Light-induced Voltage Research Articles

Light adaptation in Drosophila photoreceptors: I. Response dynamics and signaling efficiency at 25 degrees C.

Besides the physical limits imposed on photon absorption, the coprocessing of visual information by the phototransduction cascade and photoreceptor membrane determines the fidelity of photoreceptor signaling. We investigated the response dynamics and signaling efficiency of Drosophila photoreceptors to natural-like fluctuating light contrast stimulation and intracellular current injection when the cells were adapted over a 4-log unit light intensity range at 25 degrees C. This dual stimulation allowed us to characterize how an increase in the mean light intensity causes the phototransduction cascade and photoreceptor membrane to produce larger, faster and increasingly accurate voltage responses to a given contrast. Using signal and noise analysis, this appears to be associated with an increased summation of smaller and faster elementary responses (i.e., bumps), whose latency distribution stays relatively unchanged at different mean light intensity levels. As the phototransduction cascade increases, the size and speed of the signals (light current) at higher adapting backgrounds and, in conjunction with the photoreceptor membrane, reduces the light-induced voltage noise, and the photoreceptor signal-to-noise ratio improves and extends to a higher bandwidth. Because the voltage responses to light contrasts are much slower than those evoked by current injection, the photoreceptor membrane does not limit the speed of the phototransduction cascade, but it does filter the associated high frequency noise. The photoreceptor information capacity increases with light adaptation and starts to saturate at approximately 200 bits/s as the speed of the chemical reactions inside a fixed number of transduction units, possibly microvilli, is approaching its maximum.

Charge displacements in interfacial layers containing reaction centers.

Reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides were oriented in phospholipid interfacial layers adsorbed to a Teflon film separating two electrolyte-filled compartments of a Teflon cell. Light-induced voltage changes were measured as a function of time across electrodes immersed in the cell compartments. The experimental system is characterized both experimentally and theoretically to relate the measured signals to the light-induced displacement currents in the reaction centers. Mathematical relations between the measured signals and the distances and geometries of the charge-transfer reactions are derived. At pH 8.0 the reaction centers were found to be oriented with approximately 60% of the population oriented with the donor facing the aqueous phase. The density of the reaction centers in the layer was approximately 10(11) cm-2, which is close to that found in the native system. Reconstitution of the secondary quinone, QB, in 90% of the RCs was achieved with an approximately 100-fold excess of ubiquinone in the vesicle preparation.

Effect of temperature and surface potential on the electrogenic proton uptake in the Q B site of the Rhodobacter sphaeroides photosynthetic reaction center: Q A −. B −. → Q A Q B H 2 transition

Direct electrometry was used to study the light-induced voltage changes in the Rhodobacter sphaeroideschromatophores adsorbed to a phospholipid-impregnated nitrocellulose film. After the second laser flash, a fast increase in the voltage associated with charge separation was followed by a slower increase attributed to the proton uptake in the QB site of the photosynthetic reaction centers. Kinetics and relative amplitudes of these voltage changes attributed to the QA QB ! QAQBH2 transition, were measured as a function of pH and temperature between +4 and +40C. The kinetics can be approximated by a single exponent above +23C (100 sa t +25C, pH 7.2), whereas below this temperature, it was a good fit of two exponential approximation (65 s and 360s with similar contributions at +10C, pH 7.2). The faster component diminished with an apparent pK8.5, whereas the slower one was maintained at a constant level until pH9.5 and then decreased. The calculated activation energy from the temperature dependence of the slower component (55 ‐ 65 kJ/mol) was much higher than that of the faster component (< 10 kJ/mol). The two voltage components can be attributed to the transfer of the first (faster component) and the second (slower component) proton from the reaction center surface to QB. We suggested that higher activation energy of the slower component was due to a conformational change in the reaction center kinetically coupled to the second proton transfer to Q BH . The faster component diminished in the presence of 1 M KCl, with an apparent pK7.5. To explain this observation, we assume that: (i) the midpoint potential of the QA/QA redox pair was higher in 1 M KCl because of the reduced surface potential of chromatophores; (ii) the midpoint potential of the Q B /QBH redox pair was insensitive to the surface potential change; (iii) the equilibrium constant of the reaction Q A QB $ QAQBH decreased at high ionic strength.

