responses to dim lights exhibited pronounced fluctuations which are attributed in the succeeding paper (Baylor, Lamb & Yau, 1979) to the quantal nature of light.

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PubMed1979

Responses of retinal rods to single photons.

1. A suction electrode was used to record the membrane current of single rod outer segments in pieces of toad retina. During dim illumination the membrane current showed pronounced fluctuations. 2. Amplitude histograms of responses to dim flashes of fixed intensity exhibited two discrete peaks, one at 0 pA and one near 1 pA, suggesting that the response was quantized. By setting a criterion amplitude level, flash responses could be classed as 'failures' (no response) or as 'successes' (at least one quantal event). 3. The variation of fraction of successes with flash intensity was consistent with the hypothesis that each quantal electrical event resulted from a single photoisomerization. 4. The quantal event had a mean amplitude of about 1 pA (5% of the standing dark current) and a standard deviation of 0.2 pA. Dispersion in the event amplitude prevented identification of histogram peaks corresponding to two or more photoisomerizations. 5. Individual quantal responses exhibited a smooth shape very similar to that of the average quantal response. This suggests that a single photoisomerization releases many particles of transmitter and that radial diffusion of internal transmitter is not a major source of delay in the light response. 6. The 'quantum efficiency' with which an absorbed photon generated an electrical event was measured as 0.5 +/- 0.1 (S.E. of mean, n = 4). This is slightly lower than the quantum efficiency of photoisomerization obtained previously for rhodopsin in solution. 7. At wavelengths between 420 and 700 nm the quantal event was invariant in size, although the cell's sensitivity varied over a range of 10(5). 8. The power spectrum of the fluctuations in dim steady light was predicted by assuming that a random series of quantal events occurred independently. 9. In brighter light the fluctuations were faster, and the response to an incremental flash was reduced in size and duration. The power spectrum could be predicted by assuming random superposition of events with the shape of the incremental flash response.

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PubMed1979

Synaptic drive and impulse generation in ganglion cells of turtle retina.

1. Light reponses and electrical constants of ganglion cells in the retina of the turtle were examined by intracellular recording in eyecup preparations. 2. In 'on', 'off', and 'on/off' cells, the impulses produced by illumination of the centre of the receptive field arose from slow synaptic depolarizations. The ganglion cells also exhibited inhibitory synaptic potentials. 3. The synaptic depolarization evoked by a step change in light intensity rose more slowly than the response of the cones in which the excitation originated, and the depolarization then declined in spite of a well maintained cone response. This behaviour is consistent with the notion advanced previously that, during transmission to ganglion cells, receptor signals are relayed through the equivalent of a bandpass filter. 4. The e.p.s.p.s evoked by light grew when the membrane was hyperpolarized by injected current and decreased when the membrane was depolarized. The i.p.s.p.s reversed at a level slightly negative to the resting potential in darkness. 5. In neither 'on' nor 'off' ganglion cells did the synaptic potentials evoked by step changes in illumination show the hyperpolarizing phases expected of a linear filter. The absence of hyperpolarizations is consistent with a rectification which permits transmission of depolarizations but not hyperpolarizations from bipolar to ganglion cells. 6. In darkness the membrane potential of some ganglion cells showed random depolarizations which brought the potential near the threshold for impulse generation. 7. With very small spots in the receptive field centre the 'on' responses of ganglion cells to flashes and steps of light grew approximately linearly with stimulus intensity. The step reponse was not, however, related to the flash response by superposition. Larger spots in the field centre gave responses which grew non-linearly with the intensity of even dim stimuli. 8 Depolarizing current passed through the recording electrode elicited a repetitive discharge of impulses. The frequency of firing increased linearly with current strength above a rheobase value of about 10(-10) A. Accommodation occurred during steady currents, the main decline taking place with a time constant of about 15 msec. 9. Strength-latency measurements and bridge records of ganglion cell charging by constant currents gave time constants of 10--20 msec and input resistances of 100--150 M omega.

Action Potentials↗
PubMed1978

Properties of the membrane current of rod outer segments.

The membrane current of single rod outer segments in pieces of isolated toad retina was recorded with a glass suction electrode. Light evoked a slow net outward photocurrent consisting of a reduction in the steady inward dark current. In very dim light, the photocurrent broke up into discrete shot effects with a rounded shape and an amplitude of about 1 pA. These events were shown to result from photoisomerization of single rhodopsin molecules. The current in darkness showed fluctuations consisting of (a) discrete events apparently resulting from thermal isomerization of rhodopsin molecules, and (b) smaller amplitude shot effects shaped by two of the four rate processes of the light response.

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PubMed1977

Transmission from photoreceptors to ganglion cells in turtle retina.

