Actions of EGTA and high calcium on the cones in the turtle retina.

1. The effects of Ca2+ on the activity of retinal cones were investigated by recording intracellular responses from turtle retinae perfused with solutions containing different Ca2+ concentrations. 2. Lowering extracellular Ca2+ concentration with EGTA increased membrane conductance and evoked depolarization in darkness. 3. Responses to bright lights were increased by EGTA by an amount equal to the depolarization in darkness; responses to dim lights instead were usually reduced. In addition EGTA modified the time course of responses to flashes. These changes were especially evident during the later phases of the response. 4. A steady light applied during EGTA perfusion can restore the original membrane potential. The kinetics of responses in these conditions, however, differ profoundly from those prevailing Ca2+ concentrations evoked hyperpolarization of the cones and decreased the amplitude of their responses to bright lights. The time course of responses to flashes was not appreciably modified by high Ca2+. 6. A steady light which hyperpolarizes the cone membrane by the same amount as high Ca2+ has an equal effect on the amplitude of responses to bright flashes but has an entirely different action on response kinetics and on the amplitude of responses to dimmer flashes. 7. The effects of EGTA on resting potential, conductance and response amplitudecan be interpreted assuming that the drug opens channels which are normally closed by Ca2+ in darkness. Conversely, the changes caused by high Ca2+ suggest that raising the concentration of outside Ca2+ reduces the number of light-sensitive channels open in darkness. 8. The observation that Ca2+ and light have different effects on kinetics and sensitivities is difficult to reconcile with the hypothesis that Ca2+ is the substance liberated by light to cause photoresponses.

Responses of hair cells in the statocyst of Hermissenda.

Responses to mechanical stimulation were recorded from hair cells in the statocyst of Hermissenda crassicornis. The response to a brief stimulus is a depolarizing wave which reaches peak in about 25 msec and decays slowly. 2. Hyperpolarization by extrinsic currents increases the amplitude of the response; depolarization decreases it and eventually reverses its polarity. It is inferred from these results that the primary outcome of the transduction process is an increase of membrane conductance and that the voltage change (generator potential) follows as a secondary event. 3. The features of the conductance change were reconstructed from the time course of the generator potential and the passive properties of the membrane. It was found that the increase of membrane conductance develops slowly and is roughly proportional to the energy delivered by the stimulus. 4. The time course of the conductance change required to reproduce the generator potential is similar to the output of a model involving a sequence of transformations. 5. The generator potential is sensitive to temperature, becoming faster as temperature is raised. This effect is reproduced by the model if the transition rates are assumed to be temperature-dependent, with a Q10 of about 2. 6. It is concluded that a chain of temperature-sensitive processes is interposed between the stimulus and the increase of membrane conductance.

Interactions leading to horizontal cell responses in the turtle retina.

1. Small responses to large fields of dim monochromatic lights were recorded intracellularly from luminosity horizontal cells (L-cells), chromaticity horizontal cells (C-cells) and cones in the retinae of turtles, Pseudemys scripta elegans.2. Responses of cones to brief flashes applied over steady backgrounds were studied in order to interpret the corresponding responses of horizontal cells. Steady red or green backgrounds make the responses of red-sensitive cones smaller, faster and often diphasic. Green backgrounds have similar effects on the responses of green-sensitive cones to green flashes, but red backgrounds do not change them appreciably. Responses of double cones have properties intermediate between those of red and green cones.3. L-cells of both type I and type II are hyperpolarized by all visible wave-lengths, and their spectral sensitivity in the linear range resembles that of red cones. Their responses are not invariant with respect to colour, and their sensitivity to green relative to red stimuli increases during red backgrounds. These properties suggest that L-cells are activated mainly by red cones but also receive impingement from the red members of double cones.4. Spectral properties of red/green C-cells resemble those of green cones as modified by the recurrent action of L-cells. They can be explained assuming that red/green C-cells receive their principal impingement from green cones and subsidiary interactions from green/blue C-cells and the green members of double comes.5. The spectral sensitivity of the hyperpolarizing responses of green/blue C-cells is ascribed to impingement from blue cones. Their depolarizing responses have complex properties which suggest that they are brought about by the activity of both L-cells (probably through the blue cones) and red/green C-cells.6. It is concluded that the main properties of the responses of the horizontal cells can be explained by a simple circuit in which each horizontal cell is connected to a corresponding type of cone and the L-cells have a recurrent impingement on all cones. The scheme is modified by additional interactions which operate on the responses of each horizontal cell type.

Colour-dependence of cone responses in the turtle retina.