Light-induced electrogenic events associated with proton uptake upon forming Q B− in bacterial wild-type and mutant reaction centers

Light-induced voltage changes (electrogenic events) were measured in wild-type and site-directed mutants of reaction centers (RCs) from Rhodobacter sphaeroides oriented in a lipid monolayer adsorbed to a Teflon film. A rapid increase in voltage associated with charge separation was followed by a slower increase attributed to proton transfer from solution to protonatable amino-acid residues in the vicinity of the Q B site. In native reaction centers the proton-transfer voltage had a pH-dependent amplitude with two peaks at pH 4.5 and pH 9.7, respectively. In the Glu-L212→Gln RCs the high-pH peak was absent, whereas in the Asp-L213→Asn RCs the low-pH peak was absent and the high-pH peak was shifted to lower pH by about 1.3 pH units. The amplitudes of the electrogenic phases as a function of pH follow approximately the measured proton uptake from solution (P.H. McPherson, M.Y. Okamura, G. Feher, Biochim. Biophys. Acta, vol. 934, 1988, pp. 348–368) and are ascribed to proton transfer to amino acid residues upon Q B − formation. The peak around pH 9.7 is ascribed to proton uptake predominantly by Glu-L212 and the peak around pH 4.5 to proton uptake predominantly by Asp-L213 or a residue strongly interacting with Asp-L213.

Electronic processes in supported bilayer lipid membranes (s-BLMs) containing a geodesic form of carbon (fullerene C 60)

The light-induced voltage and current generated by doping a self-assembled and supported bilayer lipid membrane (s-BLM) with buckminsterfullerene C 60 were investigated. Using these s-BLMs, it is possible to construct sensor probes and to investigate redox reactions and light-induced electron transfer across the lipid bilayer. The C 60-containing s-BLM, considered basically as a “molecular device”, is a light-sensitive dipole capable of photoinduced charge separation which undergoes redox reactions across the substrate-hydrophobic lipid bilayer-aqueous solution junctions. Using the cyclic voltammetry technique, our results show that C 60 embedded in the BLM acts as an excellent electron carrier/mediator and should be useful for electrochemical biosensor and molecular electronics device development. It is concluded that supported BLMs with excellent stability, possessing the structural and dynamic properties of conventional planar lipid bilayers, are excellent models for investigating basic membrane-mediated ligand-receptor contact interactions. For practical applications, the s-BLMs are ideal systems for incorporating a host compounds, including fullerenes, semiconducting nanoparticles, receptor proteins and polymeric materials.

Effects of Anion Channel Inhibitors on Light-Induced Potential Changes in the Liverwort Conocephalum conicum

The effect of three different anion channel inhibitors, namely (5-nitro-2-3-phenylpropyl-amino)benzoic acid (NPPB), Zn 2+ and anthracene-9-carboxylic acid (A-9-C) on the action potential in the liverwort Conocephalum conicum were tested. All three caused an increase of the excitability threshold and a decrease of action potential amplitudes. This confirms the involvement of anion channels in the action potentials in Conocephalum. In plants treated with 1 or 2 mM A-9-C but not with NPPB (50 or 100 //M) and Zn 2+ (100 or 500 //M), a light-induced transient depolarization occurred. In contrast to action potentials, the amplitude of this voltage transient depended on the light intensity and on the duration of preceding dark period. Also in contrast to action potentials, which are blocked by TEA, when applied together with A-9-C, TEA even increased the amplitudes of the light-induced voltage transients to up to 170 mV. The depolarization was obviously limited by the voltage-dependent opening of K + channels in the absence of TEA. The amplitude of the light-induced voltage transients (in the presence of TEA) increased in elevated CaCI2 concentrations pointing to a Ca 2+ permeability giving rise to the depolarization. However, none of the Ca 2+ channel blockers tested, La 3+ , Gd 3+ , nifedipine, verapamil or diltiazem, had an effect. The light-induced voltage transients in A-9-C treated plants are quite different from light- and electrically triggered action potentials but share some similarities with light-induced generator potentials.

Electron and optical beam testing of integrated circuits using CIVA, LIVA, and LECIVA

Charge-Induced Voltage Alteration (CIVA), Light-Induced Voltage Alteration, (LIVA), and Low Energy CIVA (LECIVA) are three new failure analysis imaging techniques developed to quickly localize defects on ICs [1–3]. All three techniques utilize the voltage fluctuations of a constant current power supply as an electron or photon beam is scanned across an IC. CIVA and LECIVA yield rapid localization of open interconnections on ICs. LIVA allows quick localization of open-circuited and damaged semiconductor junctions. LIVA can also be used to image transistor logic states and can be performed from the backside of ICs with an infrared laser source. The physics of signal generation for each technique and examples of their use in failure analysis are described.