1. Synaptic transfer between photoreceptors and impulse-generating cells was studied in isolated eyecups from turtles. Single red-sensitive cones or rods were stimulated by current passed through an intracellular electrode, and impulses generated by the resulting synaptic action were recorded with an external micro-electrode. This technique permits study of retinal transmission without the operation of the visual transduction mechanism. Antidromic stimulation of the optic nerve indicated that most of the impulse-generating cells were ganglion cells.2. Individual ganglion cells responded transiently to changes in the membrane potential of a receptor and could be classified into three groups on the basis of the direction of the effective change in potential. Off centre ganglion cells responded selectively to depolarizations of a receptor, while on centre ganglion cells responded selectively to hyperpolarizations. On-off ganglion cells responded to both depolarizations and hyperpolarizations of a receptor.3. Ganglion cells gave the same pattern of response to electrical hyperpolarization of a receptor and to light in the centre of their receptive fields. Subthreshold depolarizing currents passed in a receptor antagonized the ganglion cell's response to light, and subthreshold hyperpolarizing currents reinforced the response. These observations are consistent with the view that the hyperpolarization generated by visual transduction is responsible for regulating the release of transmitter at the first retinal synapse.4. When a receptor was stimulated with weak current pulses of fixed intensity the number and latency of the ganglion cell impulses fluctuated randomly in successive trials. The relation between the fraction of trials yielding a response and the stimulus intensity was broad. These results indicate that the link between retinal input and output is noisy.5. In the most sensitive pairs of cells, a response of one or more impulses could be obtained in half the trials with a current of about 2 x 10(-11) A, which changed the potential of the receptor by 1-2 mV. A current of similar magnitude would be developed by about 130 photoisomerizations in a red-sensitive cone or 50 photoisomerizations in a rod.6. Dim background light producing a steady hyperpolarization of a few millivolts in the rods raised the threshold for electrically-evoked transmission from a rod to a ganglion cell. In experiments on red-sensitive cones, background light raised the threshold in the off pathway, in which depolarization was the effective stimulus, and lowered the threshold in the on pathway, in which hyperpolarization was the effective stimulus. These changes in sensitivity were not accompanied by obvious changes in the input resistance of the stimulated receptor. Regulation of retinal sensitivity in background light thus involves changes in synaptic transfer as well as changes in the sensitivity of the visual transduction mechanism.

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PubMed1977

Kinetics of synaptic transfer from receptors to ganglion cells in turtle retina.

1. Synaptic transfer between the retinal input and output was studied in turtle eyecups by injecting rectangular current pulses into a single cone or rod while recording externally from a ganglion cell.2. When a receptor was activated with weak steps of polarizing current, the probability of obtaining a ganglion cell impulse rose after an S-shaped delay to a peak at about 0.1 sec and then declined. This suggests that the transmission chain behaves like an electrical band-pass filter containing delay and differentiating elements.3. To further characterize the kinetics of excitation in the subthreshold region, the duration and polarity of the polarizing current pulses were varied while determining the magnitude of the threshold current and the delay to the ganglion cell impulses. The results of these experiments were described with linear models which assume that synaptic transfer occurs over a cascade of first-order delay stages and a single differentiating stage.4. The pathways which relay off responses to light from rods and red-sensitive cones were formally similar, but the time scale in the rod path was several times slower. The path carrying off responses from the red-sensitive cones was faster than the on path. These kinetic differences indicate that independent pathways mediate each of the three categories of response and suggest that the kinetics of each path are ;matched' to the input signals generated by light.5. The strength-latency relations for the responses of on-centre ganglion cells to flashes and steps of light were approximately predicted from the description of synaptic transfer developed here and the description of visual transduction in red-sensitive cones from a previous study.6. It is suggested that the retinal paths have kinetics which might be useful in transmitting light-evoked signals whilst attenuating noise present near the input.

Action Potentials↗
PubMed1976

Transmission of signals from photoreceptors to ganglion cells in the eye of the turtle.

Synaptic transfer between receptors and ganglion cells was studied in the retina of the turtle. In the normal operation of this pathway, signals are relayed across two or more chemical synapses. Previous work indicates that the receptors give graded hyperpolarization to light, and that the bipolar cells also respond with graded potential changes; the ganglion cells give impulses. Injection of weak electrical currents into a single receptor provides an alternative means of activating the pathway. We have used this technique to examine (1) the role of the receptors' hyperpolarization in signal transmission and (2) the sensitivity and temporal properties of the pathway. The response of a ganglion cell evoked by light on the retina can be duplicated by electrically hyperpolarizing a receptor and antagonized by electrical depolarization. This indicates that the hyperpolarization is responsible for regulating the flow of information from the receptors to the second-order retinal cells. The sensitivity of the synaptic path from cones to ganglion cells, determined from electrical stimulation of single receptors, was found to be sufficient to permit detection of about 100 photoisomerizations. Using the same technique, the kinetics of transfer were studied and found to exhibit a delay or integrating process and also a slow differentiation that blocks steady-state transmission. For the path from red-sensitive cones to ganglion cells, the apparent time constant of the delay process was of the order of 75 ms and that of the differentiation, about 100 ms. In the path from rods to ganglion cells, the differentiation was several times slower.