1. Responses to monochromatic lights were recorded intracellularly from red cones, green cones, and luminosity horizontal cells (L-cells) in the retinae of turtles.2. Both types of cones responded to small fields of illumination with graded hyperpolarizations. Red cones were only moderately more sensitive to deep red (680 nm) than to green (550 nm) light while green cones were much more sensitive to the green light than to the red. L-cells produced small responses for flashes of either colour covering small fields.3. Stimulation of large fields with monochromatic lights of moderate or high intensity evoked large L-cell responses and composite responses in cones. These latter include the hyperpolarizing action of the light absorbed by the cone itself (direct response), its enhancement by illumination of the near surround, and the depolarizing effects of L-cell feed-back.4. L-cells respond primarily to the activity of red cones; with sufficient intensity of the light, however, their responses are influenced also by green cones. As a result, if a red and a green light stimulate red cones equally, the L-cell response is larger for the green stimulus.5. Green cones were depolarized by deep red lights of moderate intensity applied over large fields. These depolarizing responses include oscillations which follow closely oscillations in L-cells. Green light applied to the same large fields produced hyperpolarization of green cones.6. Red cones were hyperpolarized by red or green light covering large fields, but the time course of their responses differed for the two colours, reflecting a corresponding difference in L-cell activity.7. Red light in the form of an annulus produced large responses in central L-cells without eliciting direct responses in central green cones. In these conditions green cones developed depolarizing waves which included a large, sharp transient.8. It is concluded from these and other results that the direct response of each cone is modified by two interactions: enhancement only from nearby cones of the same colour and depression controlled (through L-cell feed-back) by cones of all colours. In this way the response of any cone will change as the proportion of responses in cones of different colours changes, this proportion being a function of the wave-length of the light.

Analysis of responses in visual cells of the leech.

1. Potentials were recorded from the cytoplasm and from the vacuole of leech photoreceptors. Since the vacuole is lined with microvilli and is connected to the outside by narrow channels, the potential drops between vacuole and outside measure the current through the microvillar membrane.2. In darkness, the potential of the cytoplasm with respect to the outside is about - 45 mV while the potential of the vacuole is approximately zero.3. Following illumination the negativity of the cytoplasm decreases and the vacuole becomes negative relative to the outside.4. For dim intensities, the response to a flash of light may grow proportionately more than the intensity of the flash. This is probably due to development of a depolarizing local response.5. The resistance from the cytoplasm to the outside was about 150 MOmega in darkness and decreased to approximately 40 MOmega at the peak of the response to a bright flash (on average). Corresponding measurements from the vacuole gave 50 MOmega in darkness and 35 MOmega at the peak of the response.6. Charging curves produced by steps of constant currents applied to the cytoplasm or to the vacuole include two time constants (about 5 and 50 msec on average). The longer time constant decreases greatly with bright illumination.7. The results are consistent with the interpretation that the response to light is brought about by an increase of conductance of the microvillar membrane.

Responses of photoreceptors in Hermissenda.

The five photoreceptors in the eye of the mollusc Hermissenda crassicornis respond to light with depolarization and firing of impulses. The impulses of any one cell inhibit other cells, but the degree of inhibition differs in different pairs. Evidence is presented to show that the interactions occur at terminal branches of the photoreceptor axons, inside the cerebropleural ganglion. Properties of the generator potential are examined and it is shown that the depolarization develops in two phases which are affected differently by extrinsic currents. Finally, it is shown that by enhancing the differences in the responses of individual cells to a variety of stimuli, the interactions may facilitate a number of simple discriminations.

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.

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.

The site of origin of electrical responses in visual cells of the leech, Hirudo medicinalis.

In leech visual cells the presumed light-absorbing structures are microvilli arising from the membrane of what would seem to be a large intracellular vacuole. This vacuole, however, is an extracellular compartment, since it communicates with the intercellular spaces through narrow channels. Therefore, the membrane of the microvilli is-as in other invertebrate visual cells-a part of the cell membrane. Visual responses recorded with an electrode within the vacuole were compared with the intracellular recordings. Following illumination the vacuole becomes negative with respect to the outside fluid, while the cells are depolarized. This finding indicates that inward current penetrates the cell through the microvillar membrane. It is concluded, therefore, that the electrical response (receptor potential) originates as a result of changes in the properties of the light-absorbing membrane.

Responses to single photons in virual cells of limulus.

1. A system proposed in a previous article as a model of responses of visual cells has been analysed with the purpose of predicting the features of responses to single absorbed photons.2. As a result of this analysis, the stochastic variability of responses has been expressed as a function of the amplification of the system.3. The theoretical predictions have been compared to the results obtained by recording electrical responses of visual cells of Limulus to flashes delivering only few photons.4. Experimental responses to single photons have been tentatively identified and it was shown that the stochastic variability of these responses is similar to that predicted for a model with a multiplication factor of at least twenty-five.5. These results lead to the conclusion that the processes responsible for visual responses incorporate some form of amplification. This conclusion may prove useful for identifying the physical mechanisms underlying the transducer action of visual cells.