Trypsin Treatment of Reaction Centers from Rhodobacter sphaeroides in the Dark and under Illumination: Protein Structural Changes follow Charge Separation

Reaction centers from Rhodobacter sphaeroides R-26 were treated with trypsin in the dark and during illumination (in the charge-separated state). Trypsination resulted in a time-dependent modification of the reaction centers, reflected in changes in the charge recombination rate, in the inhibition of QA- to QB electron transfer, and eventually to inhibition of charge separation. Comparisons of centers with ubiquinone or anthraquinone in the QA site, in which the charge recombination pathways are different, indicate that trypsination affects charges close to the QA(-)-binding site. Studies of light-induced voltage changes from moving charges in reaction centers incorporated in lipid layers on a Teflon film, a technique which allows the discrimination of effects on donor and acceptor sides, indicate that the acceptor side is preferentially degraded by trypsin in the dark. Tryptic digestion during illumination generally resulted in a marked strengthening and acceleration of the effects seen already during dark treatment, but new effects were also detected in gel electrophoretic peptide patterns, in optical spectra, and in the kinetic measurements. Optical kinetic measurements revealed that the donor side of the reaction centers became susceptible to modification by trypsin during illumination as seen in the value of the binding constant for soluble cytochrome c2 which increased by a factor of 2, whereas it was much less affected after trypsination of reaction centers in the dark. The influence of illumination on the rate and mode by which trypsin acts on reaction centers indicates that changes in the protein conformation follow charge separation.(ABSTRACT TRUNCATED AT 250 WORDS)

Light-induced voltage changes associated with electron and proton transfer in photosystem II core complexes reconstituted in phospholipid monolayers

We have measured light-induced voltage changes (electrogenic events) in photosystem II (PSII) core complexes oriented in phospholipid monolayers. These events are compared to those measured in the functionally and structurally closely related reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides. In both systems we observed a rapid (< 100 ns) light-induced increase in voltage associated with charge separation. In PSII reaction centers it was followed by a decrease (decay) of approximately 14% of the charge-separation voltage and a time constant of approximately 500 microseconds. In bacterial reaction centers this decay was approximately 9% of the charge-separation voltage, and the time constant was approximately 200 microseconds. The decay was presumably associated with a structural change. In bacterial reaction centers, in the presence of excess water-soluble cytochrome c2+, it was followed by a slower increase of approximately 30% of the charge-separation voltage, associated with electron transfer from the cytochrome to the oxidized donor, P+. In PSII reaction centers, after the decay the voltage remained on the same level for > or = 0.5 s. In PSII reaction centers the electron transfer Q-AQB-->QA Q-B contributed with an electrogenicity of < or = 5% of that of the charge separation. In bacterial reaction centers this electrogenicity was < or = 2% of the charge-separation electrogenicity. Proton transfer to Q2-B in PSII reaction centers contributed with approximately 5% of the charge-separation voltage, which is approximately a factor of three smaller than that observed in bacterial reaction centers.

Ion Channels in Arabidopsis Plasma Membrane : Transport Characteristics and Involvement in Light-Induced Voltage Changes

White light (25 watts per square meter) induced an increase in plasma membrane K(+)-channel activity and a 30- to 70-millivolt transient membrane depolarization (completed in 2-3 minutes) in Arabidopsis thaliana leaf mesophyll cells. Transport characteristics of three types of ion channels in the plasma membrane were determined using inside-out patches. With 220 millimolar K(+) on the cytoplasmic side of the patch and 50 millimolar K(+) in the pipette, (220/50 K), the open-channel current-voltage curves of these channels were sigmoidal and consistent with an enzyme kinetic model. Two channel types were selective for K(+) over Na(+) and Cl(-). One (named PKC1) had a maximum conductance (G(max)) of 44 picosiemens at a membrane voltage (V(m)) of -65 mV in (220/50 K) and is stimulated by light. The other (PKC2) had G(max) = 66 picosiemens at V(m) = 60 millivolts in (220/50 K). The third channel type (PCC1) transported K(+) and Na(+) about equally well but not Cl(-). It had G(max) = 109 picosiemens at V(m) = 55 millivolts in (250/50 K) with 10 millimolar Ca(2+) on the cytoplasmic side. Reducing Ca(2+) to 0.1 millimolar increased PCC1 open-channel currents by approximately 50% in a voltage-independent manner. Averaged over time, PKC2 and PCC1 currents strongly outward rectified and PKC1 currents did so weakly. Reductants (1 millimolar dithiothreitol or 10 millimolar beta-mercaptoethanol) added to the cytoplasmic side of an excised patch increased the open probability of all three channel types.