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PubMed1975

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.

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PubMed1974

The electrical response of turtle cones to flashes and steps of light.

1. The linear response of turtle cones to weak flashes or steps of light was usually well fitted by equations based on a chain of six or seven reactions with time constants varying over about a 6-fold range.2. The temperature coefficient (Q(10)) of the reciprocal of the time to peak of the response to a flash was 1.8 (15-25 degrees C), corresponding to an activation energy of 10 kcal/mole.3. Electrical measurements with one internal electrode and a balancing circuit gave the following results on red-sensitive cones of high resistance: resistance across cell surface in dark 50-170 MOmega; time constant in dark 4-6.5 msec. The effect of a bright light was to increase the resistance and time constant by 10-30%.4. If the cell time constant, resting potential and maximum hyperpolarization are known, the fraction of ionic channels blocked by light at any instant can be calculated from the hyperpolarization and its rate of change. At times less than 50 msec the shape of this relation is consistent with the idea that the concentration of a blocking molecule which varies linearly with light intensity is in equilibrium with the fraction of ionic channels blocked.5. The rising phase of the response to flashes and steps of light covering a 10(5)-fold range of intensities is well fitted by a theory in which the essential assumptions are that (i) light starts a linear chain of reactions leading to the production of a substance which blocks ionic channels in the outer segment, (ii) an equilibrium between the blocking molecules and unblocked channels is established rapidly, and (iii) the electrical properties of the cell can be represented by a simple circuit with a time constant in the dark of about 6 msec.6. Deviations from the simple theory which occur after 50 msec are attributed partly to a time-dependent desensitization mechanism and partly to a change in saturation potential resulting from a voltage-dependent change in conductance.7. The existence of several components in the relaxation of the potential to its resting level can be explained by supposing that the ;substance' which blocks light sensitive ionic channels is inactivated in a series of steps.

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PubMed1974

Changes in time scale and sensitivity in turtle photoreceptors.

1. In turtle cones the steady-state relation between the internal potential and log light intensity was much flatter in the steady state than it was at 30 msec after the beginning of a step of light; this is attributed to a desensitization which develops with a delay of 50-100 msec.2. When a weak flash was superposed on a steady background light which hyperpolarized the cone by 3-6 mV the amplitude of the linear response to a flash was reduced to 1/e and the time to maximum was shortened from about 110 to 70 msec; the response also became diphasic. With stronger background lights the flash sensitivity continued to fall, but the time to maximum did not become shorter than 40-50 msec and lengthened again with very strong lights.3. In cones the flash sensitivity S(F) was reduced to half its dark value S(F) (D) by a light intensity of 1/S(F) (D)zeta where zeta is about 20 sec/V.4. At low levels of background light, about two-thirds of the change in sensitivity was time-dependent and one-third was attributable to the ;instantaneous non-linearity' described in the previous paper.5. The reduction in time to peak and the decrease in sensitivity produced by a background light which hyperpolarized by about 3 mV was little affected by changing the diameter of the area illuminated from 12 to 800 mum.6. An experiment with a rod showed that a very weak light which hyperpolarized by only 0.5 mV decreased the linear response to 1/e and shortened the time to maximum from 300 to 180 msec.7. With weak or moderate flashes the time-dependent desensitization lagged behind the potential by 50-100 msec.8. The desensitization and shortening of time scale which persisted after a flash or step were associated with an after-hyperpolarization. The relaxation of potential, sensitivity and time scale became slower as the preceding illumination was increased from 10(3) to 10(10) photons mum(-2); the increase seemed to occur in steps involving components which relaxed with time constants of the order of 0.1, 1, 10 and 100 sec. A rebound phenomenon was observed after steps longer than 30 sec and with equivalent intensities greater than 10(5) photons mum(-2) sec(-1).9. Several of the observations are explained by a hypothesis in which the central assumption is that the particles which block the ionic channels are degraded or removed by an autocatalytic reaction.

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PubMed1974

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.

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PubMed1973

Detection and resolution of visual stimuli by turtle photoreceptors.

1. Hyperpolarizing responses up to 30 mV in amplitude were recorded from cones and from certain cells believed to be rods in the isolated retina of the swamp turtle, Pseudemys scripta elegans.2. The responses evoked by weak flashes of light reach their maximum in 100-140 msec in red-sensitive cones, 140-180 msec in green-sensitive cones, and 300-600 msec in the rod-like cells (20 degrees C).3. The cone response evoked by weak flashes of light is linearly related to light intensity and obeys the superposition principle in that the response to a very weak step of light is the integral of the response to a very weak flash.4. On the basis of their spectral sensitivities cones can be divided into three distinct classes, namely red-sensitive cones whose relative quantum sensitivity is maximal at 630 nm, green-sensitive cones with a maximal sensitivity at 550 nm and blue-sensitive cones with a maximum at 460 nm.5. The difference between the spectral sensitivity of rods with a maximum at about 520 nm and green-sensitive cones (lambda(max) = 550 nm) is consistent with the view that both receptors contain a 518(2) retinal pigment as reported by Liebman & Granda, but that light is filtered by an orange oil droplet in green-sensitive cones.6. The spectral sensitivities of both red- and green-sensitive cones agree well amongst themselves at long wave-lengths but differ markedly in the extent of the reduction at short wave-lengths. This variation is attributed to differences in the extent to which light is filtered through the coloured oil droplets.7. There is a significant positive correlation between the absolute sensitivity of red- and green-sensitive cones and the reduction in sensitivity at short wave-lengths. This would be explained if a greater fraction of the light passes through the oil droplet in the most sensitive cells.8. The absolute flash sensitivities of the most sensitive receptors were about 250 muV photon(-1) mum(2) in red- and green-sensitive cones, 120 muV photon(-1) mum(2) in blue-sensitive cones, and 1300 muV photon(-1) mum(2) in rods.9. If the effective collecting area (which includes factors for absorption etc.) is taken as 10 mum(2) in a red-sensitive cone the peak hyperpolarization produced by 1 photon would average 25 muV.10. Provided that small spots of light are used, individual receptors obey the ;univariance principle' and the response produced by light of strength I', and wave-length lambda(1) can be matched by a light of strength kI' and wave-length lambda(2), where k is the same for all values of I'.11. A small proportion of cones behave like isolated units in that they have very sharp sensitivity-profiles and obey the univariance principle with respect to the position as well as to the wave-length of light.12. The majority of red and green cones have more diffuse sensitivity-profiles, sometimes with bumps on the descending limb, and behave as though cones with the same spectral sensitivity were electrically coupled to one another.13. The relation between the area of illumination and flash sensitivity agreed approximately with that calculated from the spatial profile.

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PubMed1971

Receptive fields of cones in the retina of the turtle.

1. Intracellular recordings have been made of the responses to light of single cones in the retina of the turtle. The shape of the hyperpolarizing response to a flash depends on the pattern of retinal illumination as well as the stimulus intensity.2. Although changes in the stimulus pattern can produce changes in the effective stimulus intensity, the responses to certain patterns cannot be matched by any adjustment of stimulus intensity.3. The initial portion of responses to large or small stimulating spots is proportional to light intensity; this allows comparison of responses when the amount of light on a cone is kept constant but the light on surrounding cones is changed. For equal light intensity on the cone, the response to a spot 2 or 4 mu in radius is smaller than that to a spot 70 mu in radius.4. Responses to spots 70 and 600 mu in radius coincide over their rising phases and peaks without any adjustment of stimulus intensity. The responses to the larger spot, however, contain a delayed depolarization not present with the smaller spot.5. During steady illumination of a cone with a small central spot, the response to transient illumination superimposed on the same area is greatly reduced. Illumination of cones in the near surround, however, produces a hyperpolarizing response, and illumination of cones in the more distant surround generates a delayed depolarization.6. The results described above suggested that synaptic signals might impinge on cones. This possibility was tested by electrically polarizing one retinal cell while recording from another.7. Currents passed through a cone within 40 mu of another cone can change the membrane potential of the latter. Not all cones within this distance show the interaction, however, and it has never been detected at distances greater than 50 mu.8. Hyperpolarization of a horizontal cell with applied current can produce a depolarization of a cone in the vicinity. During this depolarization, the response of the cone to a flash is reduced in size and altered in shape.9. It is concluded that the response of a cone to light may be modified by synaptic mechanisms which are activated by peripheral illumination.

Action Potentials↗
PubMed1970

Electrical responses of single cones in the retina of the turtle.

1. Intracellular recordings have been made from single photoreceptors in the retina of the turtle. Histological sections of the retina made after injection of dye through the recording electrode reveal dye in the inner segments of single cones.2. Following a brief flash of light the cone undergoes a hyperpolarization which is graded with the intensity of the flash.3. The excitatory receptive field of a receptor is probably as small as the cross-section of a single cone, but accurate measurements are rendered difficult by scattering of light within the retina.4. The voltage drop produced by a current injected into the cell is increased during the response to light. Steady hyperpolarizing currents increase the size of the response to light; depolarizing currents of increasing strength reduce and then reverse the response.5. The results are consistent with the hypothesis that light activates the visual cell by decreasing the permeability of membrane channels which in darkness act as a shunt of the membrane.

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