Photovoltaic effects in the Oriental hornet, Vespa orientalis

Photoelectric properties of the Oriental hornet ( Vespa orientalis) cuticle and the pupal silk cocoon were studied. Exposure of a part of the cuticle to light caused a sharp increase in voltage, when measured between the illuminated and the dark part of the cuticle. The direction of this voltage was reversed if the other part of the cuticle was illuminated. This voltage was found to be linearly dependent on the intensity of the incident light for relatively low light intensities of a few mW/cm 2. However, this light-induced voltage was much higher if the light beam was directed at the back part of the cuticle strip than in the case where the front part of the cuticle strip was illuminated by the same light beam. The spectral dependence of this effect was also investigated and the maximum of the relative quantum efficiency was found in the spectral range of 360–380 nm. It appears that the cuticle might act as a biological solar cell. The dependence of photo current on the exposure time to light, as well as on environmental conditions, such as temperature and relative humidity was investigated in the pupal silk cocoon. Our results indicate that the cuticle as well as the cocoon act as convertors of light to electrial energy. At present we do not know the absolute efficiency of this conversion process, nor the biological applications of this effect.

Biophysical processes in invertebrate photoreceptors: recent progress and a critical overview based on Limulus photoreceptors

Limulus ventral nerve photoreceptor, a classical preparation for the study the phototransduction in invertebrate eyes, seems to have a very complex mechanism to transform light energy into a physiological signal. Although the main function of the photoreceptor is to change the membrane conductance according to the illumination, the cell has voltage-activated conductances as well. The voltage-gated conductances are matched to the light-activated ones in the sense that they make the function of the cell more efficient. The complex mechanism of phototransduction and the presence of four different voltage-gated conductance in Limulus ventral nerve photoreceptors indicate that these cells are far less differentiated than the photoreceptor cells of vertebrates. Indications accumulated in recent years support the view that the ventral photoreceptor of Limulus has different light-activated macroscopic current components, ion channels and terminal transmitters. After conclusions from macroscopic current measurements (Payne, 1986; Payne et al. 1986 a, b), direct evidence was presented by single-channel (Nagy & Stieve, 1990 a, b; Nagy, 1990 a, b) and macroscopic current measurements (Deckert et al. 1991 a, b) for three different light-activated conductances. It has been shown that two of these conductances are stimulated by two different excitation mechanisms. The two mechanisms, having different kinetics, release probably two different transmitters. One of them might be the cGMP (Johnson et al. 1986), the other one the calcium ion (Payne et al. 1986 a, b). However, the biochemical processes which link the rhodopsin molecules and the ion channels are not known. The unknown chemical details of the phototransduction result in a delay for the mathematical description of the biophysical mechanisms. More biochemical details are known about the adaptation mechanism. It was found that inositol 1,4,5-trisphosphate is a messenger for the release of calcium ions from the intracellular stores and that calcium ions are the messengers for adaptation (Payne et al. 1986 b; Payne & Fein, 1987). Concerning the mechanism of calcium release, it was revealed that a negative feedback acts on the enzyme cascade to regulate the internal calcium level and to protect the stores against complete emptying (Payne et al. 1988, 1990). Calcium ions also play an important role in the excitation mechanism. (a) In [Ca2+]i-depleted cells the light-induced current was increased after intracellular Ca2+ injection, suggesting that calcium is necessary for the transduction mechanism (Bolsover & Brown, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)

Microwave Performance of an Optically Controlled AIGaAs/GaAs High Electron Mobility Transistor and GaAs MESFET

This paper presents for the first time extensive experimental results which show the light-induced voltage, the increase in the drain current, the RF gain, and the change in the microwave scattering parameters of an AIGaAs/GaAs high electron mobility transistor (HEMT) under optical illumination of photon energy equal to or greater than the semiconductor band gap. From the de-embedded device scattering parameters, the change in the small-signal, lumped-element equivalent circuit model is computed. These results are compared and contrasted with those of a GaAs MESFET. Lastly, three potential applications of an optically controlled AIGaAs/GaAs HEMT are demonstrated: microwave amplifier gain control, detection, and mixing with an optically coupled local oscillator signal. Preliminary experiments on the optical control of a monolithic GaAs distributed amplifier are also presented. Measurements on a HEMT show that the tight-induced voltage is 0.57 V and the increase in the drain current is 5 mA under 1.7 mW optical power at 0.83 µm. Further, the increase in magnitude of S/sub 21/ with illumination ranges from 0.5 to 2.0 dB as V/sub gs/ takes discrete values between -0.5 and -0.95 V. However, the phase of S/sub 21/ is insensitive to optical illumination. The HEMT as a photodetector has an external quantum efficiency greater than 500 percent and a dark noise current of 27 pA//spl radic/Hz. Lastly, computations show that the response speed of the HEMT to an intensity-modulated optical signal is on the order of 11 ps.

Analysis of Optically Controlled Microwave/Millimeter-Wave Device Structures

Light-induced voltage and the change in the source-to-drain channel current under optical illumination higher than the semiconductor band gap for GaAs MESFET, InP MESFET, Al/sub 0.3/Ga/sub 0.7/As/GaAs high electron mobility transistor (HEMT), and GaAs permeable base transistor (PBT) are analytically obtained. The GaAs PBT and GaAs MESFET have higher sensitivity than the InP MESFET. However, the Al/sub 0.3/Ga/sub 0.7/As/ GaAs HEMT is observed to have the highest sensitivity. Variations in the small-signal parameters, such as channel conductance, gate-to-source capacitance, and transconductance, as well as transient parameters, such as switching time and power-delay product, of GaAs MESFET with illumination are computed. The computed capacitance and transconductance are compared with the experimentally obtained values and are found to be in fair agreement. Based on these results, the design considerations for an optically controlled MESFET switch are discussed. Finally, variation in device parameter due to optical illumination and its effect on the cutoff frequencies f/sub T/ and f/sub max/ are also investigated.

Electric-field dependence of the quantum yield in reaction centers of photosynthetic bacteria

Multilayer Langmuir-Blodgett films of reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides have been fabricated with partial net orientation. The films showed substantial electrical response under pulsed illumination. From measurements of the light-induced voltage generated across the Langmuir-Blodgett film, we have succeeded in quantitating the electric-field dependence of the quantum yield of charge separation in photosynthesis. The results presented here are compared with our previous determination of the field effect on quantum yield, in which flash-activated charge separation as a function of the applied field was assayed by the extent of bacteriochlorophyll dimer, (BChl)2, oxidation measured optically at 860 nm. The two methods provided consistent dependencies of quantum yield on applied electric field. Analysis of the data reveals that the quantum yield of (BChl)⨥2BPhQ⨪A formation decreases from a value of 0.96 at zero applied field to about 0.75 for a field of 120 mV/nm vectorially directed to hinder light-activated electron transfer. For oppositely applied fields, the quantum yield saturates at unity. The source of the effects is considered to reside in the electric field dependence of the free-energy difference between the energy levels that are involved in the initial charge separation between the (BChl)2 in the first singlet excited state, (BChl)∗2, through the bacteriopheophytin, BPh, to the primary ubiquinone, QA. Possible contributions to the field-induced loss of quantum yield of (BChl)⨥2BPhQ⨪A formation are: (1) a decrease in the free-energy gap between the states (BChl)∗2 and (BChl)⨥2BPh⨪QA, leading to an increased rate of decay via the excited singlet state back to the ground state; (2) a stimulated return from (BChl)⨥2BPh⨪QA directly or via the (BChl)2 triplet state to the ground state and (3) an impeded electron transfer from (BChl)⨥2BPh⨪QA to (BChl)⨥2BPhQ⨪A. These possibilities are discussed. Correlation of the electrical response with measurements of the photo-induced absorbance change allows determination of the projection of the electron-transfer distance on the normal to the plane of the film, which is in good agreement with previous measurements using different techniques.

Fluoride blocks an inactivation step of transduction in a locust photoreceptor.

1. Photoreceptors in a superfused retina of a locust compound eye are treated with saline containing 10 mM-NaF, while their intracellular resting potential and responses are recorded using glass micropipettes.2. Treatment for two minutes with 10 mM-NaF, followed by a series of brief, bright flashes of light, results in an irreversible, noisy depolarization of approximately 10 mV. The final, stable level of depolarization is reached through the summed effect of each of the noisy, depolarizing afterpotentials that follow every response of the cell to a light flash. If kept in darkness after treatment with NaF, the noisy depolarization still develops, but more gradually, over a period of 5 min.3. The voltage noise induced by NaF mimics light-induced voltage noise when the two are compared at mean depolarizations of more than 15 mV. At very small depolarizations, however, fluoride-induced noise cannot be resolved into the large discrete events (bumps) that are typical of the response of a dark-adapted photoreceptor to a single photon.4. The complete replacement of the superfusate sodium by choline reversibly reduces the fluoride-induced noise and depolarization to the same extent as it does the light-induced noise and depolarization of an illuminated cell.5. Increasing the superfusate calcium concentration from 0.5 to 10 mM also reversibly reduces fluoride-induced noise and depolarization to the same extent as it does light-induced noise and depolarization. This action of calcium is accompanied by an increase in a cell's input resistance which opposes the reduction caused by light or fluoride treatment.6. The results confirm the proposal (Payne, 1981) that anionic metabolic inhibitors cause spontaneous activity in sodium channels that are normally opened by light. A model is proposed in which fluoride acts by blocking the inactivation of a late stage in the transduction mechanism.

Voltage noise accompanying chemically-induced depolarization of insect photoreceptors

Intracellular potentials are recorded from photoreceptors in a superfused preparation of the retina of a locust compound eye. Chloral hydrate and alkyl alcohols induce a rapid, superfusing reversible depolarization of these photoreceptors when dissolved in the saline. Analysis of voltage noise accompanying depolarization by chloral hydrate suggests that depolarizing ionic pathways are opened briefly and randomly in time in the photoreceptor membranes. This conclusion is supported by measurements of the cell resistance and of voltage noise amplitude as a function of membrane potential. Replacement of superfusate sodium by choline reversibly reduces the effects of chloral hydrate, suggesting that the ionic pathways opened are permeable by sodium. The voltage noise induced by chloral hydrate is compared to that during depolarization by steady illumination of the same cell. As the illumination intensity is increased, the amplitude and the shape of the power spectrum of light-induced voltage noise approach those of drug-induced noise at the same depolarization level. The possibility that these phenomena represent alterations in the mechanism of phototransduction is discussed.

Light-induced voltage noise in the photoreceptor of Drosophila melanogaster.

The Drosophila photoreceptor potential is thought to be composed of discrete unit potentials called bumps. The steady-state receptor potential and the accompanying voltage fluctuations were recorded intracellularly under steady illumination. The occurrence rate, effective amplitude, and duration of the bumps were deduced by assuming a shot noise model. Over a wide range of light intensity, the duration of bumps remained essentially constant (25-30 ms). Below the saturation intensity for the receptor potential, the bump rate was roughly proportional to the intensity, and the adjustment of bumps to smaller size at higher intensity was mainly responsible for the nonlinear behavior of the receptor potential. The reduction in size of bumps at increasing light intensity was found to be due mainly to the diminishing magnitude of the bump current, and not to some other secondary effects. The bump rate saturated at about 3 x 105-106 events/s.

Morphology and responses to light of the somata, axons, and terminal regions of individual photoreceptors of the giant barnacle.

1. The median eye of the giant barnacle, B. nubilus, comprises four large photoreceptor neurones which are visible under the dissecting microscope for almost their entire length. We have studied the structure of, and the responses to light recorded in, the somata, axons, and terminal regions of these neurones.2. The photoreceptor somata, each 40-70 mum in diameter, extend numerous light-sensitive dendritic processes whose membranes form rhabdomeric microvilli. Recordings from the soma show that dim light evokes a steady, noisy depolarization; brighter light elicits a transient depolarization which decays to a maintained plateau, followed by a hyperpolarization when the light is turned off.3. Light-induced voltage changes spread decrementally along the photoreceptor axons, which average 10 mm in length and 25 mum in diameter. In distal parts of the axon, near the presynaptic terminals, depolarizations and hyperpolarizations can be as large as 50% or more of their values in the soma.4. There is no demonstrable electrical coupling between photoreceptor neurones as shown by simultaneous recordings from two receptor somata or axons.5. Each photoreceptor axon enters the mid line commissure of the supraoesophageal ganglion, bifurcates, and arborizes in a restricted zone of neuropil in each hemiganglion. The large size of the terminal processes of these neurones and their characteristic cytoplasmic inclusions enable one to trace them with the electron microscope as they branch in the neuropil.6. The terminal processes subdivide and end in 1-3 mum diameter branches which are the sites of apparently chemical synapses. Vesicle-containing, presynaptic loci on these processes of the receptor cell are invariably apposed to two post-synaptic processes from cells as yet unidentified.